RECEIVER AND RECEIVING METHOD FOR HIERARCHICAL MODULATION IN SINGLE FREQUENCY
NETWORKS
Field of Invention
The present invention relates to receivers for receiving data via Orthogonal Frequency
Division Multiplexed (OFDM) symbols in which the data is provided from a plurality of
different data pipes.
Embodiments of the present invention find application in receiving data
communicated using OFDM symbols which are transmitted using communication systems
which comprise a plurality of base stations disposed throughout a geographical area. In some
embodiments the communication system is arranged to broadcast video, audio or data.
Background of the Invention
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique
which has found much favour in communication systems, such as for example those designed
to operate in accordance with the first and second generation Digital Video Broadcasting
terrestrial standards (DVB-T/T2) and is also being proposed for fourth generation mobile
communication systems which are also known as Long Term Evolution (LTE). OFDM can
be generally described as providing K narrow band sub-carriers (where K is an integer) which
are modulated in parallel, each sub-carrier communicating a modulated data symbol such as
Quadrature Amplitude Modulated (QAM) modulation symbol or Quaternary Phase-shift
Keying (QPSK) modulation symbol. The modulation of the sub-carriers is formed in the
frequency domain and transformed into the time domain for transmission. Since the data
symbols are communicated in parallel on the sub-carriers, the same modulated symbols may
be communicated on each sub-carrier for an extended period, which can be longer than the
coherence time of the radio channel. The sub-carriers are modulated in parallel
contemporaneously, so that in combination the modulated carriers form an OFDM symbol.
The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been
modulated contemporaneously with a different modulation symbol.
In the Next Generation for Hand held (NGH) television system it has been proposed to
use OFDM to transmit television signals from base stations disposed throughout a
geographical area. In some examples the NGH system will form a network in which a
plurality of base stations communicate OFDM symbols contemporaneously on the same
carrier frequency thereby forming a so-called single frequency network. As a result of some
of the properties of OFDM, a receiver may receive the OFDM signals from two or more
different base stations which can then be combined in the receiver to improve the integrity of
the communicated data.
Whilst a single frequency network has advantages in terms of operation and improved
integrity of the communicated data, it also suffers a disadvantage if data local to a part of the
geographical area is required to be communicated. For example, it is well known in the
United Kingdom that the national carrier, the BBC, broadcasts television news throughout the
entire national network but then switches, at certain times, to "local news" in which a local
news programme is transmitted which is specifically related to a local area within the national
network. However, the United Kingdom operates a multi-frequency DVB-T system so that
the insertion of local news or local content of any sort is a trivial matter because the different
regions transmit DVB-T television signals on different frequencies and so television receivers
simply tune to an appropriate carrier frequency for the region without interference from other
regions. However, providing an arrangement to insert data locally in a single frequency
network presents a technical problem.
A known technique for providing a hierarchical or multi-layer modulation scheme in a
single frequency OFDM nelw k is disclused i US 2008/0159186. The ier r lii al
modulation scheme provides a plurality of modulation layers which can be used to
communicate data from different data sources or pipes contemporaneously.
Summary of Invention
According to the present invention there is provided a receiver for receiving and
recovering local service data symbols from first Orthogonal Frequency Division Multiplexed
(OFDM) symbols in the presence of second OFDM symbols. The first and the second OFDM
symbols include a plurality of sub-carrier symbols formed in the frequency domain, the
second OFDM symbols carrying national broadcast data symbols and modulated on to the
sub-carriers of the second OFDM symbols using a first modulation scheme to form national
broadcast modulation symbols, and the first OFDM symbols carrying the national broadcast
data symbols and the local service data symbols and modulated on to the sub-carriers of the
first OFDM symbols using a second modulation scheme. The first and the second OFDM
symbols both include the same pilot sub-carrier symbols and the first OFDM symbols include
local pilot symbols. The receiver comprises a tuner which is arranged in operation to detect a
radio frequency signal representing a combination of the first and the second OFDM symbols
and to form a received base band signal representing the combined first and second OFDM
symbols, an OFDM detector which is arranged in operation to recover modulation symbols
carrying the local service data symbols from the data bearing sub-earners of the first OFDM
symbols, and a de-modulator arranged in operation to generate an estimate of the local service
data symbols from the modulation symbols carrying the local service data symbols. The
OFDM detector includes an equaliser for recovering the local service data symbols of the
second modulation scheme by
generating an estimate of a combined channel Hn (z) +Htfz)]) via which the first and
second OFDM symbols have passed using the pilot sub-carrier symbols of the first and
second OFDM symbols;
generating an estimate of the national broadcast modulation symbols from the
modulated data bearing sub-carriers of the first modulation scheme from the second OFDM
symbols ( S{z) ) ;
generating an estimate of a convolution of the combined channel and the national
broadcast modulation symbols ( S(z)[H (z) + Hl (z)] );
generating an estimate of a component of the received base band signal representing
the local service modulation symbols of the first OFDM symbols by subtracting from the
received signal the generated estimate of the national broadcast modulation symbols
convolved with the estimate of the combined channel to form an intermediate result
(D(z)H, (z) R(z) - S(z)[H„ (z) + H,(z)] );
generating an estimate of a channel via which the first OFDM symbols were received
using the local pilot symbols (H,(z) ); and
generating an estimate of local service data symbols from a combination of the
estimate of the component of the received signal representing the modulation symbols
carrying the local service data and the estimate of the channel via which the first OFDM
j ) - (z)[H„(z) + H, (z symbols were received ( (z) « " - ) .
H,(z)
According to the arrangement disclosed in US 2008/0159186 published 3 July 2008, a
single carrier frequency OFDM network is provided with a facility for communicating data
from different pipes contemporaneously by using two related modulations schemes to form a
plurality of different modulation "layers". As will be explained shortly, a first modulation
scheme is selected for communicating data from a first data pipe and a second modulation
scheme related to the first modulation scheme is selected for communicating data according to
the first and a second communications pipes. The second modulation scheme comprises an
increased number of constellation points in the complex plane than the first modulation
scheme. The data from the first pipe maybe from a national broadcast, whereas the data from
the second pipe is from a local broadcast signal, which is directed to an area which is a subset
of the area of the national broadcast signal.
According to example embodiments of the present invention, a receiver is arranged to
recover data symbols according to the local service using an equaliser, which can compute an
estimate of the local service modulation symbols from OFDM symbols which are carrying
both the local service and the national broadcast service carried using data sub-carriers
modulated with the second modulation scheme in the presence of OFDM symbols which are
only carrying the national broadcast data symbols modulated with the first modulation
scheme. This is achieved by making a first coarse estimate of the first modulation symbols
and then convolving this estimate with an estimate of the channel through which the first and
second OFDM symbols have passed. The estimate of the channel is made using the pilot subcarriers
as these coincide in the first and the second OFDM symbols. Subtracting the result of
the convolution from the received signal and dividing by the estimate of the channel through
which only the second OFDM symbol has passed provides an estimate of the modulation
symbols according to the second modulation scheme, which are carrying the local service data
symbols. The channel through which only the second OFDM symbol has passed can be
estimated using the local service insertion pilots carried on known sub-carriers of the second
OFDM symbol.
The receiver can be therefore arranged to detect and recover data from OFDM
symbols communicated by a communication system which is arranged such that one or more
base stations from a plurality of base stations which form a communications network are
selected to transmit local content via OFDM symbols which have sub-carriers modulated in
accordance with the second modulation scheme. This is because the first modulation scheme
forms a sub-set of constellation points in the complex plane of the second modulation scheme,
which can be thought of as a more coarse version of the second modulation scheme, so that
differentiation between constellation points of the first modulation symbols in the complex
plane allows the data from the national broadcast signal to be more easily recovered.
Furthermore, because other base stations may not be communicating the local insertion pipe
data, the receiver, within the geographical area in which these other base stations are
disposed, will still be able to detect the data from the national broadcast signal. Accordingly,
an effective and efficient way of inserting local content in a single frequency network is
provided.
n some examples the OFDM detector includes an equaliser which is arranged in
operation to generate the estimate of the local service data symbols from the combination of
the estimate of the component of the received base band signal representing the modulation
symbols carrying the local service data symbols and the estimate of the channel via which the
first OFDM symbols were received by dividing the estimate of the component of the received
signal representing the local service data symbols by the estimate of the local channel. An
estimate of each of the modulation symbols carrying the local service data symbols from the
first OFDM symbol is thereby recovered, and by de-mapping the modulation symbols
carrying the local service data symbols the estimate of the local service data symbols is
generated. However, whilst this provides a simple and effective equalisation technique for at
least reducing or cancelling the effects of he channel, in a multi-path fading channel,
frequency nulls can be produced in the channel, which can result in noise amplification or
cause a modulation symbol o produce an amplified value wl ic is equal to a maximum
possible real and imaginary sample components, thereby losing data which such modulation
symbols carry.
In other examples, the equaliser includes a local service equaliser/demapper which is
arranged in operation to generate the estimate of the local service data symbols from the
combination of the estimate of the component of the received base band signal representing
the modulation symbols carrying the local service modulation symbols and the estimate of the
channel via which the first OFDM symbols were received. This is achieved by calculating a
log likelihood ratio for each of the local service data symbols from the estimate of the
component of the received signal representing the modulation symbols carrying the local
service data symbols and the estimate of the local channel, and estimating the local service
data symbols from the log likelihood ratio calculations. As such, by using a log likelihood
ratio calculation for detecting the local service data symbols from the first OFDM symbols
and the estimate of the local service channel, no division by the channel occurs in the
frequency domain. Accordingly, the local service data symbols as well as the national
broadcast data symbols can be recovered in a multi-path fading channel. ;
Correspondingly in other example embodiments the OFDM detector also includes an
equaliser/demapper, which calculates a log likelihood ratio for the national broadcast data
symbols from the components of the first and second OFDM representing the national
broadcast data symbols and the combined channel estimate through which the symbols were
received.
In some examples, the equaliser can be arranged to re-generate an estimate of the
national broadcast modulation symbols, by re-generating an estimate of the component of the
received base band signal representing modulation symbols of the first OFDM symbol
carrying local service data symbols by combining the estimate of the modulation symbols
representing the local service data symbols with the channel via which the first OFDM
symbols were received, generating an estimate of a component of the received base band
signal representing the national broadcast modulation symbols by subtracting the re-generated
estimate of the component of the received base band signal representing the modulation
symbols carrying the local service data symbols from the received base band signal, and
dividing by the combined channel. Furthermore, the re-generated estimate of the national
broadcast modulation symbols may be used to make a refined estimate of the local service
modulation symbols. The equaliser can therefore be arranged to generate a refined estimate
of the local service data symbols from the local service modulation symbols. Thus in a form
of turbo detection, the re-generated estimate of the national broadcast modulation symbols can
be used to generate a further refined estimate of the local service modulation symbols and the
process of detection further repeated to generate further refined estimates.
Various further aspects and features of the present invention are defined in the
appended claims and include a method of receiving.
Brief Description of Drawings
Embodiments of the present invention will now be described by way of example only
with reference to the accompanying drawings in which like parts are referred to using the
same numerical designations and in which:
Figure 1 is a schematic representation of a plurality of base stations which form a
single frequency network for broadcasting for example video signals which may form part of
a Next Generation Hand-held (NGH) TV broadcasting system;
Figure 2 is a schematic block diagram of an example transmitter according to the prior
art;
Figure 3a is a schematic representation of a complex plane providing an illustration of
signal constellation points for a first modulation scheme of QPSK; and Figure 3b is a
schematic representation of a complex plane providing an illustration of signal constellation
points for a second modulation scheme of 16QAM according to the prior art;
Figure 4 is a schematic block diagram of part of a transmitter used in one or more of
the base stations shown in Figure 1 according to the present technique which supports SISO
or MISO;
Figure 5 is a schematic block diagram of an example modulator which forms part of
the transmitter shown in Figure 4;
Figure 6 is an illustrative representation of two neighbouring base stations forming
two cells A and B which are using a first modulation scheme of 16QAM and a second
modulation scheme of 64QAM respectively;
Figure 7 is a schematic representation showing the effects on the constellation points
as received by a mobile device at three different positions X, Y, Z between the two base
stations A and B of Figure 6;
Figure 8 is an illustrative representation of constellation points in a complex plane for
a first modulation scheme of 16QAM superimposed on a second modulation scheme of
64QAM;
Figure 9a is an illustrative representation of a cluster of four cells served by four base
stations according to the present technique; Figure 9b is a graphical representation of a plot of
frequency with respect to time providing an illustration of a time division multiplexed frame
structure; and Figure 9c is an illustrative representation of a pattern of cell clusters according
to the present technique;
Figure 10 is an illustrative representation of two neighbouring base stations forming
two cells A and B which are using a first modulation scheme of 16QAM and a second
modulation scheme of 64QAM respectively, and a mobile receiver which may be arranged to
recover local service insertion data in the presence of signals from both the first modulation
scheme and the second modulation scheme the signal from cell B transiting a channel impulse
response h (t) and the signal from cell A transiting a channel impulse response (t);
Figure 11a is a schematic representation of a complex plane providing an illustration
of signal constellation points for a first modulation scheme of QPSK; and Figure l ib is a
schematic representation of a complex plane providing an illustration of signal constellation
points for a second modulation scheme of 16QAM wherein reception is without noise and
perfect channel estimation;
Figure 12a is a schematic representation of a complex plane providing an illustration
of signal constellation points for a first modulation scheme of QPSK, when received in the
presence of the second modulation scheme; but with the signal from each cell transiting
through channels of different channel impulse responses and Figure 12b provides a
corresponding representation of the same signal after equalisation using a conventional
equaliser with perfect channel estimation;
Figure 13a is a schematic representation of a complex plane providing an illustration
of signal constellation points after subtracting Sest(z)[(Hi(z)+H (z)] and Figure 16b is the
result of dividing the signal represented in Figure 16a by ¾() assuming perfect channel
estimation in which the local service insertion channel r¾(z) is known exactly;
Figure 14a is an illustrative representation of narrow band carriers of an OFDM
symbol carrying the national broadcast signal; Figure 14b is an illustrative representation of
narruw band carriers of an OFDM symbol carrying both the national signal and the local
service insertion signal; and Figure 14c is an illustrative representation of narrow band
carriers of an OFDM symbol carrying the local service insertion signal, but adapted in
accordance with the present technique to include local pilots;
Figure 15 is a schematic block diagram of a transmitter used in one or more of the
base stations according to the present technique, which supports ;
Figure 16 is a graphical plot of bit error rate with respect to signal to noise ratio for
example of a low density parity check (LDPC) coded OFDM transmitter-receiver chain, with
error correction encoding of rate ½, 3/5, 2/3 and ¾, a first modulation scheme of 16QAM, a
second modulation scheme of 64QAM and in which the receiver is considered to be located
within coverage area of cell A and to receive OFDM symbols with 99% of the signal power
from base station A and 1% from base station B with the signal from B arriving at the
receiver 4.375us after the signal from base station A as illustrated by the example diagram
shown in Figure 6;
Figure 17 is a graphical plot of bit error rate with respect to signal to noise ratio for the
example of a LDPC coded OFDM transmitter-receiver chain, with error correction encoding
of rate ½, 3/5, 2/3 and ¾, a first modulation scheme of 16QAM, a second modulation scheme
of 64QAM and in which the receiver is considered to be located within coverage area of cell
A and to receive OFDM symbols with 80% of the signal power from base station A and 20%
from base station B with the signal from B arriving at the receiver 2.2after the signal from
base station A as illustrated by the example diagram shown in Figure 6;
Figure 8 is a graphical plot of bit error rate with respect to signal to noise ratio for
example of a LDPC coded OFDM transmitter-receiver chain, with error correction encoding
of rate ½, 3/5, 2/3 and ¾, a first modulation scheme of 16QAM, a second modulation scheme
of 64QAM and in which the receiver is considered to be located within coverage area of cell
A and to receive OFDM symbols with 99% of signal power from base station A and 1% from
base station B with zero delay between the signal times of arrival from the two cells
illustrated by the example diagram shown in Figure 6;
Figure 19 is a graphical plot of bit error rate with respect to signal to noise ratio for
example of a LDPC coded OFDM transmitter-receiver chain, with error correction encoding
of rate ½, 3/5, 2/3 and ¾, a first modulation scheme of 16QAM, a second modulation scheme
of 64QAM and in which the receiver is considered to be located within coverage area of cell
A and to receive OFDM symbols with 60% of signal power from base station A and 40%
f om base station B with zero delay between the signal times of arrival from the two cells
illustrated by the example diagram shown in Figure 6;
Figure 20 is a graphical plot of bit error rate with respect to signal to noise ratio for
example of a LDPC coded OFDM transmitter-receiver chain, with error correction encoding
of rate ½, 3/5, 2/3 and ¾, a first modulation scheme of 16QAM, a second modulation scheme
of 64QAM and in which the receiver is considered to be located within coverage area of cell
A and to receive OFDM symbols with 50% signal power from base station A and 50% from
base station B with zero delay between the signal times of arrival from the two cells
illustrated by the example diagram shown in Figure 6;
Figure 2 1 is a graphical plot of bit error rate with respect to signal to noise ratio for
example of a LDPC coded OFDM transmitter-receiver chain, with error correction encoding
of rate ½, 3/5 and 2/3, a first modulation scheme of 16QAM, a second modulation scheme of
64QAM and which the receiver is considered to be located within coverage area of cell B
and to receive OFDM symbols with 10% of signal power from base station A and 90% from
base station B with the signal from A arriving at the receiver .2 after the signal from base
station B as illustrated by the example diagram shown in Figure 6;
Figure 22 is a schematic block diagram of a receiver according to an embodiment of
the present technique;
Figure 23 is a schematic block diagram of a Physical Layer Pipe (PLP) processor
which appears in the receiver shown in Figure 22;
Figure 24a is a schematic block diagram illustrating a first example of an OFDM
detector adapted in accordance with a further example embodiment of the present invention;
Figure 24b is a schematic block diagram of an equaliser of national broadcast modulation
symbols of the OFDM detector shown in Figure 24a; and Figure 24c is a schematic block
diagram of an equaliser of local service modulation symbols of l c OFDM d tc lo show in
Figure 24a;
Figure 25 is a schematic block diagram of a second example of an OFDM detector
forming part of the receiver of Figure 22 adapted in accordance with a further example
embodiment of the present invention;
Figure 26 is a schematic block diagram of an equaliser of national broadcast
modulation symbols of the OFDM detector shown in Figure 25;
Figure 27 is a schematic block diagram of an equaliser/demapper for recovering local
service data symbols of the OFDM detector shown in Figure 25;
Figure 28 is a schematic representation of a signal constellation diagram for 16QAM
showing an example mapping of data bits to modulation symbols;
Figure 29 is a schematic block diagram of an equaliser/demapper for recovering
national broadcast data symbols of the OFDM detector shown in Figure 25; and
Figure 30 is a flow diagram illustrating an example operation of a process required to
equalise a single frequency signal which includes components from a first and a second
modulation scheme.
Description of Example Embodiments
As set out above embodiments of the present invention seek to provide, in one
application, an arrangement in which local content can be transmitted within a single
frequency network whilst allowing other parts of the network still to receive a primary
broadcast signal or their own local content. One example illustration is where local content is
required to be broadcast contemporaneously with a national broadcast television programme.
Figure 1 provides an example illustration of a network of base stations BS which are
transmitting, via transmit antennas 1, a signal in accordance with a commonly modulated
OFDM signal. The base stations BS are disposed throughout a geographical area within a
boundary 2, which may be, in one example, a national boundary. As explained above in a
single frequency network configuration the base stations BS are all broadcasting the same
OFDM signal at the same time on the same frequency. Mobile devices M may receive the
OFDM signal from any of the base stations. More particularly, the mobile devices M may
also receive the same signal from other base stations because the signal is simultaneously
broadcast from all of the base stations within the area identified by boundary 2. This socalled
transmit diversity arrangement is typical of a single frequency OFDM network. As part
of the detection of the OFDM signals in a receiver which is recovering data from OFDM
symbols, energy from the transmitted OFDM symbols which is received for each symbol
from different sources is combined in the detection process. Thus transmitting the same
signal from different base stations can improve the likelihood of correctly recovering the data
communicated by the OFDM symbols, provided that any component of the received OFDM
symbol or echo of that OFDM symbol falls within a total guard interval period allowed for the
network deployment.
As shown in Figure 1, in some examples the base stations BS may be controlled by
one or more base station controllers BSC, which may control the operation of the base
stations. In some examples the base station controllers BSC may control one or more of the
base stations within a part of the network associated with a geographical area. In other
examples the base station controllers BSC may control one or more clusters of base stations
so that the transmission of local content is arranged with respect to a time division
multiplexed frames.
As mentioned above, the area identified by the boundary 2 could correspond to a
national boundary so that the network of base stations is a national network. As such, in one
example the television signals broadcast nationally are each transmitted from the base stations
BS shown in Figure 1. However embodiments of the present technique are aimed at
addressing a technical problem associated with providing an arrangement for transmitting
locally broadcast signals from some of the base stations shown in Figure 1 but not others. An
example of such an arrangement might be if local broadcast news or traffic news which is
associated with a particular area is broadcast from some of the base stations but not others. In
a multi-frequency network this is trivial, because the signals for the local broadcast maybe
transmitted from different transmitters on different frequencies and therefore detected
independently of what is broadcast from other base stations. However in a single frequency
network a technique must be provided in order to allow for local service insertion of content
for some of the base stations but not others or different local content at different base stations.
As mentioned above prior art document US 2008/0159186 discloses a technique for
combining two modulation schemes to form a modulation layer for each of a plurality of data
sources. A transmitter which is implementing such an arrangement is shown in Figure 2. In
Figure 2 data is fed from a first data pipe 4 and second data pipe 6 to a modulator 8, which
modulates the data onto the sub-carriers to form an OFDM symbol. The modulation is
performed in such a way that the data from the first data pipe 4 can be detected separately
from e dele i i of he data fro both the fi st a d the second data pipes 4, 6. An OFDM
symbol former 10 then forms the OFDM symbol in the frequency domain as provided at the
output of the modulator 8 and converts the frequency domain OFDM symbol into the time
domain by performing an inverse Fourier transform in accordance with a conventional
operation of an OFDM modulator/transmitter. The time domain OFDM symbols are then fed
to a radio frequency modulator 12 which up converts the OFDM symbols onto a radio
frequency carrier signal so that the OFDM signal may be transmitted from an antenna 14.
The technique disclosed in US 2008/0159186 is illustrated in Figures 3a and 3b.
Figures 3a and 3b provide an illustration of signal constellation points in the complex plane
comprising in-phase I and Quadrature-phase Q components. The example signal constellation
points shown in Figure 3a is for QPSK, whereas the example shown in Figure 3b is for
16QAM. In accordance with the known technique for obtaining multi-layer modulation, data
from two sources is modulated onto the signal constellation points of a second modulation
scheme. The signal constellation points of the second modulation scheme represent the
possible modulation symbol values available for the modulation scheme. For the first
modulation scheme shown in Figure 3a, the signal constellation points for QPSK are provided
as small circles "o" 20. As such the bits from a source B that is provided from the source data
pipe 6 are mapped onto the signal constellation points as shown in Figure 3a, so that each
possible modulation symbol value represents two bits from the source bObl in conventional
manner using Grey coding for example.
The second modulation scheme shown in Figure 3b is 16QAM, which provides 16
possible signal constellation points 22 represented as "x". In addition to the modulation of the
signal by data from the first data pipe 6, which is shown as bObl a selection of one of the
constellation points from each of the four quadrants shown in Figure 3b also identifies one of
the four possible values for two bits from the second source data pipe 4 for the values aOal.
Thus detection of one of the signal points shown in Figure 3b will not only identify a value
for aOal, but also a value for bObl depending upon which of the four quadrants from which
the signal point is detected. Accordingly, a mutli-layer modulation scheme can be made.
Transmitter
Embodiments of the present technique provide an arrangement which utilises the
multi-layer modulation technique according to US 2008/0159186 to provide a local broadcast
service for local content whilst still allowing base stations in neighbouring areas to detect a
national broadcast signal.
A transmitter embodying the present technique, which might be used to insert local
content at one of the base stations shown in Figure 1 is shown in Figure 4. In Figure 4 a
plurality n of Physical Layer data Pipes (PLP) 30 are arranged to feed data for transmission to
a scheduler 34. A signalling data processing pipe 36 is also provided. Within each of the
pipes the data is received for a particular channel from an input 38 at a forward error
correction encoder 40 which is arranged to encode the data, for example, in accordance with a
Low Density Parity Check (LDPC) code. The encoded data symbols are then feed into an
interleaver 42 which interleaves the encoded data symbols in order to improve the
performance of the LDPC code used by the encoder 40.
The scheduler 34 then combines each of the modulation symbols from each of the data
pipes 30 as well as the signalling processing pipe 36 into data frames for mapping onto
OFDM symbols. The scheduled data is presented to a data slice processing unit 50, 51, 52
which includes a frequency interleaver 54, a local pilot generator 180, a modulator 182, an
optional MISO processing unit 184 and a pilot generator 56. The data slice processor arranges
the data for a given PLP in such a manner so that it will occupy only certain sub-carriers of
the OFDM symbol. The data output from the data slice processors 50, 51, 52 is then fed to a
Time Division Multiple Access (TDMA) framing unit 58. The output of the TDMA framing
unit 58 feeds an OFDM modulator 70 which generates the OFDM symbols in the time domain
which are then modulated onto a radio frequency carrier signal by an K modulator 72 and
then fed to an antenna for transmission 74.
As explained above, embodiments of the present invention provide a technique for
allowing for local content to be broadcast from one or more base stations within a local area
relating to a national area covered by the network shown in Figure 1. To this end, the
transmitter shown in Figure 4 also includes a local service insertion data slice processor 80
which includes a frequency interleaver 54 and a local pilot generator 180,. However, in
addition, according to the present technique, the modulator 44 shown in the data slice
processor 50 has a second input for receiving the data from the local service insertion data
slice processor 80. According to the present technique the modulator 44 modulates the local
service insertion data onto a related set of signal constellation points according to a second
modulation scheme. The signal constellation points of the second modulation scheme, which
is used for the local content as well as the primary data, are related to constellation points of
the first modulation scheme which is used for just communicating the primary data from the
PLP pipe n as will be explained with reference to Figures 5 and 6.
As shown in Figure 4 the modulator 44 has a first input 82 which receives data from
the data slice processor 50 and a second input 84 which receives data from the local service
insertion data slice processor 80. the following description the data from the data slice
processor 50, will be referred to as the first or primary data pipe. In one example the data
from the first data data slice processor 50 carries a national broadcast channel, which would
be communicated throughout the entire network of Figure 1.
The modulator 44 is shown in more detail in Figure 5. As shown in Figure 5 the data
from the local service insertion pipe 80 is fed from the second input 84 into a first data word
former 90. The data from the first data pipe is fed from the first input 82 into a second data
word former 92. The data from the first data pipe when received in the data word former 92
is arranged to form four groups of bits y0yly2y3 for mapping onto one of 16 possible values
of a 16QAM modulation symbol within a symbol selector 94. Similarly, the data word former
90 forms the data from the first data pipe 82 into data words comprising four bits y0yly2y3.
However, the data word former 90 also receives the data symbols from the local service
insertion pipe 80 and so appends two of the bits from the local service insertion data pipe 84
to the data bits from the first data pipe 82 to form a six bit data word y0yly2y3h0hl, which is
four bits y0yly2y3 from the symbol stream from the first data pipe 32 and two bits hOhl from
the local service insertion pipe 80, thus forming a six bit word for selecting one of 64 possible
modulation symbol values of 64QAM (2^ = 64).
A symbol selector 96 is arranged to receive the six bit word y0yly2y3h0hl and in
accordance with the value of that word select one of the 64 possible values of the 64QAM
modulation scheme to form at an output 96.1 a stream of 64QAM symbols. The respective
outputs from the symbol selectors 94, 96 are then fed to a switch unit 98 which also receives
on a control input 100 an indication as to when the local content received from the local
service insertion pipe 90 is present and is to be broadcast from the base station. If the local
service insertion data is to be broadcast from the base station then the switch 98 is arranged to
select the output 96.1 from the 64QAM symbol selector 96. If not then the switch is arranged
to select the output 94.1 from the 16QAM symbol selector 94. Modulation symbols are
therefore output from the modulator 44 for transmission on the OFDM symbols on an output
channel 102.
The control input 100 may provide, in some examples, a control signal which indicates
when local content is being transmitted from the local service insertion data slice processor
80. The control signal provided in the control input 100, may be generated from a base
station controller to which the transmitter within the base station is connected.
In other examples the signalling data processing pipe 36 may be arranged to
communicate via LI signalling data an indication to when the local service insertion pipe 80
is or will be transmitting the local data. Thus a receiver may recover may detect and recover
the LI signalling data and determine when or whether the local content is being or will be
transmitted. Alternatively, the receiver may be provided with a data providing a schedule of
when the local content data is to be transmitted, by some other means, such as by pre
programming the receiver.
Deployment of Base Stations
Figure 6 provides an example illustration of an arrangement which may be produced
within Figure 1 in which a first base station BS 110 may transmit data from the first data pipe
32 within a cell A, whereas a neighbouring base station BS 1 2 transmits data within a second
cell B, the transmitted data including data from the first data pipe 32 but also the local service
insertion data from the local service insertion pipe 80. Thus the base station 110 from the cell
A is transmitting an OFDM symbol with sub-carriers modulated using 6QAM whereas the
base station 112 from the cell B is transmitting the OFDM symbols by modulating subcarriers
with 64QAM. Thus as shown in Figure 6 as the bit ordering shows, the final two bits
hOhl are used to select a finer detail of a signal constellation point according to 64QAM
whereas the bits y0yly2y3 are used to select one of the 16QAM symbols in a coarser grid
within the complex plane.
As already explained, both of the base stations 110, 112, within the cells A and B will
be transmitting the OFDM symbols contemporaneously on the same frequency. As such a
receiver in a mobile terminal will receive a combined OFDM signal as if, in part, the signal
was being received via different paths in a multi-path environment. However, the OFDM
signal transmitted from base station 110 within cell A comprises OFDM symbols modulated
using the first modulation scheme 16QAM whereas the OFDM symbols transmitted from the
base station 112 within cell B will be modulated using the second modulation scheme
64QAM. At the receiver within the mobile terminal, a proportion of the total power with
which the OFDM symbols are received with the first modulation scheme and the second
modulation scheme will depend on the proximity of a mobile device M to each of the
transmitters within the cells A and B. Furthermore, the likelihood of correctly recovering the
data symbols from the first data pipe and the local service insertion pipe will depend o l e
extent to which the receiver can detect OFDM symbols according to the first modulation
scheme 16QAM transmitted from cell A or OFDM symbols according to 64QAM transmitted
from cell B in the presence of OFDM signals modulated with the second and the first
modulation schemes respectively.
As shown in Figure 7 three plots 120, 122, 124 of possible simulated signal
constellation values are shown for an example of 16QAM and 64QAM which are shown for
example in Figure 8 . The first left hand plot 120 provides a plot in the complex plane of
received modulation symbol values when the transmitters in the base stations 110, 112 of cells
A and B are transmitting OFDM symbols with sub-carriers modulated with 16QAM and
G4QAM modulation schemes respectively, because cell B i s transmitting local service
insertion data. The first plot 120 corresponds to a mobile device being at position X for
which it is assumed that 80% of the received signal power is from cell A and 20% of the
received signal power is from cell B. As can be seen in Figure 7 the plot 120 provides
discrete signal points in accordance with a 16QAM received signal, but with an apparent
increase in noise as a result of a spread of possible points caused by the 20% power coming
from the cell which is transmitting 64QAM modulation symbols.
Correspondingly, a middle plot 122 provides a plot of signal values in the complex
plane when the receiver is at position Y and for which it is assumed that 60% of the received
power is from cell A and 40% of the received power is from cell B. As can be seen, although
the signal constellation plots are grouped into clusters corresponding to an association with
each of the possible values of a 16QAM symbol, discrete constellation points have been
formed in accordance with a 64QAM modulation scheme. Thus it will be appreciated that if
the signal to noise ratio is high enough then a receiver at position Y can detect one of the
64QAM signal points and therefore recover the local inserted data. Correspondingly, a right
hand plot 124 illustrates the case at position Z, for which it is assumed, for example, that only
10% of the signal power comes from the cell A and 90% of the signal power comes from cell
B. Therefore, as shown in the plot 124, clearly each of the 64QAM signal constellation points
are available for detecting and recovering data, which is produced for both the first data pipe
and the local service insertion data pipe. Accordingly, it will be appreciated that depending
on the position of the receiver, a mobile terminal can recover the locally transmitted data and
the data transmitted from the first data pipe (for example the national broadcast) when in or
around cell B, whereas in cell A a receiver will still be able to recover the data f om the first
data pipe. Therefore an effect of using the layered modulation provided by the second
modulation scheme of a 64QAM signal and the first modulation scheme 16QAM will not
disrupt the reception of the nationally broadcast data when locally broadcast data is
transmitted from a neighbouring cell.
TDMA Local Service Insertion
A further enhancement which some embodiments of the present technique may use is
to distribute the capacity for local service transmission between a cluster of neighbouring cells
o the effect that the local content transmitted using the higher older (second) modulation
scheme is transmitted at different times in different cells. This technique is illustrated with
reference to Figures 9a, 9b and 9c.
In Figure 9a a cluster of four cells is shown. These are shown with different grades of
shading and are labelled respectively Txl, Tx2, Tx3, Tx4. Thus Figure 9a illustrates a cluster
of four cells. As will be appreciated in addition to receiving the data from the first data pipe,
which may be for example the national broadcast channel, a regional broadcast may also be
provided using the local data insertion pipe in combination with the higher order hierarchal
modulation technique as explained above. However as explained above when the second or
higher order modulation technique is being used, the effect is to introduce noise or
interference which reduces the signal to noise ratio for receivers receiving the data from the
first communications channel that is the national broadcast using the first or lower order
modulation scheme. More specifically, for example, if the national broadcast signal from the
first data pipe is modulated using QPSK and the combined first communications channel and
the local service insertion channel are modulated onto the second or higher order modulation
scheme of 16QAM then the 16QAM broadcast will appear as an increase in noise for a
receiver trying to receive the OFDM symbols modulated with the QPSK modulation scheme.
In order to reduce the amount of interference caused by the second/higher order
modulation scheme (16QAM) with respect to the first/lower order modulation scheme
(QPSK) the cells which broadcast the OFDM signals are clustered as shown in Figure 9a.
Furthermore the transmitters within the four cell cluster illustrated in Figure 9a take turns on a
frame by frame basis to broadcast the higher order 16QAM modulation signal providing data
symbols from the first data communications pipe and their local service insertion pipe. Such
an arrangement is illustrated Figure 9b.
In Figure 9b a TDMA frame composed of four physical layer frames is shown. The
physical layer frames are labelled frame 1, frame 2, frame 3 and frame 4. Within each
physical layer frame the OFDM signals are communicating data from various PLPs. As
explained above contemporaneously with the transmission of the data for the first data pipe
using QPSK, OFDM symbols carrying data from both the first data pipe and the local service
insertion pipe are also transmitted using for example 16QAM. However in order to reduce
the interference caused by the 16QAM modulation only one of transmitters Txl, Tx2, Tx3,
Tx4 within the cluster of four cells is allowed to transmit OFDM symbols with the higher
order 16QAM modulated sub-carriers during each physical layer frame of the TDMA frames.
Thus in physical layer frame 1, only Txl transmits the OFDM symbols with sub-carriers
modulated with 16QAM to provide data from the combined first data pipe and its local
service insertion pipe, whilst in frame 2 only transmitter Tx2 transmits the OFDM symbols
with 16QAM, and thereafter TX3 in frame 3 and TX4 in frame 4. Then the pattern repeats for
the next TDMA frame. In each case, all other transmitters are transmitting OFDM symbols
modulated with QPSK or the constellation used for carrying only the first data pipe.
As a result of time dividing the transmission of the local service insertion data
between each of the four transmitters Txl, Tx2, Tx3, Tx4, effectively the local data rate is a
quarter of that of the first data pipe. Thus each cell transmits local service insertion content
every fourth physical layer frame. However correspondingly because the higher order
modulation scheme is only transmitted from a cell once in every four frames, the effective
interference experienced by receivers located in the coverage area of the four cells that wish
to receive the first/lower order modulations scheme (QPSK) is correspondingly reduced.
Thus in a pattern of cells illustrated in Figure 9c, the interference which is caused by the local
service insertion data and would appear as increased noise to the receiver is distributed
throughout the cluster of four cells. Therefore the relative interference or increasing noise
caused by the local service insertion data is reduced. This can be considered to be the
equivalent of frequency re-use in a multi frequency network. For the example illustrated in
Figure 9a, 9b, 9c, the followmg table represents the transmission of OFDM symbols with each
of the first (16QAM) and second (64QAM) modulation schemes:
data is modulated using a second/higher modulation scheme of 64QAM and the first/lower
order modulation scheme is 16QAM for carrying data symbols from the first/national data
Pipe-
As will be appreciated, a result of allocating the transmission of the local content over
a cluster of four TDMA frames between a cluster of four base stations, may be to reduce the
bandwidth for the local content service by one quarter, if a receiver is only able to receive the
OFDM carrying signal from one base station only, which will typically be the case. The
allocation of the local content to the transmitter of the base station in each cluster may be
provided for example via signalling data provided by the signalling data pipe.
Although in the example provided above the cells are clustered into groups of four, it
will be appreciated that any number can be used. Advantageously the cells are grouped into
clusters of four to provide a balanced trade-off between an amount of baseband bandwidth
(bit rate) afforded to the local service insertion service and an amount of reduction in the
signal to noise ratio caused to the reception of data from the first data pipe using the lower
order modulation scheme by the transmission of the higher order modulation scheme carrying
data from both the first data pipe and the local service insertion channel. As such a cell
structure shown in Figure 9c can be used to transmit local content every fourth physical layer
frame for a different group of four cells and the arrangement of the cell clustering repeated
throughout to represent an equivalent arrangement of frequency re-use.
According to the present technique the transmitter within the base stations shown in
Figure 4 may be adapted to implement the TDMA frame structure illustrated above. In one
example, the scheduler 34 for forming the modulated sub-carrier signals into the OFDM
symbols and a framing unit 58 may be arranged to schedule the transmission of the OFDM
symbols according to the time divided frame illustrated in Figure 9b. The scheduler 34 and
the framing unit 58 are arranged to transmit OFDM symbols which are carrying data symbols
from both the first data pipe and the local service insertion pipe using the second modulation
scheme as illustrated in the table above.
Equalisation of Combined Local Service Insertion and National Broadcast Signals
A further aspect of the present technique will now be described with reference to
Figures 10 to 15. As explained above, data from a local service insertion channel is
transmitted with data from a national broadcast channel using a higher order modulation
scheme such as 16QAM, whereas data from the national broadcast channel is transmitted
using a lower order modulation scheme such as QPSK. A mobile receiver which is able to
detect the local service insertion data which is conveyed with the data from the national
broadcast channel by a 16QAM modulation scheme may be required to detect the 16QAM
signal in the presence of a QPSK signal, which conveys data from the national broadcast
channel only. The 16QAM modulation scheme conveying data from the national broadcast
channel and the local broadcast channel and the QPSK modulation scheme conveying the
national broadcast channel are represented in Figure 3a and 3b and described above. In the
following description the higher order modulation scheme which is conveying data according
to the national broadcast channel and the local service insertion channel will be referred to as
the local service insertion channel or data and the national broadcast channel will be referred
as the national broadcast channel, data or signal.
A further ancillary problem addressed by an embodiment of the present technique is to
provide a receiver which can equalise a signal received at the receiver which is a combination
of the local service insertion signal that is the 16QAM signal and the national broadcast signal
that is the QPSK signal for example. Equalising a signal which is a combination of a national
broadcast signal and a local service insertion signal, which is a combination of a 6QAM and
a QPSK signal is therefore addressed by a further aspect of the present technique.
As shown in Figure 10 a mobile receiver M is located at a position approximately
equi-distant from the base station transmitting the local service insertion signal 112 and a base
station transmitting the national broadcast signal 110. Thus the signal received by the mobile
receiver M is comprised of a combination of the local service insertion signal s(t)+d(t)
convolved with the channel (t) between the local service insertion base station 112 and the
mobile receiver M and the national broadcast signal s(t) convolved with a channel hn (t) from
the national broadcast base station 0 and the mobile receiver M. Thus the received signal
r(t) is represented by the following equation (where the symbol '*' represents convolution):
r(t) = hn (t)*s(t) +hi(t)*[s(t) +d(t)]
=s(t)*[hn (t) +hi +d(t)* hi(t)
Following an FFT in which the received signal is transformed into the frequency
domain, the signal formed at the output of the FFT is:
R(z) =S(z)[Hn (z) +Hi(z)] +D(z)Hi(z)
A signal constellation therefore can be represented in the complex plane for the
national broadcast signal as shown in Figure 11a, and the local insertion signal as shown in
Figure l ib; the national broadcast signal being QPSK as shown in Figure 11a and the local
service insertion signal being 16QAM shown in Figure l ib. Thus the national broadcast
signal of Figure 11a provides a lower order modulation scheme with respect to the higher
order modulation scheme of 16 QAM shown in Figure 1lb. However, the representation of
the signals shown by the constellation points of Figure 11a and lib are without noise and
moreover, without the presence of either of the other signals.
Fi ure 1 a and 12b provide a corresponding representation of the signal constellation
in the complex plane where the mobile receiver M receives a signal in the presence of both
the national broadcast signal s(t) and the locally broadcast signal s(t)+d(t) and where the
channel responses Hn (z) and \(z) are not equal. In Figure 12a the signal constellation of R(z)
the combined signal as expressed above is a combination of the national broadcast signal and
the local broadcast signal. Figure 12b shows the effect of dividing the received signal R(z) by
[Hn (z) + Hi(z)J which is a combination of the channels from the base station of the national
broadcast signal 110 and the channel of the local insertion base station 12, to produce C(z).
The diagram in Figure 12b is assuming perfect channel estimation and without noise. As can
be seen from Figure 12b only a small amount of noise will be required in order to cause a
false detection of a particular modulation symbol of the local broadcast signal. The division
of R(z) by the combined channel forms an equalised signal C(z):
C z) =-
R Z
[H z ) +H z )]
However we do not know Hn (z) and (z) separately, and so the following cannot be
computed:
H, z)
[H„(Z) + H (2)]
According to the present technique in order to recover the local insertion signal from
the national broadcast signal, it is necessary to determine either the channel Hn (z) from the
national base station 110 or the channel H[(z) from the local service insertion base station 112
separately. With knowledge of either the national broadcast channel Hn (z) or the local
insertion channel H[(z) it would be possible to compute the term D(z). Thus, first detecting
the national broadcast signal using the lower order modulation scheme and subtracting the
detected signal from the received signal it is then possible with knowledge of either the
channels from the national broadcast base station Hn (z) or the local service insertion signal
base station H\(z) to recover the local signal D(z). Thus, according to the present technique
the term Hi(z)D(z)/[ Hn (z)+ H z)] is treated as noise and the national broadcast data is
recovered by slicing S(z) to give an estimate of the national broadcast signal S z) .
Accordingly, by calculating the composite channel [ Hn (z)+ (z)] from the national
broadcast base station Hn (z) and the local service insertion signal base station (z) and
convolving with the estimate of the national broadcast signal (by multiplication in the
frequency domain) it is possible to subtract this combination from the received signal to form
an estimate of the local service insertion signal convolved with the channel from the local
service insertion base station.
Therefore to detect the local service insertion signal, the following steps are required:
1. Estimate S(z) as £(z)by considering , (z) as noise when
[H„(z) + H (z)]
slicing S(z);
2. The equaliser has already computed [Hn (z) + (z)] as the combined channel
from the national OFDM reference pilots;
3. Compute D(z)H, (z) ~ R{z) - S(z)[H„ ()+ {) } ; which provides a complex
signal as shown in the complex plane diagram in Figure 13a;
4. If some of the D(z) are known from additional pilots provided in the local
service insertion signal, then H[() can be estimated to give H,(z)
5 H . i?( ) -i(z )[H„(z) + H,(z)]
D(z)
6. Interpolation can be performed on H (z)in the frequency direction to form
Hi() and so
7 R(z)-S(z)[H z)+H,{z)]
H,(z)
Thus, by cancelling the channel from the local service insertion base station H,(z) , a
signal constellation diagram shown in Figure 1 b is formed from which the local service
insertion data D{z) can be recovered.
As will be appreciated from the above explanation in order to recover the local service
insertion signal D(z) it is necessary to estimate the local service insertion channel
H,(z) from the local service insertion base station which is separate from the channel from
the national broadcast base station Hn (z).
In a further embodiment, the computed D(z) can be used to get a better estimate of
S(z) by computing the following:
R(z) - D{z)Hi(z) = S(z)[Hn (z) +Hi(z)]
Then divide each side by [Hn (z) +H](z)] and slice again for S{z) . This kind of iteration may
be continued many times to get a continuous improvement in the estimate of D() .
The above explanation has been provided to give an explanation of the general
technique by which the data from the national broadcast signal and the local broadcast signal
can be recovered. As given above a simple technique for cancelling the effects of a channel
from a received signal is to divide the received signal by the estimate of the channel.
However there are other equalisation techniques which can be used are some are presented
below after the results section which includes generating a log likelihood ratio for the
received data symbols. This example technique avoids a potential problem caused by
cancelling by division if there are nulls in the frequency domain, which produces noise
amplification as a result of dividing by zero.
Estimating Local Channel Using Local Pilots
According to the present technique the channel from the local service insertion base
station (z) is estimated by including local service insertion pilot symbols on selected subcarriers
which are transmitting the local service insertion modulation symbols. Such an
arrangement is shown in Figure 1 a, 14b and 14c.
In Figure 14a an illustrative representation of an OFDM symbol in the frequency
domain is provided showing a plurality of subcarriers which are then designated for
conveying data according to the national broadcast signal s(t) and subcarriers which are
dedicated to transmitting pilot symbols Ps in accordance with a conventional arrangement.
Figure 14b provides an illustration of an OFDM symbol in which local service insertion
symbols are introduced on top of the national broadcasting symbols using the hierarchical
modulation scheme. However, in order to estimate the channel via which the local service
insertion symbol is broadcast, it is necessary to select some of the subcarriers which are
carrying data according to the local service insertion and replace these symbols with known
symbols which will act as pilot symbols Pd. Such an arrangement is shown in Figure 14c.
Accordingly, it will be appreciated that the local service insertion pilots Pd can be transmitted
in place of symbols which would be transmitted on subcarriers with higher order modulation
symbols which would be arranged to carry the local service insertion data but arranging for
these to be replaced by known symbols. Therefore these sub-carriers can convey a known
symbol for the higher order modulation which can act as a pilot Pd. However, as will be
appreciated in order to transmit the local service insertion signal pilots Pd. it is necessary to
accommodate the frequency interleaving which would be required for a conventional
transmission of the local service insertion data.
As shown in Figure 4, according to the present technique at the output of the
frequency interleaver 54 for each data slice processor 50, 51, the data slice processors 50, 5 1
which include local service insertion data include a block 182 for inserting the local service
insertion pilots Pd before generating the hierarchical modulation symbols as formed by the
modulators shown in Figure 4. The modulators 182 are arranged to map the data symbols
onto modulation symbols in accordance with the hierarchical modulation scheme being used.
Optionally, where a multiple input single output (MISO) scheme is being employed then
further processing of the pilots is performed as illustrated by the MISO block 184. Following
the MISO block 184, the pilot symbols are inserted on separate pilot subcarriers via the main
pilot insertion unit 56 following which the framing unit 58 forms the OFDM symbols in the
frequency domain in a combination with the OFDM block 70.
As shown in Figure 4 at the output of the frequency interleaver 54 in a branch of the
signal insertion data slice processor, the local service insertion data which is produced after
the frequency interleaver 54 is fed to the local pilots insertion block 180 in which the data
symbols for the local service insertion are replaced by the pilot symbols either by puncturing
or for example where the modulation symbols which are to be used to carry the local service
insertion of pilots are left vacant between data cells or are moved to accommodate the local
service insertion pilots. As will be appreciated the local service insertion pilots Pd a c piedesignated
and so can either be reserved for local service insertion pilots or the data can be
moved to accommodate the local service insertion pilots. Thus, the arrangement substantially
as represented in Figure 14c is produced at the output of the QAM modulator 182.
Figure 15 provides a schematic block diagram which corresponds to the schematic
block diagram shown in Figure 4 except that Figure 15 provides an example in which a
multiple-input multiple-output () transmission scheme is being used. However, a
complication with the arrangement for a MIMO scheme is that the local service insertion
pilots Pd , which a e formed as part of the hierarchical modulation structure must be inserted
before the frequency interleaver 192. This is because for a MIMO scheme, the pilots on each
version of the OFDM signal to be transmitted are adapted with respect to each other and so
each of the versions must be formed separately for each version. This applies for both the
national broadcast modulation symbols and also the local service insertion symbols.
Accordingly, it is not possible to combine the local service insertion pilots at the output of the
frequency interleaver 54.
According to the present technique, in order to accommodate an arrangement in which
the local service insertion pilots are formed in the signal before the frequency interleaver 54
then the local service insertion pilots are arranged with respect to the subcarriers which are
conveying the hierarchical modulated data in a block 190 which is then fed to a frequency deinterleaver
192 which performs an inverse of the interleaving performed by the frequency
interleaver 54. Thus, the pilot sub-carriers which include the local service insertion pilots Fd
are arranged at their desired position and the frequency de-interleaver, de-interleaves these
modulation symbols before the local service insertion data is applied by a local service
insertion data block 194. At the output of the QAM modulator 182, the modulation symbols
are formed and fed to a block 184. The frequency interleaver 54 then performs a
mapping which is a reverse of the de-interleaver mapping performed by the frequency deinterleaver
192 so that at the output of the frequency interleaver 54, the local service insertion
pilots are once again at the desired locations on the designated sub-carriers for the local
service insertion pilots. Accordingly, OFDM symbols are formed with the local service
insertion pilots Pd at their desired location. The main pilots Ps for the national broadcast
signal are then added at the sub-carrier positions concerned via the main pilot insertion block
56 before the framing unit 58 and the OFDM unit 70 form the OFDM symbols as per a
conventional arrangement.
Thus, according to the present technique the local service insertion pilots Pd are
arranged at the desired location by first arranging for them to be disposed at their desired
location and then forming an inverse of the interleaving using a de-interleaver so that when
interleaved they are once again arranged at their desired location.
A received architecture which is arranged to recover the local service insertion data or
the national broadcast data is described below with reference to Figure 24.
Results
Various results are provided in Figures 16 to 2 1 for example transmitter-receiver
chains operating with different forward error collection encoding rates of rate ½, 3/5, 2/3 and
¾, and for a first modulation scheme of 16QAM, a second modulation scheme of 64QAM.
Figures 16, 17, 18, 19, 20 and 2 1 provide examples for different ratios of the power from cell
A and cell B. For Figure 16 the fraction of the power of the received signal from cell A is
99% and 1% from cell B. The relative delay between time of arrival from cells A and B is
4.375us. For Figure 6 80% of the power is from cell A and 20% is from cell B with a 2.2
delay in time of arrival from cell B. Figure 17 provides a 99% power from cell A and 1% of
power from cell B at a 0 &delay in relative time of arrival. Figure 18 shows 60% of power
from cell A and 40% of power from cell B at a 0 relative delay and Figure 19 shows a 50%
power from base station A and 50% power from cell B at a 0 relative delay. Finally,
Figure 20 shows results in a situation where 10% of the power is from cell A and 90% is from
cell B with the signal from cell A arriving the receiver 2.2 after the arrival of the signal
from cell B.. As can be seen from the example in Figure 2 1 there is insufficient signal to
noise ratio to decode the 3/5, 2/3 rate codes. The required SN should be that enough for the
decoding of 64QAM.With respect to each of the plots is shown a signal to noise ratio value
which would correspond to a situation in which the transmitter for the same neighbouring cell
was not transmitting the local service insertion data on the higher order modulation scheme
64QAM for this example. Where appropriate some of the plots include points for each of the
respective coding rates of ½, 3/5, 2/3 and ¾ at a bit error rate of 10 as represented as a "0".
As shown in each case there is an increase in the signal to noise ratio required in order to
reach the same bit error rate value. However the performance of the scheme would still seem
to be acceptable.
Receiver
A receiver which may form part of a mobile device for receiving the signals broadcast
by any of the base stations of the network shown in Figure 1 will now be described. An
example architecture for a receiver for receiving any of the transmitted PLP pipes shown in
Figure 4 is provided in Figure 22. In Figure 22 a receiver antenna 174 detects the broadcast
radio frequency signal carrying the OFDM signals which are fed to a radio frequency tuner
175 for demodulation and analogue to digital conversion of a time domain base band signal.
A frame recovery processor 8 recovers time division multiplex physical layer frame
boundaries and OFDM symbol boundaries and feeds each of the symbols for each of each
physical layer frame to an OFDM detector 150. The OFDM detector 150 then recovers the
national broadcast data and local service insertion data from the OFDM symbols in the
frequency domain. The recovered national broadcast data and local service insertion data is
then fed to a de-scheduler 134 which divides each of these symbols into the respectively
multiplexed PLP processing pipes. Thus the de-scheduler reverses the multiplexing of
applied by the scheduler 34 shown in Figure 4 to form a plurality of data streams, which are
fed respectively to PLP processing pipes 129, 130, 136. A typical receiver would have only a
single PLP processing pipe as each PLP may carry a full broadcast service and this PLP
processing pipe processes the data from any nation broadcast PLP or any local service
insertion PLP. The processing elements forming part of the PLP processing pipes shown in
Figure 22 is shown in Figure 23.
In Figure 23 the first example PLP processing pipe 130 is shown to include a QAM
demodulator 144, a de-interleaver 142 and a forward error correction decoder 140 which are
arranged to substantially reverse the operations of the QAM modulator 44, the interleaver 42
and the FEC encoder 40 of Figure 4. Optionally, the PLP processing pipe 130 may also
include a MISO/MIMO detector 46 for performing multiple input multiple output or multiple
input signal output processing. In operation therefore modulation symbols are received at an
input 200 and fed to the MISO/MIMO processor 146 whose role is to decode the space-time
code that was used at the transmitter thereby producing one stream of modulation symbols
into a signal symbol stream which are then fed to the QAM demodulator 144. The QAM
demodulator detects one of the constellation points in the QAM modulation scheme used and
for each detected point recovers a data word corresponding to that point. Thus the output of
the QAM demodulator 144 is a data symbol stream which is fed to the de-interleaver 2 for
de-interleaving the data stream from a plurality of OFDM symbols or from within an OFDM
symbol.
Since the data symbols have been encoded in the transmitter shown in Figure 4, for
example, using a low density parity check code, the symbols are decoded by the FEC decoder
1 0 to form at an output 202 base band data stream for the PLP.
In accordance with the present technique in some embodiments, the de-scheduler 150
is arranged to apply the TDMA frame in accordance with a cluster of base stations described
above to recover OFDM symbols which have been modulated with the second modulation
scheme and transmitted on one of the physical layer frames. Thus in accordance with the
signal transmission arranged for the cell cluster the receiver tries the recovery of the OFDM
symbols with sub-carriers modulated in accordance with the second modulation scheme in
accordance with the frame timing applied by the transmitter in the base station. The
information as to which physical layer frames carry hierarchical modulation for the given PLP
is carried in the signalling PLP which the receiver first receives and decodes before any
payload carrying PLP.
Equalising Received Single Frequency Signal
First Example OFDM Detector
As explained above, according to the present technique the OFDM detector 150 shown
in Figure 22 is arranged to detect both the national signal S(z) and the local service signals
D(z) using the pilot signals transmitted with both the national and local signals and the
additional pilots transmitted with the local signal. Two example implementations of the
OFDM detector 150 will now be described, with reference to Figures 24 to 29.
Figure 24a provides a representation of a schematic block diagram of a first example
of the OFDM detector 1 0 shown in Figure 22. This can be used for a SISO, MISO or MIMO
scheme. In Figure 24 a Fast Fourier Transform FFT block 290 converts the received signal
from a time domain into the frequency domain. A national broadcast signal equaliser 292
then receives the frequency domain OFDM symbols and forms an estimate of the combined
local service insertion channel and the national broadcast channel as well as the received
national broadcast data. Blocks which make up the single frequency network equaliser 292
are shown in Figure 24b.
As shown in the Figure 24b the single frequency network equaliser comprises a pilot
separator 296 which separates the pilots from the received frequency domain signal. The
frequency domain signal is fed at an output 298 of the pilot separator 296 to a divider circuit
300. From a second output 302 of the separator 296 the pilot sub-carriers are demodulated,
interpolated in time by a time interpolation unit 304 and interpolated in frequency by a
frequency interpolation unit 308 to form at an input 310 to the divider 300 an estimate of the
combined national broadcast channel and the local service insertion channel ( (z )l +H (z))
so that the output of the divider forms a signal representative of the national broadcast signal
S z ) 312.
As shown in the receiver chain a de-mapper 314 then interprets the received
modulation signals by slicing the modulation signalling about the real and imaginary plane to
detect an estimate of the national broadcast signal S(z) . The signal representative of the
national broadcast signal S{z) 312 is then fed to a frequency de-mterleaver 6 and then to a
de-scheduler 134 as explained above for a general data recovery of the national broadcast
signal.
Figure 24c provides an example implementation of a local equaliser 320. As shown in
Figure 24c, the detected combined local service insertion channel and national broadcast
channel ( H(z), + H (z) )are fed on an output 311 to a first input of a local equaliser 320. The
estimate of the national broadcast symbols S{z) 315 is fed to a multiplier 322 which receives
on a second input the estimate of the combined local service insertion channel and the
national broadcast channel 310. A subtraction unit 324 then subtracts the multiplication of the
estimate of the national broadcast symbols multiplied with the combined local service
insertion and national broadcast channels from the received signal to form an estimate of the
local service insertion symbols which are fed to a local equaliser 320. The internal structure
of the local equaliser 320 is similar to that of the national broadcast signal equaliser. At the
output of the local service insertion pilot separator 326 the pilot signals are fed on a output
328 to a pilot demodulator 330 and then to a time interpolation unit 332 followed by a
frequency interpolation unit 334 which forms an estimate of the channel through which the
local service insertion symbols have passed. The estimate of the local service insertion data is
fed on an input 336 to divider circuit 338 which receives on a further input from the pilot
separator 326, 340 the local service insertion symbols and forms at an output 342 an estimate
of the local service insertion data symbols. A de-mapper 344 and frequency de-interleaver
346, then form an estimate of the data representing the locally inserted data which is fed to the
de-scheduler 134. Thereafter, the data recovery of the locally inserted data corresponds to
that shown with respect to the data pipe shown in Figure 23.
As will be appreciated a further aspect of the present technique provides a first
estimate of the national broadcast data, which is then refined, based on the determination of
the local service insertion symbols to form a further refined estimate of the national broadcast
symbols which may be further used to further calculate a refined estimate of the local service
insertion symbols. Thus, an iterative feedback arrangement in the form of a turbodemodulation
can be formed to provide further improvements on the estimate of the received
signals.
Second Example of the OFDM Detector
As will be appreciated by those familiar with equalisation techniques, the
implementation of the single frequency network equaliser 292 and the local insertion
equaliser 320 include a divider circuits 300, 338. Each of the divider circuits in the respective
equalisers 292, 320 cancel the effect of the respective channels by division. For example in
the single frequency equaliser 300, the channel H(z) is cancelled from the component of the
signal which includes the data for the national signal S(z) by dividing in the frequency domain
the national broadcast sign S(z) multiplied with the channel (convolved with the channel in
the time domain) by the estimate of the channel H'(z) in order to recover the national
broadcast signal. However a multi-path fading radio channel can produce nulls in the
frequency response as a result of the multiple paths interfering destructively to cancel each
other, thereby producing a zero component. As a result, a simple division of the received
signal by the channel estimate can amplify noise as a result of a division by zero. A divide by
zero (or a number close to zero) can therefore cause the estimate of a modulation symbol at a
position corresponding to the channel sample which is zero or similar to produce a maximum
of the complex real and imaginary terms. As a result information which the modulation
symbol carries, the modulating data will be lost. The second example of the OFDM detector
150 is therefore arranged to obviate this technical problem by considering the effect of the
channel in the generation of the log likelihood function at the de-mapper instead of the divide
by zero in order to recover the data from the received signal. The equaliser therefore performs
a detection of the data using a minimum mean squared error equalisation technique. The
second example of the OFDM detector 150 is shown in Figures 25, 26, 27 and 28.
Figure 25 provide the second example of the OFDM detector 150 which corresponds
to the example shown in Figure 24a and so only the differences will be described here for
brevity. An FFT processor 290, frequency de-interleavers 316, 346, a national signal
equaliser 292.2, the multiplier 322 and the subtraction circuit 324 operate to perform
corresponding functions to those of the correspondingly number units shown in Figure 24a
described above. The differences from the first example therefore reside in a national QAM
de-mapper 400, the bit slicing de-mapper 314.2 and a local equaliser/QAM de-mapper 402.
As with the first example, the FFT processor 290 recovers a frequency domain version
of the received OFDM symbol, and feeds the OFDM symbol to a first national equaliser
292.2. The national equaliser 292.2 is shown in Figure 26 and operates to generate an
estimate of the combined channel H(z) through which the national broadcast signal and the
local broadcast signal were received. The national equaliser 292.2 operates as the single
frequency equaliser 292 shown in Figure 24b to generate an estimate of the national broadcast
signal S(z) by dividing the received signal by the estimated channel. The national equaliser
292.2 therefore operates in correspondence with the national equaliser shown in Figure 24b,
but otherwise operates in the same way.
The first estimate of the national signal S (z) is fed to a bit slicing processor, which
recovers a first estimate of the data convey by the national signal S(z) using the bit slicing
processor 314.2, which operates in the same way as the QAM demapper 314 shown in Figure
24c. Correspondingly, the multiplier 322 and the subtraction circuit 324 operates as shown in
Figure 24c to form at an input to the local equaliser/QAM demapper 404, a signal which
corresponds to the local signal convolved with the local channel (D(Z)Hl(z)). Therefore
according to the operation of the FFT processor 290, the first national equaliser 292, the bit
slicing demapper 314.2, the multiplier 322 and the subtraction circuit 324, the following are
produced at the respective outputs of these circuits:
The receive signal at the output of the FFT processor 290 is:
The output from the first national equaliser 292.2 on an output channel 310 is then:
H(z)=[H^)+H w(z)]
Whereas on the output 312 from the first national equaliser, the result ofdividmg the received
signal by the channel H(z) produces:
Therefore the output 315 of the bit slicing demapper, which process C(z) by slicing
about the real and imaginary planes produces S'(z). Accordingly, R'(z) is reconstituted at the
output of the multiplier circuit 322 according to:
R z) =S z)H z)+D{z)H1{z)
So that the output of the subtraction circuit 324 becomes:
(z - S ' z) z) =D z) z)
As shown in Figure 25, the local equaliser/QAM demapper 402 operates to recover the
local signal D(z) and the H' (z) using the known local signal pilots within D(z). The local
equaliser 402 is shown in more detail in Figure 27. As shown in Figure 27, the local equaliser
402 is arranged to generate the estimate of the local channel H'i(z), using the pilot separator
326, the demodulator 330, the time interpolation circuit 332 and the frequency interplotation
circuit 334 as explained above with reference to Figure 24c. However, in Figure 27, the local
equaliser 402 includes a local 2D QAM demapper, which operates to generate an estimate of
the local signal D'(z), as explained in the ollowmg paragraphs, by using a log likelihood
function rather than a division circuit. This is achieved by using a D or a 2D dempper of
D(z)Hi(z) using the estimated H'i(z) as the channel transfer function. This gives estimate
D'(z).
Minimum Mean Square Error Local Equaliser
The local equaliser 320 shown in Figure 24c generates the estimate of the local signal
D'(z), by cancelling the effects of the local signal channel using a divider circuit and then demaps
the modulation symbols of the local signal which are produced at the output of the
divider circuit 338. In contrast, the local equaliser/demapper 402 uses a 2D or D QAM
demapper, which combines the estimate of the channel H'i(z) and a sample of the received
local signal D(z)Hi(z) to produce a log likelihood ratio for each of the estimated modulation
symbols. The log likelihood ratios (LLR) of the modulation symbols can then be used in a
subsequent error correction decoder which utilises soft decision information to recover the
data communicated from the local signal. As such, because the local equaliser/demapper does
not perform a division of the received local signal by the estimate of the local channel, the
divide by zero problem explained above is avoided or at least reduced. The operation of the
local signal demapper 404 which generates a log likelihood ratio for each estimate modulation
symbol or cell of the OFDM signal is explained as follows:
Log LikelihoodDemapper
Let a received modulation symbol estimate or cell be designated as r = + jQd. The
demapper is arranged in operation to calcualtc a soft bit in the form of a log -likelihood ratio
(LLR) for each of the bits carried by the cell. The LLR for the bit at position / of the
constellation label (£,•) can be computed as:
P , = l \ r )
where:
P b =k\ r) = P x \ r ) fork
where C(k,:) is the set of constellation points for which the value ( ,-) of the bit at
position i is Thus with 16QAM for example: C(0,3) = {0000, 0001, 0010, 0011, 0100,
0101, 0 110, 0 111} i.e. the set of constellation labels for which the MSB (b3) is zero.
In its most general form:
Where is the noise standard deviation of I and d accordingly and the p are the
respective channel fading coefficients for the I and Q channels. Note that in general, the and
p can be different if the Id and Qd come from different OFDM cells because of the use of
rotated constellations, for example. When rotated constellations are not used, the pairs of 
and p are of course the same and simplify things considerably.
The multiplier terms before the exponentials only scale and so the LL is proportional
to:
This can be simplified using the max-log approximation which postulates that:
In exp(¾) = max(a )
=
It also follows that:
Which represents a so-called 2D de-mapper, in which
where is the linear SNR/cell computed for the relevant cell in the
equalised/demapper signal and m = ½log2(M) being the number of bits conveyed per
constellation axis. Note that for rotated constellations the are different for & Q and so the
will also be different. The summation in the above equation for x 2 =— l QX \ i
2 x
the well known average energy per symbol of the particular QAM constellation.
The operation of the local equaliser/de-mapper therefore performs the following
computations for each input cell r = + jQ
1. For each point x = Ix + jQx in the constellation, calculate the distances \I -Ix \2
and \Qd-Qx \2 scaled by the relevant noise terms. If this is done in a brute force
fashion, M of these would be needed for M-QAM. However, if these are
calculated per contour of the relevant QAM then only M 2 subtractions and
square operations are needed followed by M additions.
2. For bit position i in the constellation label, find out for each constellation point
x , if bi is one or zero and consider if the I&Q distance sum of x from r
(computed in step 1) is the minimum for the relevant set. In practice, for each
constellation type and for each bit position i the indices to the points which are
members of the sets C(0,z) and C(l,z) can be pre-identified and tabulated.
3. Once the two minima are found, perform the subtraction in the equation to get
L b .
On the other hand,, the 2D-demapper LLR function can be expanded into:
÷ΐ - ! +p - Q plQ
L mm 2 2
Each component is minimised when their respective:
2 2
2<7I 2<7Q
a e maximised. Thus, another strategy is to compute and maximise only these terms
for each set C(k,:), and to compute the full LLR only for the constellation points that produce
these maxima.
1-D Demapper
The Demapper can be significantly simplified into 2 x 1-D Demappers but with some
loss in performance. In any uniform QAM half the bits for each constellation point are
completely determined by one of the axis, as illustrated for example in Figure 28. For the
example shown in Figure 28, which corresponds to the example of 16QAM we can see that
{bo, b2} are determined only by the real axis whilst {b
1 b3} are determined only by the
imaginary axis. The mapping is tabulated as the table below:
6QA bits versus axis amplitudes
This means that the soft decision values for bits {b b3} can be derived only from the
LLR of amplitudes of the imaginary axis whilst those for {b , b } can be derived only from
the LLR of amplitudes of the real axis. Thus the LLR calculation is simplified to the equation
below, which would provide a ID demapper:
This equation is difference dependent on whether the LLR for either {bo, b } or {bi,
b3} are being calculated respectively. Thus taking {bo, b2}:
where .4(k,:) is the set of I-axis amplitudes for which bt = k. Then after the maxapproximation:
As before, each component is minimised when:
is maximised. The same analysis can be done for the imaginary axis.
Thus in an alternative embodiment the local equaliser/demapper 402 can be arranged
to perform either the 2D-demapper calculation explained above, or the 1-D demapper which
typically is less complex than the 2-D demapper particularly as the number of constellation
points in each of the sets A ,: and A l ,: is rather low, which for the example of 16QAM
requires only two elements in each case as seen in Table 3. The max-log approximation is
thus often dispensed with for lower order modulation schemes and the LLR derived using
piece-wise linear approximations of the exponential functions in the ID-demapper equation
since the number of terms to sum is limited.
Second National Equaliser/De-mapper
As explained above in some example embodiments the estimate of the local signal
D'(z) can be used to re-constitute the received signal with the effect that an improved estimate
of the national signal S(z) can then be generated from that re-constituted signal. As illustrated
for the second example of an OFDM detector 150 shown in Figure 25, a second national
equaliser/demapper 400 is used to generate this improved estimate of the national signal S(z).
The second national equaliser/demapper 400 does not use a divide by zero calculation and bit
slicing as is performed by the first national equaliser 292, but like the local
equaliser/demapper 402, generates a log likelihood ratio for each modulation symbol or cell.
As shown in Figure 29, the second national equaliser/de-mapper includes a subtraction circuit
420, which receives on a first input and output from a multiplier 422. The multiplier then feds
a national signal equaliser/de-mapper 424 which operates in a similar manner to the local
equaliser/de-mapper 404 to calculate for each modulation symbol or OFDM cell a LL .
n Figure 29, the multiplier 422 receives the estimate of the local signal D'(z) on a
connecting channel 406 from the local equaliser/demapper 402 and the estimate of the local
channel l(z) on a connecting channel 408 and combines the two to form an estimate of the
local signal as received from the local channel. This resulting signal is fed to the first input of
the subtraction cirtcuit 420, which receives on a second input the originally received signal
R(z) from the FFT processor 290 as shown in Figure 25. The output of the subtraction circuit
420 therefore forms S(z)H(z) according to the equation:
R(z) - D z)H (z)=S{z)H(z)
As shown in Figure 29 the national signal demapper 424 operates substantially as
explained above for the local equaliser/de-mapper to generate a LLR value for each of the
modulation symbols or cells of the received OFDM symbol for the national signal using H(z)
received from the first national equaliser 292 as the channel transfer function to form a refined
estimate of the national signal S"(z) at an output 160.
Summary of Operation
In summary the operation of the receiver shown in Figure 24a and 25 to recover the
local data from the local service insertion symbols is illustrated at a general level by a flow
diagram shown in Figure 30 which is summarised as follows;
S2: An estimate of the national broadcast symbols S(z)is formed by regarding the
term a s n o s e an slicing the recovered signal about the real and
[H„(z) + H,(z)]
imaginary plane to form an estimate of the national broadcast data.
S4: An estimate of the combined channel which is the transmitting channel from the
nation broadcast base station and the local service insertion base station is formed using the
main pilot sub-carriers Ps to calculate an estimate of a term representing the regenerated
national broadcast signal convolved with the combined national broadcast and local service
insertion channels S(z)[H„ (z) + ,(z)] .
S6: An estimate of the local service insertion symbols convolved with the local
channel is formed by subtracting the generated term from step S4 from the received signal
R(z) (D(z)H, (z) R{z) - S z)[Hn(z) +H,(z)] ).
S8: An estimate of the channel through which the local service insertion has passed
from the base station to the receiver H,(z) is determined using the local service insertion
pilots.
S10: The local service insertion data is then estimated from the symbols produced by
dividing the recovered term by the estimate of the local channel
R(z) -S(z)[H n(z) +H! z )]
H,(z)
Various modifications maybe made to the present invention described above without
departing from the scope of the present invention as defined in the appended claims. For
example, other modulation schemes could be used other than those described above, with
appropriate adjustments being made to the receiver. Furthermore, the demodulation process
can be iterated as described above for a number of times to improve the received symbol
estimates. Furthermore, the receiver could be used in various systems, which utilise OFDM
modulation other than those defined according to the DVB-NGH standards.
CLAIMS
. A receiver for receiving and recovering local service data symbols from first
Orthogonal Frequency Division Multiplexed (OFDM) symbols in the presence of second
OFDM symbols, the first and the second OFDM symbols including a plurality of sub-carrier
symbols formed in the frequency domain, the second OFDM symbols carrying national
broadcast data symbols and modulated on to the sub-carriers of the second OFDM symbols
using a first modulation scheme to form national broadcast modulation symbols, and the first
OFDM symbols carrying the national broadcast data symbols and the local service data
symbols and modulated on to the sub-carriers of the first OFDM symbols using a second
modulation scheme, the first and the second OFDM symbols both including the same pilot
sub-carrier symbols and the first OFDM symbols including local pilot symbols, the receiver
comprising
a tuner which is arranged in operation to detect a radio frequency signal representing a
combination of the first and the second OFDM symbols and to form a received base band
signal representing the combined first and second OFDM symbols,
an OFDM detector which is arranged in operation to recover modulation symbols
carrying the local service data symbols from the data bearing sub-carriers of the first OFDM
symbols, and to generate an estimate of the local service data symbols from the modulation
symbols carrying the local service data symbols, wherein the OFDM detector includes
an equaliser for recovering the local service data symbols of the second modulation
scheme by
generating an estimate of a combined channel ( Hn (z) +Htfz)]) via which the first and
second OFDM symbols have passed using the pilot sub-carrier symbols of the first and
second OFDM symbols;
generating an estimate of the national broadcast modulation symbols from the
modulated data bearing sub-carriers of the first modulation scheme from the second OFDM
symbols S z) )
generating an estimate of a convolution of the combined channel and the national
broadcast modulation symbols ( S(z)[H (z) + H,(z)] );
generating an estimate of a component of the received base band signal representing
the local service modulation symbols of the first OFDM symbols by subtracting from the
received signal the generated estimate of the national broadcast modulation symbols
convolved with the estimate of the combined channel
( D(z)H, (z) R(z) - S z)[H (z) + H,(z)] );
generating an estimate of a channel via which the first OFDM symbols were received
using the local pilot symbols (H,(z) );
generating an estimate of local service data symbols from a combination of the
estimate of the component of the received signal representing the modulation symbols
carrying the local service data and the estimate of the channel via which the first OFDM
symbols were received ( D(z) ~ ——).
H,(z)
2. A receiver as claimed in Claim 1, wherein the second modulation scheme
provides two or more constellation points in the complex plane for each constellation point in
the complex plane of the first modulation scheme.
3. A receiver as claimed in Claim 1 or 2, wherein the first modulation scheme is
N-QAM and the second modulation second is M-QAM, where N < M and M/N is two or
more.
4. A receiver as claimed in any preceding Claim, wherein the equaliser is
arranged in operation to generate the estimate of the national broadcast modulation symbols
from the modulated data bearing sub-carriers of the first modulation scheme from the second
OFDM symbols ( S z) ) by slicing values in the complex plane about predetermined axes and
making hard decisions as to the value of the national broadcast modulation symbols.
5. A receiver as claimed in any preceding Claim, wherein the equaliser is
arranged in operation to generate the estimate of the local service data symbols from the
combination of the estimate of the component of the received base band signal representing
the modulation symbols carrying the local service data symbols and the estimate of the
channel via which the first OFDM symbols were received by
dividing the estimate of the component of the received signal representing the local
service data symbols by the estimate of the local channel to recover an estimate of each of the
modulation symbols carrying the local service data symbols from the first OFDM symbol, and
de-mapping the modulation symbols carrying the local service data symbols to
generate the estimate of the local service data symbols.
6. A receiver as claimed in any of Claims 1 to 4, wherein the equaliser includes a
local service equaliser/demapper which is arranged in operation to generate the estimate of the
local service data symbols from the combination of the estimate of the component of the
received base band signal representing the modulation symbols carrying the local service data
symbols and the estimate of the channel via which the first OFDM symbols were received by
calculating a log likelihood ratio for each of the local service data symbols from the
estimate of the component of the received signal representing the modulation symbols
carrying the local service data symbols and the estimate of the local channel to recover an
estimate of each of the local service data symbols from the first OFDM symbol, and
estimating the local service data symbols from the log likelihood calculations.
7. A receiver as claimed in Claims 5 or 6, wherein the equaliser includes a
national broadcast equaliser/de-mapper, which is arranged in operation
to re-generate an estimate of the component of the received base band signal
representing modulation symbols of the first OFDM symbol carrying local service data
symbols by combining the estimate of the modulation symbols representing the local service
data symbols with the channel via which the first OFDM symbols were received,
to form an estimate of a component of the received base band signal representing the
national broadcast modulation symbols by subtracting the re-generated estimate of the
component of the received signal representing the modulation symbols carrying the local
service data symbols from the received base band signal,
calculating a log likelihood ratio for each of the national broadcast data symbols from
the estimate of the component of the received signal representing the national broadcast
modulation symbols and the estimate of the national channel to recover an estimate of each of
the national broadcast modulation symbols of the second OFDM symbol, and
estimating the national broadcast data symbols from the log likelihood calculations.
8. A receiver as claimed in any of Claims 1 to 4, wherein the equaliser is
arranged in operation to re-generate an estimate of the national broadcast modulation
symbols, by
re-generating an estimate of the component of the received base band signal
representing modulation symbols of the first OFDM symbol carrying local service data
symbols by combining the estimate of the modulation symbols representing the local service
data symbols with the channel via which the first OFDM symbols were received,
generating an estimate of a component of the received base band signal representing
the national broadcast modulation symbols by subtracting the re-generated estimate of the
component of the received base band signal representing the national broadcast modulation
symbols from the received base band signal, and dividing by the combined channel, and
using the re-generated estimate of the national broadcast modulation symbols to make
a refined estimate of the local service modulation symbols, and the equaliser is arranged to
generate a refined estimate of the local service data symbols from the local service
modulation symbols.
9. A receiver as claimed in Claim 8, wherein the equalizer is arranged in
operation to make a further rc-gencration of the national broadcast modulation symbols from
the refined estimate of the local service data symbols by repeating the operations for
calculating the re-generated estimate of the national broadcast symbols.
10. A receiver as claimed in any preceding Claim, wherein the receiver is arranged
to receive data symbols from the OFDM symbols communicated in accordance with a Next
Generation Hand-held standard.
11. A method of receiving and recovering local service data symbols from first
Orthogonal Frequency Division Multiplexed (OFDM) symbols in the presence of second
OFDM symbols, the first and the second OFDM symbols including a plurality of sub-carrier
symbols formed in the frequency domain and modulated with data symbols being
communicated, the second OFDM symbols carrying national broadcast data symbols from a
national data pipe and modulated on to the sub-carriers of the second OFDM symbols using a
fi st modulation scheme to form national broadcast modulation symbols, and the first OFDM
symbols carrying the national broadcast data symbols from the national data pipe and the
local service data symbols from a local insertion pipe and modulated on to the sub-carriers of
the first OFDM symbols using a second modulation scheme, the first and the second OFDM
symbols both including the same pilot sub-carrier symbols and the first OFDM symbols
including local pilot symbols, the method comprising
detecting a radio frequency signal representing a combination of the first and the
second OFDM symbols to form a received base band signal representing the combined first
and second OFDM symbols,
recovering modulation symbols carrying the local service data symbols from the data
bearing sub-carriers of the first OFDM symbols, and
generating an estimate of the local service data symbols from the modulation symbols
carrying the local service data symbols, wherein the recovering the local service modulation
symbols comprises
generating an estimate of a combined channel {[IIn (z) I II[(z)J) via which the first and
second OFDM symbols have passed using the pilot sub-carrier symbols of the first and
second OFDM symbols;
generating an estimate of the national broadcast modulation symbols from the
modulated data bearing sub-carriers of the first modulation scheme from the second OFDM
symbols ( S(z) ) ;
generating an estimate of a convolution of the combined channel and the national
broadcast modulation symbols (S(z)[H (z) +H (z)] );
generating an estimate of a component of the received base band signal representing
the local service modulation symbols of the first OFDM symbol by subtracting from the
received signal the generated estimate of the national broadcast modulation symbols
convolved with the estimate of the combined channel
(D(z)H, (z) () - S(z)[H (z) +Hl (z)] );
generating an estimate of a channel via which the first OFDM symbols were received
using the local pilot symbols (H,(z) );
generating an estimate of local service data symbols from a combination of the
estimate of the component of the received signal representing the modulation symbols
carrying the local service data and the estimate of the channel via which the first OFDM
- , , R(z) -S(z)[H (z)+H,(z)]
symbols were received ( D(z) ~ " '- - ).
, z)
12. A method as claimed in Claim 11, wherein the second modulation scheme
provides two or more constellation points in the complex plane for each constellation point in
the complex plane of the first modulation scheme.
13. A method as claimed in Claim 11 or 12, wherein the first modulation scheme is
N-QAM and the second modulation second is M-QAM, where N < M and M/N is two or
more.
14. A method as claimed in Claim 11, 12 or 13, wherein the first modulation
scheme is M-QAM and the second modulation scheme is 4M-QAM and the phase rotation
which is used for both the first and the second modulation schemes is optimum for M-QAM.
15. A method as claimed in any of Claims 11 to 14, wherein the recovering the
local service modulation symbols includes
generating the estimate of the national broadcast modulation symbols from the
modulated data bearing sub-carriers of the first modulation scheme from the second OFDM
symbols ( S(z) ) by slicing values of the complex plane about predetermined axes and making
hard decisions as to the value of the national broadcast modulation symbols.
16. A method as claimed in any of Claims 11 to 1 , wherein the estimating the
local service data symbols from the combination of the estimate of the component of the
received base band signal representing the modulation symbols carrying the local service data
symbols and the estimate of the channel via which the first OFDM symbols were received
includes,
dividing the estimate of the component of the received signal representing the local
service data symbols by the estimate of the local channel to recover an estimate of each of the
modulation symbols carrying the local service data symbols from the first OFDM symbol, and
de-mapping the modulation symbols carrying the local service data symbols to
generate the estimate of the local service data symbols.
17. A method as claimed in any of Claims 1 to 1 , wherein the estimating the
local service data symbols from the combination of the estimate of the component of the
received base band signal representmg the modulation symbols carrying the local service data
symbols and the estimate of the channel via which the first OFDM symbols were received
includes
calculating a log likelihood ratio for each of the local service data symbols from the
estimate of the component of the received signal representing the modulation symbols
carrying the local service data symbols and the estimate of the local channel to recover an
estimate of each of the local service data symbols from the first OFDM symbol, and
estimating the local service data symbols from the log likelihood calculations.
18. A method as claimed in Claims 16 or 17, the method comprising
re-generating an estimate of the component of the received base band signal
representing modulation symbols of the first OFDM symbols carrying local service data
symbols by combining the estimate of the modulation symbols representing the local service
data symbols with the channel via which the first OFDM symbols were received,
forming an estimate of a component of the received base band signal representing the
national broadcast modulation symbols by subtracting the re-generated estimate of the
component of the received signal representing the modulation symbols carrying the local
service data symbols from the received base band signal,
calculating a log likelihood ratio for each of the national broadcast data symbols from
the estimate of the component of the received signal representing the national broadcast
modulation symbols and the estimate of the national channel to recover an estimate of each of
the national broadcast modulation symbols of the second OFDM symbol, and
estimating the national broadcast data symbols from the log likelihood calculations.
. A method as claimed in any of Claims 1 1 to 15, the method including
re-generating the estimate of the national broadcast modulation symbols, by
re-generating an estimate of the component of the received base band signal
representing modulation symbols of the first OFDM symbol carrying local seryice data
symbols by combining the estimate of the modulation symbols representing the local service
data symbols with the channel via which the first OFDM symbols were received,
generating an estimate of a component of the received base band signal representing
the national broadcast modulation symbols by subtracting the re-generated estimate of the
component of the received base band signal representing the national broadcast modulation
symbols from the received base band signal, and dividing by the combined channel, and
using the re-generated estimate of the national broadcast modulation symbols to make
a refined estimate of the local service modulation symbols, and the equaliser is arranged to
generate a refined estimate of the local service data symbols from the local service
modulation symbols.
20. A method as claimed in Claim 19, wherein the recovering the modulation
symbols representing the local service data symbols includes
making a further re generation of the national broadcast modulation symbols from the
refined estimate of the local service data symbols by repeating the operations for calculating
the re generated estimate of the national broadcast symbols.
21. A method as claimed in any of Claims 11 to 20, wherein the method is
arranged to receive data symbols from the OFDM symbols communicated in accordance with
a Next Generation Hand-held standard.
22. A receiver substantially as hereinbefore described with reference to Figures 1
and 4 to 30 of the accompanying drawings.
23. A method of receiving substantially as hereinbefore described with reference
to Figures 1 and 4 to 30 of the accompanying drawings.