FIELD OF THE INVENTION
The present invention relates to methods and apparatus for
performing modulation and de-modulation in multi-carrier
modulation systems (MCM systems) and, in particular, to
methods and apparatus for performing an echo* phase offset
correction when decoding information encoded onto carriers
of multi-carrier modulation symbols in multi-carrier
modulation systems.
BACKGROUND OF THE INVENTION
The present invention generally relates to broadcasting of
digital data to mobile receivers over time-variant multipath
channels. More specifically, the present invention is
particularly useful in multipath environments with low
channel coherence time, i.e. rapidly changing channels. In
preferred embodiments, the present invention can be applied
to systems implementing a multicarrier modulation scheme.
Multi-carrier modulation (MCM) is also known as orthogonal
frequency division multiplexing (OFDM).
In a MCM transmission system binary information is
represented in the form of a complex spectrum, i.e. a
distinct number of complex subcarrier symbols in the
frequency domain. In the modulator a bitstream is
represented by a sequence of spectra. Using an inverse
Fourier-transform (IFFT) a MCM time domain signal is
produced from this sequence of spectra.
Figure 7 shows a MCM system overview. At 100 a MCM trans-
mitter is shown. A description of such a MCM transmitter can
be found, for example, in William Y. Zou, Yiyan Wu, "COFDM:
AN OVERVIEW", IEEE Transactions on Broadcasting, vol. 41,
No. 1, March 1995.
A data source 102 provides a serial bitstream 104 to the MCM
transmitter. The incoming serial bitstream 104 is applied to
a bit-carrier mapper 106 which produces a sequence of
spectra 108 from the incoming serial bitstream 104. An
inverse fast Fourier transform (FFT) 110 is performed on the
sequence of spectra 108 in order to produce a MCM time
domain signal 112. The MCM time domain signal forms the
useful MCM symbol of the MCM time signal. To avoid inter-
symbol interference (ISI) caused by multipath distortion, a
unit 114 is provided for inserting a guard interval of fixed
length between adjacent MCM symbols in time. In accordance
with a preferred embodiment of the present invention, the
last part of the useful MCM symbol is used as the guard
interval by placing same in front of the useful symbol. The
resulting MCM symbol is shown at 115 in Figure 7.
A unit 116 for adding a reference symbol for each prede-
termined number of MCM symbols is provided in order to
produce a MCM signal having a frame structure. Using this
frame structure comprising useful symbols, guard intervals
and reference symbols it is possible to recover the useful
information from the MCM signal at the receiver side.
The resulting MCM signal having the structure shown at 118
in Figure 7 is applied to the transmitter front end 120.
Roughly speaking, at the transmitter front end 120, a digi-
tal/analog conversion and an up-converting of the MCM signal
is performed. Thereafter, the MCM signal is transmitted
through a channel 122.
Following, the mode of operation of a MCM receiver 130 is
shortly described referring to Figure 7. The MCM signal is
received at the receiver front end 132. In the receiver
front end 132, the MCM signal is down-converted and,
furthermore, a digital/analog conversion of the down-con-
verted signal is performed. The down-converted MCM signal is
provided to a frame synchronization unit 134. The frame
synchronization unit 134 determines the location of the
reference symbol in the MCM symbol. Based on the deter-
mination of the frame synchronization unit 134, a reference
symbol extracting unit 136 extracts the framing information,
i.e. the reference symbol, from the MCM symbol coming from
the receiver front end 132. After the extraction of the
reference symbol, the MCM signal is applied to a guard
interval removal unit 138.
The result of the signal processing performed so far in the
MCM receiver are the useful MCM symbols. The useful MCM
symbols output from the guard interval removal unit 138 are
provided to a fast Fourier transform unit 140 in order to
provide a sequence of spectra from the useful symbols.
Thereafter, the sequence of spectra is provided to a
carrier-bit mapper 142 in which the serial bitstream is
recovered. This serial bitstream is provided to a data sink
144.
As it is clear from Figure 7, every MCM transmitter 100 must
contain a device which performs mapping of the transmitted
bitstreams onto the amplitudes and/or phases of the sub-
carriers. In addition, at the MCM receiver 130, a device is
needed for the inverse operation, i.e. retrieval of the
transmitted bitstream from the amplitudes and/or phases of
the sub-carriers.
For a better understanding of MCM mapping schemes, it is
preferable to think of the mapping as being the assignment
of one ore more bits to one or more sub-carrier symbols in
the time-frequency plane. In the following, the term symbol
or signal point is used for the complex number which
represents the amplitude and/or phase modulation of a
subcarrier in the equivalent baseband. Whenever all complex
numbers representing all subcarrier symbols are designated,
the term MCM symbol is used.
DESCRIPTION OF PRIOR ART
In principle, two methods for mapping the bitstream into the
time-frequency plane are used in the prior art:
A first method is a differential mapping along the time
axis. When using differential mapping along the time axis
one or more bits are encoded into phase and/or amplitude
shifts between two subcarriers of the same center frequency
in adjacent MCM symbols. Such an encoding scheme is shown in
Figure 8. The arrows depicted between the sub-carrier
symbols correspond to information encoded in amplitude
and/or phase shifts between two subcarrier symbols.
A system applying such a mapping scheme is defined in the
European Telecommunication Standard ETS 300 401 (EU147-DAB).
A system compliant to this standard uses Differential
Quadrature Phase Shift Keying (DQPSK) to encode every two
bits into a 0, 90, 180 or 270 degrees phase difference
between two subcarriers of the same center frequency which
are located in MCM symbols adjacent in time.
A second method for mapping the bitstream into the time-
frequency plane is a non-differential mapping. When using
non-differential mapping the information carried on a
subcarrier is independent of information transmitted on any
other subcarrier, and the other subcarrier may differ either
in frequency, i.e. the same MCM symbol, or in time, i.e.
adjacent MCM symbols. A system applying such a mapping
scheme is defined in the European Telecommunication Standard
ETS 300 744 (DVB-T). A system compliant to this standard
uses 4,16 or 64 Quadrature Amplitude Modulation (QAM) to
assign bits to the amplitude and phase of a subcarrier.
The quality with which transmitted multi-carrier modulated
signals can be recovered at the receiver depends on the
properties of the channel. The most interesting property
when transmitting MCM signals is the time interval at which
a mobile channel changes its characteristics considerably.
The channel coherence time Tc is normally used to determine
the time interval at which a mobile channel changes its
characteristics considerably. Tc depends on the maximum
Doppler shift as follows:

It becomes clear from the existence of more than one
definition, that the channel coherence time Tc is merely a
rule-of-thumb value for the stationarity of the channel. As
explained above, the prior art time-axis differential
mapping requires that the mobile channel be quasi stationary
during several MCM symbols periods, i.e. required channel
coherence time Tc » MCM symbol period. The prior art
non-differential MCM mapping only requires that the mobile
channel be quasi stationary during one symbol interval, i.e.
required channel coherence time > MCM symbol period.
Thus, both prior art mapping schemes have specific
disadvantages. For differential mapping into time axis
direction the channel must be quasi stationary, i.e. the
channel must not change during the transmission of two MCM
symbols adjacent in time. If this requirement is not met,
the channel induced phase and amplitude changes between MCM
symbols will yield an increase in bit error rate.
With non-differential mapping exact knowledge of the phase
of each subcarrier is needed (i.e. coherent reception). For
multipath channels, coherent reception can only be obtained
if the channel impulse response is known. Therefore, a
channel estimation has to be part of the receiver algorithm.
The channel estimation usually needs additional sequences in
the transmitted waveform which do not carry information. In
case of rapidly changing channels, which necessitate update
of the channel estimation at short intervals, the additional
overhead can quickly lead to insufficiency of non-differen-
tial mapping.
P.H. Moose: "Differentially Coded Multi-Frequency Modulation
for Digital Communications", SIGNAL PROCESSING THEORIES AND
APPLICATIONS, 18. - 21. September 1990, pages 1807 - 1810,
Amsterdam, NL, teaches a differentially coded
multi-frequency modulation for digital communications. A
multi-frequency differential modulation is described in
which symbols are differentially encoded within each baud
between adjacent tones. At the receiver, following a
digital Fourier transform (DFT), the complex product between
the DFT coefficient of digital frequency k and the complex
conjugate of the DFT coefficient of digital frequency k-1 is
formed. Thereafter, the result is multiplied by appropriate
terms such that the differentially encoded phase bits are
realigned to the original constellations. Thus, the
constellation following the differential decoding must
correspond to the original constellation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide methods
and devices for performing an echo phase offset correction
in a multi-carrier demodulation system.
This object is achieved by methods according to claims 1 and
5 and devices according to claims 10 and 14.
In accordance with a first aspect, the present invention
provides a method of performing an echo phase offset
correction in a multi-carrier demodulation system,
comprising the steps of:
differential phase decoding phase shifts based on a phase
difference between simultaneous carriers having different
frequencies;
determining an echo phase offset for each decoded phase
shift by eliminating phase shift uncertainties related to
the transmitted information from the decoded phase shift;
averaging the echo phase offsets in order to generate an
averaged offset; and
correcting each decoded phase shift based on the averaged
offset.
In accordance with a second aspect, the present invention
provides a method of performing an echo phase offset
correction in a multi-carrier demodulation system,
comprising the steps of:
differential phase decoding phase shifts based on a phase
difference between simultaneous carriers having different
frequencies, the phase shifts defining signal points in a
complex plane;
pre-rotating the signal points into the sector of the
complex plane between -45° and +45°;
determining parameters of a straight line approximating
the location of the pre-rotated signal points in the
complex plane;
determining a phase offset based on the parameters; and
correcting each decoded phase shift based on the phase
offset.
In accordance with a third aspect, the present invention
provides an echo phase offset correction device for a
multi-carrier demodulation system, comprising:
a differential phase decoder for decoding phase shifts
based on a phase difference between simultaneous carriers
having different frequencies;
means for determining an echo phase offset for each
decoded phase shift by eliminating phase shift uncer-
tainties related to the transmitted information from the
decoded phase shift;
means for averaging the echo phase offsets in order to
generate an averaged offset; and
means for correcting each decoded phase shift based on the
averaged offset.
In accordance with a fourth aspect, the present invention
provides an echo phase offset correction device for a
multi-carrier demodulation system, comprising:
a differential phase decoder for decoding phase shifts
based on a phase difference between simultaneous carriers
having different frequencies, the phase shifts defining
signal points in a complex plane;
means for pre-rotating the signal points into the sector
of the complex plane between -45° and +45°;
means for determining parameters of a straight line
approximating the location of the pre-rotated signal
points in the complex plane;
means for determining a phase offset based on the
parameters; and
means for correcting each decoded phase shift based on the
phase offset.
The present invention provides methods and devices for
performing an echo phase offset correction suitable for
multicarrier (OFDM) digital broadcasting over rapidly
changing multipath channels, using differential encoding of
the data along the frequency axis such that there is no need
for channel stationarity exceeding one multicarrier symbol.
When using the mapping process along the frequency axis it
is preferred to make use of a receiver algorithm that will
correct symbol phase offsets that can be caused by channel
echoes.
The mapping scheme along . the frequency axis for multi-
carrier modulation renders the transmission to a certain
extent independent of rapid changes in the multipath channel
without introducing a large overhead to support channel
estimation. Especially systems with high carrier frequencies
and/or high speeds of the mobile carrying the receiving unit
can benefit from such a mapping scheme.
Thus, the mapping scheme of a differential encoding along
the frequency axis does not exhibit the two problems of the
prior art systems described above. The mapping scheme is
robust with regard to rapidly changing multipath channels
which may occur at high frequencies and/or high speeds of
mobile receivers.
The controlled respective parameters of the subcarriers are
the phases thereof, such that the information is
differentially phase encoded.
In accordance with the mapping described above, mapping is
also differential, however, not into time axis direction but
into frequency axis direction. Thus, the information is not
contained in the phase shift between subcarriers adjacent in
time but in the phase shift between subcarriers adjacent in
frequency. Differential mapping along the frequency axis has
two advantages when compared to other mapping schemes.
Because of differential mapping, no estimation of the
absolute phase of the subcarriers is required. Therefore,
channel estimation and the related overhead are not
necessary. By choosing the frequency axis as direction for
differentially encoding the information bitstream, the
requirement that the channel must be stationary during
several MCM symbols can be dropped. The channel only has to
remain unchanged during the current MCM symbol period.
Therefore, like for non-differential mapping it holds that
required channel coherence time s MCM symbol period.
The present invention provides methods and apparatus for
correction of phase distortions that can be caused by
channel echoes. As described above, differential mapping
into frequency axis direction solves problems related to the
stationarity of the channel. However, differential mapping
into frequency axis direction may create a new problem. In
multipath environments, path echoes succeeding or preceding
the main path can lead to systematic phase offsets between
subcarriers in the same MCM symbol. In this context, the
main path is thought of being the path echo with the highest
energy content. The main path echo will determine the
position of the FFT window in the receiver of an MCM system.
According to the present invention, the information will be
contained in a phase shift between adjacent subcarriers of
the same MCM symbol. If not corrected for, the path echo
induced phase offset between two subcarriers can lead to an
increase in bit error rate. Therefore, application of the
MCM mapping scheme presented in this invention will
preferably be used in combination with a correction of the
systematic subcarrier phase offsets in case of a multipath
channel.
The introduced phase offset can be explained from the
shifting property of the Discrete Fourier Transform (DFT):

Equation 3 shows, that in a multipath channel, echoes
following the main path will yield a subcarrier dependent
phase offset. After differential demapping in the frequency
axis direction at the receiver, a phase offset between two
neighboring symbols remains. Because the channel induced
phase offsets between differentially demodulated symbols are
systematic errors, they can be corrected by an algorithm.
In the context of the following specification, algorithms
which help correcting the phase shift are called Echo Phase
Offset Correction (EPOC) algorithms. Two such algorithms are
described as preferred embodiments for the correction of
phase distortions that can be caused by channel echoes.
These algorithms yield a sufficient detection security for
MCM frequency axis mapping even in channels with echoes
close to the limits of the guard interval.
In principle, an EPOC algorithm must calculate the echo
induced phase offset from the signal space constellation
following the differential demodulation and subsequently
correct this phase offset.
/ACCOMPANYING
BRIEF DESCRIPTION OF THE/DRAWINGS
In the following, preferred embodiments of the present
invention will be explained in detail on the basis of the
drawings enclosed, in which:
Figure 1 shows a schematic view representing a mapping
scheme used according to the invention;
Figure 2 shows a functional block diagram of an embodiment
of a mapping device;
Figures 3A and 3B show scatter diagrams of the output of an
differential de-mapper of a MCM receiver for
illustrating the effect of an echo phase offset
correction;
Figure 4 shows a schematic block diagram for illustrating
the position and the functionality of an echo
phase offset correction unit;
Figure 5 shows a schematic block diagram of an embodiment
of an echo phase offset correction device
according to the present invention;
Figure 6 shows schematic views for illustrating a
projection performed by another embodiment of an
echo phase offset correction device according to
the present invention;
Figure 7 shows a schematic block diagram of a generic
multi-carrier modulation system; and
Figure 8 shows a schematic view representing a prior art
differential mapping scheme.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In a preferred embodiment thereof, the present invention is
applied to a MCM system as shown in Figure 7. With respect
to this MCM system, the present invention relates to the
bit-carrier mapper 106 of the MCM transmitter 100 and the
carrier-bit mapper 142 of the MCM receiver 130, which are
depicted with a shaded background in Figure 7.
An preferred embodiment of an inventive mapping scheme used
by the bit-carrier mapper 106 is depicted in Figure 1. A
number of MCM symbols 200 is shown in Figure 1. Each MCM
symbol 200 comprises a number of sub-carrier symbols 202.
The arrows 204 in Fig. 1 illustrate information encoded
between two sub-carrier symbols 202. As can be seen from the
arrows 204, the bit-carrier mapper 106 uses a differential
mapping within one MCM symbol along the freguency axis
direction.
In the embodiment shown in Figure 1, the first sub-carrier
(k=0) in an MCM symbol 200 is used as a reference sub-
carrier 206 (shaded) such that information is encoded
between the reference sub-carrier and the first active
carrier 208. The other information of a MCM symbol 200 is
encoded between active carriers, respectively.
Thus, for every MCM symbol an absolute phase reference
exists. In accordance with Figure 1, this absolute phase
reference is supplied by a reference symbol inserted into
every MCM symbol (k=0). The reference symbol can either have
a constant phase for all MCM symbols or a phase that varies
from MCM symbol to MCM symbol. A varying phase can be
obtained by replicating the phase from the last subcarrier
of the MCM symbol preceding in time.
In Figure 2 a preferred embodiment of a device for
performing a differential mapping along the frequency axis
is shown. Referring to Figure 2, assembly of MCM symbols in
the frequency domain using differential mapping along the
frequency axis according to the present invention is
described.
Figure 2 shows the assembly of one MCM symbol with the
following parameters:
Nfft designates the number of complex coefficients of the
discrete Fourier transform, number of subcarriers
respectively.
K designates the number of active carriers. The reference
carrier is not included in the count for K.
According to Figure 2, a quadrature phase shift keying
(QPSK) is used for mapping the bitstream onto the complex
symbols. However, other M-ary mapping schemes (MPSK) like
2-PSK, 8-PSK, 16-QAM, 16-APSK, 64-APSK etc. are possible.
Furthermore, for ease of filtering and minimization of
aliasing effects some subcarriers are not used for encoding
information in the device shown in Figure 2. These sub-
carriers, which are set to zero, constitute the so-called
guard bands on the upper and lower edges of the MCM signal
spectrum.
At the input of the mapping device shown in Figure 2,
complex signal pairs bO[k], bl[k] of an input bitstream are
received. K complex signal pairs are assembled in order to
form one MCM symbol. The signal pairs are encoded into the K
differential phase shifts phi[k] needed for assembly of one
MCM symbol. In this embodiment, mapping from Bits to the 0,
90, 180 and 270 degrees phase shifts is performed using Gray
Mapping in a quadrature phase shift keying device 22 0.
Gray mapping is used to prevent that differential detection
phase errors smaller than 135 degrees cause double bit
errors at the receiver.
Differential phase encoding of the K phases is performed in
a differential phase encoder 222. At this stage of pro-
cessing, the K phases phi[k] generated by the QPSK Gray
mapper are differentially encoded. In principal, a feedback
loop 224 calculates a cumulative sum over all K phases. As
starting point for the first computation (k = 0) the phase
of the reference carrier 226 is used. A switch 228 is
provided in order to provide either the absolute phase of
the reference subcarrier 22 6 or the phase information
encoded onto the preceding (i.e. z-1, where z"1 denotes the
unit delay operator) subcarrier to a summing point 230. At
the output of the differential phase encoder 222, the phase
information theta[k] with which the respective subcarriers
are to be encoded is provided. In preferred embodiments of
the present invention, the subcarriers of a MCM symbol are
equally spaced in the frequency axis direction.
The output of the differential phase encoder 222 is
connected to a unit 232 for generating complex subcarrier
symbols using the phase information theta[k]. To this end,
the K differentially encoded phases are converted to complex
symbols by multiplication with
factor * e3*[2*pi*(theta[k] + PHI)1 (Eq.4)
wherein factor designates a scale factor and PHI designates
an additional angle. The scale factor and the additional
angle PHI are optional. By choosing PHI = 45° a rotated
DQPSK signal constellation can be obtained.
Finally, assembly of a MCM symbol is effected in an
assembling unit 234. One MCM symbol comprising Nppir sub-
carriers is assembled from NFFT-K-1 guard band symbols which
are "zero", one reference subcarrier symbol and K DQPSK
subcarrier symbols. Thus, the assembled MCM symbol 200 is
composed of K complex values containing the encoded
information, two guard bands at both sides of the Npprp
complex values and a reference subcarrier symbol.
The MCM symbol has been assembled in the frequency domain.
For transformation into.the time domain an inverse discrete
Fourier transform (IDFT) of the output of the assembling
unit 234 is performed by a transformator 236. In preferred
embodiments of the present invention, the transformator 236
is adapted to perform a fast Fourier transform (FFT).
Further processing of the MCM signal in the transmitter as
well as in the receiver is as described above referring to
Figure 7.
At the receiver a de-mapping device 142 (Figure 7) is needed
to reverse the operations of the mapping device described
above referring to Figure 2. The implementation of the
demapping device is straightforward and, therefore, need not
be described herein in detail.
However, systematic phase shifts stemming from echoes in
multipath environments may occur between subcarriers in the
same MCM symbol. This phase offsets can cause bit errors
when demodulating the MCM symbol at the receiver.
Thus, it is preferred to make use of an algorithm to correct
the systematic phase shifts stemming from echoes in multi-
path environments. Preferred embodiments of echo phase
offset correction algorithms are explained hereinafter
referring to Figures 3 to 6.
In Figures 3A and 3B, scatter diagrams at the output of a
differential demapper of a MCM receiver are shown. As can be
seen from Figure 3A, systematic phase shifts between
subcarriers in the same MCM symbol cause a rotation of the
demodulated phase shifts with respect to the axis of the
complex coordinate system. In Figure 3B, the demodulated
phase shifts after having performed an echo phase offset
correction are depicted. Now, the positions of the signal
points are substantially on the axis of the complex
coordinate system. These positions correspond to the
modulated phase shifts of 0°, 90°, 180° and 270°,
respectively.
An echo phase offset correction algorithm (EPOC algorithm)
must calculate the echo induced phase offset from the signal
space constellation following the differential demodulation
and subsequently correct this phase offset.
For illustration purposes, one may think of the simplest
algorithm possible which eliminates the symbol phase before
computing the mean of all phases of the subcarriers. To
illustrate the effect of such an EPOC algorithm, reference
is made to the two scatter diagrams of subcarriers symbols
contained in one MCM symbol in Figures 3A and 3B. This
scatter diagrams have been obtained as result of an MCM
simulation. For the simulation a channel has been used which
might typically show up in single frequency networks. The
echoes of this channel stretched to the limits of the MCM
guard interval. The guard interval was chosen to be 25% of
the MCM symbol duration in this case.
Figure 4 represents a block diagram for illustrating the
position and the functionality of an echo phase offset
correction device in a MCM receiver. The signal of a MCM
transmitter is transmitted through the channel 122 (Figures
4 and 7) and received at the receiver frontend 132 of the
MCM receiver. The signal processing between the receiver
frontend and the fast Fourier transformator 140 has been
omitted in Figure 4. The output of the fast Fourier
transformator is applied to the de-mapper, which performs a
differential de-mapping along the frequency axis. The output
of the de-mapper are the respective phase shifts for the
subcarriers. The phase offsets of this phase shifts which
are caused by echoes in multipath environments are
visualized by a block 400 in Figure 4 which shows an example
of a scatter diagram of the subcarrier symbols without an
echo phase offset correction.
The output of the de-mapper 142 is applied to the input of
an echo phase offset correction device 402. The echo phase
offset correction device 402 uses an EPOC algorithm in order
to eliminate echo phase offsets in the output of the de-
mapper 142. The result is shown in block 404 of Figure 4,
i.e. only the encoded phase shifts, 0°, 90°, 180° or 270°
are present at the output of the correction device 402. The
output of the correction device 402 forms the signal for the
metric calculation which is performed in order to recover
the bitstream representing the transmitted information.
A first embodiment of an EPOC algorithm and a device for
performing same is now described referring to Figure 5.
The first embodiment of an EPOC algorithm starts from the
assumption that every received differentially decoded
complex symbol is rotated by an angle due to echoes in the
multipath channel. For the subcarriers equal spacing in
frequency is assumed since this represents a preferred
embodiment of the present invention. If the subcarriers were
not equally spaced in frequency, a correction factor would
have to be introduced into the EPOC algorithm.
Figure 5 shows the correction device 402 (Figure 4) for
performing the first embodiment of an EPOC algorithm.
From the output of the de-mapper 142 which contains an echo
phase offset as shown for example in Figure 3A, the phase
shifts related to transmitted information must first be
discarded. To this end, the output of the de-mapper 142 is
applied to a discarding unit 500. In case of a DQPSK
mapping, the discarding unit can perform a "(.)4" operation.
The unit 500 projects all received symbols into the first
quadrant. Therefore, the phase shifts related to transmitted
information is eliminated from the phase shifts representing
the subcarrier symbols. The same effect could be reached
with a modulo-4 operation.
Having eliminated the information related symbol phases in
unit 500, the first approach to obtain an estimation would
be to simply compute the mean value over all symbol phases
of one MCM symbol. However, it is preferred to perform a
threshold decision before determining the mean value over
all symbol phases of one MCM symbol. Due to Rayleigh fading
some of the received symbols may contribute unreliable
information to the determination of the echo phase offset.
Therefore, depending on the absolute value of a symbol, a
threshold decision is performed in order to determine
whether the symbol should contribute to the estimate of the
phase offset or not.
Thus, in the embodiment shown in Fig. 5, a threshold
decision unit 510 is included. Following the unit 500 the
absolute value and the argument of a differentially decoded
symbol is computed in respective computing units 512 and
514. Depending on the absolute value of a respective symbol,
a control signal is derived. This control signal is compared
with a threshold value in a decision circuit 516. If the
absolute value, i.e. the control signal thereof, is smaller
than a certain threshold, the decision circuit 516 replaces
the angle value going into the averaging operation by a
value equal to zero. To this end, a switch is provided in
order to disconnect the output of the argument computing
unit 514 from the input of the further processing stage and
connects the input of the further processing stage with a
unit 518 providing a constant output of "zero".
An averaging unit 52 0 is provided in order to calculate a
mean value based on the phase offsets ^ determined for the
individual subcarrier symbols of a MCM symbol as follows:

In the averaging unit 520, summation over K suramands which
have not been set to zero in the unit 516 is performed. The
output of the averaging unit 520 is provided to a hold unit
522 which holds the output of the averaging unit 520 K
times. The output of the hold unit 522 is connected with a
phase rotation unit 524 which performs the correction of the
phase offsets of the K complex signal points on the basis of
the mean value-f.
The phase rotation unit 524 performs the correction of the
phase offsets by making use of the following equation:

In this equation, v^' designates the K phase corrected
differentially decoded symbols for input into the soft-
metric calculation, whereas v^ designates the input symbols.
As long as a channel which is quasi stationary during the
duration of one MCM symbols can be assumed, using the mean
value over all subcarriers of one MCM symbol will provide
correct results.
A buffer unit 527 may be provided in order to buffer the
complex signal points until the mean value of the phase
offsets for one MCM symbol is determined. The output of the
phase rotation unit 524 is applied to the further processing
stage 526 for performing the soft-metric calculation.
With respect to the results of the above echo phase offset
correction, reference is made again to Figures 3A and 3B.
The two plots stem from a simulation which included the
first embodiment of an echo phase offset correction
algorithm described above. At the instant of the scatter
diagram snapshot shown in Figure 3A, the channel obviously
distorted the constellation in a way, that a simple angle
rotation is a valid assumption. As shown in Figure 3B, the
signal constellation can be rotated back to the axis by
applying the determined mean value for the rotation of the
differentially detected symbols.
A second embodiment of an echo phase offset correction
algorithm is described hereinafter. This second embodiment
can be preferably used in connection with multipath channels
that have up to two strong path echoes. The algorithm of the
second embodiment is more complex than the algorithm of the
first embodiment.
What follows is a mathematical derivation of the second
embodiment of a method for echo phase offset correction. The
following assumptions can be made in order to ease the
explanation of the second embodiment of an EPOC algorithm.
In this embodiment, the guard interval of the MCM signal is
assumed to be at least as long as the impulse response h[q],
q = 0, 1, ..., Qh-1 of the multipath channel.
At the transmitter every MCM symbol is assembled using
frequency axis mapping explained above. The symbol of the
reference subcarrier equals 1, i.e. 0 degree phase shift.
The optional phase shift PHI equals zero, i.e. the DQPSK
signal constellation is not rotated.
Using an equation this can be expressed as

From the two possible solutions of the quadratic equation
mentioned above, Equation 23 is the one solution that cannot
cause an additional phase shift of 180 degrees.
The two plots in Figure 6 show the projection of the EPOC
algorithm of the second embodiment for one quadrant of the
complex plane. Depicted here is the quadratic grid in the
sector |arg(z)| < 7r/4 and the straight line y = f(x) = a+b-x
with a = -1.0 and b = 0.5 (dotted line). In case of a
noise-free channel, all received symbols will lie on this
straight line if 1+jO was send. The circle shown in the
plots determines the boarder line for the two cases of
Equation 23. In the left part, Figure 6 shows the situation
before the projection, in the right part, Figure 6 shows the
situation after applying the projection algorithm. By
looking on the left part, one can see, that the straight
line now lies on the real axis with 2+jO being the fix point
of the projection. Therefore, it can be concluded that the
echo phase offset correction algorithm according to the
second embodiment fulfills the design goal.
Before the second embodiment of an EPOC algorithm can be
applied, the approximation line through the received symbols
has to be determined, i.e. the parameters a and b must be
estimated. For this purpose, it is assumed that the received
symbols lie in sector |arg(z)| < n/4, if 1+jO was sent. If
symbols other than 1+jO have been sent, a modulo operation
can be applied to project all symbols into the desired
sector. Proceeding like this prevents the necessity of
deciding on the symbols in an early stage and enables
averaging over all signal points of one MCM symbol (instead
of averaging over only \ of all signal points).
For the following computation rule for the EPOC algorithm"of
the second embodiment, x^ is used to denote the real part of
the i-th signal point and y^ for its imaginary part,
respectively (i = 1, 2,..., K) . Altogether, K values are
available for the determination. By choosing the method of
least sguares, the straight line which has to be determined
can be obtained by minimizing
can be applied. However, the trade-off will be a much higher
computational complexity.
To avoid problems with the range in which the projection is
applicable, the determination of the straight line should be
separated into two parts. First, the cluster's centers of
gravity are moved onto the axes, following, the signal space
is distorted. Assuming that a and b are the original
parameters of the straight line and a is the rotation angle,
fK(.) has to be applied with the transformed parameters
Besides the two EPOC algorithms explained above section,
different algorithms can be designed that will, however,
most likely exhibit a higher degree of computational
complexity.
The new mapping method for Multicarrier Modulation schemes
presented herein consists in principal of two important
aspects. Differential mapping within one MCM symbol along
the frequency axis direction and correction of the channel
echo related phase offset on the subcarriers at the receiver
side. The advantage of this new mapping scheme is its
robustness with regard to rapidly changing multipath
channels which may occur at high frequencies and/or high
speeds of mobile receivers.
WE CLAIM:
1. A method of performing an echo phase offset correction
in a multi-carrier demodulation system, comprising the
steps of:
differential phase decoding (142) phase shifts based on
a phase difference between simultaneous carriers having
different frequencies;
determining an echo phase offset for each decoded phase
shift by eliminating (500) phase shift uncertainties
related to the transmitted information from said decoded
phase shift;
averaging (520) said echo phase offsets in order to
generate an averaged offset; and
correcting (524) each decoded phase shift based on said
averaged offset.
The method according to claim 1, wherein said step of
differential phase decoding comprises the step of
differential phase decoding phase shifts based on a
phase difference between simultaneous carriers which are
adjacent in the frequency axis direction.
The method according to claim 1 or 2, wherein said step
of differential phase decoding comprises the step of
differential phase decoding phase shifts based on phase
differences between at least three simultaneous carriers
which are equally spaced in the frequency axis
direction.
The method according to one of claims 1 to 3, which
involves a step of comparing (516) an absolute value
of a symbol associated with a respective decoded phase
shift with a threshold, wherein only phase shifts having
associated therewith symbols having an absolute value
exceeding said threshold are used in said step of
averaging said echo phase offsets.
5. A method of performing an echo phase offset correction
in a multi-carrier demodulation system, comprising the
steps of:
differential phase decoding phase shifts based on a
phase difference between simultaneous carriers having
different frequencies, said phase shifts defining signal
points in a complex plane;
pre-rotating said signal points into the sector of said
complex plane between -45° and +45°;
determining parameters (a, b) of a straight line
approximating the location of said pre-rotated signal
points in said complex plane;
determining a phase offset based on said parameters (a,
b); and
correcting each decoded phase shift based on said phase
offset.
6. The method according to claim 5, wherein said
simultaneous carriers are equally spaced in the
frequency axis direction.
7. The method according to claim 5 or 6, wherein said step
of determining said parameters (a, b) comprises a least
squares method for selecting those parameters which
minimize the deviations of said pre-rotated signal
points from said straight line.
8. The method according to claim 7, wherein said parameters
(a, b) are determined as follows:
10. An echo phase offset correction device for a multi-
carrier demodulation system, comprising:
a differential phase decoder (142) for decoding phase
shifts based on a phase difference between simultaneous
carriers having different frequencies;
means for determining an echo phase offset for each
decoded phase shift comprising means (500) for
eliminating phase shift uncertainties related to the
transmitted information from said decoded phase shift;
means (520) for averaging said echo phase offsets in
order to generate an averaged offset; and
means (524) for correcting each decoded phase shift
based on said averaged offset.
11. The device according to claim 10, wherein said
differential phase decoder is adapted for decoding said
phase shifts based on a phase difference between
simultaneous carriers which are adjacent in the
frequency axis direction.
12. The device according to claim 10 or 11,
having means (516) for comparing an absolute value
of a symbol associated with a respective decoded phase
shift with a threshold, wherein said means for averaging
said phase offsets only uses phase shifts having
associated therewith symbols having an absolute value
exceeding said threshold.
13. The device according to one of claims 10 to 12, wherein
said differential phase decoder is adapted for decoding
said phase shifts based on phase differences between at
least three simultaneous carriers which are equally
spaced in the frequency axis direction.
14. An echo phase offset correction device for a multi-
carrier demodulation system, comprising:
a differential phase decoder for decoding phase shifts
based on a phase difference between simultaneous
carriers having different frequencies, said phase shifts
defining signal points in a complex plane;
means for pre-rotating said signal points into the
sector of said complex plane between -45° and +45°;
means for determining parameters (a, b) of a straight
line approximating the location of said pre-rotated
signal points in said complex plane;
means for determining a phase offset based on said
parameters (a, b); and
means for correcting each decoded phase shift based on
said phase offset.
15. The device according to claim 14, wherein said
differential phase decoder comprises means for decoding
phase shifts of at least three simultaneous carriers
which are equally spaced in the frequency axis
direction.
16. The device according to claim 14 or 15, wherein said
means for determining said parameters (a, b) comprises
means for performing a least squares method for
selecting those parameters which minimize the deviations
of said pre-rotated signal points from said straight
line.
17. The device according to claim 16, wherein said means for
determining said parameters (a, b) calculates said
parameters (a, b) as follows:
19. A method for performing an echo phase offset corrections in a multi-carrier
demodulation system, substantially as herein described.
20. An echo phase offset correction device for a multi-carrier demodulation
system, substantially as herein described with particular reference to the
accompanying drawings.

A method of mapping information onto at least two simul-
taneous carriers (202, 206, 208) having different fre-
quencies in a multi-carrier modulation system involves the
step of controlling respective parameters of the at least
two carriers such that the information is differential
encoded. A method of de-mapping information based on at
least two simultaneous encoded carriers having different
frequencies in a multi-carrier demodulation system comprises
the step of recovering the information by differential
decoding (14 2) of respective parameters of the at least two
carriers. In a method of performing an echo phase offset
correction in a multi-carrier demodulation system, phase
shifts are differential phase decoded (142) based on a phase
difference between simultaneous carriers having different
frequencies. An echo phase offset is determined for each
decoded phase shift by eliminating (500) phase shift
uncertainties corresponding to codeable phase shifts from
the decoded phase shift. The echo phase offsets are averaged
(520) in order to generate an averaged offset. Finally, each
decoded phase shift is corrected (524) based on the averaged
offset.