Description
Title of Invention:
PILOT PATTERNS FOR OFDM SYSTEMS WITH MULTI PLE ANTENNAS
Technical Field
[0001] The present invention relates to OFDM (orthogonal frequency-division multiplexing)
communication systems with a plurality of (e.g., four) transmit antennas and one or
more receive antennas, and in particular to methods for inserting scattered pilots into
the transmit signals of such OFDM systems, for estimating channel properties on the
basis of the scattered pilots, a multi-antenna OFDM transmitter, and an OFDM
receiver.
Background Art
[0002] Orthogonal Frequency-Division Multiplexing (OFDM) is a digital multi-carrier
modulation scheme, which uses a large number of closely-spaced orthogonal sub-
carriers. Each sub-carrier is modulated with a conventional modulation scheme (such
as quadrature amplitude modulation) at a low symbol rate, maintaining data rates
similar to conventional single-carrier modulation schemes in the same bandwidth.
[0003] The primary advantage of OFDM over single-carrier schemes is its ability to cope
with severe channel conditions, for example, attenuation of high frequencies at a long
copper wire, narrowband interference and frequency-selective fading due to multipath,
without complex equalization filters. Channel equalization is simplified because
OFDM may be viewed as using many slowly-modulated narrowband signals rather
than one rapidly-modulated wideband signal. Low symbol rate makes the use of a
guard interval between symbols affordable, making it possible to handle time-
spreading and eliminate inter-symbol interference.
[0004] In OFDM communications, scattered pilots (SP) are typically used for channel es-
timation and equalization. Scattered pilots are complex OFDM cells with known phase
and amplitude arranged in frequency and time according to a defined pattern. The
pilots are typically selected from a binary alphabet, e.g., {+1; -1} and are boosted in
power compared to the data cells.
[0005] Figure 1 shows an example of such a pattern in the form of a diagonal grid, as it is
used in DVB-T (digital video broadcasting terrestrial), i.e., a digital broadcasting
standard based on OFDM (cf. the ETSI Standard ETS 300 744, "Digital Broadcasting
Systems for Television, Sound and Data Services; framing structure, channel coding
and modulation for digital terrestrial television"). The pilots are indicated by the black
circles, whereas the data cells are indicated by open circles.
[0006] The scattered pattern shown in Fig. 1 is characterized by two parameters: Ds is the
distance between SPs that are adjacent along the time axis in a pilot-bearing subcarrier,
and Dk is the distance between two SP-bearing subcarriers that are adjacent along the
frequency axis. These two parameters are also referred to as the SP spacing in time and
frequency, respectively. In this type of pattern, SPs are present in every OFDM symbol
and the distance between two SPs in a symbol is DsDk. The present invention builds on
such a SP pattern. In DVB-T, Ds = 4 and Dk = 3, as shown in Fig. 1.
[0007] Since the channel (the state of the channel) usually varies (fades) in time, due to
Doppler variations, and in frequency, due to multi-path delay, the SP pattern must be
dense enough to sample the channel variations along both axes as required by the
sampling theorem. The Ds parameter defines the sampling along the time axis, whereas
the Dk parameter defines the sampling along the frequency axis.
[0008] The channel estimation process consists of two steps. First, the channel is estimated
at the SP positions by dividing the received value by the known pilot value (the
reference signal). Second, the channel estimates for the other cells are computed by in-
terpolating between the estimates at the SP positions. The interpolation is conceptually
two-dimensional, but can be practically performed by first interpolating in time then in
frequency, as shown in Fig. 9. Moreover, interpolation can be combined with noise
reduction in order to improve the accuracy of the estimate.
[0009] In order to increase the communication reliability, multiple transmitters operating in
parallel in the same frequency band can be used. This is referred to in the art as
multiple-input single-output (MISO), when there is one receiver, or multiple-input
multiple-output (MIMO), when there are multiple receivers. As in the single-
transmitter case, channel estimation is required for coherent demodulation.
[0010] In the general MIMO case, the channel between each transmitter and each receiver
must be estimated. A MIMO configuration for 4 transmitters and 2 receivers is shown
in Fig. 10 as an example. We can express the received signal vector y as a function of
the transmitted signal vector x and the channel matrix H as shown in the following
Math. 1.
[0011]
[Math.1]

wherein N is the number of transmitters, e.g., N = 4, and M is the number of receivers.
All quantities are complex valued.
[0012] A channel estimation process is performed by each receiver independently. For
channel estimation purposes the number of receivers is therefore irrelevant. The signal
seen by a receiver can be written as shown in the following Math. 2.
[0013] [Math.2]

[0014] Each receiver produces estimates of the N channel components h1,..., hN at each
OFDM cell based on the values received at the SP locations.
[0015] Any implementation of an MISO or MIMO system based on OFDM thus has to
define (i) how the scattered pilots are to be encoded so that the N channel components
can be easily estimated in the receiver and (ii) how the scattered pilots are to be
arranged in time and frequency.
[0016] The key idea for estimating the channel components is to employ different SPs for
different transmitters. In order to be able to estimate the individual channel
components, the SPs are partitioned into as many subsets as there are transmitters. All
pilots belonging to a subset are multiplied by a constant coefficient that depends on the
subset and the transmitter (transmit antenna). In the four-transmitter (four-transmit
antenna) case, there are 16 coefficients, which can be expressed as a 4 x 4 matrix, as
shown in Figs. 12A and 12B. The rows correspond to the transmitters (transmit
antennas) and the columns to the SP subsets.
[0017] According to Fig. 12A, a pilot that is transmitted by a transmit antenna n and belongs
to subset m is multiplied by a coefficient Cmn. For example, a result of multiplying a
pilot by one of the four coefficients C11 (for the transmit antenna 1), Cl2 (for the
transmit antenna 2), C13 (for the transmit antenna 3), and Cl4 (for the transmit antenna
4) is used as an SP to be allocated to signals transmitted in subset 1.
[0018] Regarding the values of the coefficients, there is one necessary and sufficient
condition that must be met in order to be able to separate the channel components in
the receiver: the coefficient matrix must be full rank, i.e. invertible. The justification is
readily apparent if we write the received values at the pilot locations as in the
following Math. 3.
[0019] [Math.3]

[0020] In the above Math. 3, pm is the original value (before multiplication) of a pilot in
subset m, ym is the received value at the location of the said pilot, hn is the channel
between transmitter (transmit antenna) n and receiver, and cmn is the constant pilot co-
efficient for subset m and transmitter (transmit antenna) n. For simplicity the channel
noise has not been considered here.
[0021] The original pilot values pm are irrelevant. Denoting the ym / pm ratio by em, the
equation of the above Math. 3 can be expressed in matrix form shown in the following
Math. 4.
[0022] [Math.4]

[0023] The channel estimates for the channels h1 to hN can be computed by left multiplying
the em estimates by the inverse of the coefficients matrix as shown in the following
Math. 5.
[0024]
[Math.5]

[0025] As apparent from the above Math. 5, an inverse of the coefficients matrix is in-
evitable for channel estimation. This is the reason why the coefficients matrix must be
full rank.
[0026] Although any full-rank complex matrix will do, the following two matrices are
typically used in the art because of their simplicity:
[0027] Unitary diagonal matrix (exists for any N)
[0028] [Math.6]

[0029] Hadamard matrix (exists only for N = 2 or a multiple of 4)
[0030] [Math.7]

[0031] Physically, using a unitary diagonal matrix means that for antenna n only the pilots in
subset n are non-zero. This means that the received values at those positions corre-
sponding to pilots of subset n can be used for computing the estimate of channel
component hn for that position, with no further signal processing.
[0032] Using a Hadamard matrix for pilot encoding requires a multiplication with this
matrix to be performed in the receiver for each cell. Such a multiplication is also
referred to as a Hadamard transform. The schematic of an optimized implementation,
known in the art as the fast Hadamard transform, is shown in Fig. 13.
[0033] The remaining question is how to arrange the scattered pilots in time and frequency.
One option is to keep the same SP pattern like in the single-transmitter case, in which
case the placement is known and only the partitioning into subsets has to be clarified.
[0034] Patent Citation 1 provides one possible solution to the question of how to arrange the
scattered pilots in time and frequency. According to Patent Citation 1, the scattered
pilots are arranged in accordance with the same pattern as in the single-transmitter case
in time and frequency, and different subsets of pilots are allocated to different pilot-
bearing subcarriers. In other words, the scattered pilots are partitioned into subsets
according to their subcarrier index. Thus, the N subsets of scattered pilots are evenly
interleaved in frequency, i.e. along the subcarrier axis. Figures 2 and 3 illustrate
examples of subset arrangement patterns as taught by Patent Citation 1 for two and
four transmitters, respectively.
[0035] An alternative approach is known from Patent Citation 2, according to which the N
subsets of scattered pilots are evenly (equally-spaced) interleaved in time, i.e. along the
symbol axis. Figures 4 and 5 illustrate examples of subset arrangement patterns as
taught by Patent Citation 2 for two and four transmitters, respectively.
[0036] Instead of keeping the pilot pattern of the single-transmitter case, pilots of the N
subsets may be grouped, as indicated in Fig. 6. This approach is called "grouped in-
terleaving" and is to be contrasted to the approach of "equally-spaced interleaving" or
"even interleaving" illustrated in Figs. 2 to 5. In the case of four transmitters (transmit
antennas), there may be groups of four pilots (from subsets 1/2/3/4) or groups of two
pilots (from subsets 1/2 and 3/4 for example). Fig. 6 shows the former case. Referring
to Fig. 6, the groups themselves are arranged so as to be scattered in time and
frequency, just like the individual scattered pilots in the single-transmitter case.
[0037] In general, interleaving N pilot subsets in one direction (time or frequency) increases
the effective pilot distance in said direction by a factor of N. In order to compensate for
this effect and preserve the effective distance, the physical pilot distance must be
decreased by the same factor. Thus, if 4 subsets are interleaved in frequency, as in
Patent Citation 1, Dk must be reduced by a factor of 4. Likewise, if the 4 subsets are
multiplexed in time, as in Patent Citation 2, Ds must be reduced by the same factor.
[0038] Since the physical Dk and Ds must be integers, the effective distances Dk,eff and Ds,eff
will always be multiples of 4 when interleaving the subsets in one direction only. Such
a granularity of the subsets may be too coarse for some applications.
Citation List
Patent Literature
[0039] PTL 1:GB 2449470A
PTL 2: WO 2009/001528 A1
Summary of Invention
Technical Problem
[0040] It is therefore an object of the present invention to provide an SP pattern with a finer
granularity without increasing the total number of pilots in OFDM systems with a
plurality of (e.g., four) transmit antennas. It is also an object of the present invention to
provide a method used by a receiver compatible with multi-antenna transmitters to
perform channel estimation in OFDM systems with a plurality of (e.g., four) transmit
antennas.
Solution to Problem
[0041] The above objects are achieved by the features as set forth in the independent claims.
[0042] Preferred embodiments are the subject matter of dependent claims.
[0043] It is the particular approach of the present invention to keep the same SP pattern like
in the single-transmitter case, to partition the pilots into as many subsets as there are
transmit antennas, and to interleave these subsets both in time and in frequency.
[0044] According to a first aspect of the present invention, a multi-antenna OFDM
transmitter is provided. The multi-antenna OFDM transmitter has N antennas, N being
an integer greater than or equal to two. The multi-antenna OFDM transmitter
comprises: a multi-antenna encoder for generating a plurality of data streams, one for
each of the N transmit antennas, each data stream consisting of a succession of OFDM
symbols, each OFDM symbol consisting of a plurality of OFDM cells, each OFDM
cell being associated with one of a plurality of subcarriers; a pilot generation unit for
generating, for each of the plurality of data streams, a plurality of scattered pilots, said
plurality of scattered pilots being partitioned into M subsets, each of the scattered
pilots being encoded on the basis of the subset to which the scattered pilot belongs and
the data stream into which the scattered pilot is to be inserted, and a plurality of pilot
inserting units, each pilot inserting unit for inserting one of the plurality of scattered
pilots into a corresponding one of the plurality of data streams in accordance with a
predefined periodic pattern in which a temporal spacing between two OFDM symbols
having scattered pilots in OFDM cells associated with the same subcarrier is equal to D
s and a frequency spacing of two subcarriers bearing scattered pilots in any of the
OFDM symbols is equal to Dk, each of Ds and Dk being an integer greater than or equal
to two, wherein M is greater than or equal to N and satisfies a relationship M = MsMk,
each of Ms and Mk being an integer greater than or equal to two, and each of the
plurality of pilot inserting units inserts the scattered pilots in such a manner that a
temporal spacing between two OFDM symbols having scattered pilots of the same
subset in OFDM cells associated with the same subcarrier is equal to DSMS and a
frequency spacing of two subcarriers bearing scattered pilots of the same subset in any
of the OFDM symbols is equal to DkMk.
[0045] In the above multi-antenna OFDM transmitter, each of M and N may be equal to
four, and each of Ms and Mk may be equal to two.
[0046] In the above multi-antenna OFDM transmitter, Dk may be equal to two, three, or
four.
[0047] In the above multi-antenna OFDM transmitter, the pilot generation unit may encode
the scattered pilots by, for each subset, multiplying all scattered pilots of the subset
with a constant coefficient that depends on the subset and the data stream into which
said all scattered pilots of the subset are to be inserted.
[0048] In the above multi-antenna OFDM transmitter, a matrix formed by the constant coef-
ficients used for multiplying the scattered pilots may be invertible, in particular a
unitary diagonal matrix or a Hadamard matrix.
[0049] According to a further aspect of the present invention, an OFDM receiver is
provided. The OFDM receiver comprises: an OFDM demodulator for obtaining a data
stream consisting of a succession of OFDM symbols, each OFDM symbol consisting
of a plurality of OFDM cells, each OFDM cell being associated with one of a plurality
of subcarriers; a pilot extraction unit for (i) extracting scattered pilots from the data
stream in accordance with a predefined periodic pattern in which a temporal spacing
between two OFDM symbols having scattered pilots in OFDM cells associated with
the same subcarrier is equal to Ds and a frequency spacing of two subcarriers bearing
scattered pilots in any of the OFDM symbols is equal to Dk, each of Ds and Dk being an
integer greater than or equal to two, and (ii) partitioning the extracted scattered pilots
into M subsets; and a channel estimation unit for estimating a plurality of channel
components from the M subsets of scattered pilots, each channel component rep-
resenting a channel condition between one of a plurality of transmitters and the OFDM
receiver, wherein M satisfies a relationship M = MsMk, each of Ms and Mk being an
integer greater than or equal to two, and a temporal spacing between two OFDM
symbols having scattered pilots of the same subset in OFDM cells associated with the
same subcarrier is equal to MsDs and a frequency spacing of two subcarriers bearing
scattered pilots of the same subset in any of the OFDM symbols is equal to MkDk.
[0050] In the above OFDM receiver, M may be equal to four, and each of Ms and Mk may be
equal to two.
[0051] In the above OFDM receiver, Dk may be equal to two, three, or four.
[0052] In the above OFDM receiver, the pilot extraction unit may extract, for each OFDM
symbol, at least one continual pilot from OFDM symbols associated with predefined
subcarriers and partition the extracted continual pilots into the M subsets, and the
channel estimation unit may estimate the plurality of channel components from the M
subsets of scattered pilots and continual pilots.
[0053] In the above OFDM receiver, the predefined subcarriers may be the subcarriers
bearing scattered pilots.
[0054] In the above OFDM receiver, the predefined subcarriers may be distinct from the
subcarriers bearing scattered pilots.
[0055] In the above OFDM receiver, continual pilots extracted from the same subcarrier
may be partitioned into the same subset.
[0056] In the above OFDM receiver, continual pilots extracted from the same subcarrier
may be partitioned into at least two different subsets.
[0057] According to a further aspect of the present invention, a method for inserting
scattered pilots into transmit signals is provided. The method is used by a multi-
antenna transmitter with N transmit antennas for inserting scattered pilots into transmit
signals. The method comprises the steps of: generating a plurality of data streams, one
for each of the N transmit antennas, each data stream consisting of a succession of
OFDM symbols, each OFDM symbol consisting of a plurality of OFDM cells, each
OFDM cell being associated with one of a plurality of subcarriers; generating, for each
of the plurality of data streams, a plurality of scattered pilots, said plurality of scattered
pilots being partitioned into M subsets, each of the scattered pilots being encoded on
the basis of the subset to which the scattered pilot belongs and the data stream into
which the scattered pilot is to be inserted, and inserting one of the plurality of scattered
pilots into a corresponding one of the plurality of data streams in accordance with a
predefined periodic pattern in which a temporal spacing between two OFDM symbols
having scattered pilots in OFDM cells associated with the same subcarrier is equal to D
s and a frequency spacing of two subcarriers bearing scattered pilots in any of the
OFDM symbols is equal to Dk, each of Ds and Dk being an integer greater than or equal
to two, wherein M is greater than or equal to N and satisfies a relationship M = MsMk,
each of Ms and Mk being an integer greater than or equal to two, and in the inserting
step, the scattered pilots are inserted in such a manner that a temporal spacing between
two OFDM symbols having scattered pilots of the same subset in OFDM cells as-
sociated with the same subcarrier is equal to DsMs and a frequency spacing of two sub-
carriers bearing scattered pilots of the same subset in any of the OFDM symbols is
equal to DkMk.
[0058] In the above method, each of M and N may be equal to four, and each of Ms and Mk
may be equal to two.
[0059] According to a further aspect of the present invention, a method for estimating, at an
OFDM receiver, channel properties between the OFDM receiver and each of N
transmit antennas is provided. The method comprises the steps of: obtaining a data
stream consisting of a succession of OFDM symbols, each OFDM symbol consisting
of a plurality of OFDM cells, each OFDM cell being associated with one of a plurality
of subcarriers; extracting scattered pilots from the data stream in accordance with a
predefined periodic pattern in which a temporal spacing between two OFDM symbols
having scattered pilots in OFDM cells associated with the same subcarrier is equal to D
s and a frequency spacing of two subcarriers bearing scattered pilots in any of the
OFDM symbols is equal to Dk, each of Ds and Dk being an integer greater than or equal
to two, and partitioning the extracted scattered pilots into M subsets; and estimating a
plurality of channel components from the M subsets of scattered pilots, each channel
component representing a channel condition between one of a plurality of transmitters
and the OFDM receiver, wherein M satisfies a relationship M = MsMk, each of Ms and
Mk being an integer greater than or equal to two, and a temporal spacing between two
OFDM symbols having scattered pilots of the same subset in OFDM cells associated
with the same subcarrier is equal to MsDs and a frequency spacing of two subcarriers
bearing scattered pilots of the same subset in any of the OFDM symbols is equal to Mk
Dk.
[0060] In the above method, each of M and N may be equal to four, and each of M£ and Mk
may be equal to two.
[0061] The above and other objects and features of the present invention will become more
apparent from the following description and preferred embodiments given in con-
junction with the accompanying drawings.
Brief Description of Drawings
[0062] [fig. l]Fig. 1 shows a conventional single-transmitter SP pattern as it is used in the
European digital broadcasting standard DVB-T.
[fig.2]Fig. 2 shows a conventional two-transmitter (two-transmit antenna) SP pattern
with equally-spaced interleaving in frequency.
[fig.3]Fig. 3 shows a conventional four-transmitter (four-transmit antenna) SP pattern
with equally-spaced interleaving in frequency.
[fig.4]Fig. 4 shows a conventional two-transmitter (two-transmit antenna) SP pattern
with equally-spaced interleaving in time.
[fig.5]Fig. 5 shows a conventional four-transmitter (four-transmit antenna) SP pattern
with equally-spaced interleaving in time.
[fig.6]Fig. 6 shows a conventional SP pattern for four transmitters (transmit antennas)
and grouped interleaving in time and frequency.
[fig.7]Fig. 7 shows an SP pattern for four transmitters (transmit antennas) and equally-
spaced interleaving in time and frequency in accordance with an embodiment of the
present invention.
[fig.8]Fig. 8 shows another SP pattern for four transmitters (transmit antennas) and
equally-spaced interleaving in time and frequency in accordance with an embodiment
of the present invention.
[fig.9]Fig. 9 illustrates the channel estimation process for the single-transmitter case,
using separable interpolation in time and frequency.
[fig.10]Fig. 10 shows the receivers, transmitters, and the eight channel components in a
4x2 MIMO configuration.
[fig. 11]Fig. 11 shows an exemplary block diagram of a multi-antenna OFDM
transmitter.
[fig. 12A]Fig. 12A shows the 16 pilot-multiplication coefficients for the 4-transmitter
case.
[fig. 12B]Fig. 12B shows the preferred realization of Fig. 12A as a Hadamard matrix.
[fig.13]Fig. 13 illustrates the fast Hadamard transform.
[fig.14]Fig. 14 is an illustration of the channel estimation process in a receiver for
4-transmitter OFDM.
[fig.15]Fig. 15 shows a block diagram of a receiver for 4-transmitter OFDM, corre-
sponding to the process of Fig. 14.
[fig.l6]Fig. 16 shows a receiver configuration for 4 x 2 MIMO, where each OFDM
receiver provides its own channel estimates to the MIMO decoding stage.
[fig.17]Fig. 17 shows an example of an inventive SP pattern with additional continual
pilots located on SP-bearing subcarriers.
[fig. 18]Fig. 18 shows another example of an inventive SP pattern with additional
continual pilots located on non-SP-bearing subcarriers.
[fig.19]Fig. 19 shows an inventive SP pattern of signals transmitted from the first
antenna and coefficients by which the SPs are multiplied.
[fig.20]Fig. 20 shows an inventive SP pattern of signals transmitted from the second
antenna and coefficients by which the SPs are multiplied.
[fig.21]Fig. 21 shows an inventive SP pattern of signals transmitted from the third
antenna and coefficients by which the SPs are multiplied.
[fig.22]Fig. 22 shows an inventive SP pattern of signals transmitted from the fourth
antenna and coefficients by which the SPs are multiplied.
[fig.23]Fig. 23 shows an inventive SP pattern where there are six subsets, with equally-
spaced interleaving in both time and frequency.
[fig.24]Fig. 24 shows an inventive SP pattern where there are six subsets, with equally-
spaced interleaving in both time and frequency.
[fig.25]Fig. 25 shows an inventive SP pattern where there are six subsets, with equally-
spaced interleaving in both time and frequency.
[fig.26]Fig. 26 shows an inventive SP pattern where there are six subsets, with equally-
spaced interleaving in both time and frequency.
[fig.27]Fig. 27 shows an inventive SP pattern for each subset, which is distinct from
the SP pattern shown in Fig. 1.
[fig.28]Fig. 28 shows an exemplary configuration of a digital broadcast system
pertaining to an embodiment of the present invention.
[fig. 29] Fig. 29 is a functional structural diagram showing an exemplary structure of a
receiver pertaining to an embodiment of the present invention.
Description of Embodiments
[0063] The present invention provides methods for inserting scattered pilots (SPs) into the
transmit signals of multi-antenna OFDM systems, for estimating channel properties on
the basis of the scattered pilots, a multi-antenna OFDM transmitter, and a corre-
sponding OFDM receiver. The inventive methods and apparatuses achieve a high
granularity of the scattered pilots in both the time and the frequency directions, which
is a prerequisite for efficiently estimating channel properties in apparatuses that receive
signals into which scattered pilots have been inserted.
[0064] To this end, it is the particular approach of the present invention to keep the same
scattered pilot pattern like in the single-transmitter case, to partition the pilots into as
many subsets as there are transmit antennas, and to interleave these subsets both in
time and in frequency. In this manner, the granularity of the subsets is reduced
compared to conventional technologies since, in the four transmitter case, Dk,eff and D
s,eff are multiples of 2 instead of 4. This offers increased flexibility in designing the
scattered pilot patterns.
[0065] Fig. 7 shows a scattered pilot pattern for four transmitters and equally-spaced in-
terleaving in time and frequency in accordance with an embodiment of the present
invention. Each circle represents one OFDM cell, each row of circles corresponds to
one OFDM symbol, and each column represents one subcarrier. Pilots are indicated by
large circles, whereas data cells are indicated by small circles.
[0066] The scattered pilots are divided into four subsets denoted by numerals 1, 2, 3, and 4,
respectively. The number of subsets corresponds to the number of distinct transmitter
antennas, i.e., four in this case.
[0067] As it is apparent from a comparison of Fig. 1 and Fig. 7, the scattered pilots are
arranged in accordance with the same pattern as in the conventional single-transmitter
case (DVB-T), i.e., in the form of a diagonal grid. The pilot cells, whatever subset they
belong to, are interleaved with the data cells such that the distance between two
adjacent pilot cells in a pilot-bearing subcarrier in the time direction is Ds = 4 and the
distance between two adjacent SP-bearing subcarriers in the frequency direction is Dk
= 3.
[0068] The pilot cells of any given subset, on the other hand, are interleaved with the pilot
cells of the other subsets both in time and frequency. In other words, the subset to
which a pilot belongs alternates both in the frequency and the time directions. This is
to be contrasted with the conventional multi-antenna SP patterns of Figs. 2 and 3,
wherein a certain subcarrier or a certain OFDM symbol carries only pilots of one and
the same subset.
[0069] With the inventive SP pattern of Fig. 7, the effective distance in the time direction D
s,eff between two pilots of the same subset equals 2Ds, i.e., twice the SP spacing in the
time direction Ds, and the effective distance in the frequency direction Dk,eff between
two pilots of the same subset equals 2Dk, i.e., twice the SP spacing in the frequency
direction Dk.
[0070] This is to be contrasted with the conventional multi-antenna SP patterns of Fig. 3,
wherein the effective distance in the frequency direction Dk,eff between two pilots of the
same subset equals 4Dk, i.e., four times the SP spacing in the frequency direction Dk.
This is also to be contrasted with the conventional multi-antenna SP patterns of Fig. 5,
wherein the effective distance in the time direction Ds,eff between two pilots of the same
subset equals 4Ds, i.e., four times the SP spacing in the time direction Ds.
[0071] Hence, the inventive SP pattern of Fig. 7 provides a finer granularity in the dis-
tribution of scattered pilots of the same subset than the conventional SP patterns shown
in Figs. 3 and 5.
[0072] Fig. 8 shows another SP pattern for four transmitters and equally-spaced interleaving
in time and frequency in accordance with another embodiment of the present
invention. In this SP pattern, similar granularity is maintained both in time and
frequency. This SP pattern is in many aspects similar to that of Fig. 7 and achieves the
same advantages. A repetition of the detailed explanations provided in connection with
Fig. 7 is therefore omitted.
[0073] The advantages of the inventive SP patterns become even more apparent by
comparing "unit cells" of the SP lattices. A unit cell, originally defined in the context
of crystallography, is the smallest unit of a lattice from which the entire (periodic)
lattice may be reconstructed by means of translations only. Unit cells of the SP patterns
are indicated by dashed lines in Figs. 2 to 8. Obviously, the unit cells of the SP patterns
shown in Figs. 7 and 8 are more compact than those of Figs. 3 and 5, which are more
extensive in either the frequency (Fig. 3) or the time direction (Fig. 5).
[0074] Figure 11 shows a block diagram for a multi-antenna OFDM transmitter using
scattered pilots. The bits to be transmitted are fed at the input of an encoder 1110. In
the encoder 1110, these bits undergo a BICM (bit-interleaved coding and modulation)
encoding, which produces complex symbols at its output. The BICM encoding consists
of three basic steps: 1) FEC (forward error correction) encoding, 2) bit interleaving,
and 3) modulation. Such a process is well known in the art. The FEC code is typically
an LDPC (low-density parity check) code or a Turbo code, and the modulation is
typically QAM (quadrature amplitude modulation).
[0075] The complex symbols produced by the BICM encoder 1110 are fed to a multi-
antenna processor 1120, wherein they undergo a multi-antenna encoding, whereby the
input stream is encoded to generate multiple parallel streams of equal data rate, one for
each transmitter/antenna. The output streams carry exactly the same information as the
input streams. Typically an STBC (space-time block code) is used for this purpose.
There are many STBC variants known in the art.
[0076] The complex symbols produced by the STBC encoding are then mapped onto the
time-frequency OFDM grid, according to a specific algorithm, which does not make
the object of the present invention. The output of the mapping process consists of
OFDM symbols, which in turn consist of complex OFDM cells. The cells are then in-
terleaved by a set of symbol interleavers (1130-1, 1130-2, 1130-3, 1130-4) in order to
improve the frequency diversity of the encoded data. Such interleaving is well known
in the art and is also referred to as frequency interleaving since the OFDM symbols
span the available frequency bandwidth. The mapping and interleaving are identical for
all transmitters/antennas.
[0077] Following the OFDM symbol interleaving, the scattered pilots (SP) are generated by
pilot generation unit 1140 and inserted by a set of pilot insertion units (1150-1, 1150-2,
1150-3, 1150-4). The SPs are not the same for all transmitters/antennas. Each of the
pilot insertion units inserts the input SPs into the OFDM signals so as to achieve, for
example, the symbol patterns shown in Figs. 7 and 8. In order to be able to estimate the
individual channel components, the SPs are partitioned into as many subsets as there
are transmitters/antennas. All pilots belonging to a subset are multiplied by a constant
coefficient that depends on the subset and the transmitter (transmit antenna). In the
four-transmitter case, there are 16 coefficients, which can be expressed as a 4 x 4
matrix, as shown in Figs. 12A and 12B. The rows correspond to the transmitters
(transmit antennas) and the columns to the SP subsets.
[0078] The resulting OFDM symbols including data cells and pilot cells are then fed to the
OFDM modulators (1160-1, 1160-2, 1160-3, 1160-4), followed by up-converters
(1170-1, 1170-2, 1170-3, 1170-4), RF-amplifiers (1180-1, 1180-2,1180-3, 1180-4),
and finally transmitted via transmit antennas.
[0079] On the receiver side, the channel estimation process is similar to that used in the
single-transmitter case in Fig. 9. Instead of one channel estimation process, however,
four will be performed in parallel, one for each SP subset, as shown in Fig. 14. If a
pilot encoding is used, e.g. Hadamard, an additional processing step, e.g. Hadamard
transform, is required in order to separate the four channel components. This
processing step is performed for each OFDM cell independently.
[0080] A possible receiver block diagram for the OFDM-specific part is shown in Fig. 15. A
pilot extraction unit 1540, 2D interpolation units (1550-1, 1550-2, 1550-3, 1550-4),
and a Hadamard transform unit 1560 shown in Fig. 15 perform the process of Fig. 14.
[0081] An RF-frontend 1510 receives an RF signal, which is fed to a down converter 1520.
The down converter 1520 performs downconversion on the RF signal, which is then
fed to OFDM demodulator 1530. This RF signal fed from the down converter 1520 is
demodulated by the OFDM demodulator 1530. From the demodulated signal, the pilots
are extracted by pilot extraction unit 1540. The pilot extraction unit 1540 is adapted for
partitioning the extracted pilots into subsets in accordance with the inventive SP
patterns described above. Signals other than the pilots are output as data signals. The
pilots of each of these subsets are then fed to a corresponding one of a set of the 2D in-
terpolation units (1550-1, 1550-2, 1550-3, 1550-4). It is to be noted that the two
distinct interpolation steps are merged into a single block "2-D interpolation". The
signals obtained through the interpolation performed by the 2D interpolation units
(1550-1, 1550-2, 1550-3, 1550-4) are fed to the transform unit 1560 and converted into
channels. Depending on how the pilots of different subsets are encoded, a Hadamard
transform is applied in the transform unit 1560 in order to extract the channel estimates
h1,..., h4. A delay compensation unit 1580 is provided on the data path, and com-
pensates for a group delay introduced by the interpolation process based on the pilots
in each subset. By thus compensating for a data delay, the delay compensation unit
1580 realigns data and channels in each of symbol deinterleavers (1570-1, 1570-2,
1570-3, 1570-4).
[0082] The MIMO and BICM decoding are not OFDM specific, but require the data and the
associated channel estimates from all receivers/antennas. The actual decoding ar-
chitecture strongly depends on the particular STBC, as well as on the desired reception
performance. Optimal results are obtained when the complex symbols encoded in an
STBC block are decoded and demodulated jointly. Fig. 16 shows an exemplary block
diagram containing two OFDM receivers 1610-1 and 1610-2 feeding a common
MIMO and BICM decoding stage 1620.
[0083] According to a further aspect of the present invention, modified continual pilots (CP)
are inserted into the SP pattern. Conventional CPs are pilots that are present in every
symbol on a given subcarrier. They can be located on an SP-bearing subcarrier or on a
non-SP-bearing subcarrier and are generally not subject to any additional processing.
According to the present invention, however, the CPs are also partitioned into subsets
like the SPs.
[0084] Fig. 17 shows an example of an inventive SP pattern with additional continual pilots
located on SP-bearing subcarriers. The CPs on a given SP-bearing subcarrier are par-
titioned into the two subsets the SPs of that subcarrier belong to. In the figure, the
continual pilots are represented by rectangles and scattered pilots by large circles.
OFDM cells that are used both for scattered and continual pilots are represented by a
combination of a circle and a rectangle. Numerals indicate the corresponding subset.
[0085] The CP partitioning is performed so that those CPs that are also SPs will not change
their subset. Moreover, the CP partitioning must be balanced between the two subsets,
and the number of transitions in subsets to which the CPs belong must be minimized
along the time axis (symbol direction). These constraints lead to a partitioning
consisting of alternating contiguous groups of Ds CPs, as in Fig. 17 (subcarriers 6, 9,
24, 27). The main feature is that, with respect to the CPs, there is a subset change every
Ds pilots. The locations where the subsets of the CPs change are not limited to the ones
shown in Fig. 17. There are Ds possibilities to choose from as candidates for such
positions.
[0086] Fig. 18 shows, in a manner similar to Fig. 17, an example of an inventive SP pattern
with additional continual pilots located on non-SP-bearing subcarriers.
[0087] For the CPs on non-SP-bearing subcarriers there are two possibilities. First, all CPs
on the same subcarrier may be kept in one subset, as it is illustrated in Fig. 18 for sub-
carriers 5, 7, 14, and 16. Second, two subsets of pilots may be alternated on each
subcarrier in a manner similar to that for SP-bearing subcarriers, as it is illustrated in
Fig. 18 for subcarriers 23, 28, 31, and 34. Preferably, the CPs are evenly distributed
among the four subsets.
[0088] Summarizing, the present invention relates to orthogonal frequency-division mul-
tiplexing (OFDM) communication systems with multiple (e.g., four) transmit antennas
and one or more receivers/antennas for transmitting and receiving OFDM signals, and
in particular to methods for inserting scattered pilots (SPs) into the transmit signals of
such OFDM systems, for estimating channel properties on the basis of the scattered
pilots, a multi-antenna OFDM transmitter, and an OFDM receiver. In this context, it is
the particular approach of the present invention to keep the same SP pattern like in the
single-transmitter case, to partition the pilots into as many subsets as there are transmit
antennas, and to interleave these subsets both in time and in frequency. In this manner,
the granularity of pilots of the same subset is reduced. This offers increased flexibility
in designing the scattered pilot patterns and greater accuracy of the estimated channel
properties.
(Supplementary Notes)
[0089] The implementation methods pertaining to the present invention are not limited to
those described in the above embodiments. The following explains variations of con-
ceptions of the present invention.
[0090] (1) The above embodiments have not provided detailed descriptions of the signals
transmitted by each antenna. Described below are SPs in the signals transmitted by
each antenna.
[0091] Figs. 19 to 22 each show the SP pattern of the signals transmitted by each antenna in
the case of four-antenna transmission. Here, in encoding the SPs, the SPs are mul-
tiplexed by the coefficients shown in the matrix of Fig. 12A.
[0092] Fig. 19 shows a symbol pattern of the signals transmitted by a first antenna (so
named for convenience), which is one of the four antennas. As is apparent from the
comparison between Fig. 19 and Fig. 7, which shows the symbol pattern pertaining to
the present invention, the SPs belonging to the subsets indicated by the numbers 1, 2, 3
and 4 in Fig. 7 are respectively multiplied by the coefficients C11, C12, C13 and C14 cor-
responding to the first antenna as shown in Fig. 12A.
[0093] Meanwhile, Figs. 20, 21 and 22 respectively show symbol patterns of the OFDM
signals to be transmitted by the second antenna, the third antenna and the fourth
antenna.
[0094] As is apparent from the comparison between Figs. 19 and 20, SPs that are multiplied
by the coefficient C12 in the signals transmitted by the second antenna are located in the
positions of SPs that are multiplied by the coefficient C11 in the signals transmitted by
the first antenna. Pilots that are multiplied by the coefficients C13 and C14 in the signals
transmitted by the third and fourth antennas (respectively shown in Figs. 21 and 22)
are also located in such positions.
[0095] As indicated by the signals shown in Figs. 19 to 22, which are respectively
transmitted by the four antennas, the SPs included in the signals correspond to a
different one of the antennas, and are periodically arranged in such a manner that two
adjacent SPs belonging to one subset are spaced with an SP belonging to another
subset arranged therebetween, both in time and in frequency.
[0096] (2) The above embodiments have described the case where the number of subsets is
four. However, the number of subsets is not limited to four, but may be any number
that is a product of numbers Ms (an integer greater than or equal to two) and Mk (an
integer greater than or equal to two).
[0097] At this time, the distance between two adjacent pilots belonging to the same subset in
one subcarrier is Ds x Ms, based on Ds and Ms described in the above embodiments.
The distance between two adjacent subcarriers including pilots belonging to the same
subset is Dk x Mk.
[0098] As one example, Ms = 2 and Mk = 3. In this case, the SP pattern is the same as, for
instance, the one shown in Fig. 23. In other words, SPs belonging to each subset
should be arranged as shown in Fig. 23.
[0099] As another example, in a case where Ms = 2 and Mk = 3, SPs belonging to each
subset may be arranged as shown in, for example, Figs. 24 to 26. In this case, it is
preferable that information indicating which one of the SP patterns should be used be
either preset in each receiver, or notified to each receiver by the transmitters. Figs. 25
and 26 show examples of an SP pattern with Ms = 3 and Mk = 2.
[0100] (3) In the above embodiments, methods of encoding the SPs using the Hadamard
transform, as shown in Fig. 12, have been described. However, the SPs may be
encoded using any orthogonal transform method with use of a unitary diagonal matrix
as has been described in Background Art, the Fourier transform matrix shown in the
following Math. 8, and the like. It should be noted that since an inverse of the or-
thogonal transform matrix must exist, the orthogonal transform matrix must be full
rank.
[0101] [Math.8]

[0102] In a case where SPs to be transmitted by N transmit antennas are encoded after being
partitioned into N subsets, the Fourier transform matrix shown in Math. 9 may be used.
[0103]
[Math.9]

[0104] (4) In the above embodiments, the number of the transmitters (transmit antennas) has
been described as the same as the number of subsets, i.e., four. It should be noted,
however, that the number of the transmitters (transmit antennas) may be smaller than
or equal to the number of the subsets.
[0105] As one example, the number of subsets and the number of transmitters (transmit
antennas) may be four and three, respectively. As another example, the number of
subsets and the number of transmitters (transmit antennas) may be six and five, re-
spectively. That is to say, the number of subsets and the number of transmitters
(transmit antennas) may be arbitrary, as long as each receiver can distinguish the
signals transmitted by the respective transmitters (transmit antennas).
[0106] For instance, in a case where the number of subsets and the number of transmitters
(transmit antennas) are four and three, respectively, provided that the matrix shown in
Fig. 12A is used, the coefficients in a column corresponding to one of the antennas are
not used, and three other antennas transmit the OFDM signals in which are arranged
SPs multiplied by the coefficients in the columns corresponding to these three other
antennas. For example, assume a case where the fourth antenna shown in Fig. 12A
does not exist. In this case, (i) the first antenna transmits the OFDM signals in which
are arranged SPs multiplied by the coefficients C11, C21, C31 and C41, (ii) the second
antenna transmits the OFDM signals in which are arranged SPs multiplied by the coef-
ficients C12, C22, C32 and C42, and (iii) the third antenna transmits the OFDM signals in
which are arranged SPs multiplied by the coefficients C13, C23, C33 and C43. Here, none
of the transmitters (transmit antennas) transmits the OFDM signals in which are
arranged SPs multiplied by the coefficients C14, C24, C34 and C44. Each receiver that has
received the OFDM signals transmitted in the above manner performs demodulation
by estimating channels between the three transmit antennas based on the SPs belonging
to the four subsets. In this case, out of all the coefficients shown in the matrix of Fig.
12A, the coefficients in the column corresponding to the fourth antenna need not be
used.
[0107] In the above (3) of Supplementary Notes, the number of subsets is described as a
product of numbers Ms (an integer greater than or equal to two) and Mk (an integer
greater than or equal to two). Put another way, the number of subsets may be a
composite number that is greater than or equal to the number of transmitters.
[0108] (5) The SP pattern shown in Fig. 1 pertaining to the above embodiments is merely an
example. For example, in DVB-T2, other SP patterns are permitted, one example of
which is an SP pattern where SPs are inserted into every sixth subcarrier as shown in
Fig. 27. An SP pattern similar to that of Fig. 27 may be employed in transmitters of the
present invention. In this case, the SPs should be inserted according to the SP insertion
methods described in the above embodiments to realize the pattern of pilots belonging
to each subset as shown in Fig. 27. That is to say, provided that the distance between
two adjacent SP symbols in an SP-bearing subcarrier is Ds and the distance between
two adjacent SP-bearing subcarriers is Dk, (i) the distance between SPs belonging to
the same subset should be greater than or equal to 2DS in one subcarrier along the
symbol direction, (ii) the distance between subcarriers bearing SPs belonging to the
same subset should be greater than or equal to 2Dk, and (iii) between two SPs
belonging to a certain subset, another SP belonging to a subset other than said certain
subset is arranged, both in time and frequency. In the example of Fig.27, Ds = 2 and Dk
= 6.
[0109] (6) The above embodiments and variations may be combined partially.
[0110] (7) The communication systems pertaining to the present invention with a plurality of
transmitters and one or more receivers may each be the MIMO system or the MISO
system, as long as each transmitter (transmit antenna) is configured to transmit the
signals in which are arranged SPs belonging to subsets corresponding to the respective
transmitters (see, for example, Fig. 7).
[0111] (8) It is possible to provide a control program that is composed of programming
codes such as machine language and high-level language and that causes a processor in
each transmitter, or various circuits connected to that processor, to execute the process
for inserting pilots into the OFDM transmit signals described in the above em-
bodiments. Such a control program may be recorded on a recording medium, or may
be distributed/disseminated via various types of communication chancels. Furthermore,
it is also possible to provide a control program that is composed of programming codes
such as machine language and high-level language and that causes a processor in each
transmitter, or various circuits connected to that processor, to execute the process for
estimating channel properties of the OFDM reception signals described in the above
embodiments. Such a control program may be recorded on a recording medium, or
may be distributed/disseminated via various types of communication chancels.
Examples of such a recording medium include an IC card, a hard disk, an optical disc,
a flexible disk, ROM, and flash memory. The distributed/disseminated control program
is stored in memory or the like that can be read by the processor so as to be provided
for use. Each of the functions described in the above embodiments can be realized by
the processor executing the control program. The processor may directly execute the
control program, or execute the control program after compiling the same, or execute
the control program with an interpreter.
[0112] (9) Each of the functional constituent elements included in each transmitter and each
receiver described in the above embodiments (the Hadamard transform unit, the pilot
generation unit, etc.) may be realized as a circuit for executing its functions, or may be
realized by one or more processors executing a program, or may be configured as a
packaged integrated circuit such as an IC and an LSI. Such a packaged integrated
circuit is built in each device to be provided for use. This way, each device can realize
the functions described in the above embodiments.
[0113] (10) The following describes an exemplary application of the transmission/reception
methods explained in the above embodiments, as well as an exemplary configuration
of a system using such transmission/reception methods.
[0114] Fig. 28 shows an exemplary configuration of a system including devices that perform
the transmission/reception methods explained in the above embodiments. The
transmission/reception methods explained in the above embodiments are implemented
in a digital broadcast (or communication) system 2800 shown in Fig. 28 that includes a
broadcast station (or a base station) 2801 and various types of receivers, such as a TV
(television) 2811, a DVD recorder 2812, an STB (Set Top Box) 2813, a computer
2820, an in-vehicle TV 2841, and a cell phone 2830. More specifically, the broadcast
station (base station) 2801 transmits a transmission data stream (e.g., a multiplexed
data stream obtained by multiplexing a video data stream, an audio data stream, etc.) to
a predetermined transmission band by using the transmission methods explained in the
above embodiments.
[0115] The signals transmitted from the broadcast station (base station) 2801 are received by
antennas (e.g., antennas 2810 and 2840) that are either built in the receivers, or po-
sitioned outside the receivers while being connected to the receivers. Each receiver
performs the reception operations explained in the above embodiments on the signals
received by its antenna, and obtains the received data stream. This way, the digital
broadcast system 2800 can achieve the effects of the present invention described in the
above embodiments.
[0116] The video data stream included in the multiplexed data stream has been encoded with
a video encoding method conforming to such standards as MPEG (Moving Picture
Experts Group)-2, MPEG-4 AVC (Advanced Video Coding), and VC-1. The audio
data stream included in the multiplexed data stream has been encoded with an audio
encoding method such as Dolby AC (Audio Coding)-3, Dolby Digital Plus, MLP
(Meridian Lossless Packing), DTS (Digital Theater Systems), DTS-HD, and linear
PCM (Pulse-Code Modulation).
[0117] Fig. 29 shows the structure of a receiver 2900 used in a digital broadcast system, as
one example of devices that perform the reception methods explained in the above em-
bodiments. As shown in Fig. 29, one exemplary structure of the receiver 2900 is such
that a modem portion is constituted by one LSI (or one chipset), and a codec portion is
constituted by another LSI (or another chipset). The structure of the receiver 2900
shown in Fig. 29 is equivalent to that of such devices as the TV (television) 2811, the
DVD recorder 2812, the STB (Set Top Box) 2813, the computer 2820, the in-vehicle
TV 2841, and the cell phone 2830 shown in Fig. 28. The receiver 2900 includes a tuner
2901 and a demodulation unit 2902. The tuner 2901 converts radio frequency signals
received by an antenna 2960 into baseband signals. The demodulation unit 2902
obtains a multiplexed data stream by performing the reception operations explained in
the above embodiments on the baseband signals. As a result, the effects of the present
invention described in the above embodiments can be achieved.
[0118] The receiver 2900 also includes a stream input/output unit 2903, a signal processing
unit 2904, an AV (Audio and Visual) output unit 2905, an audio output unit 2906, and
a video display unit 2907. The stream input/output unit 2903 demultiplexes the video
data stream and the audio data stream from the multiplexed data stream obtained by the
demodulation unit 2902. The signal processing unit 2904 decodes the video data
stream into a video signal by using a video decoding method corresponding to the de-
multiplexed video data stream, and decodes the audio data into an audio signal by
using an audio decoding method corresponding to the demultiplexed audio data stream.
The AV output unit 2905 outputs the decoded audio signal to the audio output unit
2906, and outputs the decoded video signal to the video display unit 2907. Alter-
natively, the AV output unit 2905 outputs the decoded audio and video signals to an
AV (Audio and Visual) output IF (interface) 2911. The audio output unit 2906 (e.g., a
speaker) outputs the decoded audio signal. The video display unit 2907 (e.g., a display)
displays the decoded video signal.
[0119] By way of example, a user transmits information on a selected channel (a selected
(TV) program, selected audio broadcasting, etc.) to an operation input unit 2910 with
use of a remote controller 2950. Thereafter, the receiver 2900 obtains the multiplexed
data stream corresponding to the selected channel by performing demodulation, error
correction decoding, and the like on the received signals that correspond to the selected
channel. At this time, the receiver 2900 selects a reception method being appropriate to
the selected channel according to information of a transmission method (for example,
the SP patterns, the number of subsets, and a modulation method and an error
correction method performed on the data stream transmitted by the data cells, which
are discussed in the above embodiments) obtained from control symbols included in
the received signals. This way, the receiver 2900 can obtain data contained in the data
cells transmitted from the broadcast station (base station). In the example described
above, the user selects a channel with use of the remote controller 2950. However, the
above-described operations are performed also when a channel is selected with use of a
channel selection key built in the receiver 2900.
[0120] With the above structure, the user can view the program that has been received by the
receiver 2900 using the reception methods described in the above embodiments.
[0121] Assume a case where the above-described receiver 2900 pertaining to the present
invention is built in a TV, a recording device (e.g., a DVD recorder, a Blu-ray
recorder, an HDD recorder, and an SD card), and a cell phone. In this case, if the mul-
tiplexed data obtained through demodulation and error correction decoding by the de-
modulation unit 2902 includes (i) data for correcting a default (bug) in software used to
cause the TV and the recording device to operate, or (ii) data for correcting a default
(bug) in software used to prevent leaks of personal information and recorded data, then
a default in software provided in the TV and the recording device may be corrected by
installing such data. If the multiplexed data obtained through demodulation and error
correction decoding by the demodulation unit 2902 includes data for correcting a
default (bug) in software provided in the receiver 2900, then a default in the receiver
2900 may be corrected with such data. This way, the TV, the recording device and the
cellular phone in which the receiver 2900 is built can operate in a more stable manner.
[0122] The receiver 2900 of the present embodiment also includes a recording unit (drive)
2908 that records the following (i) through (iii) on a recording medium such as a
magnetic disk, an optical disc and a nonvolatile semiconductor memory: (i) part of the
multiplexed data stream obtained through demodulation and error correction decoding
by the demodulation unit 2902 (In some occasions, error correction decoding is not
performed on signals obtained through demodulation by the demodulation unit 2902.
Also, the receiver 2900 may perform other signal processing after error correction
decoding. These are true of the following descriptions that use similar expressions
about error correction decoding to those used in this section.); (ii) data corresponding
to the data of (i), such as data obtained by compressing the data of (i); and (iii) data
obtained by processing video and audio. Here, the optical disc is, for example, a
recording medium on/from which information is recorded/read using laser light, such
as DVD (Digital Versatile Disc) and BD (Blu-ray Disc). The magnetic disk is, for
example, a recording medium that stores information by magnetizing a magnetic
material using magnetic flux, such as FD (Floppy Disk) (registered trademark) and a
hard disk. The nonvolatile semiconductor memory is, for example, a recording medium
made up of semiconductor elements, such as flash memory and ferroelectric random
access memory. Examples of the nonvolatile semiconductor memory include an SD
card and a flash SSD (Solid State Drive) that incorporate flash memory. It should be
noted that the above-listed types of recording media are merely examples. It goes
without saying that the recording may be performed with use of a recording medium
other than the above-listed recording media.
[0123] With the above structure, the user can record and store the program that has been
received by the receiver 2900 by using the reception methods described in the above
embodiments. Accordingly, the receiver 2900 can read the recorded data and the user
can view the program corresponding to the recorded data at any time after the
broadcast time of the program.
[0124] It has been described above that in the receiver 2900, the recording unit 2908 records
the multiplexed data stream obtained through demodulation and error correction
decoding by the demodulation unit 2902. Alternatively, part of the multiplexed data
stream may be extracted and recorded. For instance, when the multiplexed data stream
obtained by the demodulation unit 2902 includes contents etc. that are provided from
data broadcast services and that are different from the video data stream and the audio
data stream, the recording unit 2908 may record a new multiplexed data stream that is
obtained by extracting and multiplexing the video data stream and the audio data
stream in the multiplexed data stream obtained by the demodulation unit 2902. Alter-
natively, the recording unit 2908 may record a new multiplexed data stream that is
obtained by multiplexing one of the video data stream and the audio data stream
included in the multiplexed data stream obtained by the demodulation unit 2902. In
addition, the recording unit 2908 may record the aforementioned contents that are
included in the multiplexed data and that are provided from the data broadcast services.
[0125] As one example, the stream input/output unit 2903 performs the processing for ex-
tracting and multiplexing part of plural data pieces included in the multiplexed data
obtained through demodulation and error correction decoding by the demodulation unit
2902. More specifically, with an instruction from a controller such as CPU (not il-
lustrated), the stream input/output unit 2903 generates a new multiplexed data stream
by (i) demultiplexing the multiplexed data stream obtained by the demodulation unit
2902 into demultiplexed data streams, such as a video data stream, an audio data
stream, and other contents provided from data broadcast services, and (ii) extracting
and multiplexing only a specified data stream out of the demultiplexed data streams.
Which data stream should be extracted from the demultiplexed data streams may be
determined by the user, or may be predetermined for each type of recording media.
[0126] With the above structure, the receiver 2900 can extract and record only the data
required to view the recorded program. This can reduce the data size of the data to be
recorded.
[0127] It has been described above that the recording unit 2908 records the multiplexed data
obtained through demodulation and error correction decoding by the demodulation unit
2902. Alternatively, the recording unit 2908 may perform the recording in the
following steps: (i) converting the original video data stream included in the mul-
tiplexed data stream obtained by the demodulation unit 2902 into a new video data
stream, which has been encoded with a video encoding method that is different from a
video encoding method performed on the original video data stream, so that the data
size or bit rate of the new video data stream is smaller/lower than the data size or bit
rate of the original video data stream; and (ii) recording a new multiplexed data stream
obtained by multiplexing the post-conversion new video data stream. The video
encoding methods that are respectively performed on the original video data stream
and the post-conversion new video data stream may conform to different standards, or
may conform to the same standard but use different parameters for encoding. In the
similar manner, the recording unit 2908 may perform the recording in the following
steps: (i) converting the original audio data stream included in the multiplexed data
stream obtained by the demodulation unit 2902 into a new audio data stream, which
has been encoded with an audio encoding method that is different from an audio
encoding method performed on the original audio data stream, so that the data size or
bit rate of the new audio data stream is smaller/lower than the data size or bit rate of
the original audio data stream; and (ii) recording a new multiplexed data stream
obtained by multiplexing the post-conversion new audio data.
[0128] As one example, the stream input/output unit 2903 and the signal processing unit
2904 perform the processing for converting the original video data stream and audio
data stream included in the multiplexed data stream obtained by the demodulation unit
2902 into a new video data stream and a new audio data stream that have different data
sizes or bit rates from the original video data stream and audio data stream. More
specifically, with an instruction from the controller such as CPU, the stream input/
output unit 2903 demultiplexes the multiplexed data stream obtained by the de-
modulation unit 2902 into demultiplexed data streams, such as a video data stream, an
audio data stream, and other contents provided from data broadcast services. With an
instruction from the controller, the signal processing unit 2904 performs (i) processing
for converting the demultiplexed original video data stream into a new video data
stream that has been encoded with a video encoding method that is different from a
video encoding method performed on the original video data stream, and (ii)
processing for converting the separated original audio data stream into a new audio
data stream that has been encoded with an audio encoding method that is different
from an audio encoding method performed on the original audio data stream. With an
instruction from the controller, the stream input/output unit 2903 generates a new mul-
tiplexed data stream by multiplexing the post-conversion new video data stream and
the post-conversion new audio data stream. With the instruction from the controller,
the signal processing unit 2904 may perform the conversion processing on one or both
of the original video data stream and the original audio data stream. Furthermore, the
data sizes or bit rates of the post-conversion new video data stream and the post-
conversion new audio data stream may be determined by the user, or may be prede-
termined for each type of recording media.
[0129] With the above structure, the receiver 2900 can perform the recording after changing
the data sizes or bit rates of the video data stream and the audio data stream in ac-
cordance with the data size of data that can be recorded on the recording medium, or in
accordance with the speed at which the recording unit 2908 records/reads data. This
way, the recording unit can record a program even when the data size of data that can
be recorded on the recording medium is smaller than the data size of the multiplexed
data stream obtained by the demodulation unit 2902, or when the speed at which the
recording unit records/reads data is slower than the bit rate of the multiplexed data
stream obtained by the demodulation unit 2902. Consequently, the receiver 2900 can
read the recorded data and the user can view the program corresponding to the
recorded data at any time after the broadcast time of the program.
[0130] The receiver 2900 further includes a stream output IF (interface) that transmits the
multiplexed data stream obtained by the demodulation unit 2902 to an external device
via a communication medium 2930. One example of the stream output IF 2909 is a
wireless communication device that transmits the demodulated multiplexed data to the
external device via a wireless medium (equivalent to the communication medium
2930), by using a wireless communication method conforming to the wireless commu-
nication standards such as Wi-Fi (registered trademark) (e.g., IEEE 802.1 la, IEEE
802.11b, IEEE 802.11g, and IEEE 802.11n), WiGig, WirelessHD, Bluetooth, and
ZigBee. Alternatively, the stream output IF 2909 may be a wired communication
device that transmits the demodulated multiplexed data stream to the external device
via a wired communication channel (equivalent to the communication medium 2930)
connected to the stream output IF 2909, by using a communication method conforming
to the wired communication standards such as Ethernet, USB (Universal Serial Bus),
PLC (Power Line Communication), and HDMI (High-Definition Multimedia
Interface).
[0131] With the above structure, the user can use, on the external device, the multiplexed
data stream that has been received by the receiver 2900 using the reception methods
described in the above embodiments. Note that the use of the multiplexed data by the
user includes (i) real-time viewing of the multiplexed data stream by using the external
device, (ii) recording of the multiplexed data stream with a recording unit provided in
the external device, and (iii) transmission of the multiplexed data stream from the
external device to yet another external device.
[0132] It has been described above that in the receiver 2900, the output IF 2909 outputs the
multiplexed data stream obtained by the demodulation unit 2902. Alternatively, part of
the multiplexed data may be extracted and output. For instance, when the multiplexed
data stream obtained by the demodulation unit 2902 includes contents etc. that are
provided from data broadcast services and that are different from the video data stream
and the audio data stream, the stream output IF 2909 may output a new multiplexed
data stream that is obtained by extracting and multiplexing the video data stream and
audio data stream in the multiplexed data stream obtained by the demodulation unit
2902. Alternatively, the stream output IF 2909 may output a new multiplexed data
stream that is obtained by multiplexing one of the video data stream and audio data
stream included in the multiplexed data stream obtained by the demodulation unit
2902.
[0133] As one example, the stream input/output unit 2903 performs the processing for ex-
tracting and multiplexing part of the multiplexed data stream obtained by the de-
modulation unit 2902. More specifically, with an instruction from the controller such
as CPU (Central Processing Unit, not illustrated), the stream input/output unit 2903
generates a new multiplexed data stream by (i) demultiplexing the multiplexed data
stream obtained by the demodulation unit 2902 into demultiplexed data streams, such
as a video data stream, an audio data stream, and other contents provided from data
broadcast services, and (ii) extracting and multiplexing only a specified data stream out
of the demultiplexed data streams. Which data stream should be extracted from the de-
multiplexed data streams may be determined by the user, or may be predetermined for
each type of the stream output IF 2909.
[0134] With the above structure, the receiver 2900 can extract and output only the data
required by the external device. This can reduce the communication band consumed by
outputting the multiplexed data.
[0135] It has been described above that the stream output IF 2909 outputs the multiplexed
data stream obtained by the demodulation unit 2902. Alternatively, the stream output
IF 2909 may perform the output in the following steps: (i) converting the original
video data stream included in the multiplexed data stream obtained by the de-
modulation unit 2902 into a new video data stream, which has been encoded with a
video encoding method that is different from a video encoding method performed on
the original video data stream, so that the data size or bit rate of the new video data
stream is smaller/lower than the data size or bit rate of the original video data stream;
and (ii) outputting a new multiplexed data stream obtained by multiplexing the post-
conversion new video data stream. The video encoding methods that are respectively
performed on the original video data stream and the post-conversion new video data
stream may conform to different standards, or may conform to the same standard but
use different parameters for encoding. In the similar manner, the stream output IF 2909
may perform the output in the following steps: (i) converting the original audio data
stream included in the multiplexed data stream obtained by the demodulation unit 2902
into a new audio data stream, which has been encoded with an audio encoding method
that is different from an audio encoding method performed on the original audio data
stream, so that the data size or bit rate of the new audio data stream is smaller/lower
than the data size or bit rate of the original audio data stream; and (ii) outputting a new
multiplexed data stream obtained by multiplexing the post-conversion new audio data
stream.
[0136] As one example, the stream input/output unit 2903 and the signal processing unit
2904 perform the processing for converting the original video data stream and audio
data stream included in the multiplexed data stream obtained by the demodulation unit
2902 into new video data and audio data that have different data sizes or bit rates from
the original video data stream and audio data stream. More specifically, with an in-
struction from the controller, the stream input/output unit 2903 demultiplexes the mul-
tiplexed data stream obtained by the demodulation unit 2902 into demultiplexed data
streams, such as a video data stream, an audio data stream, and other contents provided
from data broadcast services. With an instruction from the controller, the signal
processing unit 2904 performs (i) processing for converting the demultiplexed original
video data stream into a new video data stream that has been encoded with a video
encoding method that is different from a video encoding method performed on the
original video data stream, and (ii) processing for converting the demultiplexed
original audio data stream into a new audio data stream that has been encoded with an
audio encoding method that is different from an audio encoding method performed on
the original audio data stream. With an instruction from the controller, the stream
input/output unit 2903 generates a new multiplexed data stream by multiplexing the
post-conversion new video data stream and the post-conversion new audio data stream.
With the instruction from the controller, the signal processing unit 2904 may perform
the conversion processing on one or both of the original video data stream and the
original audio data stream. Furthermore, the data sizes or bit rates of the post-
conversion new video data stream and the post-conversion new audio data stream may
be determined by the user, or may be predetermined for each type of the stream output
IF 2909.
[0137] With the above structure, the receiver 2900 can perform the output after changing the
bit rates of video data and audio data in accordance with the speed at which commu-
nication is performed with the external device. This way, the stream output IF can
output a new multiplexed data stream to the external device even when the speed at
which the communication is performed with the external device is slower than the bit
rate of the multiplexed data stream obtained by the demodulation unit 2902. Con-
sequently, the user can use the new multiplexed data stream on another communication
device.
[0138] The receiver 2900 further includes the AV output IF 2911 that outputs, to the
external device and an external communication medium, a video signal and an audio
signal decoded by the signal processing unit 2904. One example of the AV output IF
2911 is a wireless communication device that transmits the modulated video signal and
audio signal to the external device via a wireless medium, by using a wireless commu-
nication method conforming to the wireless communication standards such as Wi-Fi
(registered trademark) (e.g., IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, and IEEE
802.11n), WiGig WirelessHD, Bluetooth, and ZigBee. Alternatively, the AV output IF
2911 may be a wired communication device that transmits the modulated video signal
and audio signal to the external device via a wired communication channel connected
to the AV output IF 2911, by using a communication method conforming to the wired
communication standards such as Ethernet, USB, PLC and HDMI. Alternatively, the
AV output IF 2911 may be a terminal for connecting to a cable that outputs the video
signal and audio signal as-is, i.e., as analog signals.
[0139] With the above structure, the user can use the video signal and audio signal decoded
by the signal processing unit 2904 on the external device.
[0140] The receiver 2900 further includes an operation input unit 2910 that receives input of
a user operation. The receiver 2900 switches between various operations based on a
control signal that is input to the operation input unit 2910 in accordance with the user
operation. For example, the receiver 2900 switches between (i) ON and OFF of the
power, (ii) channels to be received, (iii) display and non-display of subtitles, (iv)
languages to be displayed, (v) volumes of audio to be output from the audio output unit
2906. The receiver 2900 also changes various settings, such as channels that can be
received.
[0141] The receiver 2900 may have the function of displaying an antenna level indicating
the reception quality of signals that are being received by itself. The antenna level is an
index showing the reception quality calculated based on, for example, RSSI (Received
Signal Strength Indication/Indicator, which indicates the strength of the received
signals), a reception electric field strength, C/N (carrier-to-noise power ratio), BER
(Bit Error Rate), a packet error rate, a frame error rate, and channel state information of
the signals received by the receiver 2900. The antenna level is a signal indicating the
signal level and the quality (superior, inferior, etc.) of the received signals. In this case,
the demodulation unit 2902 has functions of a reception quality measurement unit that
measures RSSI, the reception electric field strength, C/N, BER, the packet error rate,
the frame error rate, the channel state information, etc. of the received signals. The
receiver 2900 displays the antenna level (the signal indicating the signal level and the
quality (superior, inferior, etc.) of the received signals) on the video display unit 2907
in format that can be distinguished by the user. The display format of the antenna level
(the signal indicating the signal level and the quality (superior, inferior, etc.) of the
received signals) may display numerical values corresponding to RSSI, the reception
electric field strength, C/N, BER, the packet error rate, the frame error rate, the channel
state information, etc., or may display different images in accordance with RSSI, the
reception electric field strength, C/N, BER, the packet error rate, the frame error rate,
the channel state information, etc.
[0142] The following explains an exemplary method of calculating an antenna level from
the signals received by the receiver 2900 that uses the transmission methods described
in the above embodiments. By using the methods described in the above embodiments,
a 2D interpolation unit 1550 (not illustrated in Fig. 29) of the receiver 2900 in-
terpolates signals of pilots that have been detected according to the SP pattern for each
subset. The reception quality measurement unit of the receiver 2900 calculates an in-
terpolation error, which is an error between the interpolated values and the signals of
the CPs that have been actually received, by using (i) the signals of the received CPs at
CP-bearing cells, and (ii) the interpolated values of subsets to which these CPs belong.
It is considered that the smaller the calculated interpolation error, the higher the
reception quality. Thus, the receiver 2900 generates an index indicating the reception
quality based on the calculated interpolation error, and displays the index as an antenna
level. At this time, as the index indicating the reception quality, the receiver 2900 may
use an average value or the largest value of interpolation errors calculated from the
CPs within a predetermined unit of time. In addition, the interpolation error may be
expressed using an absolute value or a value normalized by the reception power.
[0143] When the Hadamard transform is used to encode pilot signals, the reception quality
measurement unit of the receiver 2900 may be configured as follows. By using (i)
values of signals separated with use of channel components h1, ..., hn, which are
calculated by performing the Hadamard transform (where n is an integer greater than
or equal to two and is the same as the number of transmit antennas), and (ii) values of
known CPs transmitted from antennas of each transmitter, the reception quality mea-
surement unit of the receiver 2900 calculates an error included in the separated signals.
The receiver 2900 generates an index indicating the reception quality based on the
calculated error included in the separated signals, and displays the index as an antenna
level. At this time, as the index indicating the reception quality, the receiver 2900 may
use an average value or the largest value of errors calculated within a predetermined
unit of time as included in the separated signals.
[0144] With the above structure, in a case where signals are received by using the reception
methods described in the above embodiments, the user can grasp the antenna level (the
signal indicating the signal level and the quality (superior, inferior, etc.) of the received
signals) either numerically or visually.
[0145] Furthermore, regarding methods of displaying the antenna level, the receiver 2900
may be configured as follows. Although it is not necessary to combine the following
several methods of displaying the antenna level with the SP pattern described in the
above embodiments, it goes without saying that such a combination is expected to
improve the reception quality.
[0146] For example, the receiver 2900 may have functions of (i) calculating indices in-
dicating reception qualities of the separated signals, respectively, and (ii) displaying
the indices as multiple antenna levels (signals indicating the signal levels and the
qualities (superior, inferior, etc.) of the respective separated signals), either all at once
or by switching from display of one index to display of another index. Alternatively,
the receiver 2900 may have functions of (i) calculating an index indicating the
reception quality of a group including all or some of the separated signals, and (ii)
displaying the index as the antenna level (the signal indicating the signal level and the
quality (superior, inferior, etc.) of the respective separated signals).
[0147] Furthermore, the receiver 2900 may be configured as follows in a case where the
broadcast station (base station) 2801 incorporates multiple transmission modes such as
MISO and SISO (Signal Input Single Output) other than MIMO explained in the above
embodiments and performs the transmission while switching from one transmission
mode to another over time. For example, the receiver 2900 may have functions of (i)
calculating indices indicating reception qualities of the multiple transmission modes,
respectively, and (ii) displaying the indices as multiple antenna levels (signals in-
dicating the signal levels and the qualities (superior, inferior, etc.) of the respective
received signals), either all at once or by switching from display of one index to
display of another index. Alternatively, the receiver 2900 may have functions of (i)
calculating an index indicating the reception quality of a group including all or some of
the multiple transmission modes, and (ii) displaying the index as the antenna level (the
signal indicating the signal level and the quality (superior, inferior, etc.) of the re-
spective received signals).
[0148] Furthermore, the receiver 2900 may be configured as follows in a case where the
broadcast station (base station) 2801 groups a plurality of data streams that constitute a
program (e.g., a video data stream and an audio data stream) into a plurality of hier-
archical layers and performs the transmission by using a hierarchical transmission
method in which a transmission mode, a modulation method, error correction
encoding, an encoding rate, etc. are independently configurable for each hierarchical
layer. For example, the receiver 2900 may have functions of (i) calculating indices in-
dicating the reception qualities of hierarchical layers, respectively, and (ii) displaying
the indices as multiple antenna levels (signals indicating the signal levels and the
qualities (superior, inferior, etc.) of the received signals), either all at once or by
switching from display of one index to display of another index. Alternatively, the
receiver 2900 may have functions of (i) calculating an index indicating the reception
quality of a group including all or some of the multiple hierarchical layers, and (ii)
displaying the index as the antenna level (the signal indicating the signal level and the
quality (superior, inferior, etc.) of the received signals).
[0149] With the above structure, in a case where signals are received by using the reception
methods described in the above embodiments, the user can grasp the antenna level (the
signal indicating the signal level and the quality (superior, inferior, etc.) of the received
signal), either numerically or visually, in units of reception that can be distinguishable
(e.g., the separated signals, multiple transmission mode, and multiple hierarchical
layers).
[0150] In an exemplary case described above, the receiver 2900 includes the audio output
unit 2906, the video display unit 2907, the recording unit 2908, the stream output IF
2909, and the AV output IF 2911. However, the receiver 2900 need not include all of
these structural elements. As long as the receiver 2900 includes at least one of these
structural elements, the user can use the multiplexed data stream obtained through de-
modulation by the demodulation unit 2902 and error correction decoding. Therefore,
each receiver may include any combination of the above structural elements depending
on how it is used.
Industrial Applicability
[0151] The present invention is useful in a communication system where multiple transmit
antennas transmit signals at the same time in the same frequency band and the
transmitted signals are received and demodulated.
We claim:
[Claim 1] A multi-antenna OFDM transmitter having N antennas, N being an
integer greater than or equal to two, the multi-antenna OFDM
transmitter comprising:
a multi-antenna encoder for generating a plurality of data streams, one
for each of the N transmit antennas, each data stream consisting of a
succession of OFDM symbols, each OFDM symbol consisting of a
plurality of OFDM cells, each OFDM cell being associated with one of
a plurality of subcarriers;
a pilot generation unit for generating, for each of the plurality of data
streams, a plurality of scattered pilots, said plurality of scattered pilots
being partitioned into M subsets, each of the scattered pilots being
encoded on the basis of the subset to which the scattered pilot belongs
and the data stream into which the scattered pilot is to be inserted, and
a plurality of pilot inserting units, each pilot inserting unit for inserting
one of the plurality of scattered pilots into a corresponding one of the
plurality of data streams in accordance with a predefined periodic
pattern in which a temporal spacing between two OFDM symbols
having scattered pilots in OFDM cells associated with the same
subcarrier is equal to Ds and a frequency spacing of two subcarriers
bearing scattered pilots in any of the OFDM symbols is equal to Dk,
each of Ds and Dk being an integer greater than or equal to two, wherein
M is greater than or equal to N and satisfies a relationship M = MsMk,
each of Ms and Mk being an integer greater than or equal to two, and
each of the plurality of pilot inserting units inserts the scattered pilots in
such a manner that a temporal spacing between two OFDM symbols
having scattered pilots of the same subset in OFDM cells associated
with the same subcarrier is equal to DsMs and a frequency spacing of
two subcarriers bearing scattered pilots of the same subset in any of the
OFDM symbols is equal to DkMk.
[Claim 2] A multi-antenna OFDM transmitter according to claim 1, wherein
each of M and N is equal to four, and
each of Ms and Mk is equal to two.
[Claim 3] A multi-antenna OFDM transmitter according to claim 2, wherein
Dk is equal to two, three, or four.
[Claim 4] A multi-antenna OFDM transmitter according to claim 2 or 3, wherein
the pilot generation unit encodes the scattered pilots by, for each subset,
multiplying all scattered pilots of the subset with a constant coefficient
that depends on the subset and the data stream into which said all
scattered pilots of the subset are to be inserted.
[Claim 5] A multi-antenna OFDM transmitter according to claim 4, wherein
a matrix formed by the constant coefficients used for multiplying the
scattered pilots is invertible, in particular a unitary diagonal matrix or a
Hadamard matrix.
[Claim 6] An OFDM receiver comprising:
an OFDM demodulator for obtaining a data stream consisting of a
succession of OFDM symbols, each OFDM symbol consisting of a
plurality of OFDM cells, each OFDM cell being associated with one of
a plurality of subcarriers;
a pilot extraction unit for (i) extracting scattered pilots from the data
stream in accordance with a predefined periodic pattern in which a
temporal spacing between two OFDM symbols having scattered pilots
in OFDM cells associated with the same subcarrier is equal to Ds and a
frequency spacing of two subcarriers bearing scattered pilots in any of
the OFDM symbols is equal to Dk, each of Ds and Dk being an integer
greater than or equal to two, and (ii) partitioning the extracted scattered
pilots into M subsets; and
a channel estimation unit for estimating a plurality of channel
components from the M subsets of scattered pilots, each channel
component representing a channel condition between one of a plurality
of transmitters and the OFDM receiver, wherein
M satisfies a relationship M = MsMk, each of Ms and Mk being an
integer greater than or equal to two, and
a temporal spacing between two OFDM symbols having scattered
pilots of the same subset in OFDM cells associated with the same
subcarrier is equal to MsDs and a frequency spacing of two subcarriers
bearing scattered pilots of the same subset in any of the OFDM
symbols is equal to MkDk.
[Claim 7] An OFDM receiver according to claim 6, wherein
M is equal to four, and
each of Ms and Mk is equal to two.
[Claim 8] An OFDM receiver according to claim 7, wherein
Dk is equal to two, three, or four.
[Claim 9] An OFDM receiver according to claim 7 or 8, wherein
the pilot extraction unit extracts, for each OFDM symbol, at least one
continual pilot from OFDM symbols associated with predefined sub-
carriers and partitions the extracted continual pilots into the M subsets;
and
the channel estimation unit estimates the plurality of channel
components from the M subsets of scattered pilots and continual pilots.
[Claim 10] An OFDM receiver according to claim 9, wherein
the predefined subcarriers are the subcarriers bearing scattered pilots.
[Claim 11] An OFDM receiver according to claim 9, wherein
the predefined subcarriers are distinct from the subcarriers bearing
scattered pilots.
[Claim 12] An OFDM receiver according to claim 11, wherein
continual pilots extracted from the same subcarrier are partitioned into
the same subset.
[Claim 13] An OFDM receiver according to claim 10 or 11, wherein
continual pilots extracted from the same subcarrier are partitioned into
at least two different subsets.
[Claim 14] A method used by a multi-antenna transmitter with N transmit antennas
for inserting scattered pilots into transmit signals, said method
comprising the steps of:
generating a plurality of data streams, one for each of the N transmit
antennas, each data stream consisting of a succession of OFDM
symbols, each OFDM symbol consisting of a plurality of OFDM cells,
each OFDM cell being associated with one of a plurality of subcarriers;
generating, for each of the plurality of data streams, a plurality of
scattered pilots, said plurality of scattered pilots being partitioned into
M subsets, each of the scattered pilots being encoded on the basis of the
subset to which the scattered pilot belongs and the data stream into
which the scattered pilot is to be inserted, and
inserting one of the plurality of scattered pilots into a corresponding
one of the plurality of data streams in accordance with a predefined
periodic pattern in which a temporal spacing between two OFDM
symbols having scattered pilots in OFDM cells associated with the
same subcarrier is equal to Ds and a frequency spacing of two sub-
carriers bearing scattered pilots in any of the OFDM symbols is equal
to Dk, each of Ds and Dk being an integer greater than or equal to two,
wherein
M is greater than or equal to N and satisfies a relationship M = MsMk,
each of Ms and Mk being an integer greater than or equal to two, and
in the inserting step, the scattered pilots are inserted in such a manner
that a temporal spacing between two OFDM symbols having scattered
pilots of the same subset in OFDM cells associated with the same
subcarrier is equal to DsMs and a frequency spacing of two subcarriers
bearing scattered pilots of the same subset in any of the OFDM
symbols is equal to DkMk.
[Claim 15] A method according to claim 14, wherein
each of M and N is equal to four, and
each of Ms and Mk is equal to two.
[Claim 16] A method for estimating, at an OFDM receiver, channel properties
between the OFDM receiver and each of N transmit antennas, said
method comprising the steps of:
obtaining a data stream consisting of a succession of OFDM symbols,
each OFDM symbol consisting of a plurality of OFDM cells, each
OFDM cell being associated with one of a plurality of subcarriers;
extracting scattered pilots from the data stream in accordance with a
predefined periodic pattern in which a temporal spacing between two
OFDM symbols having scattered pilots in OFDM cells associated with
the same subcarrier is equal to Ds and a frequency spacing of two sub-
carriers bearing scattered pilots in any of the OFDM symbols is equal
to Dk, each of Ds and Dk being an integer greater than or equal to two,
and partitioning the extracted scattered pilots into M subsets; and
estimating a plurality of channel components from the M subsets of
scattered pilots, each channel component representing a channel
condition between one of a plurality of transmitters and the OFDM
receiver, wherein
M is greater than or equal to N and satisfies a relationship M = MsMk,
each of Ms and Mk being an integer greater than or equal to two, and
a temporal spacing between two OFDM symbols having scattered
pilots of the same subset in OFDM cells associated with the same
subcarrier is equal to MsDs and a frequency spacing of two subcarriers
bearing scattered pilots of the same subset in any of the OFDM
symbols is equal to MkDk.
[Claim 17] A method according to claim 16, wherein
each of M and N is equal to four, and
each of Ms and Mk is equal to two.

ABSTRACT

The present invention relates to orthogonal frequency- division multiplexing
(OFDM) communication systems with multiple transmit antennas receive antennas,
and in particular to methods for inserting scattered pilots (SPs) into the transmit
signals of such OFDM systems, for estimating channel properties on the basis of the
scattered pilots, a multi -antenna OFDM transmitter, and an OFDM receiver. In this
context, it is the particular approach of the present invention to keep the same SP
pattern like in the single- transmitter case, to partition the pilots into as many
subsets as there are transmitters (transmit antennas), and to interleave these
subsets both in time and in frequency. In this manner, the granularity of pilots of the
same subset is reduced. This offers increased flexibility in designing the scattered
pilot patterns and greater accuracy of the estimated channel properties.