U.S. patent application number 12/490096 was filed with the patent office on 2009-10-22 for constellation rearrangement for arq transmit diversity schemes.
This patent application is currently assigned to PANASONIC CORPORATION. Invention is credited to Alexander Golitschek Edler von Elbwart, Eiko Seidel, Christian WENGERTER.
Application Number | 20090262858 12/490096 |
Document ID | / |
Family ID | 32103874 |
Filed Date | 2009-10-22 |
United States Patent
Application |
20090262858 |
Kind Code |
A1 |
WENGERTER; Christian ; et
al. |
October 22, 2009 |
CONSTELLATION REARRANGEMENT FOR ARQ TRANSMIT DIVERSITY SCHEMES
Abstract
An ARQ (re-) transmission method of transmitting data in a
wireless communication system wherein data packets are transmitted
from a transmitter to a receiver, using a first transmission and a
second transmission based on a repeat request. The method comprises
the steps of modulating data at the transmitter using a first
signal constellation pattern to obtain a first data symbol. The
first data symbol is transmitted as the first transmission to the
receiver using a first diversity branch. Further, the data is
modulated at the transmitter using a second signal constellation
pattern to obtain a second data symbol. Then, the second data
symbol is transmitted as the second transmission to the receive
over a second diversity branch. Finally, the received first and
second data symbol data symbol are diversity combined at the
receiver. The invention further relates to a transmitter and a
receiver embodied to carry out the method of the invention.
Inventors: |
WENGERTER; Christian;
(Kleinheubach, DE) ; Golitschek Edler von Elbwart;
Alexander; (Darmstadt, DE) ; Seidel; Eiko;
(Dermstadt, DE) |
Correspondence
Address: |
Dickinson Wright PLLC;James E. Ledbetter, Esq.
International Square, 1875 Eye Street, N.W., Suite 1200
Washington
DC
20006
US
|
Assignee: |
PANASONIC CORPORATION
Osaka
JP
|
Family ID: |
32103874 |
Appl. No.: |
12/490096 |
Filed: |
June 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11633421 |
Dec 5, 2006 |
7567622 |
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12490096 |
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10501906 |
Dec 6, 2004 |
7154961 |
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PCT/EP2002/011694 |
Oct 18, 2002 |
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11633421 |
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Current U.S.
Class: |
375/267 ;
375/299 |
Current CPC
Class: |
H04L 1/0003 20130101;
H04B 7/0842 20130101; H04L 1/1845 20130101; H04L 1/0071 20130101;
H04L 1/08 20130101; H04L 27/3472 20130101; H04L 1/1816 20130101;
H04L 1/1819 20130101; H04L 1/0016 20130101; H04L 25/067 20130101;
H04L 27/34 20130101 |
Class at
Publication: |
375/267 ;
375/299 |
International
Class: |
H04B 7/02 20060101
H04B007/02; H04L 27/00 20060101 H04L027/00 |
Claims
1. A transmission method for use in an ARQ retransmission technique
in a wireless communication system wherein data packets are
transmitted to a receiver using a higher order modulation scheme
wherein more than two data bits are mapped onto one data symbol to
perform a first transmission and at least a second transmission
based on a repeat request received from a receiver, the
transmission method: modulating data packets using a first mapping
of said higher order modulation scheme to obtain first data
symbols; performing, by a transmitter, the first transmission by
transmitting the first data symbols over a first diversity branch
to the receiver; receiving the repeat request issued by the
receiver to retransmit the data packets in case the data packets of
the first transmission have not been successfully decoded, wherein
said modulating includes, in response to the received repeat
request, modulating said data packets using a second mapping of
said higher order modulation scheme to obtain second data symbols,
and wherein the transmission method further comprises: performing,
in response to the received repeat request, the second transmission
by transmitting the second data symbols over a second diversity
branch to the receiver, and pre-storing the first mapping and the
second mapping of said higher order modulation scheme in a memory
table.
2. The transmission method according to claim 1, wherein properties
of the first and second mappings are obtained by (a) interleaving
positions of the bits, in the bit sequence of the modulation scheme
or (b) inverting bit values of the bits in the bit series of the
modulation scheme.
3. A transmission method for use in an ARQ retransmission technique
in a wireless communication system wherein data packets are
transmitted to a receiver using a higher order modulation scheme
wherein more than two data bits are mapped onto one data symbol to
perform a first transmission and at least a second transmission
based on a repeat request received from a receiver, the
transmission method comprising: modulating data packets using a
first mapping of said higher order modulation scheme to obtain
first data symbols; performing the first transmission by
transmitting the first data symbols over a first diversity branch
to the receiver; receiving the repeat request issued by the
receiver to retransmit the data packets in case the data packets of
the first transmission have not been successfully decoded, wherein
said modulating includes, in response to the received repeat
request, modulating said data packets using a second mapping of
said higher order modulation scheme to obtain second data symbols,
and wherein the transmission method further comprises: performing
in response to the received repeat request, the second transmission
by transmitting the second data symbols over a second diversity
branch to the receiver; and signaling the first mapping and the
second mapping of said higher order modulation scheme to the
receiver.
4. The transmission method according to claim 3, wherein properties
of the first and second mappings are obtained by (a) interleaving
positions of the bits, in the bit sequence of the modulation scheme
or (b) inverting bit values of the bits in the bit series of the
modulation scheme.
Description
[0001] This is a continuation of application Ser. No. 11/633,421
filed Dec. 5, 2006, which is a continuation of application Ser. No.
10/501,906 filed Jul. 20, 2004, which is a national phase
application under 35 USC 371 of PCT/EP2002/011694 filed Oct. 18,
2002, the entire contents of each of which are incorporated by
reference herein.
[0002] The present invention relates generally to ARQ (re-)
transmission techniques in wireless communication systems and in
particular to a method, transceiver and receiver using transmit
diversity schemes wherein data packets are transmitted using a
first and a second transmission based on a repeat request, and the
bit-to-symbol mapping is performed differently for different
transmitted diversity branches. The invention is particularly
applicable to systems with unreliable and time-varying channel
conditions resulting in an improved performance avoiding
transmission errors.
[0003] There exist several well known transmit diversity techniques
wherein one or several redundancy versions relating to identical
data are transmitted on several (at least two) diversity branches
"by default" without explicitly requesting (by a feedback channel)
further diversity branches (as done in an ARQ scheme by requesting
retransmissions). For example the following schemes are considered
as transmit diversity: [0004] Site Diversity: The transmitted
signal originates from different sites, e.g. different base
stations in a cellular environment. [0005] Antenna Diversity: The
transmitted signal originates from different antennas, e.g.
different antennas of a multi-antenna base station. [0006]
Polarization Diversity: The transmitted signal is mapped onto
different polarizations. [0007] Frequency Diversity: The
transmitted signal is mapped e.g. on different carrier frequencies
or on different frequency hopping sequences. [0008] Time Diversity:
The transmitted signal is e.g. mapped on different interleaving
sequences. [0009] Multicode Diversity: The transmitted signal is
mapped on different codes in e.g. a CDMA (Code Division Multiple
Access) system.
[0010] There are known several diversity combining techniques. The
following three techniques are the most common ones: [0011]
Selection Combining: Selecting the diversity branch with the
highest SNIP for decoding, ignoring the remaining ones. [0012]
Equal Gain Combining: Combining received diversity branches with
ignoring the differences in received SNR. [0013] Maximal Ratio
Combining: Combining received diversity branches taking the
received SNR of each diversity branch into account. The combining
can be performed at bit-level (e.g. LLR) or at modulation symbol
level.
[0014] Furthermore, a common technique for error
detection/correction is based on Automatic Repeat reQuest (ARQ)
schemes together with Forward Error Correction (FEC), called hybrid
ARQ (HARQ). If an error is detected within a packet by the Cyclic
Redundancy Check (CRC), the receiver requests the transmitter to
send additional information (retransmission) to improve the
probability to correctly decode the erroneous packet.
[0015] in WO-02/607491 A1 a method for hybrid ARQ transmissions has
been disclosed which averages the bit reliabilities over
successively requested retransmissions by means of signal
constellation rearrangement.
[0016] As shown therein, when employing higher order modulation
formats (e.g. M-PSK, M-QAM with log.sub.2(M)>2), where more than
2 bits are mapped onto one modulation symbol, the bits have
different reliabilities depending on the chosen mapping. This leads
for most FEC (e.g. Turbo Codes) schemes to a degraded decoder
performance compared to an input of more equally distributed bit
reliabilities.
[0017] In conventional communication systems the modulation
dependent variations in bit reliabilities are not taken into
account and, hence, usually the variations remain after combining
the diversity branches at the receiver.
[0018] The object of the invention is to provide an ARQ (re-)
transmission method, a transmitter and a receiver which show an
improved performance with regard to transmission errors. This
object is solved by a method, transmitter and receiver as set forth
in the independent claims.
[0019] The invention is based on the idea to improve the
performance at the receiver by applying different signal
constellation mappings to the available distinguishable transmit
diversity branches and ARC (re-) transmissions. The invention is
applicable to modulation formats, where more than 2 bits are mapped
onto one modulation symbol, since this implies a variation in
reliabilities for the bits mapped onto the signal constellation.
The variations depend on the employed mapping and on the actually
transmitted content of the bits.
[0020] Depending on the employed modulation format and the actual
number of bits mapped onto a single modulation symbol, for a given
arbitrary number (N>1) of available diversity branches and
required retransmissions the quality of the averaging process is
different. Averaging in the sense of the present invention is
understood as a process of reducing the differences in mean
combined bit reliabilities among the different bits of a data
symbol. Although it might be that only after using several
diversity branches or paths a perfect averaging with no remaining
differences is achieved, averaging means in the context of the
document any process steps in the direction of reducing the mean
combined bit reliability differences. Assuming on average an equal
SNR for all available diversity branches and ARQ transmissions, for
16-QAM 4 mappings (4 diversity branches) would be needed to
perfectly average out the reliabilities for all bits mapped on any
symbol. However, not always the number of available transmit
diversity branches and/or the number of ARQ transmissions is
sufficient to perform a perfect averaging. Hence, the averaging
should then be performed on a best effort basis as shown in the
example below.
[0021] The present invention will be more readily understood from
the following detailed description of preferred embodiments with
reference to the accompanying figures which show:
[0022] FIG. 1 an example for a 16-QAM signal constellation;
[0023] FIG. 2 an example for a different mapping of a 16-QAM signal
constellation;
[0024] FIG. 3 two further examples of 16-QAM signal
constellations;
[0025] FIG. 4 an exemplary embodiment of a communication system
according to the present invention;
[0026] FIG. 5 details of a table for storing a plurality of signal
constellation patterns; and
[0027] FIG. 6 shows the communication system according to the
present invention with an interleaver/inverter section.
[0028] The method described here performs a combined averaging of
bit reliabilities considering the transmit diversity branches. The
following detailed description is shown for a square 16-QAM with
Gray mapping. However, without loss of generality the shown example
is extendable to other M-QAM and M-PSK (with log.sub.2(M)>2)
formats. Moreover, the examples are shown for transmit diversity
and HARQ schemes transmitting an identical bit-sequence on both
branches and all HARQ transmissions (single redundancy version
scheme). Then again, an extension to a transmit diversity and HARD
scheme transmitting only partly identical bits on the diversity
branches and HARQ transmissions can be accomplished. An example for
a system using multiple redundancy versions is described in
copending EP 01127244, filed on Nov. 16, 2001. Assuming a Turbo
encoder, the systematic bits can be averaged on a higher level as
compared to the parity bits.
[0029] Although the below examples give details of an embodiment
with the special case of hybrid ARQ (HARD), it should be noted that
the inclusion of an FEC code is not necessary for the present
invention to show performance gains. However the highest
performance gains can be achieved with the use of HARQ.
[0030] The following example describes a method with two diversity
branches and HARQ.
1.sup.st Transmission:
[0031] Assuming a transmit diversity scheme with two generated
diversity branches, which are distinguishable at the receiver (e.g.
by different spreading or scrambling codes in a CDMA system) and a
transmission of the same redundancy version, usually the received
diversity branches are combined at the receiver before applying the
FEC decoder. A common combining technique is the maximal ratio
combining which can be achieved by adding the calculated
log-likelihood-ratios LLRs from each individual received diversity
branch.
[0032] The log-likelihood-ratio LLR as a soft-metric for the
reliability of a demodulated bit b from a received modulation
symbol r=x/jy is defined as follows:
LLR ( b ) = ln [ Pr { b = 1 r } Pr { b = 0 r } ] ( 1 )
##EQU00001##
[0033] As can be seen from FIG. 1 (bars indicate rows/columns for
which the respective bit equals 1), the mappings of the in-phase
component bits and the quadrature component bits on the signal
constellation are orthogonal (for M-PSK the LLR calculation cannot
be simplified by separating into complex components, however the
general procedure of bit-reliability averaging is similar).
Therefore, it is sufficient to focus on the in-phase component bits
i.sub.1 and i.sub.2. The same conclusions apply then for q.sub.1
and q.sub.2.
[0034] Assuming that Mapping 1 from FIG. 1 is applied for the
bit-to-symbol mapping for the 1.sup.st diversity branch, the
log-likelihood-ratio LLR of the most significant bit (MSB) i.sub.1
and the least significant bit (LSB) i.sub.2 yields the following
equations for a Gaussian channel:
LLR ( i 1 ) = ln [ - K ( x + x 0 ) 2 + - K ( x + x 1 ) 2 - K ( x -
x 0 ) 2 + - K ( x - x 1 ) 2 ] ( 2 ) LLR ( i 2 ) = ln [ - K ( x - x
1 ) 2 + - K ( x + x 1 ) 2 - K ( x - x 0 ) 2 + - K ( x + x 0 ) 2 ] (
3 ) ##EQU00002##
where x denotes the in-phase component of the normalized received
modulation symbol r and K is a factor proportional to the
signal-to-noise ratio. Under the assumption of a uniform signal
constellation (x.sub.13x.sub.0) equations (2) and (3) can be fairly
good approximated, as shown in S. Le Goff, A. Giavieux, C. Berrou,
"Turbo-Codes and High Spectral Efficiency Modulation," IEEE
SUPERCOMM/ICC '94, Vol. 2, pp. 645-649, 1994, and Ch. Wengerter, A.
Golitschek Edler von Etbwart, E. Seidel, G. Velev, M. P. Schmitt,
"Advanced Hybrid ARQ Technique Employing a Signal Constellation
Rearrangement," IEEE Proceedings of VTC 2002 Fall, Vancouver,
Canada, September 2002, by
LLR(i.sub.1).apprxeq.-4Kx.sub.0x (4)
LLR(i.sub.2).apprxeq.-4Kx.sub.0(2x.sub.0-|x|) (5)
[0035] The mean LLR for i.sub.1 and i.sub.2 for a given transmitted
modulation symbol yields the values given in Table 1 (substituting
4Kx.sub.0.sup.2 by .LAMBDA.). Mean in this sense, refers to that
the mean received value for a given transmitted constellation
point, exactly matches this transmitted constellation point.
Individual samples of course experience noise according to the
parameter K. However, for a Gaussian channel the mean value of the
noise process is zero. In case of transmitted modulation symbols
Oq.sub.11q.sub.2 and 1q.sub.11q.sub.2, where q.sub.1 and q.sub.2
are arbitrary, the magnitude of the mean LLR (i.sub.1) is higher
than of the mean LLR (i.sub.2). This means that the LLR for the MSB
i.sub.1 depends on the content of the LSB i.sub.2; e.g. in FIG. 1
i.sub.1 has a higher mean reliability in case the logical value for
i.sub.2 equals 1 (leftmost and rightmost columns). Hence, assuming
a uniform distribution of transmitted modulation symbols, on
average 50% of the MSBs i.sub.1 have about three times the
magnitude in LLR of i.sub.2.
TABLE-US-00001 TABLE 1 Mean LLRs for bits mapped on the in-phase
component of the signal constellation for Mapping 1 in FIG. 1
according to equations (4) and (5). Symbol Mean Mean Mean
(i.sub.1q.sub.1i.sub.2q.sub.2) value of x LLR (i.sub.1) LLR
(i.sub.2) 0q.sub.10q.sub.2 x.sub.0 -4Kx.sub.0.sup.2 = -.LAMBDA.
-4Kx.sub.0.sup.2 = -.LAMBDA. 0q.sub.11q.sub.2 x.sub.1
-12Kx.sub.0.sup.2 = -3.LAMBDA. 4Kx.sub.0.sup.2 = .LAMBDA.
1q.sub.10q.sub.2 -x.sub.0 4Kx.sub.0.sup.2 = .LAMBDA.
-4Kx.sub.0.sup.2 = -.LAMBDA. 1q.sub.11q.sub.2 -x.sub.1
12Kx.sub.0.sup.2 = 3.LAMBDA. 4Kx.sub.0.sup.2 = .LAMBDA.
[0036] If now adding a 2.sup.nd transmit diversity branch
transmitting e.g. an identical bit sequence prior art schemes would
employ an identical mapping to the 1.sup.st diversity branch. Here,
it is proposed to employ a 2.sup.nd signal constellation mapping
(Mapping 2) according to FIG. 2, which yields the mean LLRs given
in Table 2.
TABLE-US-00002 TABLE 2 Mean LLRs for bits mapped on the in-phase
component of the signal constellation for Mapping 2 in FIG. 2.
Symbol Mean Mean Mean (i.sub.1q.sub.1i.sub.2q.sub.2) value of x LLR
(i.sub.1) LLR (i.sub.2) 0q.sub.10q.sub.2 x.sub.0 -.LAMBDA.
-3.LAMBDA. 0q.sub.11q.sub.2 x.sub.1 -.LAMBDA. 3.LAMBDA.
1q.sub.10q.sub.2 -x.sub.0 .LAMBDA. -.LAMBDA. 1q.sub.11q.sub.2
-x.sub.1 .LAMBDA. .LAMBDA.
[0037] Comparing now the soft-combined LLRs of the received
diversity branches applying the constellation rearrangement
(Mapping 1+2) and applying the identical mappings (Mapping 1+1,
prior art), it can be observed from Table 3 that the combined mean
LLR values with applying the constellation rearrangement have a
more uniform distribution (Magnitudes: 4.times.4.LAMBDA. and
4.times.2.LAMBDA. instead of 2.times.6.LAMBDA. and
6.times.2.LAMBDA.). For most FEC decoders (e.g. Turbo Codes and
Convolutional Codes) this leads to a better decoding performance.
Investigations have revealed that in particular Turbo
encoding/decoding systems exhibit a superior performance. It should
be noted, that the chosen mappings are non exhaustive and more
combinations of mappings fulfilling the same requirements can be
found.
TABLE-US-00003 TABLE 3 Mean LLRs (per branch) and combined mean
LLRs for bits mapped on the in-phase component of the signal
constellation for the diversity branches when employing Mapping 1
and 2 and when employing 2 times Mapping 1. Constellation Prior Art
Rearrangement No Rearrangement Transmit (Mapping 1 + 2) (Mapping 1
+ 1) Diversity Symbol Mean Mean Mean Mean Branch
(i.sub.1q.sub.1i.sub.2q.sub.2) LLR (i.sub.1) LLR (i.sub.2) LLR
(i.sub.1) LLR (i.sub.2) 1 0q.sub.10q.sub.2 -.LAMBDA. -.LAMBDA.
-.LAMBDA. -.LAMBDA. 0q.sub.11q.sub.2 -3.LAMBDA. .LAMBDA. -3.LAMBDA.
.LAMBDA. 1q.sub.10q.sub.2 .LAMBDA. -.LAMBDA. .LAMBDA. -.LAMBDA.
1q.sub.11q.sub.2 3.LAMBDA. .LAMBDA. 3.LAMBDA. .LAMBDA. 2
0q.sub.10q.sub.2 -.LAMBDA. -3.LAMBDA. -.LAMBDA. -.LAMBDA.
0q.sub.11q.sub.2 -.LAMBDA. 3.LAMBDA. -3.LAMBDA. .LAMBDA.
1q.sub.10q.sub.2 .LAMBDA. -.LAMBDA. .LAMBDA. -.LAMBDA.
1q.sub.11q.sub.2 .LAMBDA. .LAMBDA. 3.LAMBDA. .LAMBDA. Combined
0q.sub.10q.sub.2 -2.LAMBDA. -4.LAMBDA. -2.LAMBDA. -2.LAMBDA. 1 + 2
0q.sub.11q.sub.2 -4.LAMBDA. -4.LAMBDA. -6.LAMBDA. 2.LAMBDA.
1q.sub.10q.sub.2 2.LAMBDA. -2.LAMBDA. 2.LAMBDA. -2.LAMBDA.
1q.sub.11q.sub.2 4.LAMBDA. 2.LAMBDA. 6.LAMBDA. 2.LAMBDA.
2.sup.nd and Further Transmissions:
[0038] In case the 1.sup.st transmission has not been successfully
decoded the receiver requests a retransmission (2.sup.nd
transmission). Assuming for 2.sup.nd transmission also 2 transmit
diversity branches are available, the 2 additional mappings
(mapping 3 and mapping 4 in FIG. 3) are employed to further improve
the averaging of the bit reliabilities as shown in Table 4. In this
example (assuming an equal SNR for all received signals) the
averaging is performed perfectly after receiving 2 transmit
diversity branches times 2 transmissions (possibility to employ 4
different mappings--sufficient for 16--QAM). Table 4 compares the
LLRs with and without applying the proposed Constellation
Rearrangement. Having a closer look at the combined LLRs, it can be
seen that with application of the Constellation Rearrangement the
magnitude for all bit reliabilities results in 6.LAMBDA..
[0039] It should be noted again, that the chosen mappings are non
exhaustive and more combinations of mappings fulfilling the same
requirements can be found.
TABLE-US-00004 TABLE 4 Mean LLRs (per branch) and combined mean
LLRs for bits mapped on the in-phase component of the signal
constellation for the diversity branches and (re-) transmissions
when employing Mappings 1 to 4 and when employing 4 times Mapping
1. Constellation Prior Art Rearrangement No Rearrangement Transmit
(Mapping 1 + 2 + 3 + 4) (Mapping 1 + 1 + 1 + 1) Diversity
Transmission Symbol Mean Mean Mean Mean Branch Number
(i.sub.1q.sub.1i.sub.2q.sub.2) LLR (i.sub.1) LLR (i.sub.2) LLR
(i.sub.1) LLR (i.sub.2) 1 1 0q.sub.10q.sub.2 -.LAMBDA. -.LAMBDA.
-.LAMBDA. -.LAMBDA. 0q.sub.11q.sub.2 -3.LAMBDA. .LAMBDA. -3.LAMBDA.
.LAMBDA. 1q.sub.10q.sub.2 .LAMBDA. -.LAMBDA. .LAMBDA. -.LAMBDA.
1q.sub.11q.sub.2 3.LAMBDA. .LAMBDA. 3.LAMBDA. .LAMBDA. 2 1
0q.sub.10q.sub.2 -.LAMBDA. -3.LAMBDA. -.LAMBDA. -.LAMBDA.
0q.sub.11q.sub.2 -.LAMBDA. 3.LAMBDA. -3.LAMBDA. .LAMBDA.
1q.sub.10q.sub.2 .LAMBDA. -.LAMBDA. .LAMBDA. -.LAMBDA.
1q.sub.11q.sub.2 .LAMBDA. .LAMBDA. 3.LAMBDA. .LAMBDA. 3 2
0q.sub.10q.sub.2 -.LAMBDA. -.LAMBDA. -.LAMBDA. -.LAMBDA.
0q.sub.11q.sub.2 -.LAMBDA. .LAMBDA. -3.LAMBDA. .LAMBDA.
1q.sub.10q.sub.2 .LAMBDA. -3.LAMBDA. .LAMBDA. -.LAMBDA.
1q.sub.11q.sub.2 .LAMBDA. 3.LAMBDA. 3.LAMBDA. .LAMBDA. 4 2
0q.sub.10q.sub.2 -3.LAMBDA. -.LAMBDA. -.LAMBDA. -.LAMBDA.
0q.sub.11q.sub.2 -.LAMBDA. .LAMBDA. -3.LAMBDA. .LAMBDA.
1q.sub.10q.sub.2 3.LAMBDA. -.LAMBDA. .LAMBDA. -.LAMBDA.
1q.sub.11q.sub.2 .LAMBDA. .LAMBDA. 3.LAMBDA. .LAMBDA. Combined
0q.sub.10q.sub.2 -6.LAMBDA. -6.LAMBDA. -4.LAMBDA. -4.LAMBDA. 1 + 2
+ 3 + 4 0q.sub.11q.sub.2 -6.LAMBDA. 6.LAMBDA. -12.LAMBDA. 4.LAMBDA.
1q.sub.10q.sub.2 6.LAMBDA. -6.LAMBDA. 4.LAMBDA. -4.LAMBDA.
1q.sub.11q.sub.2 6.LAMBDA. 6.LAMBDA. 12.LAMBDA. 4.LAMBDA.
[0040] If the constellation rearrangement is performed by applying
different mapping schemes, one would end up in employing a number
of different mappings as given in FIG. 1, FIG. 2 and FIG. 3. If the
identical mapper (e.g. FIG. 1) should be kept for all transmit
diversity branches, e.g. mapping 2 can be obtained from mapping 1
by the following operations: [0041] exchange positions of original
bits i.sub.1 and i.sub.2 [0042] exchange positions of original bits
q.sub.1 and q.sub.2 [0043] logical bit inversion or original bits
i.sub.1 and q.sub.1
[0044] Alternatively, those bits that end in positions 1 and 2 can
also be inverted (resulting in a different mapping with an
identical bit-reliability characteristics). Accordingly, mapping 2
can be obtained from mapping 1, using an interleaver/inverter
section 14 (see FIG. 6) which performs interleaving and/or
inverting of the bits.
[0045] Therefore, the following table provides an example how to
obtain mappings 1 to 4 (or mappings with equivalent bit
reliabilities for i.sub.1, i.sub.2, q.sub.1 and q.sub.2), where the
bits always refer to the first transmission, and a long dash above
a character denotes logical bit inversion of that bit:
TABLE-US-00005 TABLE 5 Alternative implementation of the
Constellation Rearrangement by interleaving (intra-symbol
interleaving) and logical inversion of bits mapped onto the
modulation symbols. Mapping Interleaver and Inverter No.
functionality 1 i.sub.1q.sub.1i.sub.2q.sub.2 2 .sub.2 q.sub.2
.sub.1 q.sub.1 or i.sub.2q.sub.2 .sub.1 q.sub.1 3 .sub.2
q.sub.2i.sub.1q.sub.1 or i.sub.2q.sub.2i.sub.1q.sub.1 4
i.sub.1q.sub.1 .sub.2 q.sub.2 or .sub.1 q.sub.1 .sub.2 q.sub.2
[0046] Generally at least 2 different mappings should be employed
for N>1 diversity branches, where the order and the selection of
the mappings is irrelevant, as long as the bit-reliability
averaging process, meaning the reduction in differences in bit
reliabilities) is maintained.
[0047] Preferred realizations in terms of number of employed
mappings [0048] M-QAM [0049] Employing log.sub.2(M) different
mappings [0050] Employing log.sub.2(M)/2 different mappings [0051]
M-PSK [0052] Employing log.sub.2(M) different mappings [0053]
Employing log.sub.2(M)/2 different mappings [0054] Employing 2
log.sub.2(M) different mappings
[0055] The applied signal constellation mappings for modulation at
the transmitter and demodulation at the receiver need to match for
each individual transmit diversity branch. This can be achieved by
appropriate signalling of parameters indicating the proper mapping
or combination of mappings to be applied for the diversity branches
and HARQ transmissions. Alternatively the definition of the
mappings to be applied for transmit diversity branches and HARQ
transmissions may be system predefined.
[0056] FIG. 4 shows an exemplary embodiment of a communication
system according to the present invention. More specifically, the
communication system comprises a transmitter 10 and a receiver 20
which communicate through a communication channel consisting of a
plurality of diversity branches 40A, 40B and 40C. Although three
diversity branches are illustrated in the figure, it becomes clear
to a person skilled in the art that an arbitrary number of branches
may be chosen. From a data source 11, data packets are supplied to
a FEC encoder 12, preferably a FEC Turbo encoder, where redundancy
bits are added to correct errors. The bits output from the FEC
encoder are subsequently supplied to a mapping unit 13 acting as a
modulator to output symbols formed according to the applied
modulation scheme stored as a constellation pattern in a table 15 .
Subsequently the data symbols are applied to a transmission unit 30
for transmission over the branches 40A-C. The receiver 20 receives
the data packets by the receiving unit 35. The bits are then input
into a demapping unit 21 which acts as a demodulator using the same
signal constellation pattern stored in the table 15 which was used
during the modulation of these bits.
[0057] The demodulated data packets received over one diversity
branch are stored in a temporary buffer 22 for subsequent combining
in a combining unit 23 with the data packets received over at least
one other diversity branch.
[0058] A retransmission is launched by an automatic repeat request
issued by an error detector (not shown) and communicated by a
communication section 57 of receiver 20 to a receiving section 55
of transmitter 10 with the result that an identical data packet is
transmitted from transmitter 10. In combining unit 23, the
previously received erroneous data packets are soft-combined with
the retransmitted data packets. Then a decoder decodes the bits and
outputs a measure for the transmission quality, e.g. the bit-error
rate BER.
[0059] As illustrated in FIG. 5, table 15 stores a plurality of
signal constellation patterns #0 . . . #n which are selected for
the individual transmissions over the individual diversity branches
and HARQ transmissions according to a predetermined scheme. The
scheme, i.e. the sequence of signal constellation patterns used for
modulating/demodulating are either pre-stored in the transmitter
and the receiver or are signalled by transmitter to the receiver
prior to usage.
* * * * *