LOW COMPLEXITY MAXIMUM LIKELIHOOD DETECTION OF CONCATENATED SPACE CODES FOR WIRELESS APPLICATIONS

DETECTION PAR PROBABILITE MAXIMALE A FAIBLE COMPLEXITE DE CODES D'ESPACES CONCATENES POUR APPLICATIONS SANS FIL

English Abstract

Good transmission characteristics are achieved in the presence of fading with a transmitter that employs a trellis coder followed by a block coder. Correspondingly, the receiver comprises a Viterbi decoder followed by a block decoder. Advantageously, the block coder and decoder employ time-space diversity coding which, illustratively, employs two transmitter antennas and one receiver antenna.

French Abstract

On a obtenu de bonnes caractéristiques de transmission en présence d'évanouissement avec un émetteur employant un codeur par treillis suivi d'un codeur de blocs. De même, le récepteur comprend un décodeur Viterbi suivi d'un décodeur de blocs. Le codeur et le décodeur de blocs emploient avantageusement un codage en diversité temps-espace qui, à titre d'illustration, emploie deux antennes d'émission et une antenne de réception.

Claims

We Claim:


1. A transmitter comprising:

a trellis encoder, wherein the trellis encoder generates a first symbol so and
a
second symbol S1, and

a block encoder responsive to the trellis encoder and adapted to feed two
antennas, wherein the block encoder generates a block including the first
symbol, the
second symbol, a third symbol generated using a complex conjugation of the
first
symbol, and a fourth symbol generated using a negative complex conjugation of
the
second symbol, and wherein in a first time period the first and second symbol
s0, s1 are
forwarded to said two antennas respectively, and wherein in a second time
period said
fourth and third symbols are forwarded to said two antennas, respectively.

2. The transmitter of claim 1 wherein the block encoder is a multi-branch
block
encoder.

3. The transmitter of claim 1 wherein the block encoder is a space-time block
encoder.

4. A receiver for receiving blocks of symbols sent by the transmitter of
claims 1-3,
two said receiver comprising:
a space block combiner configured for receiving data transmitted from the two
transmitting antennas, and
a Viterbi decoder responsive to output signals of the space block combiner.
5. The receiver of claim 4 wherein the combiner combines a frame of received
symbols, wherein the frame consists of n time slots and in each time slot
concurrently
provides m symbols to the combiner.

6. The receiver of claim 5 wherein n=m.

-7-



7. The receiver of claim 6 wherein n=m=2.

8. The receiver of claim 4 wherein the Viterbi decoder develops the metric
Image , wherein S i is a
hypothesized signal at a first time interval, s i is a hypothesized signal at
a second time
interval, s 0 is a transmitted signal at the first time interval, s1 is a
transmitted signal at the
second time interval, ~0 is an estimate of channel characteristics between a
transmitting
antenna that transmits signal s0 and a receiving antenna of the receiver, and
~1 is an
estimate of channel characteristics between a transmitting antenna that
transmits signal s1
and the receiving antenna of the receiver.

9. The receiver of claim 4 wherein the Viterbi decoder develops the metric
Image to recover the symbol s0, and the metric
Image to recover the symbol s1, where s j is a

hypothesized signal at a first time interval, s i is a hypothesized signal at
a second time
interval, s0 is a transmitted signal at the first time interval, s1 is a
transmitted signal at the
second time interval, ~0 is an estimate of channel characteristics between a
transmitting
antenna that transmits signal s0 and a receiving antenna of the receiver, and
~1 is an
estimate of channel characteristics between a transmitting antenna that
transmits signal s1
and the receiving antenna of the receiver.

10. The receiver of claim 4, wherein the Viterbi decoder develops the metric
M[(s0,s1 ),(s i,s j)]=M(s0,s i)+M(s1,s j) where

Image

-8-




where s i is a hypothesized signal at a first time interval, s j is a
hypothesized signal at a
second time interval, s0 is a transmitted signal at the first time interval,
s1 is a transmitted
signal at the second time interval, ~0 is an estimate of channel
characteristics between a
transmitting antenna that transmits signal s0 and a receiving antenna of said
receiver, ~1
is an estimate of channel characteristics between a transmitting antenna that
transmits
signal s1 and the receiving antenna of the receiver, ~0 is one signal
developed by the
combiner, and ~1 is another signal developed by the combiner.

11. The receiver of claim 4 wherein the combiner creates signals
* * *
~0= ~0r0 +~1r1* and ~1 = ~l*r0 - ~0r1* , where r0 is a received signal at one
time
interval, r1 is a received signal at another time interval, ~0 is an estimate
of channel
characteristics between a transmitting antenna that transmits signal s0 and a
receiving
antenna of said receiver, and it, ~1 is an estimate of channel characteristics
between a
transmitting antenna that transmits signal s1 and the receiving antenna of the
receiver.
12. In a receiver, a method for linking trellis codes with block codes, the
method
comprising:
receiving on a single receiver antenna, a first baseband signal and a second
baseband signal, wherein the first and second baseband signals were
transmitted using
space-time coding;
determining channel estimates for the received first and second baseband
signals;
using the determined channel estimates, generating a first contained signal
based on the first received baseband signal and a second combined signal based
on the
second received signal, the first and second combined signals including a
complex
multiplicative distortion factor;


-9-



based on the first generated combined signal, building a first metric
corresponding to a first hypothesized symbol, the first hypothesized symbol
replicating
the received first baseband signal prior to transmission; and
based on the second generated combined signal, building a second metric
corresponding to a second hypothesized symbol, the second hypothesized symbol
replicating the received second baseband signal prior to transmission.

13. The method of claim 12 wherein the first baseband signal and the second
baseband signal include noise and interference, including multipath fading.

14. The method of claim 12 wherein the first baseband signal is received at a
first
time and wherein the second baseband signal is received at a second time.

15. The method of claim 12 wherein the first and second combined signals are
generated using a multiple branch space block combiner.

16. In a receiver, an apparatus for linking trellis codes with block codes,
the apparatus
comprising:
means for receiving on a single receiver antenna, a first baseband signal
and a second baseband signal, wherein the received first and second baseband
signals
were encoded at a transmitter having at least two transmit antennas coupled to
an at least
two branch trellis encoder;
means, coupled to the means for receiving, for determining channel
estimates for the received first and second baseband signals;
means, coupled to the means for determining, for using the determined
channel estimates and generating a first combined block signal based on the
first received
baseband signal and a second combined block signal based on the second
received signal,
wherein the first and second combined signals include a complex multiplicative
distortion
factor; and
means for building a combined metric for a hypothesized branch symbol,
the hypothesized symbol replicating both the received first and second
symbols.


-10-



17. A transmitter comprising:
a trellis encoder, wherein the trellis encoder generates a first symbol and a
second symbol, and
a block encoder responsive to the trellis encoder and adapted to feed a
plurality of antennas, wherein the block encoder generates a block including
the first
symbol, the second symbol, a third symbol generated using a complex
conjugation,
negation, or negative complex conjugation of the first symbol, and a fourth
symbol
generated using a complex conjugation, negation, or negative complex
conjugation of the
second symbol.

18. The transmitter of claim 17 further comprising the plurality of antennas.

19. The transmitter of claim 17 wherein the trellis encoder is a multiple
trellis code
modulation encoder.

20. The transmitter of claim 17 wherein the block encoder is a multi-branch
block
encoder.

21. The transmitter of claim 17 wherein the block encoder is a space-time
block
encoder.

22. In a transmitter, a method for linking trellis codes with block codes, the
method
comprising:

receiving input data;
trellis encoding the received data, including generating complex numbers,
each complex number representing a constellation symbol;
receiving the generated complex numbers;
block encoding the received complex numbers, including generating a
block of symbols, wherein the block of symbols includes the generated complex
numbers



-11-



and at least one of a complex conjugation, negation, or negative complex
conjugation of
at least some of the generated complex numbers; and
outputting the blocks of symbols for transmission by two or more
transmitting antennas.

23. The method of claim 22 wherein the received data is binary data.

24. The method of claim 22 wherein the block encoding includes taking a first
complex number and a second complex number as input and generating a complex
conjugate of the first complex number and a negative complex conjugate of the
second
complex number.

25. The method of claim 22 wherein the block encoding includes ordering the
block
to provide a first complex number, a second complex number, a negative complex

conjuration of the second complex number and a complex conjugation of the
first
complex number.

26. The method of claim 22, further comprising, transmitting the blocks of
symbols
over the two or more antennas.

27. In a transmitter, an apparatus for generating encoded symbols for
transmission
over a wireless link, the apparatus comprising:
means for receiving input data;
means, coupled to the means for receiving input data, for generating a first
symbol and a second symbol; and
means, coupled to the means for generating first and second symbols, for
generating a block, the block including the first symbol, the second symbol, a
third
symbol generated using a complex conjugation, negation, or negative complex
conjugation of the first symbol, and a fourth symbol generated using a complex

conjugation, negation, or negative complex conjugation of the second symbol.



-12-



28. The apparatus of claim 27 further comprising:
means for sending, at a first time, the first symbol from a first antenna and
the second symbol from a second antenna; and

means for sending, at a second time, the forth symbol from the first
antenna and the third symbol from the second antenna.

29. The apparatus of claim 27 further comprising:
means for sending, on a first carrier the first symbol from a first antenna
and the second symbol from a second antenna; and
means for sending, on a second carrier, the third symbol from a second
antenna and the fourth symbol from a first antenna.

30. The apparatus of claim 27 wherein the block is generated using a two-
branch
space block encoder.

31. The apparatus of claim 27 wherein the block is generated using a multiple-
branch
space block encoder.

32. The receiver of claim 5 wherein the combiner develops n signals that
represent
estimates of signals transmitted by a transmitter.

33. The receiver of claim 4 wherein the Viterbi decoder generates a separate
metric
for soft decision of a transmitted symbol.

34. The receiver of claim 4 wherein the Viterbi decoder is a multiple trellis
code
modulation decoder.

35. A method for use in a transmitter for linking trellis codes with block
codes, the
method comprising:
receiving input data;



-13-



trellis encoding the received data, including generating a first symbol so and
a
second symbol s1;

block encoding the received symbols including generating a block of symbols,
wherein the block of symbols includes the first symbol, the second symbol a
third symbol
generated using a complex conjugation of the first symbol, and a fourth symbol
using
negative complex conjugation of the second symbol.

36. A method for use in a receiver for receiving blocks of symbols transmitted
in
accordance with the method of claim 35, the method comprising:
receiving on a single receiver antenna, a first signal and a second signal,
wherein
the first and second signals were transmitted using space-time coding;
determining channel estimates for the received first and second signals;
using the determined channel estimates, generating a first combined signal
based
on the sum of the first and second received signals and a second combined
signal based
on the difference of the first and second received signals, the first and
second combined
signals including a distortion component; and

based on the first generated combined signal, building a first metric
corresponding
to a first hypothesized symbol, the first hypothesized symbol replicating the
received first
signal prior to transmission; and

based on the second generated combined signal, building a second metric
corresponding to a second hypothesized symbol, the second hypothesized symbol
replicating the received second signal prior to transmission.

37. The method of claim 36 wherein the first signal and the second signal
include
noise and interference, including multipath fading.

38. In a data communication system employing space diversity via two or more
transmitting antennas, a receiver comprising:

a space block combiner configured for receiving data transmitted from the two
or
more transmitting antennas,
a Viterbi decoder responsive to output signals of the space block combiner;



-14-



wherein the Viterbi decoder is a multiple trellis code modulation detector;
and
wherein the Viterbi decoder develops the metric

Image wherein s i is a
hypothesized signal at a first time interval, s j is a hypothesized signal at
a second time
interval, s0 is a transmitted signal at the first time interval, s1 is a
transmitted signal at the
second time interval, ~ is an estimate of channel characteristics between a
transmitting
antenna that transmits signal s0 and a receiving antenna of the receiver, and
~ is an
estimate of channel characteristics between a transmitting antenna that
transmits signal s1
and the receiving antenna of the receiver.

39. The receiver of claim 38 wherein the combiner combines a frame of received

symbols, wherein the frame consists of n time slots and in each time slot
concurrently
provides m symbols to the combiner.

40. The receiver of claim 39 wherein n=m.
41. The receiver of claim 40 wherein n=m=2.

42. The receiver of claim 39 wherein the combiner develops n signals that
represent
estimates of signals transmitted by a transmitter.

43. The receiver of claim 38 wherein the Viterbi decoder generates a separate
metric
for soft decision of a transmitted symbol.

44. In a data communication system employing space diversity via two or
more transmitting antennas, a receiver comprising:

a space block combiner configured for receiving data transmitted from the two
or
more transmitting antennas,
a Viterbi decoder responsive to output signals of the space block combiner;



-15-



wherein the Viterbi decoder is a multiple trellis code modulation detector;
and wherein
the Viterbi decoder develops the metric

Image to recover the symbol s0, and the metric
Image to recover the symbol s1, where s i is a
hypothesized signal at a first time interval, s j is a hypothesized signal at
a second time
interval, s0 is a transmitted signal at the first time interval, s1 is a
transmitted signal at the
second time interval, ~ is an estimate of channel characteristics between a
transmitting
antenna that transmits signal s0 and a receiving antenna of said receiver, and
~ is an
estimate of channel characteristics between a transmitting antenna that
transmits signal s1
and the receiving antenna of the receiver.

45. In a data communication system employing space diversity via two or
more transmitting antennas, a receiver comprising:
a space block combiner configured for receiving data transmitted from the two
or
more transmitting antennas,

a Viterbi decoder responsive to output signals of the space block combiner;
wherein the Viterbi decoder is a multiple trellis code modulation detector;
and wherein
the Viterbi decoder develops the metric

M[(s0,s1),(s i,s j)]=M(s0,s i)+M(s1,s j) where
Image
where s i is a hypothesized signal at a first time interval, s j is a
hypothesized signal at a
second time interval, s0 is a transmitted signal at the first time interval,
s1 is a transmitted
signal at the second time interval, ~ is an estimate of channel
characteristics between a
transmitting antenna that transmits signal s0 and a receiving antenna of said
receiver, ~



-16-




is an estimate of channel characteristics between a transmitting antenna that
transmits
signal s1 and the receiving antenna of the receiver, ~0 is one signal
developed by the
combiner, and ~1, is another signal developed by the combiner.

46. In a data communication system employing space diversity via two or
more transmitting antennas, a receiver comprising:
a space block combiner configured for receiving data transmitted from the two
or
more transmitting antennas,
a Viterbi decoder responsive to output signals of the space block combiner;
wherein the Viterbi decoder is a multiple trellis code modulation detector;
and

wherein the combiner creates signals Image, where
r0 is a received signal at one time interval, r1 is a received signal at
another time interval,
~0 is an estimate of channel characteristics between a transmitting antenna
that transmits
signal s0 and a receiving antenna of said receiver, and it, ~1 is an estimate
of channel
characteristics between a transmitting antenna that transmits signal s1 and
the receiving
antenna of the receiver.

47. The apparatus of claim 16 wherein the first baseband signal and the second

baseband signal include noise and interference, including multipath fading.

48. The apparatus of claim 16 wherein the first baseband signal is received at
a first
time and wherein the second baseband signal is received at a second time.

49. The apparatus of claim 16 wherein the first baseband signal is received
via a first
frequency and wherein the second baseband signal is received via second
frequency.

50. A mobile device comprising:
a space block combiner and a decoder to decode incoming codes generated based
on concatenated trellis and space block coding; and

-17-



wherein the incoming codes include a first code and a second code, wherein the

first code includes a first symbol and a second symbol and wherein the second
code
includes a third symbol generated using a complex conjugation, negation, or
negative
complex conjugation of the first symbol and a fourth symbol generated using a
complex
conjugation, negation, or negative complex conjugation of the second symbol.

51. The mobile device of claim 50 wherein the block coding includes space-time
block
coding.

52. The mobile device of claim 50 wherein the block coding includes space-
frequency
coding.

53. The mobile device of claim 50, wherein the processor decodes the incoming
codes
using at least a space block combiner and a Viterbi decoder.

54. In a wireless communication system, a method of communicating information
over a wireless link, the method comprising:
generating complex numbers that represent constellation symbols; and

encoding adjacent pairs of symbols, including the generated complex numbers,
to produce a set of concatenated codes, wherein the set of concatenated codes
include a
first code including a first symbol and a second symbol and a second code
including a
third symbol generated using a complex conjugation, or negative complex
conjugation of
the first symbol and a fourth symbol generated using a complex conjugation, or
negative
complex conjugation of the second symbol.

55. The method of claim 54 wherein the complex numbers that represent
constellation symbols are generated, at least in part, using a trellis code
modulation
(TCM) encoder.

56. The method of claim 54 wherein the adjacent pairs of symbols are encoded
using a block encoder.

-18-



57. The method of claim 54 wherein the first code is sent at a first time and
the
second code is sent at a second time that is distinct from the first time.

58. In a wireless communication system, a method of communicating information
over a wireless link, the method comprising:
receiving incoming codes generated based on concatenated trellis and space
block coding, wherein the incoming codes include a first code and a second
code, wherein
the first code includes a first symbol and a second symbol and wherein the
second code
includes a third symbol generated using a complex conjugation, or negative
complex
conjugation of the first symbol and a fourth symbol generated using a complex
conjugation, or negative complex conjugation of the second symbol;

performing a first decoding of the incoming codes, wherein the first decoding
includes combining at least the first incoming code and the second incoming
code
together to form one or more signals; and
performing a second decoding, wherein the second decoding includes applying
Viterbi decoding to the one or more signals.

59. The method of claim 58 wherein the first incoming code is received at a
first
time, and wherein the second incoming code is received at a second time
distinct from
the first time.

60. The method of claim 58 wherein the first incoming code is received after
the
first symbol is transmitted over a first channel and the second symbol is
transmitted
over a second channel that is distinct from the first channel.

61. The method of claim 58 where the Viterbi decoding includes generating a
separate metric for soft decision of an incoming code.

-19-

Description

CA 02411286 2002-12-18
Low Complexity Maximum Likelihood Detection Of Concatenated
Space Codes For Wireless Applications
Background of the Invention
This invention relates to wireless communication and, more
particularly, to techniques for effective wireless communication in the
presence of fading and other degradadons.
The most effective technique for mitigating multipath fading in a
wireless radio channel is to cancel the effect of fading at the transmitter by
controlling the transmitter's power. That is, if the channel conditions are
known at the transmitter (on one side of the link), then the transmitter can
pre-
distort the signal to overcome the effect of the channel at the receiver (on
the
other side). However, there are two fundamental problems with this approach.
The first problem is the transmitter's dynamic range. For the transmitter to
overcome an x dB fade, it must increase its power by x dB which, in most
cases, is not practical because of radiation power limitations, and the size
and
cost of amplifiers. The second problem is that the transmitter does not have
any knowledge of the channel as seen by the receiver (except for time division
duplex systems, where the transmitter receives power from a known other
transmitter over the same channel). Therefore, if one wants td control a
transmitter based on channel characteristics, channel information has to be
sent
from the receiver to the transmitter, which results in throughput degradation
and added complexity to both the transmitter and the receiver.

CA 02411286 2002-12-18
Other effective techniques are time and frequency diversity. Using time
interleaving together with coding can provide diversity improvement. The same
holds for frequency hopping and spread spectrum. However, time interleaving
results in unnecessarily large delays when the channel is slowly varying.
Equivalently, frequency diversity techniques are ineffective when the
cohetentx
lo bandwidth of the channel is large (small delay spread).
It is well known that in most scattering environments antenna diversity is the
most practical and effective technique for reducing the effect of multipath
fading.
The classical approach to antenna diversity is to use multiple antennas at the
receiver
and perform combining (or selection) to improve the quality of the received
signal.
is The major problem with using the receiver diversity approach in current
wireless communication systems, such as IS-136 and GSM, is the cost. size and
power consumption constraints of the receivers. For obvious reasons, small
size,
weight and cost are paramount. The addition of multiple antennas and RF chains
(or
selection and switching circuits) in receivers is presently not be feasible.
As a result,
2o diversity techniques have often been applied only to improve the up-link
(receiver to
base) transn~issiori quality with multiple antennas (and receivers) at the
base station.
Since a base station often serves thousands of receivers, it is more
economical to add
equipment to base stations rather than the receivers
Recently, some interesting approaches for transmitter diversity have been
z5 suggested. A delay diversity scheme was proposed by A. Wittneben in ''Base
Station Modulation Diversity for Digital SIMULCAST," Proceeding of the 1941
IEEE Vehicular Technology Conference (VTC 41st), PP. 848-853, May 1991, and
in "A New Bandwidth Efficient Transmit Antenna Modulation Diversity Scheme
Far Linear Digital Modulation," in Proceeding of the 1993 IEEE International
3o Conference on Comraunications (IICC'93), pF.1630-ib34, May 1993. The
proposal is for a base station to transmit a sequence of symbols through one
antenna,
and the same sequence of symbols -but delayed - through another antenna.
U.S. patent 5,479,448, issued to Nambirajan Seshadri on December 26,
1995, discloses a similar arrangement where a sequence of codes is transmitted
33 through two antennas. The sequence of codes is routed through a cycling
switch that
directs each code to the various antennas, in succession. Since copies of the
same
2

CA 02411286 2002-12-18
symbol are transmitted through multiple antennas at different times, both
space
and time diversity are achieved. A maximum likelihood sequence estimator
(MLSE) or a minimum mean squared error (MMSE) equalizer is then used to
resolve multipath distortion and provide diversity gain. See also N. Seshadri,
J.H. Winters, "Two Signaling Schemes for Improving the Error Performance
of FDD Transmission Systems Using Transmitter Antenna Diversity,"
Proceeding of the 1993 IEEE Vehicular Technology Conference (VTC 43rd),
pp. 508-511, May 1993; and J.H. Winters, "The Diversity Gain of Transmit
Diversity in Wireless Systems with Rayleigh Fading," Proceeding of the 1994
ICClSUPERCOMM, New Orleans, Vol. 2, pp. 1121-1125, May 1994.
Still another interesting approach is disclosed by Tarokh, Seshadri,
Calderbank and Naguib in U.S. Patent No. 6,115,427, issued September 5,
2000, where symbols are encoded according to the antennas through which
they are simultaneously transmitted, and are decoded using a maximum
likelihood decoder. More specifically, the process at the transmitter handles
the information in blocks of M1 bits, where M1 is a multiple of M2., i.e.,
Ml=k*M2. It converts each successive group of M2 bits into information
symbols (generating thereby k information symbols), encodes each sequence
of k information symbols into n channel codes (developing thereby a group of
n channel codes for each sequence of k information symbols), and applies each
code of a group of codes to a different antenna.
Yet another approach is disclosed by Alamouti and Tarokh in U.S.
Patent No. b,185,258, issued February 6, 2001, and titled "Transmitter
Diversity Technique for Wireless Communications" where symbols are
encoded using only negations and conjugations, and transmitted in a manner
that employs channel diversity.
Still another approach involves dividing symbols into groups, where
each group is transmitted over a separate group of antennas and is encoded
with a group code C that is a member of a product code.

CA 02411286 2002-12-18
Summary
An advance in the art is realized with a transmitter that employs a trellis
coder followed by a block coder. Correspondingly, the receiver comprises a
Viterbi decoder followed by a block decoder. Advantageously, the block coder
and decoder employ time-space diversity coding which, illustratively, employs
two transmitter antennas and one receiver antenna.
In accordance with one aspect of the present invention there is provided
a transmitter comprising: a trellis encoder that encodes incoming digital data
to
generate complex numbers representing constellation symbols defined as so
and s~, wherein the trellis encoder transmits by a first antenna and a second
antenna, respectively, during a first time or frequency interval; a space-
block
encoder responsive to the constellation symbols to encode two adjacent
constellation symbols as a block comprising two trellis-coded symbols and two
parity symbols chosen from a group consisting of negated trellis-coded
symbols, complex conjugates of the trellis-coded symbols, and negative
complex conjugates of the trellis-coded symbols, wherein the space-block
encoder is adapted to feed two antennas such that a different symbol is
transmitted by each antenna; and wherein the symbols sl* and so* are
generated by the space-block encoder and transmitted by the first antenna and
the second antenna, respectively, during a second time or frequency interval,
wherein s;* is defined as a complex conjugate of a symbol s~.
Brief Description of the DraWlriE
FIG. 1 presents a block diagram of an embodiment in conformance
with the principles of this invention.
4

CA 02411286 2002-12-18
Detail Descriution
FIG. 1 presents a block diagram of an arrangement comporting with the
principles of this invention. It comprises a trellis code modulation (TCM)
encoder 10 followed by a two-branch space block encoder 20. The output is
applied to antenna circuitry 30, which feeds antenna 31, and antenna 32 FIG.1
shows only two antennas, but this is merely illustrative. Arrangements can be
had with a larger number of antennas, and it should be understood that the
principles disclosed herein apply with equal advantage to such arrangements.
TCM encoder 10 generates complex numbers that represent
constellation symbols, and block encoder 20 encodes (adjacent) pairs of
symbols in the manner described in the aforementioned U.S. Patent
No. 6,115,427. That is, symbols so and s,, forming a pair, are sent to antenna
31 and antenna 32, respectively, and in the following time period symbols -s~
and so* are sent to antennas 31 and 32, respectively. Thereafter, symbols s2
and s3 are sent to antenna 31 and 32, respectively, etc. Thus, encoder 20
creates
channel diversity that results from
4a

CA 02411286 2002-12-18
s signals traversing from the transmitter to the receiver at different times
aad over
different channels.
The signals transmitted by antennas 31 and 32 are received by a receiver
after traversing the airlink and suffering a multiplicative distortion and
additive
noise. Hence, the received signals at the two consecutive time intervals
duriag
t o which the signals s~, s!,
-s,*, and so* are sent correspond to:
ro(t) = hflsfl +h,s, +no,
(1)
and r, (t)=h,so-hos; +n,,
l s (2)
where l~ represents the channel from antenna 31, h, . represents the channel
from
antenna 32, no is the received noise at the first time interval, and n, is the
received
noise at the second time interval.
The receiver compzises a receive antenna 40, a two-branch space block
2o combiner 50, and a Viterbi decoder 60. The receiver also includes a channel
estimator, but since that is perfectly conventional and does not form a part
of the
invention, FIG. 1 does not explicitly show it. The following assumes that the
receiver possesses ha and h, , which are estimates of ho and h, ,
respectively. Thus,
the received signals at the first and second time intervals arc combined in
element 50
25 to form signals
so = ho ro -!' ~r1
(3)
and s, = h,~ro -h~r't ,
(4}
3o and those signals are applied to Viterbi decoder 60.
- The Viterbi decoder builds the following metric for the hypothesized branch
symbol s, corresponding to the first transmitted symbol so:
s

CA 02411286 2002-12-18
M(so~sr)~d'Iso~(~o~'~'~I:)srJ~
(s)
Similarly, the Viterbi decoder builds the following metric for the
hypothesized
branch symbol s, corresponding to the first transmitted symbol s~:
.M(s~.sr) =d2I~~~~x +~~ )sr,
io (6)
(Additional metrics are similarly constructed in arrangements that employ a
larger
number of antennas and a correspondingly larger constellation of signals
transmitted
at any one time.) If Trellis encoder 10 is a multiple TCM encoder, then the
Viterbi
decoder builds the following metric:
is ~(so~s,),(sr~s;)l= M{so.s,)+ M{s"s~) .
('~
or equivalently,
_ ~(SO~se)~(stns;)~=d~(TO,I~s;+h,sl)+il=(r,,h,s; -hosJ).
(x)
2o The Viterbi decoder outputs estiraates of the transmitted sequence of
signals.
The above presented an illustrative embodiment. However, it should be
understood that various modifications and alternations might be made by a
skilled
artisan without departing from the spirit and scope of this invention.
6

Representative Drawing