Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-08-22
2004-10-05
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S347000, C375S240000, C375S260000, C375S299000, C375S259000, C375S267000, C375S246000, C370S310000, C370S334000
Reexamination Certificate
active
06801579
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to modulation schemes for wireless communication. More particularly, the invention relates to the construction of signal constellations for use in unitary space-time modulation of wireless signals.
ART BACKGROUND
When wireless communication signals are in transit between a transmit antenna and a receive antenna, they are generally subject to destructive interference and other physical effects that vary in time. As a consequence, the received signal arrives with an attenuation and phase delay that will also generally vary in time. This effect is referred to as fading. Where, e.g., attention is confined to a sufficiently narrow bandwidth, the attenuation and phase delay of the received signal can be described by a complex-valued coefficient h often referred to as the fading coefficient.
Practitioners in the field of wireless communications have recognized that by using multiple antennas for transmission, reception, or both, it is possible to mitigate some of the undesirable consequences of fading, and to achieve certain other benefits as well. For example, the use of multiple antennas affords alternate transmission paths, some of which may, at a given time, be less subject to fading than others. The use of multiple antennas also provides a mechanism for sending redundant signals, the better to understand them at the receiving end, even in the face of severe fading. Even if redundancy is not a primary objective, the use of multiple antennas can provide a mechanism for increasing total transmission rates in a given frequency channel, by simultaneously transmitting multiple, independent signals that can be separated at the receiving end.
FIG. 1
is a simplified, schematic diagram of a wireless communication system having two transmit antennas
10
,
15
, and three receive antennas
20
,
25
,
30
. As indicated at block
40
, baseband-level signals generated at block
35
are modulated onto a carrier wave, which is shown symbolically in the figure as generated at oscillator
50
.
It will be seen from the figure that each of the receive antennas receives transmission from, e.g., antenna
10
. Provided there is sufficient spatial separation, exemplarily spatial separation, between the receive antennas, the transmitted signals will bear distinct fading effects when they are received. (In this regard, it should be noted that diversity of fading effects can in at least some cases also be achieved by using receive antennas that are selectively receptive to diverse polarizations of the incoming signal, even if the antennas are not substantially separated in space.) A separate fading coefficient h
mn
accounts for the fading effects, in the physical propagation channel, between each transmit antenna m and each of the receive antennas n. As shown in the figure, m=1 for antenna
10
and m=2 for antenna
15
. Similarly, n=1 for antenna
20
, n=2 for antenna
25
, and n=3 for antenna
30
. All six of the fading coefficients h
mn
are arranged in a matrix H, denoted by block
55
of the figure.
It will be clear from the foregoing that each receive antenna receives, during a given time interval, a total signal that is a weighted sum of the transmissions from the respective transmit antennas. The weight coefficients of that sum are the fading coefficients. The received signal is also typically corrupted by additive noise, which is not indicated in the figure. Because each of the receive antennas is typically receiving a different weighted sum of the transmitted signal, it is theoretically possible under certain conditions to recover the transmitted baseband-level signals by taking appropriate weighted combinations of the demodulated, received signals. One condition necessary for such recovery is that there must be at least as many receive antennas as there are transmit antennas. Another such condition is that the additive noise must not be excessive relative to the signal strength. (It should be noted in this regard that practical methods of signal recovery often imply indirect methods such as maximum-likelihood detection, which is described below.)
Turning again to
FIG. 1
, demodulation of the received signals is indicated at blocks
60
-
70
. Signal recovery is indicated at block
75
. The recovered signals are indicated at block
80
.
A new kind of transmitted signal, referred to as a space-time signal has been shown to offer potential improvements in both fading performance and transmission rate. In space-time modulation, each signal that is sent is selected from a finite set, or constellation, of predetermined signal matrices. Thus, if there are a total of L such matrices, each individual matrix that is transmitted conveys log
2
L bits of information. Advantageously, the signal matrices are unitary. In a unitary matrix, the columns or rows (whichever are longer) are mutually orthogonal and have unit norm. The individual elements of a unitary matrix are complex numbers; i.e., numbers that are real, imaginary, or sums of real and imaginary components. When unitary matrices are used, the modulation method is referred to as unitary space-time modulation (USTM).
The method of transmitting space-time matrices will now be explained with reference to
FIG. 1. A
2×2 space-time signal matrix is represented in block
35
as the matrix:
(
s
11
s
12
s
21
s
22
)
.
Each element of this matrix is a complex number. Such an element is exemplarily modulated onto the carrier wave by subjecting the carrier wave to a suitable pulse-shaping function of the corresponding complex amplitude and having a width appropriate to the length of a transmission time interval. Each such transmission time interval is referred to as a channel use. During the first channel use, element s
11
is transmitted by antenna
10
and element s
12
is transmitted by antenna
15
. During the second channel use, element s
21
is transmitted by antenna
10
, and element s
22
is transmitted by antenna
15
. More generally, each row of a space-time matrix corresponds to a respective channel use, and each column corresponds to a respective transmit antenna. Thus, the entry in row p and column q is the complex amplitude transmitted during the p'th channel use by the q'th antenna. The length of a channel use is generally chosen to be no longer than a fading interval; i.e., a length of time over which the fading coefficients can be assumed constant.
At the receiving end, the transmitted signal matrix is recovered as the matrix
(
s
^
11
s
^
12
s
^
21
s
^
22
)
,
as indicated at block
80
of FIG.
1
. There are various methods for recovering an estimate of the transmitted signal matrix, based on the received matrix. According to one such method, referred to as maximum likelihood (ML) detection, a likelihood score is computed for each candidate signal matrix that might have been transmitted, and that candidate which maximizes the likelihood score is identified as the transmitted matrix. Typically, the likelihood score is the probability of the raw, basebanded signal amplitudes that are received, given that the candidate matrix was transmitted.
When each signal matrix is transmitted as a matrix drawn from the signal constellation, the ML detector must typically be provided with values of the fading coefficients. Such values may, for example, be measured using appropriate pilot signals. However, there are alternative transmission techniques, referred to as unknown channel techniques, that do not require the ML detector to know the (approximate) fading coefficients, provided the fading coefficients do not change substantially over at least as many, typically at least twice as many, channel uses as there are transmit antennas. One class of unknown channel techniques is referred to as differential modulation. An example of differential modulation is described in greater detail below. Briefly, each signal to be transmitted is the product of the previously transmitted signal, times a new signal matrix selected from the signal constellation. In that case,
Hassibi Babak
Hochwald Bertrand M
Shokrollahi Mohammad Amin
Sweldens Wim
Chin Stephen
Finston Martin A.
Lucent Technologies - Inc.
Munoz Guillermo
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