Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-04-02
2002-09-17
Maung, Nay (Department: 2681)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S335000, C375S141000, C375S145000
Reexamination Certificate
active
06452916
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the use of spreading codes in CDMA systems for wireless communication. More particularly, the invention relates to methods for coding messages for transmission on the downlink, so as beneficially to use multiple transmitting antennas for improved reception in fading environments.
ART BACKGROUND
The quality of reception in wireless communication systems may suffer as a result of fluctuations in the propagation channel between the transmitting and receiving antennas. This phenomenon is referred to as “fading.” In theory, reception in fading environments can be improved by using multiple antennas at the transmitting or receiving end, or both, of the communication link. Multiple antennas can help by providing multiple, independent paths between the ends of the link. The existence of such independent paths is referred to as “diversity.”
There has been recent interest in applying such diversity to boost the capacity and data rate of CDMA systems.
In regard to mobile telephone systems in general, and CDMA systems in particular, spatial as well as economic limitations make it more practical to install multiple antennas at the base station rather than the mobile stations. On the uplink of CDMA systems (i.e., from the mobile to the base station), multiple base-station antennas have in fact been advantageously used to improve data rates and reduce error probabilities. However, it has proven more difficult to attain the desired benefits of diversity in the downlink direction. Previously proposed schemes have tended to provide relatively little diversity gain (i.e., improved reception due to improvement in the statistical distribution of the instantaneous signal-to-noise ratio at the mobile), or they have called for the consumption of too many resources, or they have entailed substantial changes to existing CDMA standards.
Some of the limitations of these previously proposed schemes will be illustrated in the following example. In this example, we will describe, in simplified terms, the processing of data symbols at the baseband level, in accordance with CDMA procedures. We will focus on the modulation of spreading codes by the data symbols, in accordance with well-known procedures. We will not discuss the precise spreading codes to be used, nor will we discuss the gains or pulse shapes. Those skilled in the art will appreciate that these details, as well as methods for placing the coded signals onto carrier waves and transmitting them, are well known.
In our simplified example, there are only two users. We treat the sending of one respective real-valued, scalar data symbol to each user over a physical propagation channel that is free of multipath effects, so that it is adequately modeled by one fading (i.e., propagation) coefficient for each user.
Thus, with reference to
FIG. 1
, base station
10
is to transmit real-valued scalar data symbols b
1
and b
2
to users U
1
(reference numeral
15
.
1
) and U
2
(reference numeral
15
.
2
), respectively. There are provided two orthonormal spreading codes, denoted by vectors c
1
and c
2
. In our simplified example, data symbol b
1
multiplies code c
1
, and data symbol b
2
multiplies code c
2
.
In typical CDMA communications, a single base-station antenna transmits the vector sum b
1
c
1
+b
2
c
2
, where the scalar elements of the vector sum are transmitted consecutively at the rate commonly known as the chip rate. Then the received baseband signal at U
1
is given by r
1
=h
1
(b
1
c
1
+b
2
c
2
)+n
1
, and similarly, at U
2
, r
2
=h
2
(b
1
c
1
+b
2
c
2
)+n
2
, where h
1
and h
2
are the respective fading (i.e., propagation) coefficients (with subscripts that relate to the respective users), and n
1
and n
2
are respective components of additive receiver noise.
Despreading of the received signal by each mobile station is represented mathematically as left-multiplication by the complex transpose of the respective spreading code belonging to that base station. After this operation, the respective received signals d
1
and d
2
are given by: d
1
=h
1
b
1
+&ngr;
1
, d
2
=h
2
b
2
+&ngr;
2
, where &ngr;
1
=c
1
†
n
1
, &ngr;
2
=c
2
†
n
2
, and “†” denotes conjugate transposition.
For effective recovery of the data symbols, it is advantageous for the mobile receiver to know the pertinent fading coefficient from, e.g., measurement of a pilot signal. It is also desirable, for this purpose, to have a relatively high channel gain (i.e., the absolute magnitude of the pertinent fading coefficient). This condition cannot be guaranteed, in general. Below, we describe an exemplary scheme for using diversity to increase the likelihood that the instantaneous signal-to-noise ratio will be high enough to support reliable communication. For simplicity, we assume that there are only two transmitting antennas, and that they are separated by a distance of several wavelengths, so that their respective paths to users are, to a substantial degree, statistically independent.
It should be noted in this regard that the fading coefficients are not fixed quantities, but instead are described probabilistically, in terms of appropriate statistical distributions. Here, we will present certain analytical results assuming that the fading coefficients are complex-Gaussian variables having Rayleigh-distributed amplitude and uniformly distributed phase. We also assume that the additive receiver noise is zero-mean complex-Gaussian.
Under these assumptions, the squared magnitude of the individual fading coefficient h
1
or h
2
has a chi-square distribution with two degrees of freedom, which arise from the squares of the respective real and imaginary parts of the fading coefficients. In general, the sum of squares of M independent, zero-mean, Gaussian random variables, each with unit variance, has the chi-square distribution with M degrees of freedom. If the squared magnitude of the effective fading coefficient is proportional to a chi-square random variable with 2M degrees of freedom, we say that the diversity is “M-fold.” Diversity is useful in the context of CDMA transmission because increased diversity results in a probability density for the signal-to-noise ratio that is more sharply peaked about its mean value. As a consequence, there is less likelihood that the signal-to-noise ratio will fall into the range of values so low that dependable communication is precluded.
Continuing our simplified model, suppose now that each antenna of a two-antenna array transmits the baseband signal
(
1
2
)
(
b
1
c
1
+
b
2
c
2
)
,
where the normalizing factor of
(
1
2
)
signifies that the total transmitted power is the same as for the single-antenna case. The signal d
k
received at the k'th mobile (k=1,2), after despreading, is given by
d
k
=
(
1
2
)
(
h
1
(
k
)
+
h
2
(
k
)
)
b
k
+
v
.
In this expression, h
1
(k)
and h
2
(k)
are the complex-Gaussian channel gains from antenna
1
and antenna
2
, respectively, to user (i.e., mobile station) k, assuming, again, that there are no multipath effects. To simplify notation, we drop the subscript k (i.e., the user index) from the pre-despreading received noise terms n and the post-despreading noise terms &ngr;. It should be noted that here, and in the following discussion, the subscripts of the fading coefficients relate to the respective antennas, and not to the respective users.
If the two antennas are widely separated, the respective fading coefficients are statistically independent, and therefore their sum
(
normalized
by
a
factor
of
1
2
)
has the same statistical distribution as either of them individually. As a consequence, little or no diversity gain is afforded by this scheme.
Next, with reference to
FIG. 2
, suppose that user
1
(denoted in the figure by reference numeral
20
.
1
) and user
2
(denoted by referenc
Hochwald Bertrand M.
Marzetta Thomas Louis
Papadias Constantinos Basil
Finston Martin I.
Lucent Technologies - Inc.
Maung Nay
LandOfFree
Space-time spreading method of CDMA wireless communication does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Space-time spreading method of CDMA wireless communication, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Space-time spreading method of CDMA wireless communication will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2868557