Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-03-12
2002-07-02
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C335S301000, C335S301000, C375S148000
Reexamination Certificate
active
06414949
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a CDMA receiver, i.e. a receiver with Code Division Multiple Access. It finds application in telecommunications and notably in mobile radio systems.
STATE OF THE PRIOR ART
The advantages of CDMA communications no longer need to be demonstrated. It is known that this technique consists of spreading a signal by a pseudo-random sequence, allocating to each user his own sequence, the allocated sequences being orthogonal to one another, transmitting all of the signals thus spread and then, on reception, narrowing down the received signal with the help of the sequences used at transmission and finally reconstituting the signals pertinent to each user.
If the various sequences of the spreading were strictly orthogonal to each other and if the propagation signals were perfect, the signals pertinent to each user would be reconstituted without any error. However, in reality, things are not so simple and each user interferes to a greater or lesser extent with the others.
In order to reduce these effects, referred to as multiple access interference it has been necessary to envisage correction means arranged in the receiver.
By way of example, one may describe the receiver that is the subject of U.S. Pat. No. 5,553,062.
FIG. 1
appended substantially reproduces
FIG. 1
of the document quoted. The circuit shown allows one to supply a nominal signal designated {circumflex over (d)}
1
, belonging to a first user. To do this, it uses an input circuit
41
, receiving a signal R(t) from the multipliers
51
,
61
, . . . ,
71
connected to the generators of the pseudo-random sequences (PRS)
52
,
62
, . . . ,
72
, which reproduce the sequences used for transmission by the various users, delay circuits
53
,
63
, . . . ,
73
, amplifiers (AMP)
54
,
64
, . . . ,
74
, and multipliers
55
,
65
, . . . ,
75
. If one assumes that there are K users, there are K channels of this type arranged in parallel. The last K−1 channels permit the extraction of K−1 signals pertinent to K−1 channels and then to respread these K−1 signals by the corresponding pseudo-random sequences. One can then subtract from the general input signal R(t) all of these respread K−1 signals. To do this, a delay line
48
is provided in order to hold back the input signal R(t) for the duration of the formation of the respread K−1 signals, the respread K−1 signals and the delayed input signal being subsequently applied to a subtractor
150
. This supplies a global signal from which the signals belonging to the K−1 users other than the first one, have been removed. One can then correlate this signal with the pseudo-random sequence belonging to the first user in a multiplier
147
, which receives the pseudo-random sequence supplied by the generator
52
, a sequence suitably delayed by a delay line
53
. An amplifier
146
then supplies the estimated data {circumflex over (d)}
1
, pertinent to the first user.
This structure can be repeated K times in order to process the K signals pertinent to the K users. Hence K first estimations of the data {circumflex over (d)}
1
, {circumflex over (d)}
2
, . . . , {circumflex over (d)}
k
are obtained.
This procedure can be repeated in a second interference suppression stage and so on.
FIG. 2
appended shows, diagrammatically s stages, the first E
0
being, strictly speaking, an ordinary correlation stage, the others E
1
, . . . , E
i
, . . . , E
s−1
being interference suppression stages.
One can still further improve the performance of such a receiver by using, not the last estimation obtained, but a weighted mean of the various estimates provided. This amounts to allocating a weight w
i
to the estimation {circumflex over (d)}
1
, and forming the sum of the W
i
{circumflex over (d)}
i
signals. In
FIG. 2
, it can be seen that all the signals supplied by stage E
0
are multiplied by a coefficient w
0
in a multiplier M
0
, all the signals supplied by stage E
1
are multiplied by a coefficient w
1
in a multiplier M
1
, all the signals supplied by stage E
i
are multiplied by a coefficient w
i
in a multiplier M
i
, and all the signals supplied by the final stage E
s−1
by a coefficient w
s−1
in a multiplier M
s−1
. An adder ADD then forms the sum of the signals supplied by the multipliers.
FIG. 2
also allows one to make clear certain notations appropriate to this technique. At the output of a stage, K signals are to be found corresponding to K users. Rather than individually marking these signals one can consider, in a more synthetic way, that they are the K components of a “vector”. At the output of stage E
i
one will find K signals which are the K components of a vector designated {overscore (Z)}
i
. At the output of the multipliers M
0
, M
1
, . . . , M
i
, . . . , M
s−1
, one will therefore find, with this synthetic notation, vectors w
0
{overscore (Z)}
0
, . . . , w
1
{overscore (Z)}
1
, . . . , w
i
{overscore (Z)}
i
, . . . , w
s−1
{overscore (Z)}
s−1
. The output from the adder will therefore be designated {overscore (Z)} and one can write:
Z
_
=
∑
i
=
0
s
-
1
w
i
Z
_
i
(
1
)
There remains the question of determining the weighting coefficients wi. In the document U.S. Pat. No. 5,553,062 already mentioned, an empirical law w
i
=1/2
i
was proposed (column 12, line 30) without any justification. This amounts to weighting the outputs from the stages in a decreasing way.
Furthermore, the article by S. MOSHAVI et al., entitled “Multistage Linear Receivers for DS-CDMA Systems” published in the magazine “International Journal of Wireless Information Networks”, vol. 3, No. 1, 1996, pages 1-17, takes up certain ideas from the patent U.S. Pat. No. 5,553,062 already mentioned and develops the theory of this type of receiver. It also proposes an optimization of the weighting coefficients.
Without going into the details of this theory, which is complex and would depart from the context of this invention, one can summarize it in the following way.
If each user only transmitted a single binary signal (or bit), one would find, in the receiver, signals including, for each user, the bit which is pertinent to him, to which would be added the parasitic interference signals due to the presence of other users. At the output from each stage, one would find a group of K bits that can be considered as the K components of a vector. At the output from the following stage, one would again find K signals and one would be able to characterize the transfer function of the stage by a matrix of K lines by K columns, the diagonal elements of this matrix would be the autocorrelation coefficients and the other elements, intercorrelation coefficients between different users.
In practice, however, the transmitted signal does not comprise one single bit but N bits, so that the values in question are no longer of dimension K but of dimension NK. The transfer matrix is then a matrix of NK lines and NK columns.
If one designates {overscore (d)} the data vector (which has NK components), and the vector at the output from the first decorrelation stage (or matched filtering stage) is designated {overscore (y)}
mf
(the index mf referring to the matched filtering function), one may write, ignoring the noise,
{overscore (y)}
mf
=R{overscore (d)}
where R is an NK by NK correlation matrix. The matrix R can be broken down into K by K sub-matrices, all of which are identical if the users use the same broadcasting code for all the bits of the total message. However, this is not necessarily so in all cases.
In a precise way, for a bit in row i, the coefficient for line j and for column k of the correlation matrix is of the form:
P
j.k
(
i
)=∫
r
j
T
b
−&tgr;
j
&agr;
j
(
t−&tgr;
j
)
a
k
(
t−&tgr;
k
−iT
b
)
dt
where a
j
and a
k
are the values (+1 or −1) of the pulses (or “chip”) of the pseudo-random sequences, T
b
is the duration of a bit, &tgr;
j
and &tgr;
k
are delays.
Each stage of interference suppression repe
Boulanger Christophe
Ouvry Laurent
Commissariat a l'Energie Atomique
Yin Lu
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