Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-09-10
2003-09-16
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000
Reexamination Certificate
active
06621856
ABSTRACT:
TECHNOLOGICAL FIELD
The subject of this invention is a method of receiving CDMA signals with parallel interference suppression, a corresponding stage and a corresponding receiver.
It finds application notable in radiocommunication with mobiles.
STATE OF THE PRIOR TECHNOLOGY
The technology of spectrum spreading by a direct sequence consists, schematically of multiplying an information symbol (for example a binary element) by a pseudo-random sequence (also called a code) made up of a sequence of elements called “chips”. This operation has the effect of spreading the spectrum of the signal. On reception, the received signal is processed by correlation (or matched filtering) with a pseudo-random sequence identical to that of the transmission, which has the effect of reducing (or correlating) the spectrum. The signal correlated in this way, is processed in order to recover the information symbol.
This technique allows several users to access a single radiocommunications system, with the condition that they use distinct codes. One is then speaking of “Code Division Multiple Access” or CDMA for short.
Despite offering numerous advantages, communications by spectrum spreading with code division multiple access are of limited capacity in terms of the number of users. This limitation is due to interference occurring between signals coming from different users. The more numerous the users are, the more important this interference phenomenon becomes.
Various solutions have been proposed to remedy this disadvantage and, notably, the suppression (or at the very least the reduction) of interference. Hence, in American patent U.S. Pat. No. 5,218,619, for example, sequential suppression of the interference is recommended proceeding by decreasing order of power of the signals from the various users. In American patent U.S. Pat. No. 5,363,403 contrary to this, parallel suppression of these interference signals is recommended. As this invention again takes up this latter technique, we can break off there and illustrate the general structure of a receiver of this type.
The receiver illustrated in the appended
FIG. 1
comprises a general input E receiving a composite signal r(t) formed from a plurality of signals corresponding to different information symbols S
1
, S
2
, S
3
which have been spread by a plurality of pseudo-random codes C
1
, C
2
, C
3
. The receiver shown is assumed to work with three codes but in practice, obviously, this number is higher.
The receiver firstly comprises an input stage with means
101
,
102
,
103
capable of receiving the composite signal and of supplying a signal correlated by the code C
1
, C
2
or C
3
appropriate to each channel; these means can consist of a correlator or a matched filter.
Next the receiver comprises a parallel interference suppression stage
100
which comprises :
means
111
,
112
,
113
of receiving the correlated signal and supplying an estimation Ŝ
1
, Ŝ
2
, or Ŝ
3
of the corresponding information symbol; these means can comprise an integrator and a decision circuit
means
121
,
122
,
123
capable of respreading the estimated symbol Ŝ
1
, Ŝ
2
, or Ŝ
3
using the code C
1
, C
2
, C
3
appropriate to the channel, to supply the respread signals s
1
, s
2
or s
3
.
means
131
,
132
,
133
to subtract from the signal applied to the input of the channel (after a suitable delay produced by a delay circuit
161
,
162
,
163
), the sum &Sgr;
1
, &Sgr;
2
, &Sgr;
3
of the respread signals coming from the other channels; in other words, the signal &Sgr;
1
is formed by the sum s
2
+s
3
, the signal &Sgr;
2
by s
1
+s
3
and the signal &Sgr;
3
by s
1
+s
2
. The means
131
,
132
,
133
supply, in each channel, a new signal r
1
, r
2
, r
3
which, at least in part, has been cleared of multiple access interference corresponding to other channels.
After the parallel interference suppression stage, there are three matched filters
201
,
202
,
203
working respectively with the codes C
1
, C
2
, C
3
and correlating the signals r
1
, r
2
, r
3
then an output stage
200
with three decision circuits
211
,
212
,
213
supplying the three data S
1
, S
2
and S
3
.
Although giving satisfaction in certain regards, such receivers do not eliminate the risks of error. The suppression of interference, if it is carried out without precautions, can even increase this risk. The purpose of this invention is precisely to reduce this risk (in other words to reduce the bit error rate), by improving the means of reconstructing the signals before the actual interference suppression itself. With the invention, a single parallel interference suppression stage offers better performance than the traditional two suppression stages.
In order to obtain this result in the interference suppression stage and to estimate the received data, the invention provides for the use of a particular criterion which is called “The Maximum Likelihood” criterion. This criterion is known of itself in CDMA techniques. One may find a description for example in the work by J. G. PROAKIS entitled “Digital Communications” McGRAW-HILL Inc., 3
rd
edition, 1995, Chapter 5-1-4. However, in the prior art, this criterion is used in an ordinary receiver, and not in a means of parallel suppression of multiple access interference. Furthermore, in the prior art, this criterion is used with the aid of an algorithm called Viterbi's Algorithm, which allows one to find, through a lattice representing all possible configurations, a sequence of data which minimizes a quantity called the “Euclidean distance metric”. This technique, which takes into account the whole of the data transmitted by all users, is often very complex. This invention adapts this technique notably by simplifying it. Furthermore it defines a metric which is particularly suitable for the parallel suppression of multiple access interference.
DESCRIPTION OF THE INVENTION
Put precisely, the subject of this invention is a method of receiving CDMA signals with parallel interference suppression in which:
a composite signal is received comprising a plurality of K signals corresponding to information symbols which have been spread in frequency by K different pseudo-random sequences,
these K signals are correlated using said K sequences
the corresponding K symbols are estimated,
the K correlated signals are reconstructed in frequency by respreading said estimated symbols using the corresponding pseudo-random sequences,
the contributions of the other signals are subtracted from a respread signal to provide K new signals, spread in frequency but cleared, at least in part of the interference,
this method being characterized in that:
all the possible hypotheses possible are formulated on the signs of the NK correlated signals, where N is a whole number equal to 1 or to a few units,
for each hypothesis, one calculates the distance metric between the group of correlated signals undergoing processing and the corresponding signals before processing,
the hypothesis for which the metric is the smallest is retained, being the hypothesis which has a maximum likelihood,
only those signals corresponding to this maximum likelihood hypothesis are reconstructed.
Another subject of the invention is a parallel interference suppression stage that implements this method, this stage comprising:
K inputs receiving signals correlated in frequency,
K means of estimating K symbols corresponding to these K signals,
K means of reconstructing signals respread in frequency using the corresponding pseudo-random sequences,
means of parallel interference suppression comprising K channels in parallel capable of subtracting from one respread signal, the contributions of the other respread signals,
K outputs supplying K signals spread in frequency, cleared, at least in part of the interference,
this stage being characterized in that it comprises
means placed between the estimation means and the reconstruction means and capable of formulating all the possible hypotheses on the signs of NK correlated signals, where N is a
Boulanger Christophe
Fort Charles
Ouvry Laurent
Piaget Bernard
Chin Stephen
Commissariat a l'Energie Atomique
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Williams Lawrence
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