Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-04-26
2002-06-11
Le, Thanh Cong (Department: 2684)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C375S148000
Reexamination Certificate
active
06404759
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) multi-user receiving apparatus including an interference canceller, and more particularly to a CDMA multi-user receiving apparatus including an interference canceller which an interference removing process is carried out at each of stages to output demodulation signals from all users.
2. Description of the Related Art
Conventionally, a CDMA receiving apparatus spreads is used for a communication system using a CDMA system, in which an information signal is spread and modulated with the signal ten times to hundred times faster than the information signal.
For such a CDMA receiver, a single user receiver is generally used to receive a signal from one user. However, it is requested in recent years to efficiently extract a desired user signal, while removing interference with signals from other users. In this case, the interference changes depending upon time.
To respond to this request, a multi-user type interference canceller is known as described in, for example, the technical report of the electronic, information, and communication engineers, RCS
96
-
100
. In the reference, replicas (spread signals) of other user signals are reproduced from a reception signal by use of a determined symbol, a transmission path data and a spreading code, and the reproduced spread signals are subtracted from the reception signal. Then, a desired user signal is demodulated by use of the subtracted signal once again.
The technique described in the above reference is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-51353), and will be described with reference to a block diagram of the CDMA multi-user receiving apparatus shown in FIG.
1
.
Referring to
FIG. 1
, the receiving apparatus is composed of three receiving apparatus stage
1
to
3
. In each of the receiving apparatus stages
1
to
3
, a demodulating process and an interference removal process are carried out regardless of the reception signal level. Each of the receiving apparatus stages
1
and
2
is composed of three interference estimating units (IEUs)
101
-
1
to
101
-
6
, multipliers
102
-
1
to
102
-
6
, delay units
106
-
1
and
106
-
2
and adders
107
-
1
and
107
-
2
. Each of the interference estimating units
101
-
1
and
101
-
4
,
101
-
2
and
101
-
5
, and
101
-
3
and
101
-
6
corresponds to
3
user signals. Each of the delay units
106
-
1
and
106
-
2
corresponds to the multipliers
102
-
1
to
102
-
3
, or
102
-
4
to
102
-
6
. Each of the adders
107
-
1
and
107
-
2
corresponds the delay unit
106
-
1
or
106
-
2
. The adders
107
-
1
and
107
-
2
input the outputs of the delay units
106
-
1
and
106
-
2
and output an adding result to three interference estimating units
101
-
4
to
101
-
6
, and
101
-
7
to
101
-
9
. The respective interference estimating units
101
-
1
to
101
-
3
,
101
-
4
to
101
-
6
, and
101
-
7
to
101
-
9
at each stage input a reception signal and interference removal residual signals obtained from the adders
107
-
1
and
107
-
2
at the previous stage, respectively. Also, the interference estimating units
101
-
4
to
101
-
6
and
101
-
7
to
101
-
9
input reception symbol replicas of the corresponding user signals which have been estimated in the interference estimating units
101
-
1
to
101
-
3
and
101
-
4
to
101
-
6
at the previous stage. The interference estimating units
101
-
1
to
101
-
3
and
101
-
4
to
101
-
6
re-estimate the reception symbol replicas of the current stage to output to the interference estimating units
101
-
4
to
101
-
6
and
101
-
7
to
101
-
9
for the same user signals at the next stage. Also, the interference estimating units
101
-
1
to
101
-
3
, and
101
-
4
to
101
-
6
output spreading process results of the difference between the reception symbol replicas at the current stage and the reception signal or the interference removal residual signals (hereinafter, to be referred to as error signals).
In each of the interference estimating units
101
-
7
to
101
-
9
at the last stage
3
, it is not necessary to re-estimate interference replicas at the current stage
3
. Therefore, demodulation result are outputted just as they are, as demodulation signals A to C for the respective user signals.
The interference removal residual signal inputted to each of the interference estimating units
101
-
1
to
101
-
3
at the first stage
1
is the reception signal itself. At the last stage
3
, the delay unit
106
and the adder
107
are omitted.
The delay unit
106
delays the interference removal residual signal which is also supplied to the interference estimating unit
101
by the time until a reproduction signal is outputted from the multiplier
102
. The delayed signal is supplied to the adder
107
. The error signal to be supplied to the adder
107
is obtained by multiplying a weighting factor &agr; with the output of the interference estimating unit
101
. The interference removal residual signal is obtained by subtracting three error signals of the interference estimating units
101
from the output of the delay unit
106
and is outputted to the next stage.
In this way, when there are many user signals which are to be processed in parallel so that the characteristic sometimes degrades, the reliability of the interference removal residual signal obtained through the interference removing process is sometimes decreased, if the multiplication of the interference removal suppression weighting coefficient &agr; is carried out. Therefore, the weighting coefficient &agr; below “1” is multiplied to prevent the decrease of the reliability of the interference removal residual signal and the degradation of the interference removal characteristic.
However, as seen from the above description, the interference removal suppression coefficient &agr; is fixed. Therefore, there is a problem in that the interference removal suppression coefficient &agr; is not always optimal to the number of user signals which can be transmitted at the same time, when the receiving states of the spread signal change randomly. Consequently, there is a problem in that the change of the operation point of the interference removal suppression coefficient &agr; prevents the convergence operation of a multi-user type interference canceller so that the operation of the interference canceller diverges.
Also, there is another problem in that the interference removal suppression coefficient &agr; is initially set to a small value so that the ability of the interference removing process can be suppressed to a low level, in order to operate the interference canceller in a stable state under any reception situation.
In conjunction with the above description, a receiving apparatus is described in Japanese Laid Open Patent Application (JP-A-Heisei 7-273713). In this reference, a sync signal is detected by a sync signal detecting section (
10
) and amplitude information (A
1
to A
3
) and phase information (&thgr;
1
to &thgr;
3
) of a main wave and a delay wave are generated from the reproduced sync signal. A sync signal removing section (
9
) removes the sync signal from a signal supplied from a radio demodulating section (
8
) by use of the reproduced sync signal. A reception signal r obtained thus is supplied to an other station interference removing sections (
11
). The other station interference removing sections (
11
to
1
K) estimate the first to N-th station signal while removing the interference between the stations from the reception signal by use of the amplitude information (A
1
to A
3
) and the phase information (&thgr;
1
to &thgr;
3
). A correlation detecting operation is performed to the estimated signals (r
1
K to rNK) by correlation detecting section (
21
to
2
N). Thus, the demodulation signals (S
1
to SN) are obtained for the first to N-th stations.
Also, a DS-CDMA interference canceller is described in Japanese Laid Open Patent Application (JP-A-Heisei 9-331274). In this reference,
Cong Le Thanh
NEC Corporation
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