Communications: radio wave antennas – Antennas – With coupling network or impedance in the leadin
Reexamination Certificate
1999-01-08
2001-04-24
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
With coupling network or impedance in the leadin
C370S342000, C375S349000
Reexamination Certificate
active
06222498
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (code-division multiple-access) multiuser receiver which combines directivity control of an array antenna and interference canceling operations. The CDMA multiuser receiver according to the present invention features a small size and excellent interference cancellation. By way of example, the present invention may be applicable to the receiver installed in the base station of a cellular mobile communications system.
2. Description of the Related Art
It is expected that CDMA is able to markedly increase a subscriber's capacity and thus find an extensive application in a cellular mobile communications system (for example). However, the mobile communications utilizing CDMA (viz., spread-spectrum) techniques have suffered, at a receiver side, the problems of interference caused by delayed signals due to multiple transmission paths and concurrently communicating other party's signals.
As is known in the art, an array antenna is able to suppress and cancel interference through directivity control. On the other hand, a multiuser receiver is a receiver which demodulates all the user's signals by implementing mutual interference cancellation using all the user's spreading codes and channel characteristics. The multiuser receiver itself is known in the art. One example of such a receiver is disclosed in a paper by M. K. Varanasi and B. Aazhang, entitled “Multistage Detection in Asynchronous Code-Division Multiple-Access Communications”, IEEE Transactions on Communications, Vol. 38, No. 4, April 1990, pp. 509-519 (Prior Paper 1). Another example of a conventional multiuser receiver is disclosed in a paper by M. Sawahashi, et al., entitled “Serial Canceler Using Recursive Channel Estimation by Pilot Symbols for DS-CDMA”, Electronics Information Communications Association of Japan, Technical Report RCS95-50,July 1995 (Prior Paper 2).
According to the apparatus disclosed in the aforesaid Prior Paper 1, all the user's signals are demodulated at an initial stage of the apparatus, after which an interfering replica of each user becomes produced. Subsequently, interference cancellation is implemented by reducing an interference replica of each of the users other than a desired user from a received signal. At the next stage, the signal, which has been obtained through the interference cancellation, is again demodulated in connection with the desired (intended) user and therefore, the signal quality of the demodulation result at the second stage is higher than that at the first stage. Thus, the conventional technique, disclosed in Prior Paper 1, is to improve the interference cancellation by repeating a series of signal processes using multi-stage configuration.
Channel estimation is necessary to demodulate the signal of each user and produce an interference replica. The aforesaid Prior Paper 2 discloses that a channel (viz., transmission path) is recursively estimated at each stage thereby to prevent deterioration of the interference cancellation characteristics due to channel estimation error.
Another example of the multiuser receiver is disclosed in a paper by Yoshida and Ushirokawa, entitled “CDMA Multi-Stage Interference Canceler with Recursive Channel Estimation Based on Symbol Replica Processing”, the Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE, A. p96-157, EMCJ96-92, RCS96-171, February 1997 (Prior Paper 3).
The above-mentioned Prior Paper 3 discloses a multi-stage type CDMA multiuser receiver. According to this known technique, the size of the apparatus can be reduced through the use of symbol replica processing. At the same time, it is possible to realize interference cancellation at the unit of multi-path in the case of implementing recursive channel estimation thereby to improve interference cancellation in the case of multi-path transmission.
FIG. 1
is a drawing showing a CDMA multiuser receiver that is based on the known techniques disclosed in Prior Paper 3. The CMDA multiuser receiver of FIG.
1
is comprised of three-stage interference cancelers
10
-
1
to
10
-
3
. At the first two stages of interference cancelers
10
-
1
and
10
-
2
, the signals of all the users, the number of which is assumed three, are demodulated and then subjected to interference cancellation. That is, this means that the multiuser interference cancellation is implemented.
As shown in
FIG. 1
, the interference canceler
10
-
1
at the first stage is provided with a delay unit
12
, three EIUs (interference estimation units)
14
a
-
14
c,
an adder
16
, and another adders
18
a
-
18
c.
The interference canceler
10
-
2
is configured in the same manner as the canceler
10
-
1
and is comprised of three IEUs (interference estimation units)
14
a
′-
14
c
′, an adder
16
′, and another adders
18
a
′-
18
c′.
On the other hand, the interference canceler
10
-
3
at the final stage is provided with IEUs
20
a
-
20
c
each of which differs from those provided at the first and second stages.
A received signal is directly applied to the first stage (viz., interference canceler
10
-
1
). The interference canceler
10
-
3
at the final stage is not provided with any delay unit and any adder. The IEUs
20
a
-
20
c
generate demodulated signals respectively corresponding to the first to third users.
The operations of the interference cancelers
10
-
1
and
10
-
2
, which are respectively provided at the first and second stages, are identical with each other and thus, there will be described the operation of the first stage. The three IEUs
14
a
-
14
c
respectively output estimated interference spread signals that are applied to the adder
16
. The delay unit
12
operates such as to delay the incoming signal by the time for which each of the IEUs
14
a
-
14
c
estimates the interference and outputs the result thereof, and applies the output thereof to the adder
16
and the delay unit
12
′ of the second stage. The adder
16
subtracts the outputs of the IEUs
14
a
-
14
c
from the output of the delay unit
12
, and applies the output thereof to the adders
18
a
-
18
c
that are respectively assigned to the users. Each of the adders
18
a
-
18
c
sums the output of the adder
16
and the output of the corresponding IEU (
14
a,
14
b,
or
14
c
), and applies the resultant sum to the second stage.
The IEUs
14
a
-
14
c
of the first stage and the IEUs
14
′
a
-
14
c
′ of the second stage are substantially identical with each other in terms of configuration as well as operations, and accordingly there will be described only the IEU
14
a
of the first stage.
The IEU
14
a
of
FIG. 2
is configured under the assumption that the number of paths of the incoming signal is three (3). In the drawing, the circuits prepared for first to third propagation paths are depicted by P
1
-P
3
. Since the circuits for the multiple paths are identical with each other, the description is made with reference to the circuit P
1
for the first path. The IEU shown in the drawing is generally comprised of a front section (stage) S
1
, an intermediate section S
2
, and a rear section S
3
. More specifically, the front section S
1
comprises a spread-spectrum despreader
22
and a detector
24
, while the intermediate section S
2
comprises an adder
25
and a discriminator
26
. Finally, the rear section S
3
comprises a multiplier
27
, a spread-spectrum modulator
28
and an adder
29
. Further, the detector
24
comprises a channel estimator
24
a,
a complex conjugate generator
24
b,
and a multiplier
24
c.
The received signal (incoming signal) is split and applied to the circuits P
1
-P
3
prepared for the three transmission paths. The despreader
22
despreads the incoming signal using the first user's spreading code at the timing in synchronism with the spreading code transmitted via the first path, and outputs the operation result.
The detector
24
is supplied with the output of the despreader
22
, implem
Ishii Naoto
Maruta Yasushi
Ushirokawa Akihisa
Yoshida Shousei
Clinger James
NEC Corporation
Scully Scott Murphy & Presser
Wong Don
LandOfFree
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