Multiplex communications – Channel assignment techniques – Combining or distributing information via code word channels...
Reexamination Certificate
1999-06-22
2003-06-24
Ton, Dang (Department: 2666)
Multiplex communications
Channel assignment techniques
Combining or distributing information via code word channels...
C370S332000, C370S342000, C370S535000
Reexamination Certificate
active
06584115
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DS-CDMA (Direct Sequence-Code Division Multiple Access) system which is one of CDMA communication systems, and more particularly to a multiuser interference canceler, which is used in a DS-CDMA system, for processing a received signal containing spread signals from a plurality of users to remove signals of other users from a signal of each user and thereafter decode the signal of each user.
2. Description of the Related Art
In recent years, CDMA communication systems that are resistant to interference and jamming have attracted much attention as mobile communication systems. In a CDMA communication system, a transmitting station spreads a user signal to be transmitted with a spreading code and transmits the spread user signal, and a receiving station despreads the received user signal with a spreading code which is the same as the spreading code in the transmitting station for thereby obtaining the original user signal.
In the CDMA communication system, a plurality of transmitting stations spread user signals with different orthogonal spreading codes, and a plurality of receiving stations select respective spreading codes for use in despreading the received user signals for thereby specifying the respective user signals. Consequently, a common frequency band can be used to transmit a plurality of the user signals from the transmitting stations to the receiving stations.
However, because not all spreading codes are made orthogonal to each other, the signal of a user may possibly serve as noise with respect to another user, resulting in interference. Therefore, if a plurality of users use a link with a common frequency within one cell, the signals of the users tend to interfere with each other, resulting in a reduction in the bit energy to noise ratio (Eb/No) which is the ratio of the desired reception wave power to interfering wave power. Because the Eb/No determines the quality of communications, it is necessary to attain an Eb/No having at least a certain value in order to obtain a desired quality of communications. Such an Eb/No having at least a certain value is referred to as a required Eb/No.
The number of links that can use a common frequency within one cell is limited to a number that can maintain the required Eb/No. For increasing a system capacity for subscribers, any interference between communication links must be lowered.
One proposed method of reducing any interference within cells, increasing a system capacity for subscribers, or improving the quality of communications in the DS-CDMA communication system is a multiuser interference canceling method which employs a multiuser interference canceler.
The multiuser interference canceling method is a method in which when a received signal containing the signals of a plurality of users is to be demodulated by a spreading code of a certain user, an interference replica component which is the same as a signal component other than the user is generated and subtracted from the received signal to be demodulated in a multiplicity of stages for thereby reducing interference from the other users. There are two types of multiuser interference cancelers, i.e., a serial multiuser interference canceler and a parallel multiuser interference canceler. The principles of the serial multiuser interference canceler are described in Technical Report (RC95-50) of the Institute of Electronics, Information and Communication Engineers, “Sequential channel estimating serial canceler using a pilot symbol in DS-CDMA”, for example. Japanese laid-open patent publication No. 09-270736 (JP, 09270736, A) entitled “DC-CDMA multiuser serial interference canceler device” discloses a type of serial multiuser interference canceler.
The conventional serial multiuser interference canceler disclosed in the above publication is shown in FIG.
1
.
The serial multiuser interference canceler shown in
FIG. 1
is designed for three users and comprises first, second, and third stages for effecting an interference cancellation process with an interference replica signal and three decoders
19
a
-
19
c
. Each of the first, second, and third stages comprises three blocks. Therefore, the serial multiuser interference canceler has a total of nine blocks.
These nine blocks have respective interference canceler units (ICUs)
61
a
,
61
b
,
61
c
,
62
a
,
62
b
,
62
c
,
63
a
,
63
b
,
63
c
. Each of the blocks also has, in addition to the ICU, a delay memory (D) and a subtractor and/or an adder. For example, the first block of the first stage comprises an ICU
61
a
, a delay memory
3
, and a subtractor
4
, and the first block of the second stage comprises an ICU
62
a
, delay memories
5
and
7
, an adder
8
, and a subtractor
6
. Each of the second and third blocks of the first stage is identical in structure to the first block of the first stage. Each of the blocks of the second and third stages, except the third block of the third stage, is identical in structure to the first block of the second stage.
Each of the ICUs
61
a
,
62
a
,
63
a
generates an interference replica signal which is the same as the signal component of the first user. Each of the ICUs
61
b
,
62
b
,
63
b
generates an interference replica signal which is the same as the signal component of the second user. Each of the ICUs
61
c
,
62
c
,
63
c
generates an interference replica signal which is the same as the signal component of the third user.
The arrangement of the ICU
61
a
, as an example of the ICUs
61
a
-
63
c
, will be described below with reference to FIG.
2
.
The ICU
61
a
comprises a plurality of despreaders −
71
1
-
71
n
, a rake combiner
26
, a decision unit
27
, and a plurality of respreaders
72
1
-
72
n
. Each of the despreaders
71
1
-
71
n
comprises a pair of multipliers
22
and
25
, an integrator
23
, and a transmission path estimator
24
. A received signal
1
that is supplied to the ICU
61
a
is multiplied by a spreading code Ca by the multiplier
22
, and then integrated by the integrator
23
, which produces a correlated value. The transmission path estimator
24
determines a transmission path fading vector &xgr; from the correlated value determined by the integrator
23
. The multiplier
45
multiplies the correlated value from the integrator
23
by the inverse &xgr; of the transmission path fading vector &xgr; from the transmission path estimator
24
for thereby performing phase correction for the received signal.
The received signals from paths which have been corrected in phase by the multipliers
25
of the despreaders
71
1
-
71
n
are combined by the rake combiner
26
, and the combined signal is decoded into an original symbol sequence by the decision unit
27
. Since the rake combiner
26
and the decision unit
27
are of general nature in the CDMA communication system and do not have direct bearing on the operation of the multiuser interference canceler, they will not be described in detail below. However, those skilled in the art will be able to construct the rake combiner
26
and the decision unit
27
with ease.
Each of the respreaders
72
1
-
72
n
comprises a pair of multipliers
28
and
29
. In each of the respreaders
72
1
-
72
n
, the multiplier
28
multiplies the original symbol sequence by the transmission path fading vector &xgr; of one of the paths to impart original transmission path characteristics to the original symbol sequence. Thereafter, the original symbol sequence is spread with the spreading code Ca by the multiplier
49
. The signals from the respreaders
72
1
-
72
n
are combined into a chip-rate interference replica signal
81
a
, which is outputted from the ICU
61
a.
In
FIG. 2
, the signal supplied from the rake combiner
26
to the decision unit
27
is branched and outputted to an external circuit. Such a signal branching arrangement is included in only the ICUs
63
a
,
63
b
,
63
c
in the third stage. From the ICUs
63
a
,
63
b
,
63
c
, the signals are supplied to the decoders
19
a
,
19
b
,
19
c
, respe
Dickstein Shapiro Morin & Oshinsky LLP.
NEC Corporation
Ton Dang
Vu Thong N.
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