Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-04-05
2003-07-29
Hsu, Alpus H. (Department: 2665)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S342000, C375S148000, C375S150000, C375S346000, C455S063300
Reexamination Certificate
active
06600729
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a DS-CDMA multi-user interference canceller and a DS-CDMA communication system in a DS-CDMA (Direct Sequence—Code Division Multiple Access) communication system.
2. Description of the Related Art
Recently, the demand for portable mobile devices has been remarkably increased, and a DS-CDMA communication system or an FH (Frequency Hopping)-CDMA communication system are expected as a system for satisfying this demand. Further, it is being promoted to use base stations and a plurality of portable mobile equipment serving as mobile stations which use the above system.
According to the DS-CDMA communication system, voice signals and signal data is modulated by the DS (Direct Sequence)-based SS (Spread Spectrum) system. This system has various advantages of multi-user performance, high resistance to interference, high secrecy performance, etc., and high-quality telephone communications and data communications can be satisfied by the base stations and the plural mobile devices. However, when a number of users communicate with a base station in the same frequency band, the communication of a user may be disturbed due to interference of another user's communication. As compared with a frequency modulation system or a time-division multiplexing system, the DS-CDMA communication system has higher resistance to interference, however, restriction is imposed on the number of users which can communicate in the same frequency band.
In view of the foregoing problem of the DS-CDMA communication system, a multi-user serial interference canceller has been proposed, and also a method of reducing the interference in regionally-sectioned area cells and increasing the capacity for subscribers or enhancing the communication quality (QOS: Quality Of Service) has been proposed for the multi-user serial interference canceller. According to this method, before the signal of a user k (1≦k≦K: K represents the number of all users in a cell) is decoded, interference replica components of users other than the user k which have been decoded until that time are generated and subtracted from the signal of the user k to decode the signal of the user k. This processing is carried out at I times (I represents the stage number of multi-stages), thereby reducing interference effects of the other users. The interference replica components generated with the other reception signals are delayed by the processing time corresponding to the number of the other users, and the interference components of the other users are subtracted from the reception signal of the user k concerned. As the number of the other users increases, the delay time is increased more and more, resulting in occurrence of such a problem that the capacity of a memory for holding the interference replica components which have been detected until the delay time is increased and the processing time is lengthened because the processing must wait during the delay time.
In order to resolve this problem, a multi-user serial interference canceller has been proposed. The principle of the multi-user serial interference canceller is described in “SEQUENTIAL CHANNEL ESTIMATION TYPE SERIAL CANCELLER USING PILOT SYMBOL IN DS-CDMA” of the technical reports of ELECTRONIC INFORMATION COMMUNICATION ASSOCIATE (RCS95-50), and an example of a fabricating method thereof is disclosed in “DS-CDMA MULTI-USER SERIAL INTERFERENCE CANCELLER” of Japanese Laid-open Patent Application No. Hei-09-270736.
FIG. 4
shows the method disclosed in Japanese Laid-open Patent Application No. Hei-09-270736. In
FIG. 4
, reference numeral
31
represents a reception signal, reference numeral
32
represents a interference-cancelled reception signal transmission line for transferring an interference-cancelled reception signal to an adjacent stage, reference numeral
33
represents an interference canceller unit ICU, reference numeral
34
represents a delay memory, reference numeral
35
represents a subtracter for subtracting interference replica components from the reception signal, reference numeral
36
represents an interference replica signal transferring line for transferring the interference replica signal of each user to its own user (present user) of a succeeding stage, reference numeral
37
represents an adder for adding a (interference) replica signal of the present user of a previous stage again (because the signal components of all the users of the first stage are subtracted), reference numeral
38
represents a delay (memory) for holding the (interference) replica signal of the present user of the previous stage, and reference numeral
39
represents a decoder for performing the final decoding.
Next, the operation of the multi-user serial interference canceller shown in
FIG. 4
will be described.
A reception signal r
31
is first input to ICU#
11
(Interference Cancellation Unit)
33
of a first user in a first stage, and an interference replica signal S
11
on the line
36
thereof is generated in ICU#
11
. Subsequently, in a subtracter
35
, the interference replica signal S
11
generated from the reception signal r
31
is subtracted from the signal r
31
which is delayed by the delay
34
for delaying the signal by the processing time in ICU#
11
, and the subtraction result is input to ICU#
12
of a second user in the first stage. A signal [r
31
−S
11
] is input to ICU#
12
, and an interference replica signal S
12
on the line
36
is output. Subsequently, the signal of a k-th user in an i-th stage which is represented by the following equation is generally transmitted to an adder
37
which is connected to ICU#k of the k-th user in the i-th stage:
y
k,1
(
m
)=∫
mT+&tgr;
k,1
(m+1)T+&tgr;
k,1
Ck
(
t−&tgr;
k,1
)
C
k
*(
t−&tgr;
k,1
) (1)
As is apparent from the equation (1), the equation (1) contains the reception signal r
31
at all times, and thus it is unnecessary to transfer the reception signal by another system and holds the reception signal r
31
. Further, the signal in ICU#ik is a residual signal obtained by subtracting the components of all the users from the reception signal r
31
. Therefore, the signal S
(i−1)k
, that is, the interference replica signal of the present user obtained at the previous stage is added again and then input to ICU before the processing for the k-th user in the i-th stage is carried out. AR of these signals are chip rate signals.
FIG. 5
shows the internal construction of ICU#ik based on the conventional system.
A reception signal r(t) input is despreaded by a spread code Ck(t) of the present path of the present user, and then integrated in an integrator
42
to examine a correlation therebetween. A transmission path fading vector &xgr; is calculated from the integrated signal in a transmission estimator
43
, and then the inverse &xgr;′ of the transmission path fading vector is multiplied by the integrated signal in a multiplier
54
to perform phase correction. The signal which is subjected to the phase correction every path is subjected to RAKE combining in a RAKE combiner
45
. The signal output from the RAKE combiner
45
is output to a decoder and also is used to decode an original symbol sequence in a data-decision unit
46
. Thereafter, the original symbol sequence is multiplied by the transmission path fading vector &xgr; every path in a multiplier
47
to be returned to an original transmission characteristic. The original transmission characteristic (Ck(t)) is multiplied/spread in a multiplier
48
to reproduce the interference replica signal of the chip rate, and then the interference replica signal is transferred to a succeeding user or a succeeding stage.
As shown in
FIG. 4
, in the final stage, the interference-cancelled signal is input to a decoder
39
, and the final decoding result is output.
Japanese Laid-open Patent Application No. Hei-9-270766 discloses a transmission system for interference replica signals of DS-CDMA m
Dickstein Shapiro Morin & Oshinsky LLP.
Hsu Alpus H.
NEC Corporation
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