Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1997-12-24
2001-04-24
Pham, Chi H. (Department: 2731)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C375S143000
Reexamination Certificate
active
06222833
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a device and method for a multiuser detection in a DS-CDMA (Direct Sequence-Code Division Multiple Access) system, and more particularly, to a device and method for a decorrelating multiuser detection in a DS-CDMA system which allows real time removal of an MAI (Multiple Access Interference) occurred in the DS-CDMA system.
2. Discussion of the Related Art
The DS-CDMA system is a system in which a transmitter modulates and transmits its message by making a direct band spreading with a pseudo noise (PN) code or a spreading signal (code) assigned to multiusers individually for distinguishing between the multiusers, and a receiver restores an originally transmitted message by receiving the transmitted signal and by despreading the transmitted signal with the spreading code. Since the DS-CDMA system has many merits, such as strong in multipath fading, good utilization of voice activity cycles, availability of soft handoff between base stations, strong in jamming, and reuse of one frequency band, which reuse allows the DS-CDMA system to have a greater capacity over conventional systems, the DS-CDMA systems has been spot lighted in implementation of cellular and personal communications, recently. Despite the aforementioned merits of the DS-CDMA system, because its performance is restricted by MAI occurred at multiuser reception or great SNR (signal to noise ratio), ceaseless efforts have been concentrated on removing the MAI, and thus various multiuser detectors have been proposed up to now.
As a typical one of the multiuser detector, there is a decorrelating multiuser detector as shown in
FIG. 1
, provided with a matched filter block
10
having a plurality of multipliers (CO
1
-CO
K
), a plurality of integrators (I
1
-I
K
), and a plurality of switches (SW
1
-SW
K
) for despreading a received signal r(t) with multiuser' spreading codes to provide a sample of each message of the multiuser, a R
−1
filter
11
for filtering the samples in the matched filter block
10
to remove an MAI signal included in each of the samples of the multiuser's messages, and a binary data determining part
12
having a plurality of determiners (DC
1
-DC
K
) for comparing plural outputs (Z
1
-Z
K
) corresponding to the multiuser of the R
−1
filter
11
with a threshold voltage to determine binary outputs (b
1
-b
K
).
The operation of the background art decorrelating multiuser detection will be described.
The received signal r(t) input to the decorrelating multiuser detector is despread through the plurality of matched filters in the matched filter block
10
having the plurality of multipliers (CO
1
-CO
K
), integrators (I
1
-I
K
) and switches (SW
1
-SW
K
) and each of the original messages of the multiuser is recovered. The outputs (y
1
-y
K
) of the matched filter block
10
can be expressed as a matrix as follows.
y
=
RAb
+
n
,
R
=
&LeftDoubleBracketingBar;
1
P
12
P
13
⋯
P
1
K
P
21
1
P
23
⋯
P
2
K
⋮
⋮
⋮
⋮
⋮
P
K1
P
K2
⋯
⋯
1
&RightDoubleBracketingBar;
where
,
y
-
(
y
1
,
y
2
,
…
,
y
K
)
T
A
=
&LeftDoubleBracketingBar;
a
1
0
⋯
0
0
a
2
⋮
⋮
⋮
⋮
⋰
⋮
0
⋯
0
a
K
&RightDoubleBracketingBar;
b
=
(
b
1
,
b
2
,
…
,
b
K
)
T
n
=
(
n
1
,
n
2
,
…
,
n
K
)
T
(
1
)
Where, a
i
is a received amplitude of an ith multiuser, A is an amplitude matrix of the received signal, b is a bit vector of a transmitted data, n is a Gaussian noise vector, and an element P
ij
in the R matrix represents a cross-correlation coefficient between ith and jth user spreading codes.
If an output y from the matched filter block
10
as expressed in equation (1), is provided to the R
−1
filter
11
, an output Z expressed as the following equation (2) can be obtained.
Z=R
−1
y=Ab+R
−1
n
(2),
Z=(Z
1
, Z
2
, . . . Z
K
,)
The output Z of the R
−1
filter
11
, expressed as equation (2), sends to the binary data determining part
12
having the binary determiners DC
1
-DC
K
where the binary data of multiuser is recovered.
The background art decorrelating multiuser detector can completely remove the MAI signal caused by the cross-correlation value (not 0) between multiuser's spreading codes and included in the output y of the matched filter
10
by using the R
−1
filter and thus can improved quality of the received signal, However, the R
−1
filter is required to compute an inverted matrix of R as shown in the following equation (3). This computation becomes the more complex as the dimension of the matrix becomes the greater as a number of the users increases.
R
-
1
-
1
det
(
R
)
&LeftDoubleBracketingBar;
b
11
b
12
⋯
b
1
K
b
21
b
22
⋯
b
2
K
b
K1
b
K2
⋯
b
KK
&RightDoubleBracketingBar;
(
3
)
In this equation (3), the diagonal element b
11
is expressed as b
11
=1−(K−1)(K−2)/2 second order term of the cross-correlation coefficient+O(P
3
), the diagonal element b
22
is expressed as b
11
=1−(K−1)(K−2)/2 second order term of the cross-correlation coefficients+O(P
3
) , and the other diagonal elements b
ii
are expressed in the same manner. The non-diagonal element b
12
is expressed as b
12
=−P
12
+(K−2) second order term of the cross-correlation coefficients+O(P
3
) and the non-diagonal element b
13
is expressed as b
13
=−P
13
+(K−2) second order term of the cross-correlation coefficients+O(P
3
). The other non-diagonal element b
ij
is expressed in the same manner. The O(P
3
) denotes a polynomial of the cross-correlation coefficients having a third order term and greater. Therefore, the R
−1
filter has a problem in that a circuit can not be realized actually due to the excessive amount of computation.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a device and method for a multiuser detection in a DS-CDMA system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a device and method for a multiuser detection in a DS-CDMA system, which largely reduces an amount of data computation while reducing an MAI so as to be actually applied to the system.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for multiuser detection in a DS-CDMA system includes the steps of receiving a band-spread signal by a plurality of spreading codes respectively transmitted by each of multiusers, obtaining a plurality of sampled outputs from each of multiuser's messages by despreading the received signal with a matched filter, obtaining an MAI signal included in the plurality of sampled outputs by applying a minus (−) value of a cross-correlation value between the multiuser's spreading codes to the plurality of sampled outputs, removing the MAI signal from the sampled outputs by subtracting the MAI signal from the sampled outputs, and generating a binary data by comparing the sampled outputs from which the MAI signal is to be removed with a threshold value to recover the multiuser's messages.
In another aspect, a device for a multiuser detection in a DS-CDMA system according to the present invention includes a matched filter
Fleshner & Kim LLP
LG Communications & Information, Ltd.
Pham Chi H.
Yao Kwang B.
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