Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1997-12-19
2001-02-20
Tse, Young T. (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S140000, C375S346000
Reexamination Certificate
active
06192067
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multistage interference canceller to be used in Code Division Multiple Access (CDMA) communication systems. More particularly, the present invention relates to digital mobile radio communication systems that use a Direct Sequence Code Division Multiple Access (DS-CDMA) communication system. More particularly, the present invention relates to a method of tentative decision in the multistage interference canceller.
2. Description of the Related Art
The CDMA communication system is used for digital mobile radio communication systems in various countries. In this system, it is very important to improve the Signal-to-Interference Ratio (SIR) to decode received signals much more accurately. SIR can be adversely affected, for example, by interference from other users caused by the correlation between spreading codes.
The multistage interference canceller, which generates and removes interference replicas from received signals in multiple stages, is generally expected to improve SIR.
FIG. 1
shows a configuration example of a conventional multistage interference canceller. In this example, stages 1 to m are linked longitudinally. Each stage has interference canceller units (ICU) (
71
) and synthesis units (
72
). The subscripts attached to the names of interference canceller units (
71
) ICU
1, 1
~ICU
1,k
, ICU
2, 1
~ICU
2,k
, . . . , and ICU
m, 1
~ICU
m,k
correspond to stage numbers “
1
” to “m” and user numbers “
1
” to “k.”
In stage 1, received signal R
0
is input to interference canceller units ICU
1,1
to ICU
1,k
(which correspond to users). The interference canceller units then output interference replica signals S
1,1
to S
1,k
and estimated interference residual signals d
1,1
to d
1,k
. The synthesis unit (
72
) synthesizes estimated interference residual signals d
1,1
to d
1,k
, removes them from received signal R
0
, and then outputs an error signal e
1
.
In stage 2, error signal e
1
from the synthesis unit (
72
) in stage 1 and interference replica signals S
1,1
to S
1,k
from interference canceller units ICU
1,1
to ICU
1,k
in stage 1 are input to interference canceller units ICU
2,1
to ICU
2,k
. Next, the interference canceller units output interference replica signals S
2,1
to S
2,k
and estimated interference residual signals d
2,1
to d
2,k
. The synthesis unit synthesizes estimated interference residual signals d
2,1
to d
2,k
, removes them from error signal e
1
input from stage 1, and then outputs error signal e
2
.
Similarly, in stage m, error signal e
m−1
(from the synthesis unit of the previous stage) and interference replica signals S
m−1,1
to S
m−1,k
(from the interference canceller units of the previous stage) are input. The interference canceller units in stage m then output interference replica signals S
m,1
to S
m,k
and estimated interference residual signals d
m,1
to d
m,k
. Thus, the interference replica signals from which the interference between users and the multipath interference is removed can be obtained by the processing in each stage.
FIG. 2
shows the configuration of each interference canceller unit (
71
) shown in FIG.
1
. In this example, the interference canceller unit has a three-finger structure for rake (RAKE) synthesis. In
FIG. 2
, “
81
” indicates a despread unit, “
82
” a synthesizer, “
83
” a decision unit, “
84
” a spreading unit, “
85
” a synthesizer, and “
86
” a despreader. Also, “
87
” indicates an adder, “
88
” a multiplier, “
89
” channel estimation, “
90
” a multiplier, “
91
” an adder, and “
92
” a spreader. In the following explanation, symbol “{circumflex over ( )}” represents an estimated value and symbol “*” represents a complex conjugate number.
Error signal e
m−1
from the previous stage (received signal R
0
if stage m is stage 1) and interference replica signal S
m−1,k
from the previous stage (zero if stage m is stage 1) are input to the despread unit (
81
) corresponding to the delay profile (path) of the received signal. The despreader (
86
) demodulates error signal e
m−1
input from the previous stage in reverse according to a spreading code by despread. Note that in stage 1, received signal R
0
is input to the interference canceller in synchronization with the spreading code.
The signal spread in reverse and demodulated by the above despreader is added to the interference replica signal input from the previous stage by the adder (
87
). Received symbol vector R
i
is then generated for path i. Received symbol vector R
i
for path is input to channel estimation (
89
), which outputs an estimated value &xgr;i{circumflex over ( )} of the channel (phasing vector) of path i. Channel estimation (
89
) estimates the value by using a pilot symbols included in the received signal. For instance, the estimated value of the phasing vector may refer to an error in the signal phase or amplitude caused by phasing in a radio channel.
The multiplier (
88
) multiplies received symbol vector R
i
by using complex conjugate number &xgr;i{circumflex over ( )}* of estimated channel value &xgr;i{circumflex over ( )} for weighting and phase compensation in proportion to the amplitude of estimated channel value &xgr;i{circumflex over ( )}. The synthesizer (
82
) synthesizes the signal output from the multiplier (
88
) corresponding to the path at the maximum ratio.
The decision unit (
83
) temporarily evaluates synthesized received symbol vector &Sgr; R
i
&xgr;
i
{circumflex over ( )}*. The synthesizer (
83
) outputs estimated information symbol vector Zs{circumflex over ( )} following hard decision of synthesized received symbol vector &Sgr; R
i
&xgr;
i
{circumflex over ( )}*.
Output estimated information symbol vector Zs{circumflex over ( )} is input to the spreading unit (
84
). The multiplier (
90
) multiplies estimated information symbol vector Zs{circumflex over ( )} by estimated channel value &xgr;i {circumflex over ( )} to generate interference replica signal S
m,k
for each path, then outputs the generated signal to the next stage.
The adder (
91
) subtracts interference replica signal S
m−1,k
from interference replica signal S
m,k
for each path, then outputs the result to the spreader (
92
). The spreader (
92
) despreads the signal input from the adder (
91
) according to the spreading code, then outputs the spread signal for each path to the synthesizer (
85
). The synthesizer (
85
) synthesizes the signals input from the synthesizer (
85
) and outputs estimated interference residual signal d
m,k
.
As the above operation is executed for users in more stages, error signal em becomes closer to noise only, resulting in higher interference replica signal accuracy. Thus, a received signal (from which the interference between users and multipath interference is removed) can be obtained after rake reception processing using the error signal and interference replica signal in the final stage.
FIG. 3
shows an example of the signal space where the decision unit (
83
) executes hard decision of the received signal to which QPSK modulation is applied.
FIG. 3
shows in detail the first quadrant of the signal space enclosed by Q channels. In this example, received symbol vector &Sgr; R
i
&xgr;
i
{circumflex over ( )}* is subject to hard decision to confirm that it is estimated information symbol vector Zs{circumflex over ( )}. The phase of received symbol vector &Sgr; R
i
&xgr;
i
{circumflex over ( )}* is compensated so that it becomes a normal vector signal. Estimated information symbol vector Zs{circumflex over ( )} is then output as the signal after tentative decision at a level equivalent to the total amplitude of estimated channel value &xgr;i{circumflex over ( )}.
As previously described, the interference canceller unit corresponding to each user of a conventional multistage interference canceller has a decision unit (
83
). The decision unit (
83
) inputs and evaluates received symbol vector &Sgr; R
i
&xgr;
i
{circumflex over ( )}* output after rake synthesis by the synthesizer
Kobayakawa Shuji
Seki Hiroyuki
Tanaka Yoshinori
Toda Takeshi
Tsutsui Masafumi
Fujitsu Limited
Helfgott & Karas P.C.
Tse Young T.
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