Adaptive DS-CDMA multi-user receiver with diversity combining for interference cancellation
View Patent ↗A digital receiver detects symbols in a baseband signal in a DS-CDMA network. The receiver includes multiple spaced apart antennas. A time-frequency rake receiver is connected to each of the antennas. An interference canceller is connected to each output of each of the rake receiver. Each interference canceller produces a contributing symbol in parallel. A diversity combiner determines a decision symbol corresponding to the baseband signal from the contributing symbols.
1. A digital receiver for detecting symbols in a baseband signal in a DS-CDMA network, comprising:
a plurality of spaced apart antennas;
a time-frequency rake receiver connected to each of the antennas, wherein a frequency offset is estimated by identifying a location of contributing symbols with a smallest error signal;
an interference canceller connected to each output of each of the rake receiver, each interference canceller producing a contributing symbol in parallel, wherein the outputs of each interference canceller include an error signal and one contributing symbol; and
a diversity combiner to determine a decision symbol from the plurality of contributing symbols, the decision symbol corresponding to the baseband signal.
2. The receiver of claim 1 wherein the antennas are spaced about three to five times the wavelength of the baseband signals.
3. The receiver of claim 1 wherein each rake receiver includes a plurality of rake fingers, and wherein the baseband signal received at each antenna is modulated to a plurality of different frequencies, one frequency for each of the plurality of rake fingers.
4. The receiver of claim 3 wherein each rake finger has a different time delay.
5. The receiver of claim 4 wherein the output of each rake finger is sampled at symbol times T b to form a down-sampled signal for each interference canceller.
6. The receiver of claim 5 wherein each interference canceller comprises:
an adaptive filter to receive a real part (Re(*)) of the down-sample signal u i,j , the adaptive filter including a plurality of taps, each tap having a tap weight, and wherein the tap weights are update every symbol time T b according to a least mean square process.
7. The receiver of claim 6 wherein a sign of an output of the adaptive filter is a reference signal subtracted by the adaptive filter.
8. The receiver of claim 7 wherein the reference signal is a training signal during an initial training stage.
9. The receiver of claim 8 wherein the training signal is a predetermined random sequence generated by using a polynomial known to the receiver.
10. The receiver of claim 8 wherein a transmitter periodically transmits the training signal to establish initial tap weights for adaptive filter of each interference canceller.
11. The receiver of claim 1 wherein the diversity combiner combines all contributing symbols C i,j with different weights according to the error signals E i,j , and the decision symbol d is defined by
d
=
sgn
{
∑
i
=
1
M
∑
j
=
1
N
α
i
,
j
C
i
,
j
}
,
where α i is a weighting factor
α
i
,
j
=
∑
i
=
1
M
∑
j
=
1
N
E
i
,
j
E
i
,
j
,
where M is the number of antennas, and N is the number of frequency shifts at each antenna.
12. The receiver of claim 1 wherein the decision signal has the smallest error signal.
13. The receiver of claim 1 wherein the decision signal has a highest signal-to-noise ratio.