Method and system for canceling multiple access interference in CDMA wireless communication system

• US 20020067761A1
• Filed: 12/01/2000
• Published: 06/06/2002
• Est. Priority Date: 12/01/2000
• Status: Active Grant

• Current Assignee: Chibo Capital, LLC
• Original Assignee: EAnywhere Tech, Inc.
• Inventors: Kong, Ning, Zhou, Xue-Song
• Granted Patent: US 6,882,678 B2
• Time in Patent Office: Days
• US Class Current: 375/148
• CPC Class Codes: H04B 1/7107 Subtractive interference cancellation
• CPC Class Codes: H04B 1/712 Weighting of fingers for combining, e.g. amplitude control or phase rotation using an inner loop
• CPC Class Codes: H04B 7/0845 per branch equalization, e.g. by an FIR-filter or RAKE receiver per antenna branch rake receivers as such H04B1/7115
• CPC Class Codes: H04B 7/0857 using maximum ratio combining techniques, e.g. signal-to- interference ratio [SIR], received signal strenght indication [RSS]
• CPC Class Codes: H04B 7/0897 using beamforming per multi-path, e.g. to cope with different directions of arrival [DOA] at different multi-paths

Abstract

A receiver for the third-generation (“3G”) CDMA wireless system is disclosed. The receiver is adapted to mitigate the correlated interference by adaptive antenna process which will maximize the signal-to-interference-plus-noise ratio. After the mitigation of correlated interference from first step space process, the second step is to counter fading with MRC which is optimum after the mitigation of correlated interference. After these first two steps of interference mitigation by adaptive antennas and fading compensation through MRC, the decision signal for the first stage of MPIC will have the strongest desired part and least interference compared to any MPIC available. As can be appreciated, the first stage BER of the present receiver will be the lowest. Then, this low BER will result in a more accurate MAI regeneration, which will further cause a lower BER for the next stage and any subsequent stages. Therefore, the BER of the receiver of the present invention is more likely to converge to the BER of a single user, i.e. MAI-free, faster than those in the conventional systems. In other words, for a given BER requirement, the receiver of the present invention will take the least number of iterations to achieve.

Claims

1. A system for a CDMA wireless communication system, comprising:

a plurality of antennas adapted to receive CDMA multipath signals;

a plurality of adaptive filters coupled to said plurality of antennas, each of said filters adapted to weigh and combine said multipath signals by maximizing their SINR;

a plurality of rake processors coupled to said adaptive filters, each rake processor adapted to combine said multipath signals by maximal ratio combining to generate estimated bits.

2. The system according to claim 1, further comprising:

interference generate unit adapted to generate multiple access interference (“

MAI”

) using the estimated bits from said rake processors.

interference subtract unit adapted to subtract the MAI from the received multipath signals.

3. A method of canceling multiple access interference from a desired user'"'"'s signal in a CDMA receiver, said receiver coupled to a plurality of antennas, said receiver adapted to receive multipath signals from a plurality of users, comprising the steps of:

a) for each user, weighting multipath signals received from the plurality of antennas to maximize SINR;

b) for each user, combining the adaptive filter outputs from all fingers with MRC to generate estimated bits of a desired user;

c) based on the estimated bits, generating MAI for all users;

d) subtracting the MAI from the received signals to derive the desired user'"'"'s signal.

4. The method according to claim 3, further comprising:

e) demodulating the desired user'"'"'s signal;

f) generating the MAI for further cancellation.

5. A CDMA receiver, comprising:

a plurality of antennas, each of the plurality of antennas to receive signals from multiple users;

a plurality of adaptive filters coupled to the antennas, the filters canceling multiple access interference by maximizing signal to interference plus noise ratio to generate a MAI-free signal with fading;

a plurality of rake processors coupled to the adaptive filters, each rake processor adapted to minimize fading from the MAI-free signal with fading using maximal ratio combining to generate an estimated bit of a desired user.

6. The CDMA receiver according to claim 5, further comprising:

a multi-stage parallel interference cancel circuit (“

MPIC”

) coupled to the rake processors to cancel the MAI from the multiple users signals, in at least one stage, to generate a better desired signal until a predetermined BER is achieved.

7. A CDMA receiver, comprising:

a plurality of antennas, the antennas receiving multiple users'"'"' signals of a first frequency, each user'"'"'s signal comprising a unique pilot signal;

a plurality of RF receivers, down-converting the multiple users'"'"' signals to baseband analog signals;

a plurality of analog-to-digital converters, converting the baseband analog signals to baseband composite digital signals;

a plurality of despreaders, each despreader correlating the baseband composite digital signals with pseudonoise of a desired user with the right timing tracking the corresponding multipath signal to generate the desired user'"'"'s signal, which includes MAI;

space process means, for each multipath, for weighting the desired user'"'"'s signal and the MAI from all antennas, such that the SINR of the combined weighted signal is maximized to mitigate MAI;

maximum ratio combiner (“

MRC”

), combining all the weighted outputs from each multipath, each weighted output being weighted again by a complex conjugate of the channel gain derived from the desired user'"'"'s pilot signal to generate maximal signal-to-noise ratio combined signal for the desired user;

first estimator, for each user, estimating a bit based on the MRC signal;

interference generator, for each user, generating other users'"'"' spreaded signals with their channel variation derived from their respective pilot signals;

cancellation means, subtracting other users'"'"' spreaded signals from the baseband composite digital signal to generate the desired user'"'"'s multipath signals;

MRC, weighting multipath signals by their respective complex conjugate of the channel gain derived from the desired user'"'"'s pilot signal to generate maximal ratio combined signal for the desired user;

second estimator, for each user, estimating a user'"'"'s bit based on the MRC signal.

8. In a CDMA wireless system with multiple users, a method of receiving a desired user'"'"'s signal by canceling multiple access interference (“

MAI”

) from other users, each user'"'"'s signal propagating to an antenna though a plurality (“

L”

) of different paths and resulting in multi-path components from each user on each antenna, the method comprising the steps of;

(a) receiving composite RF signals from the multiple users simultaneously, at a plurality (“

M”

) of spatially separated antennas;

(b) down-converting the composite RF signals to base-band signals and converting the base-band signals to digital signals;

(c) separating the digital signals at each antenna by multi-path components and by users;

(d) for each user, weighting and combining the same multi-path components from all “

M”

antennas such that the desired user'"'"'s signal-to-interference-plus-noise ratio is maximized to generate an adaptively filtered output;

(e) for each user, weighting the adaptively filtered output by a channel complex conjugate gain and combining all “

L”

adaptively filtered outputs to counter fading;

(f) estimating a first-stage bit for each user using the combined adaptively filtered outputs;

(g) generating MAI from other user'"'"'s bit estimation, their channel gain, and spreading signal;

(h) subtracting MAI for each multi-path from the composite signals and coherently combining the L×

M cleaned multi-path signals for the desired user; and

(i) estimating the desired user'"'"'s bit.

9. The method according to claim 8, wherein the steps (g) through (i) are repeated until a desired bit-error ratio is reached.