Method and apparatus for cancellation of multiple access interference in a code division multiple access (CDMA) communication system

• US 6,546,043 B1
• Filed: 06/29/2000
• Issued: 04/08/2003
• Est. Priority Date: 06/29/2000
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: TRW Limited (Zeppelin - Stiftung der Stadt Friedrichshafen)
• Inventors: Kong, Ning
• Primary Examiner(s): Bocure, Tesfaldet
• Application Number:
  US09/606,806
• Time in Patent Office: 1,013 Days
• Field of Search: 375/285, 375/346, 375/347, 375/267, 375/316, 375/148, 375/147, 455/132-139, 370/335, 370/342, 370/441
• US Class Current: 375/148
• CPC Class Codes: H04B 1/7103 the interference being multiple access interference
• CPC Class Codes: H04B 1/71075 Parallel interference cancellation
• CPC Class Codes: H04B 1/712 Weighting of fingers for combining, e.g. amplitude control or phase rotation using an inner loop

Abstract

Cancellation of multiple access interference (MAI) in a CDMA receiver is effected more reliably by the combination of conventional multistage parallel interference cancellation (MPIC) apparatus and the exploitation of space and time diversity to obtain accurate first estimates of multiple user signals received simultaneously at the receiver. The signals are received at multiple, spatially separated receiver antennas (12) providing spatial diversity and over multiple propagation paths, or multipaths, providing time diversity of the received signals. The signals are separated by user in despreaders (60). A processing module (62) selects an optimum subset of the resolvable multipath signal components for each user, to maximize the signal-to-noise ratio (SNR). The selected signals are coherently combined for each user, providing a reliable set of user signal estimates, which are cascaded to the MPIC apparatus for iterative calculation of MAI for each user. Using space and time diversity to obtain the initial set of user signal estimates results in a higher signal-to-noise ratio (SNR) and a lower bit error rate (BER) for every cancellation stage, more reliable operation of the MPIC apparatus, and more rapid convergence of calculated MAI values.

Claims

1. A method for cancellation of multiple access interference (MAI) in a code division multiple access (CDMA) communication system, the method comprising the steps of:

(a) receiving signals from multiple users of a CDMA communication system simultaneously, at a plurality (M) of spatially separated antennas, wherein at least some of the signals reach the antennas as multipath components traversing a plurality of (L) of different propagation paths;

(b) converting-the received signals to digital form;

(c) separating signals received at each antenna by multipath component and by user, to provide as many as M×

L signals for each user;

(d) for each user, selecting a number, n, of signals from the M×

L signals provided in step (c), wherein n is less than or equal to M×

L, and wherein the n signals are selected as having relatively high signal-to-noise ratio;

(e) coherently combining the n signals selected in step (d), to produce a high signal-to-noise ratio (SNR) combined signal for each user; and

(f) performing multistage parallel interference cancellation on the user combined signals obtained in step (e);

whereby the method employs both time and space diversity to provide a more accurate estimate of user signals, and multiple access interference is more reliably canceled.

2. A method as defined in claim 1, wherein the step (e) of coherently combining is immediately followed by an additional step of determining, at each interval of a time clock, the binary value of the single combined signal for each user.

3. A method as defined in claim 1, wherein the step (f) of performing multistage parallel Interference cancellation includes:

spreading the separate combined user signals, using spreading codes uniquely associated with the users;

computing a set of estimated MAI cancellations, obtained as partial sums of the spread signals, wherein the partial sum for each user is computed as the sum of all of the other user'"'"'s signals;

subtracting each partial sum from the received signal, to obtain a first estimate of spread user signals compensated for MAI;

despreading the spread user signals compensated for MAI in the preceding step; and

repeating the preceding steps as desired to obtain more accurate estimates of the user signals compensated for MAI.

4. A method for cancellation of multiple access interference (MAI) in a code division multiple access (CDMA) communication system, the method comprising the steps of:

simultaneously receiving, at spatially separated multiple antennas, signals transmitted from multiple users, the signals from at least some of the users following multiple propagation paths and arriving at the antennas as resolvable multipath components;

for each user signal, selecting from among all of the multipath components received at all of the antennas a subset of resolvable multipath components of the user signal that provides the highest signal-to-noise ratio;

coherently combining the selected subset of multipath components to obtain for each user a single combined received signal for which signal-to-noise ratio has been maximized by use of space and time diversity;

estimating multiple access interference (MAI) for each user; and

subtracting the estimates from the received signals to obtain estimates of the user signals compensated for MAI.

5. A method as defined in claim 4, wherein the steps of generating estimates of MAI and subtracting them from the received signals are performed by multistage parallel interference cancellation.

6. Receiver apparatus for cancellation of multiple access interference (MAI) in a code division multiple access (CDMA) communication system, the apparatus comprising:

multiple antennas for receiving signals from multiple users transmitting simultaneously to the receiver apparatus, the signals from at least some of the users being received as multipath components traversing direct and indirect paths;

multiple radio-frequency (RF) receivers coupled to respective antennas;

signal processing means coupled to each antenna, for separating the received signals by user and by multipath component;

means for selecting for each user a subset of signals from all of the multipath components received through all of the antennas, wherein the selection is made on the basis of maximizing signal-to-noise ratio;

means for coherently combining the selected signals for each user, to obtain an initial set of estimated user signals with high signal-to-noise ratios; and

a multistage parallel interference cancellation (MPIC) circuit, connected in a cascade arrangement with the foregoing apparatus elements, to provide cancellation of multiple access interference cancellation that operates to reduce bit error rate for each user, whereby the initial set of estimated user signals derived by the apparatus provides more efficient reliable operation of the MPIC circuit.

7. A method for cancellation of multiple access interference (MAI) in a code division multiple access (CDMA) communication system, the method comprising the steps of:

simultaneously receiving, at spatially separated multiple antennas, multiple user signals associated with multiple users, wherein at least some of the signals follow multiple propagation paths and arrive at the antennas as resolvable multipath components;

for each user signal, selecting from among all of the multipath components received at all of the antennas a subset of resolvable multipath components of the user signal that provides the highest signal-to-noise ratio;

coherently combining the selected subset of multipath components to obtain for each user a single combined received signal for which signal-to-noise ratio has been maximized by use of space and time diversity;

estimating multiple access interference (MAI) from the single combined received signals obtained in the coherently combining step; and

using the estimates of MAI obtained in the estimating step, to compensate for MAI.