Multiple access interference cancellation for CDMA demodulator

Claim 1

A demodulator for block-spreading code-division multiple-access communications, in which blocks of transmit data are converted to codewords taken from a set of orthogonal codewords common to all transmitting stations, then spread by spreading codes assigned to individual transmitting stations, comprising: a memory (6) divided into separate memory areas for different transmitting stations, each memory area storing received chip data and updated chip data for a plurality of blocks including at least a newest block and an oldest block; a product codeword generator (9) for generating a spreading code used by a designated transmitting station, and using said spreading code to spread all codewords in said set of orthogonal codewords, thereby producing corresponding product codewords; a correlator (8) coupled to correlate data in each of said plurality of blocks in a designated memory area in said memory (6) with said product codewords, to select for each of said blocks a particular product codeword which gives a maximum correlation value, and to output, for said oldest block, said maximum correlation value and information identifying a codeword which was spread to produce said particular product codeword; an interference estimator (18) coupled to use said particular product codeword and said maximum correlation value to calculate interference correction data, said interference correction data being an estimate of interference that was generated by said designated station and has not yet been canceled; an interference subtractor (20) coupled to update all data stored in said memory (6), except data stored in said designated memory area, by subtracting said interference correction data, thereby canceling interference; and a controller (22) for recognizing block boundaries and, when a transmitting station completes transmission of a block, designating that station as said designated station and designating the corresponding memory area in said memory (6) as said designated memory area.

Claim 11

A method of demodulating a block-spreading code-division multiple-access signal received from a plurality of transmitting stations, comprising the steps of: storing received chip data and updated chip data in a memory (6) divided into separate memory areas, each memory area corresponding to a different transmitting station, and each memory area storing data for a plurality of blocks including at least a newest block and an oldest block; designating a transmitting station and its corresponding memory area whenever said transmitting station finishes transmitting a block, thus obtaining a designated transmitting station and designated memory area; spreading a plurality of orthogonal codewords by a spreading code used by said designated transmitting station, thereby obtaining, for each block, a plurality of product codewords; correlating the data stored for each of said blocks in the designated memory area with all of the product codewords obtained for that block; selecting, for each block, a particular product codeword that gives a maximum correlation value; outputting, for said oldest block, said maximum correlation value and information identifying a codeword that was spread to generate said particular product codeword; using the particular product codeword and maximum correlation value for each block to estimate interference that was generated by said designated transmitting station and has not yet been subtracted from data of other transmitting stations, thereby obtaining interference correction data; and updating all data stored in said memory (6), except data stored in said designated memory area, by subtracting said interference correction data, thereby canceling interference.