Code-division multiple-access system with improved utilization of upstream and downstream channels

A code-division multiple-access communication system for communication between a base station and a plurality of mobile stations, comprising a base-station apparatus at said base station and a mobile-station apparatus at each of said mobile stations, wherein:

• said base-station apparatus encodes a plurality of audio signals to generate symbols to be transmitted to respective mobile stations, and has
• a plurality of downstream spreading-code generators for generating different downstream spreading codes for different mobile stations, and dividing each downstream spreading code into a first part and a second part;
• a plurality of first spreaders coupled to respective downstream spreading-code generators, for spreading the symbols to be transmitted to respective mobile stations by first parts of respective downstream spreading codes, thereby generating first baseband signals;
• a plurality of second spreaders coupled to respective downstream spreading-rode generators, for spreading said symbols by second parts of respective downstream spreading codes, thereby generating second baseband signals;
• a base-station downstream carrier generator for generating two mutually orthogonal carrier signals;
• at least two product modulators coupled to said base-station downstream carrier generator, for modulating one of said two orthogonal carrier signals by said first baseband signals, and modulating another one of said two orthogonal carrier signals by second baseband signals;
• a base-station radio transceiver coupled to said product modulators, for transmitting the two orthogonal carrier signals thus modulated to all of said mobile stations as a downstream signal, and receiving an upstream signal in which transmit signals transmitted from all of said mobile stations are combined;
• a demodulator coupled to said base-station radio transceiver, for demodulating said upstream signal to obtain an upstream received baseband signal;
• and
• an interference canceler coupled to said demodulator, for recognizing symbol boundaries in said upstream signal, despreading and respreading said upstream signal to generate an estimated interference signal, and canceling said estimated interference signal from said upstream signal, the operations of despreading, respreading, and canceling being performed each time a symbol boundary is recognized;
• and the mobile-station apparatus at each mobile station has
• a transmit processor for encoding an audio input signal to generate symbol data, spreading said symbol data by an upstream spreading code, and modulating an upstream carrier signal by the symbol data thus spread, thereby obtaining a transmit signal;
• a mobile-station radio-transceiver coupled to said transmit processor, for transmitting said transmit signal to said base station as part of said upstream signal, and receiving said downstream signal from said base station;
• a mobile-station downstream carrier generator for generating two local carrier signals;
• a pair of product demodulators coupled to said mobile-station radio transceiver and said mobile-station downstream carrier generator for demodulating said downstream signal by multiplication with said two local carrier signals, thereby obtaining two downstream received baseband signals;
• a mobile-station downstream spreading-code generator for generating the downstream spreading code of said mobile station and dividing said spreading code into a first part and a second part as was done in said base-station apparatus;
• a pair of correlators for correlating one of said two downstream received baseband signals with said first part of said downstream spreading code, and correlating another one of said two downstream received baseband signals with said second part of said downstream spreading code;
• and
• an adder for adding outputs of said pair of correlators.

A method of communication between a base station and a plurality of mobile stations, employing a single downstream channel for transmission from said base station to all of said mobile stations and a single upstream channel for transmission from all of said mobile stations to said base station, comprising the steps of:

• (a) assigning a different upstream spreading code to each mobile station among said plurality of mobile stations;
• (b) assigning a different downstream spreading code to each mobile station among said plurality of mobile stations;
• (c) dividing the downstream spreading code assigned to each mobile station into two parts;
• (d) at said base station, spreading data comprising symbols to be transmitted to each mobile station by both of said parts of the downstream spreading code assigned to that mobile station, thereby generating, for each mobile station, a first downstream baseband signal and a second downstream baseband signal derived from identical data;
• (e) modulating a first carrier signal by the first downstream baseband signal of each mobile station;
• (f) modulating a second carrier signal by the second downstream baseband signal of each mobile station;
• (g) combining said first carrier signal and said second carrier signal to create a downstream signal;
• (h) transmitting said downstream signal to all of said mobile stations;
• (i) in each individual mobile station, receiving and demodulating said downstream signal by two locally generated carrier signals to generate two received baseband signals;
• (j) despreading said two received baseband signals by respective parts of the downstream spreading code assigned to said individual mobile station, thereby obtaining pairs of correlated values;
• (k) additively combining respective pairs of correlated values;
• (l) spreading data comprising symbols to be transmitted from each mobile station to said base station by the upstream spreading code assigned to said mobile station;
• (m) transmitting the data thus spread from said mobile station to said base station, by modulation of an upstream carrier signal;
• (n) receiving at said base station an upstream signal in which modulated upstream carrier signals transmitted from all of said mobile stations in said step (m) are additively combined;
• (o) demodulating said upstream signal to obtain an upstream received baseband signal;
• (p) storing at least one copy of said upstream received baseband signal in a shift register;
• (q) as soon as reception of a symbol from any one mobile station is completed, despreading said upstream received baseband signal by correlation with the upstream spreading code assigned to said one mobile station, thereby deriving an estimated symbol value;
• (r) respreading said estimated symbol value by said upstream spreading code, thereby obtaining an estimated interference signal;
• and
• (s) subtracting said estimated interference signal from at least one stored copy of said upstream received baseband signal, so that said estimated interference signal will be excluded when estimated symbol values of mobile stations other than said one mobile station are derived in subsequent repetitions of said step (q).