Multiuser detection and interference suppression techniques for direct sequence spread spectrum systems in which all users employ same spreading code

A method for differentiating between signals transmitted by different users in Direct Sequence Spread Spectrum wireless communication system where all users employ same spreading code, the spreading code being modulated by information signal to be transmitted by users, the method comprising the steps of:

• (a) correlating a received signal against an expected training signal, the training signal consisting of training symbols modulating the spreading code, each training symbol consisting of chips;
• and (b) separating multipath components belonging to different users;
• wherein the step of separating multipath components belonging to different users comprises the steps of:
• (a) identifying a plurality of significant multipath components based on synchronization enhancement using successive interference suppression;
• and (b) separating the significant multipath components according to different users from the identified significant multipath components:
• wherein the step of separating significant multipath components according to different users comprises the steps of:
• (a) generating a sequence of demodulation statistics corresponding to each delay in the active set of delays;
• (b) computing a demodulation statistic vector corresponding to each delay in the active set of delays;
• (c) computing correlation statistic between all demodulation statistic vectors considering a pair of delays in the active set of delays at a time;
• (d) comparing the correlation statistic with a predetermined threshold;
• and (e) associating the pair of delays with same symbol of same user if the correlation statistic is greater than the predetermined threshold.

A method for differentiating between signals transmitted by different users in Direct Sequence Spread Spectrum wireless communication system where all users employ same spreading code, the spreading code being modulated by information signal to be transmitted by users, the method comprising the steps of:

• (a) selecting a plurality of good hypotheses;
• (b) computing a correlator locked to one user corresponding to each selected good hypothesis;
• (c) identifying correlators that provide useful data;
• and (d) separating multipath components belonging to different users;
• wherein the step of selecting a plurality of good hypotheses comprises the steps of:
• (a) computing an empirical autocorrelation matrix for the received vector;
• (b) estimating a desired vector under the hypothesized phase of the training sequence;
• (c) computing a plurality of approximations to linear minimum mean squared error correlators where one correlator correspond to one hypothesis, (d) selecting the hypotheses that have values of qj higher than a predetermined threshold, as good hypotheses.

A method for differentiating between signals transmitted by different users in Direct Sequence Spread Spectrum wireless communication system where all users employ same spreading code, the spreading code being modulated by information signal to be transmitted by users, the method comprising the steps of:

• (a) selecting a plurality of good hypotheses;
• (b) computing a correlator locked to one user corresponding to each selected good hypothesis;
• (c) identifying correlators that provide useful data;
• and (d) separating multipath components belonging to different users;
• wherein the step of computing a correlator locked to one user corresponding to each selected good hypothesis comprises the steps of:
• (a) computing inner product of the linear minimum mean squared error correlator with spreading sequence shifted by d chips, where d ranges over all possible chip offsets that may correspond to a complete symbol falling into observation interval;
• (b) Identifying delay for which output of the inner product is maximized;
• (e) identifying all delays, as part of an active set of delays, for which output of the inner product is more than a predetermined threshold;
• (d) associating the delays that are within a predetermined number of chips apart from the delay for which the output of the inner product is maximized with primary user;
• estimating interference subspace spanned by the delays that are farther than the predetermined number of chips apart from the delay for which the output of the inner product is maximized, the interference subspace comprising of contributions from secondary users;
• and (e) recomputing correlator locked to the primary user that minimizes mean squared error, subject to the constraint that the correlator is orthogonal to the in interference subspace.