Capacity of space-time block-coded CDMA systems: comparison of unitary and complementary codes

Two major categories of CDMA signature codes, namely unitary codes and complementary codes, are of interest. All currently available 2-3G CDMA systems make use of the unitary codes, such as Gold codes, m-sequences, Walsh-Hadamard sequences and OVSF codes, all of which work on a one-code-per-channel basis. On the other hand, a CDMA system based on complementary codes works on a flock-of-codes basis. Each user in such a CDMA system is assigned a flock of codes which should be sent to a receiver via different carriers to reconstruct perfect autocorrelation and crosscorrelation functions. A comprehensive analytical study of a STBC-CDMA system using different spreading codes is given and their bit error rate performance is evaluated and compared with both analysis and simulation under Rayleigh fading and multiple-access interference. The results reveal that a STBC-CDMA system based on complementary codes offers a capacity similar to that achievable in a noise-limited system, while a STBC-CDMA system based on traditional unitary codes suffers serious interference-limited problems even with full STBC diversity gain.