On the spectral efficiency of space-time spreading schemes for multiple antenna CDMA systems

We have previously introduced a particular space-time spreading (STS) scheme that can be used in order to provide important diversity gains in CDMA systems that use multiple antenna transceivers. While such simple STS schemes are simpler to implement (especially in terms of error-correction coding) than the BLAST (Bell-labs LAyered Space-Time) architecture, it is of interest to quantity the expected penalties paid in terms of spectral efficiency. We derive capacity expressions for such STS systems and compare them to the corresponding expressions of related BLAST systems. Interestingly enough, it turns out that for small numbers of antennas, the deviations in performance are not so high. These results suggest that, when small numbers of antennas are available, space-time spreading is a viable option that provides a good tradeoff between ease of implementation and increased spectral efficiency of direct-spreading CDMA systems.