On the Performance of Open-Loop Transmit Diversity Techniques for IS-2000 Systems: A Comparative Study

Third generation CDMA cellular systems incorporate a downlink transmission technique called Transmit Diversity (TD). Transmit Diversity has been studied in various forms by several researchers. This paper provides a comprehensive investigation of the performance and practical implementation issues of open-loop transmit diversity schemes for the IS-2000(1) third generation cellular CDMA standard. Discussed in detail are orthogonal transmit diversity (OTD) and space time spreading (STS) diversity schemes. Space time spreading is a transmit diversity technique that is motivated by space-time coding principles originally described for narrowband systems. ITD is a TD technique that obtains diversity not at the symbol level, but in the decoding process and has performance that is general lower bounded by STS, which obtains diversity combining prior to decoding. Thus, STS always outperforms OTD, with the improvement particularly significant in the presense of weak convolutional codes. Probability of error analysis is performed for STS under the assumptions of imperfect channel estimates, correlation between antennas, and unequal pilot power allocations. Simulation studies are performed to detail the performance of both open-loop transmit diversity schemes with convolutional coding and closed-loop power control. Many of these results were generated in the course of the IS-2000 standardization procedure. Performance is studied in radio environments which experience flat Rayleigh fading, frequency selective Rayleigh fading, spatially selective fading, as well as Ricean fading with various K-factors. Some additional results are presented for cases where mobile receivers have two antennas. Implementation issues are also considered including the impact of antenna delay differences on performance, transmitter and receiver architectures, and computational complexity.