On the forward link capacity of a wide band DS-CDMA system with transmit diversity

The capacity gain expected from next-generation CDMA (IS-2000, UMTS) networks over current (IS-95) networks is attributed to fast forward power control and transmit diversity. We study the forward link capacity of a 3.6864 Mcps direct spread (DS) CDMA system for 9600 bit/s voice traffic. Three transmit diversity schemes are considered, orthogonal transmit diversity (OTD), phase sweeping transmit diversity (PSTD), and space-time spreading (STS). The study indicates IS-2000 radio configuration 7 (RC7) with STS will maximize capacity. On comparing STS to the no-transmit diversity cases, the gain is 3.35 dB when averaged over the two IMT-2000 pedestrian channel models and 0.45 dB when averaged over the two IMT-2000 vehicular models. Due to lower rate convolutional coding, RC6 users will require lower forward link power than RC7 users. However, the fewer available Walsh codes in the RC6 system will likely lower RC6 Erlang capacity below RC7 Erlang capacity at typical system operating levels - blocking 1% to 5%.