Optimal transmission scheme for a distributed antenna in CDMA system

Distributed antenna diversity employs separately installed, but cooperating, antennas within a single base-station; it has been proved that the uplink capacity (per cell) in terms of maximum achievable SIR (signal-to-interference ratio) is linear with the number of deployed antenna units. While in the downlink, because of the multiple-to-one propagation topology, equal power (transmission power at each antenna) allocation (providing equal SIR) in the base-station results in fixed SIR at the mobile terminal, irrespective of the number of base-station antennas used. A transmission scheme, using optimal power allocation and SIR-balanced power control, is proposed to increase the SIR by exploiting multiple base-station antennas. The downlink diversity (called antenna-multipath diversity) transfers the antenna diversity to multipath diversity by utilizing the spread-spectrum signal property. If optimal, rather than equal, power allocation is employed with antenna-multipath diversity, the SDMA advantage can be exploited. The optimisation result shows that, for a particular user, transmitting a signal from one antenna, instead of all base-station antennas, gives better SIR performance. The SIR CDF has been examined by simulation to verify this scheme. Compared to sending from all antennas, the result shows, that for 8 users, this scheme yields improved SIR by 3 dB with 5 antennas. The SIR advantage increases with increasing numbers of antennas and decreases, however, with increasing numbers of users in the cell.