Optimum Power Allocation for Distributed Antenna Systems with Large-scale Fading-only Feedback

We propose an optimum power allocation scheme for distributed antenna systems (DAS) in time-varying Rayleigh and Ricean fading channels. In the conventional power allocation schemes for DAS, channel state information (CSI) for feedback includes the fast small-scale fading. This causes two practical problems. First, tracking fast fading requires considerable extra bandwidth and processing, which becomes even larger as the number of the distributed antennas increases. Secondly, there is non-negligible power control latency due to the high Doppler of small-scale fading and the feedback loop delay. In this paper, we consider a DAS system which determines the transmission power at each distributed base stations (antennas) only based on the slowly varying large-scale fading excluding the fast small-scale fading and we derive the optimum power allocation (PA) rule for minimum BER under total power constraint. In case of even small variation in fast fading during feedback loop delay, the conventional optimum PA scheme, which includes fast small-scale fading in CSI feedback, degrades significantly and the proposed scheme achieves much lower bit error rates. Even in the case with no latency, the proposed scheme loses only about 1 dB SNR compared to the optimum scheme, which includes fast fading feedback. Finally, though the proposed power allocation formula is derived assuming Rayleigh fading, we show that it can be employed to Ricean fading channels with a small line-of-sight (LOS) component with a negligible degradation.