Downlink distributed antenna systems: Optimal beamforming designs and capacity behavior

In this paper, we investigate the outage and ergodic capacity of downlink distributed antenna systems (DAS) where each distributed antenna unit (DAU) has multiple antennas with per-DAU power constraint. We first derive the optimal beamforming vector in a closed form by applying a matrix minor condition to relax the positive semi-definite constraint. We observe that our derived solution has a form of maximum ratio transmission per each DAU with full power according to each DAU power constraint. Based on the derived optimal beamforming, the outage and ergodic capacity under Rayleigh fading channels are analyzed. To this end, we show that a distribution of the received signal-to-noise ratio is characterized as a Gamma distribution by approximating a sum of non-identical independent Nakagami-m random variables as a Nakagami-m random variable based on the moment matching method. Then, a formula of the outage and ergodic capacity is presented in a closed form. Simulations confirm that our analysis is accurate and matches well with the simulation results.