On optimizing transmit antenna placement for downlink distributed antenna systems with zero-forcing beamforming

We consider a downlink distributed antenna system (DAS) in which transmit antennas are placed in a circular layout and each mobile user is equipped with a single receive antenna. Assuming zero-forcing beamforming transmission, a base station randomly selects users to which it transmits data. We derive the upper bound on a sum of all users´ achievable rates and use the bound to estimate the optimal radius of the antenna layout that maximizes the sum rate. Numerical examples show that the optimal radius depends on the number of transmit antennas, signal-to-noise ratio, and variance of user density. The sum rate of DAS with optimized placement of transmit antennas can be significantly higher than that of a co-located antenna system (CAS).