System capacity analysis of downlink distributed antenna systems over composite channels

This paper focuses on the downlink system capacity in distributed antenna systems (DAS). Firstly, this paper establishes a composite channel model which takes path loss, lognormal shadowing and Rayleigh fading into consideration. Then, to facilitate the analysis, the distribution of the output SNR is approximated by a lognormal distribution. After that, by making use of selective diversity (SD) scheme for distributed antennas, an approximate analytical expression of the capacity for a mobile station (MS) over a given position is derived. Further, considering the distribution of MSs in the system, a closed-form expression of the system capacity is obtained. Numerical results show that the closed-form expression can provide sufficient precision for evaluating the system capacity performance of DAS.