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Ergodic capacity analysis of downlink distributed antenna systems using stochastic geometry

This paper studies the ergodic capacity of a multicell distributed antenna system (DAS), where remote antenna ports are spread within each cell to cooperatively transmit to user terminals. Unlike most prior studies which assume the antenna ports to be deployed at fixed locations, this paper assumes the antenna ports to be distributed as a spatial Poisson point process (PPP) to account for the fact that in practice the antenna ports are randomly placed to cover wherever the dead spots are. We first model DAS within each cell as a downlink multiple-input single-output (MISO) channel with per-antenna power constraint while accounting for inter-cell (inter-cluster) interference. Two DAS layouts are considered: the “regular” layout where the antenna ports are randomly deployed within regular cellular boundary to serve a given user, and the “user-centric” layout where the antenna ports are distributed over a wide area and the users choose the surrounding antenna ports to form a “virtual cell” as its own serving antenna subset. Using the tool of stochastic geometry, we analytically derive efficiently computable ergodic capacity expressions for the two layouts of DAS. Using these expressions, the cell-edge capacity of DAS under the regular layout is shown to be upper-bounded by α/2, where α is the pathloss exponent. Numerical results show that the proposed analytical model can accurately model the first layout, and can well approximate the second layout when the serving radius of users is not large. Compared to the traditional cellular system where all antennas are co-located at the cell center, DAS has better cell-edge performance. Further, the user-centric DAS has higher capacity than the DAS under regular layout.