On the Rate Gap Between Multi- and Single-Cell Processing Under Opportunistic Scheduling

Base station (BS) coordination is a key technique to handle intercell interference (ICI) in cellular networks. Nevertheless, recent work on scheduling indicates that the value of coordination is less prominent when the number of users grows large. More specifically, the loss in sum rate due to ICI in uncoordinated networks can be made arbitrarily small as the number of users goes to infinity. However, the gap in performance for a finite number of users has remained unknown so far. From this perspective we study the gains of multicell zero-forcing beamforming (ZFBF) on the downlink of a Wyner-type network. We first identify the beamforming weights and the optimal scheduling policy under a per-base power constraint. To compare ZFBF with single-cell processing (SCP) we focus on the extra number of users that is needed per cell to compensate for ICI. Specifically, we find the number of users n₁ with ZFBF and n₂ with SCP that gives the same mean postscheduling signal-to-interference-plus-noise ratio (SINR) as an interference free network with n users. The results show that the ratio [(n₂)/(n₁)] grows logarithmically with n. Finally, we demonstrate that the difference in sum-rate between SCP and multicell ZFBF goes to zero as O([(lnlnn)/(lnn)]). As a consequence of the slow convergence there is a significant gain with multicell ZFBF for all practical numbers of users.