Optimum Downlink Beamwidth Estimation in mmWave Communications

With increasing density of data-hungry devices per unit area, allocating single highly-directed beam per user in millimeter-wave communications is not practical. Therefore, the requirement is to serve multiple users over a single beam. Considering a single-cell scenario with a fixed number of users, this paper addresses the problem of selection of optimal beamwidth depending on user density and distribution. First, by considering fixed beam service time in each sector, optimal beamwidth is estimated using exhaustive search for average long-run user rate and base station energy efficiency maximization. Based on the results of the average long-run user rate maximization using an exhaustive search, another method of reduced complexity is proposed to find sub-optimal beamwidth. Subsequently, optimum beamwidth is estimated with user density dependent variable time scheduling in a sector, that offers improved performances over fixed time scheduling. An efficient algorithm on variable time scheduling is also provided. Finally, the effect of localization error on optimal beamwidth estimation is investigated. The numerical results show that using the narrowest beam does not necessarily result in achieving a better average long-run user rate. Further, localization error does not affect the selection of optimal beamwidth, however, user Quality-of-Service degrades.