Joint Long-Term Admission Control and Beamforming in Green Downlink Networks: Offline and Online Approaches

Admission control is an effective solution to managing a wireless network with a large user set. It dynamically rejects users in bad conditions, and thereby guarantees high quality-of-service (QoS) for the others. Unfortunately, a frequently-varying admissible user set requests remarkable power to continually re-establish the transmission link. In this work, we explore the stability of admissible user set, and propose a joint long-term admission control and beamforming problem for a network with one multi-antenna base station (BS) and multiple single-antenna users. We consider the downlink transmission in a time period comprised of multiple time slices. By jointly optimizing the admissible users and the BS transmit beamformers in different time slices, we aim to balance the size of user set and the network total power cost for both QoS guarantee and user status switching. We develop two (offline and online) algorithms to find some efficient suboptimal solutions of this NP-hard problem. The offline algorithm requires all channel state information (CSI), and then solves the admissible users and beamformers in different time slices in one shot, based on successive upper-bound minimization (SUM). To support real-time data transmission, we further design an alternating direction method of multipliers (ADMM)-based online algorithm, which outputs the admissible users and beamformers in different time slices successively, utilizing the previous admissible user set, the actual value of current CSI, and the distribution of future CSI. Simulations validate the performance of the two algorithms, and show that the online algorithm is an efficient practical alternative to the offline algorithm.