Multiuser downlink beamforming under quality of service (QoS) constraints has attracted considerable interest in recent years, because it is particularly appealing from a network operator's perspective (e.g., UMTS, 802.16e). When there are many co-channel users and/or the service constraints are stringent, the problem becomes infeasible and some form of admission control is necessary. We advocate a cross-layer approach to joint multiuser transmit beamforming and admission control, aiming to maximize the number of users that can be served at their desired QoS. It is shown that the core problem is NP-hard, yet amenable to convex approximation tools. Two computationally efficient convex approximation algorithms are proposed: one is based on semidefinite relaxation of an equivalent problem reformulation; the other takes a penalized second-order cone approach. Their performance is assessed in a range of experiments, using both simulated and measured channel data. In all experiments considered, the proposed algorithms work remarkably well in terms of the attained performance-complexity trade-off, consistently exhibiting close to optimal performance at an affordable computational complexity.
Bibliographical noteFunding Information:
L. Tassiulas is with the Department of Computer Engineering and Telecommunications, University of Thessaly, 38221 Volos, Greece (e-mail: firstname.lastname@example.org). Supported in part by ARO under Grant W911NF-04-1-0306, and the EC under projects Netrefound (IST-034413-2) and WIP. Digital Object Identifier 10.1109/TWC.2008.070104.
E. Matskani and N. D. Sidiropoulos (corresponding author) are with the Department of Electronic and Computer Engineering, Technical University of Crete, 73100 Chania - Crete, Greece (e-mail: email@example.com). Supported in part by ARL/ERO contract N62558-06-0340.
Z.-Q. Luo is with the Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, USA (e-mail: firstname.lastname@example.org). Supported in part by U.S. NSF grants DMS-0312416 and DMS-0610037.
- Admission control
- Convex approximation
- Downlink beamforming
- Semidefinite relaxation