Adaptive induced fluctuations for multiuser diversity

This paper investigates multiuser diversity in the context of cellular networks, with emphasis on the gains that can he achieved by adaptively inducing fluctuations in the environment. In a cellular system that at any time schedules for service the user with the best channel at that time, the expected service rate increases with the variability of the channel. Controlling the fluctuations using available feedback can further increase the expected service rate. This paper proposes a scheme for controlling fluctuations using only the feedback required to exploit multiuser diversity. Fluctuations are induced by introducing at the base station another transmit antenna that sends out the same signal but at a different phase from the first one, and then adaptively varying the phase difference. The performance of the scheme (for the same total transmitted power) is evaluated when the users are infinitely back-logged or have finite queues, and when the channels are Rayleigh or Ricean distributed. Fairness issues and performance of the scheme under an additional fairness mechanism are also investigated in the context of users with finite queues. In all scenarios the performance is better when fluctuations are adaptively induced than when the fluctuations are randomly induced or not induced at all.