Exploiting Time Coherence in Opportunistic Beamforming for Slow Fading Channels

Opportunistic beamforming has been proposed to exploit multiuser diversity in slow fading channels. The benefit of multiuser diversity is obtained using the proportional fair scheduling algorithm in combination with multiple transmit antennas. The antenna gains (power and phase) are randomly varied to induce fast fluctuation in slow fading channels. In this paper, we present an enhancement of this technique, named Time Coherent-Opportunist Beamforming, which exploits the time coherence existing in the slow fading channels to yield improved throughput. The proposed scheme is compared to opportunistic beamforming. Computer simulation results indicate a favorable throughput performance. Analysis in terms of scheduling jitter and frame size is also carried out. The average throughput of the proposed scheme approaches the average throughput of the optimal beamforming independently of the number of users. For a fixed number of users, the proposed scheme converges to the optimal beamforming as either the frame size or the scheduling jitter goes to infinity. Furthermore, the implementation requires only base station updates, facilitating the adoption of the proposed scheme.