A simplified opportunistic feedback and scheduling scheme for OFDM

Opportunistic beamforming schedules users when they experience a high instantaneous signal-to-noise ratio. Multiple antennas at the transmitter can he used to induce temporal fading to ensure that all users fade at a rate fast enough to ensure fairness. Because feedback is required, the fading rate must be fast enough to ensure fairness among users, but slow enough so that the feedback information is not out of date. An OFDM system with opportunistic beamforming has the advantage that multiple users can be scheduled at the same time in a frequency-selective channel, thus allowing a slower fading rate. However, the overhead cost of feeding back every sub-carrier for every user is very high. We propose a simplified opportunistic feedback scheme that divides the OFDM symbol into clusters. Each user feeds back a figure-of-merit listing its strongest clusters. This scheme greatly reduces the feedback overhead, without sacrificing performance significantly. In addition, the scheme has an inherent on/off waterfilling property. We compare this simplified feedback scheme in a HIPERLAN/2 scenario to a feeding back of all subcarriers and also to a smart antenna system and show that, when there are many users, it outperforms the smart antenna system.