Opportunistic Feedback Protocol for Achieving Sum-Capacity of the MIMO Broadcast Channel

This paper presents an opportunistic feedback (OF) protocol that achieves the asymptotic sum-capacity of the fading broadcast channel (BC) with a limited amount of feedback. The OF protocol associates a channel value instead of a user to each feedback slot. The users access the slots opportunistically based on their channel state measurement. This allows the base-station to deduce the channel state from the slot index without explicit channel-state feedback. This protocol is then extended to the MIMO BC by using zero-forcing beamforming and random beamforming. The OF protocol is analytically shown to (a) require finite number of feedback slots upper bounded by a small number, (b) be fully distributed, (c) need minimal transmission energy during feedback and (d) asymptotically achieve the sum-capacity of the MIMO BC. Comparison with other opportunistic feedback schemes shows that the proposed scheme has substantially larger sum-rate with a lower feedback overhead.