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A random-access-based feedback protocol with a reservation-based (RB) channel is proposed for multiuser diversity in a wireless time-division-duplex system. The feedback channels consist of one RB channel and several contention-based (CB) channels. The users randomly access the CB channels if their channel states exceed a threshold, and a base station (BS) schedules a user who reports the highest signal-to-noise ratio from multiple users that successfully provide feedback. If there is no successful feedback in the CB channels, the BS uses the feedback information that a designated user reports through the RB channel. The proposed feedback protocol achieves an almost ideal sum-rate capacity with a fixed number of feedback channels regardless of the number of users.