User Identification for Opportunistic OFDM-Based Multiuser Wireless Communications

Multiuser diversity can be employed in wireless communications to significantly improve system performance by scheduling the channel to the user with the best instantaneous channel-state information (CSI). Conventionally, all users' CSI must be available at the base station (BS) to achieve the benefit of multiuser diversity, thereby inducing enormous system overhead. To solve the overhead issue, we propose a user identification approach (UIDA) in a generic wireless network and demonstrate its application in the orthogonal frequency-division multiplexing (OFDM) system and the multiple-input multiple-output OFDM (MIMO-OFDM) system. In the UIDA, to find the user with the best CSI, all active users simultaneously send response messages to the BS following a broadcast probing message from the BS, and only the selected user's CSI needs to be available at the BS, thus reducing the system overhead. We present a theoretical upper bound of the throughput of the system with the UIDA over general channels, and its tightness over Rayleigh fading channels is verified by simulations. In addition, a modified UIDA is proposed to effectively trade off throughput and fairness performances, where only the active users whose signal-to-noise ratios (SNRs) are above their predetermined thresholds will send response messages. Compared with medium access diversity and multiuser diversity with capture (two alternative approaches in the literature for exploiting multiuser diversity) and with a similar system setup, simulation results show that the UIDA achieves better throughput performance when there are a reasonably large number of users.