Taking Advantage of Multi-User Diversity in OFDM Systems

In a frequency-selective multi-antenna wireless channel, we aim at improving the utilization of the channel by applying multi-user diversity techniques. In the case of frequency-selective channels, Orthogonal Frequency Division Multiplexing (OFDM) is used to subdivide the channel into overlapping, orthogonal subchannels. Due to the random channel fading that degrades the signal-to-noise ratio (SNR) at the receiver, different techniques have been developed to combat fading and improve the performance of OFDM systems. We consider a multi-user diversity approach, called Opportunistic Interference Management (OIM) that takes advantage of fading to improve performance. OIM is applied to each consecutive group of OFDM subchannels that are highly correlated. We use OIM to transmit information in Q_g consecutive subchannels to d users in parallel. In order to understand the performance of this technique, a mathematical model is constructed. The expected parallel transmissions per group of subchannels is then derived mathematically. Results obtained via the mathematical model and by Matlab simulation of the channel, show that the maximum expected parallel transmissions is achieved almost surely when enough mobile users exist. We then demonstrate a method to achieve a specified number of parallel transmissions while requiring less users by adding some feedback. With this work, we then hope to be able to show the practicality of the technique in improving the throughput capacity of wireless networks.