Exploiting Multiuser Diversity in OFDMA Systems with Limited Feedback

In this paper, we extend the results of [12] by proposing a joint polling and contention scheduling scheme that reduces the feedback load in orthogonal frequency division multiple access (OFDMA) systems employing adaptive modulation. This feedback reduction is achieved by grouping users into categories, and requesting feedback from users that belong to a certain category only. The feedback process comprises of two modes. The first mode is a polling only mode and the second mode consists of a contention stage followed by a polling stage. The contention stage is used to find the i-th category where the best user is likely to be found. All users belonging to that category are polled sequentially until an acceptable user with channel quality above a predefined threshold is found. Numerical results show that the proposed scheme reduces the feedback load and the guard-time without any loss in the spectral efficiency when compared to the optimal selective diversity with full feedback algorithm shown in [11]. We, further, derive closed-form expressions for the feedback load, average guard time and the spectral efficiency and show that the proposed scheme improves the system spectral efficiency, the feedback load and the average guard time when compared to the schemes proposed in [12].