From cell capacity to subcarrier allocation in OFDMA

In this paper, we show how a fundamental result from information theory can be used to calculate the aggregate channel capacity in the uplink of an access point. The fundamental limits for this cell capacity provide an upper bound for any practically implementable multiple-access scheme. Based on the famous iterative water-filling algorithm, we derive an adaptive method for maximising the sum rate in the uplink of a multi-user OFDM system. This approach results in a surprisingly simple algorithm for the subcarrier allocation in OFDMA. We also evaluate the possible gains that can be obtained by sharing a subcarrier by multiple users and compare its performance to the theoretical limit as well as to a low-complexity solution. Finally, we show how these methods can be implemented using coded QAM with iterative decoding and we outline how they can be combined with a scheduling strategy.

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