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In a downlink Orthogonal Frequency Division Multiple Access (OFDMA) system, optimally allocating sub-channels across mobile users can require excessive feedback of Channel State Information (CSI). The authors consider an OFDMA model in which the feedback overhead is explicitly taken into account, given a fixed feedback rate and finite coherence time. The tradeoff between feedback rate and sum capacity is studied for two limited feedback schemes: a sequential scheme in which the users send compressed feedback bits over consecutive time slots, and a contention scheme in which users send their feedback via a random access protocol.
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