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In a frequency-division-duplex (FDD) system, uplink and downlink are allocated equal bandwidths. Due to the relative sparseness of uplink Internet traffic, the uplink spectrum is usually under-utilized. In this paper, we propose dynamic spectrum access algorithms allowing relay-assisted downlink to opportunistically transmit over the uplink spectrum, thereby maximizing its usage efficiency. Specifically, uplink sub-carrier allocation and scheduling based on spectrum sensing are jointly designed for minimizing received interference and thereby enhancing throughput under the constraint of interference. The proposed algorithms support both centralized and distributed implementation. Simulation results demonstrate that the proposed cognitive algorithms contribute significant capacity gains compared with existing designs.