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We propose an optimal asymmetric resource allocation scheme with limited feedback for wireless relay systems based on orthogonal frequency division multiplexing (OFDM) modulation. Unlike the previous work, we assume that the bits transmitted over the same subchannel from the source to the relay may be distributed over different subchannels when being forwarded from the relay to the destination. This not only increases the degree of freedom for transmission, but also reduces the transmission overhead significantly because the amount of feedback information required is independent of the number of subchannels. Furthermore, the transmission durations at the source and the relay are designed to be asymmetric, which also enhances the degree of freedom for transmission. Simulation results show that the proposed optimal asymmetric resource allocation scheme achieves a higher system capacity, and has less sensitivity to channel estimation errors than the previous work. It also demonstrates a fast convergence speed.