This paper studies a multi-carrier based spectrum sharing system, in which two users iteratively update their respective transmit power allocation over parallel subcarriers (SCs) to maximize the individual transmit rate. Unlike the conventional iterative water-filling (IWF) algorithm that adopts the single-user detection (SD) at each user's receiver by treating the interference from all other users as additive noise, this paper proposes a new decentralized resource allocation scheme, namely iterative spectrum shaping (ISS), in which each receiver applies multiuser detection to opportunistically cancel the co-channel interference over selected SCs, thus termed opportunistic multiuser detection (OMD). Two coding schemes are investigated to exploit different forms of "frequency diversity" gains with OMD: carrier independent coding, which applies independent codebooks for different SCs to maximize the interference-decoding diversity; and carrier joint coding, which uses one single codebook across all the SCs to achieve the full coded diversity. For each coding scheme, the optimal transmit power allocation strategy is derived to maximize one user's transmit rate, with the power and rate allocation of the other user being fixed. Simulation results show that the converged system throughput by the proposed ISS algorithm with OMD is significantly improved over that by the conventional IWF with SD, in cognitive radio (CR) based wireless spectrum sharing networks.
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