Abstract: An improved cooperative spectrum sharing scheme is proposed for a cognitive radio system，where the primary transmission is assisted by the secondary user.Decode-reencode-forward protocol and Dirty Paper Coding（DPC） are employed at the secondary transmitter to eliminate the interference from the primary user.In order to protect the outage performance of the primary user，the secondary user allocates a fraction of power to help the primary signal transmission.A critical distance radius is derived in which the secondary user can share the spectrum with the primary user.Simulation results show that the spectrum sharing region for the secondary user is enlarged in the proposed scheme.Furthermore，the outage performance of the primary user and secondary user are both improved.

Key words: cognitive radio, cooperative spectrum sharing, dirty paper coding, outage probability

摘要： 针对认知无线电系统，提出一种改进的协作频谱共享方案，其中，次用户充当中继，协助主用户传输。改进方案采用解码-再生编码-转发中继协议，通过污纸编码有效地在发射端消除来自主用户信号的干扰，并且分配一部分功率协助传输主用户信号，保证主用户的中断性能。通过理论推导，得出次用户离主用户发射机的距离半径表达式。仿真结果表明，改进方案能有效扩大次用户的频谱共享范围，还提高了主用户和次用户的中断性能。

关键词: 认知无线电, 协作频谱共享, 污纸编码, 中断概率