OFDM调制的物理层网络编码中的载波频偏问题研究

Physical-layer network coding (PNC) is a multi-user transmission technique that can boost performance of relay networks significantly. Its combination with OFDM modulation effectively increases its robustness against multipath channels. However, the signal frequency offset caused by the local oscillator instability and/or the Doppler shift has a big impact on PNC. This project adopts OFDM subcarrier suppression and power control to mitigate the impact of the frequency offset. Due to the extreme difficulty in deriving the theoretical BER at the relay in PNC, we will first establish a BER expression from simulation results based on an advanced signal detection scheme at the relay; this BER expression serves as the foundation of the following studies. First, we will set up a linear program to study the power control problem at full transmission rate of the user. Second, we will formulate the subcarrier-suppression merged power control as a nonlinear program. With these two programs and considering multiple users involved in PNC system, this project is tasked to design low-complexity and operable power allocation algorithms as well as subcarrier-suppression merged power allocation algorithms, and then analyze and evaluate their performances. With the research in this project, an effective mitigation of the frequency offset impact on OFDM based PNC will be achieved, thus ensuring a notable network throughput improvement.

物理层网络编码是一种能显著提升中继网络性能的多用户传输技术，与OFDM的结合能有效减轻多径信道对它的影响。然而，由晶振不稳定性和多普勒频移导致的信号载波频偏会严重影响物理层网络编码的性能。本项目拟采用OFDM子载波抑制与功率控制来减小载波频偏的影响。考虑到在载波频偏下获得中继处理论误码率极其困难，拟针对中继处一种先进的信号检测方法，首先从仿真数据中建立相应的误码率数学表达式，在此基础上，开展以下研究内容。第一，拟通过建立线性优化模型来研究用户在全速率发送下的子载波功率控制问题；第二，拟通过建立非线性优化模型来研究融合子载波抑制的功率控制问题。第三，基于以上优化模型，考虑到物理层网络编码具有多用户的特点，拟设计具有低复杂度、易实现的功率分配算法、融合子载波抑制的功率分配算法，并分析及评估它们的性能。开展此项目的研究，对有效减小载波频偏对物理层网络编码的影响并显著提高网络吞吐量具有重要的意义。

本项目针对使用OFDM和物理层网络编码（PNC）技术的双向中继信道（TWRC）提出了一套功率分配算法，能有效减轻载波频偏（CFO）对上行链路信号检测/解调的影响，不仅降低了误码率，而且提升了网络吞吐量。首先，本项目理论分析和仿真验证了中继节点处一种最优的CFO补偿机制；在该机制基础上，建立起BPSK调制下中继节点处的误码率近似表达式；基于该表达式，提出了TWRC中全速率数据发送时的功率分配算法，能有效降低端到端的误码率。其次，在以上功率分配的基础上，提出了一种参数化的导频信号插入或子载波抑制方法来进一步降低误码率，通过仿真验证了：对于BPSK和QPSK调制，在不同信噪比和上行链路CFO值的条件下，可通过设置合理的参数值来提升网络吞吐量；特别地，对于由子载波抑制而节省的功率，提出了一种有效的、参数化的功率二次分配方法。最后，本项目考虑信道编码来对抗CFO影响，针对BPSK和QPSK调制，分别提出了中继节点处多种信号解调及信道解码算法。本项目所提出的CFO补偿机制、功率分配算法、子载波抑制方法和信道解码算法是一个整体，能确保TWRC有效对抗CFO影响，进而为其成为其他网络的组建单元奠定了基础。