SER optimization in transparent OFDM relay systems in the presence of dual nonlinearity

Transparent relay is a frequently used solution to enhance signal coverage with low implementation cost. However, the nonlinear distortion caused by cascaded amplification in the relay systems degenerates the system performance, increases the analysis complexity, and brings challenges to the system-level optimization. This paper analytically studies the system-level optimization of symbol error rate (SER) performance in nonlinear OFDM relay systems, where both power amplifiers (PAs) in transmitter and transponder show clear nonlinearity. Firstly, the system SER is expressed as a function of receiver signal-to-noise ratio (SNR), and the latter is further simplified to a polynomial-based function of PA parameters and system operating points. Secondly, the asymptotic behavior and Hessian Matrix (HM) of the receiver SNR are analyzed, and it is found that the SNR has an upper bound when dual nonlinearity of the transmitter PA and transponder PA are considered. Thirdly, this paper derives the analytical expressions of the optimal PA operating points corresponding to the optimal SER performance for the first time. Finally, the expression of the SNR upper bound is presented, and the derived functions are found to have the merit of low complexity, which suits the application in transparent relay systems with limited computational resources.