Peak-to-Average Power Ratio Reduction In OFDM Systems

Orthogonal Frequency Division Multiplexing(OFDM) has drawn explosive attentions in a number of current and future standard systems, due to the advantages of high spectral efficiency, robustness to frequency selective fading and low complexity. However, OFDM suffers from a high peak-to-average power ratio(PAPR). When the peak amplitude of the OFDM signals exceeds the linear region of high power amplifiers(HPAs), undesirable nonlinear distortions will occur, which leads to performance degradation. To reduce the nonlinearity impact on OFDM signals, it is desirable to accommodate the entire signal amplitude in the linear region of HPAs. However, it results in lower power efficiency of HPAs. On the other hand, as energy consumption has been becoming a key issue in mobile communication system, HPA, being a major contributor for the energy consumption, should be operated with power efficiency as high as possible. Therefore, the contradicted demands of the system performance and the power efficiency of HPAs become prominent. This dissertation focuses mainly on the PAPR reduction of OFDM signals, which is organized as follows:1. To alleviate the performance degradation caused by the companding distortion, the effects of companding distortion on bit error rate(BER) is investigated. With a theoretical analysis of the expressions for companding distortions, the general design criteria for companding transform are proposed. Based on the design criteria, two companding schemes, named as the piecewise linear companding scheme and the piecewise exponential companding scheme, respectively, are proposed. By carefully designing the companding transform parameters, the two schemes both can improve the BER performance, while reducing PAPR effectively. In addition, with the introduction of an inflexion point, the two schemes can offer more design freedom and flexibility to satisfy various design requirements.2. To reduce the overwhelmingly high complexity of the ideal tone injection(TI) scheme, a sequential search based suboptimal TI scheme with low complexity is introduced. By determining the subcarrier perturbation sequence based on the information carried by the distortion caused by HPA, the proposed scheme reduces the original two-dimensional joint search problem on the subcarrier set and theextended constellation to a one-dimensional sequential search problem on the extended constellation. Therefore, the proposed scheme saves complexity greatly. Moreover, with the analysis of the effect of each equivalent constellation point on PAPR reduction, the proposed scheme narrows the size of the equivalent constellation point set, which further reduces the complexity. Simulation results demonstrate that the proposed scheme achieves significant PAPR reduction while maintaining low complexity.3. To enhance the power efficiency of HPAs with the demand of reducing energy consumption, tone reservation(TR) technique is adopted to make OFDM signals less sensitive to nonlinearity of HPAs. From the point of signal-to-noise-ratio(SNR) loss at receiver, theoretical analysis demonstrates that nonlinearity impact on OFDM signals can be effectively evaluated by normalized correlation coefficient(NCC) of amplifier input and output signals. Then, with the introduction of NCC, the TR scheme is formulated as an optimization problem. By adopting an adaptive scaling based gradient-project approach, the proposed scheme can achieve improved BER and power spectral density(PSD) performance.