Research And Simulation Of Multiple Access Techniques Based On Constant Envelope OFDM

Orthogonal Frequency Division Multiplexing(OFDM)is widely used in wireless communication networks,which provides a relatively straightforward way to accommodate high data rate links over harsh wireless channels characterized by severe multipath fading.One of the drawbacks of OFDM system,however,is that the modulated waveform has high amplitude fluctuations that produce large Peak to Average Power Ratio(PAPR).The high PAPR reduces the power efficiency of high power amplifier(HPA).Constant Envelope OFDM(CE-OFDM)transforms the high PAPR OFDM signal to a constant envelope signal by way of phase modulation.Such a signal can be amplified with minimal power backoff,which maximizes HPA efficiency.In this thesis,based on the existing research on single-user CE-OFDM,we study the multiplexing technique for Constant Envelop Orthogonal Frequency Division Multiple Access(CE-OFDMA)system,as well as the signal detection and pilot transmission schemes for uplink and downlink transmission respectively.The details are as follows.We first present a downlink CE-OFDM system model,and evaluate its performance based on different parameter settings.Based on the performance evaluation,a near maximum likelihood detection method is proposed,which serially utilizes a simplified maximum likelihood detection based on a modified candidate constellation space.Simulation results approves that the proposed low-complexity CE-OFDMA downlink receiver can achieve a compromise between complexity and performance,compared to the conventional maximum likelihood(ML)receiver.Furthermore,we present an uplink CE-OFDM system model,and study the performance of different receivers.To reduce the inter-user interference caused by phase modulation in the uplink CE-OFDM system,a new multiple access method is proposed.By the user carrier offset,the method alleviates data aliasing among users and effectively improves the system detection performance.In addition,for the phase wrapping problem caused by phase demodulation,an improved uplink CE-OFDM transceiver design combining clipping and noise recovery is proposed which further improve detection performance of the uplink CE-OFDM.Finally,we study the pilot transmission technology in multi-user CE-OFDM system.For the block-type pilot,we evaluate the channel estimation performance and the PAPR characteristic based on different pilot sequences settings in multi-user CE-OFDM system.For the comb-type pilot,to reduce the interference between data and pilot,a receiver design for interference cancellation is proposed,which improves the channel estimation performance of multi-user CE-OFDM system.