The Design Of OFDM Signals With High Energy And Spectral Efficiencies

Orthogonal frequency division multiplexing(OFDM)has been widely deployed in the Fourth Generation(4G)mobile communications due to its high data rate,immunity to frequency selective channels,high spectral efficiency,flexibility of spectrum resource allocation,and simple combination with multiple-input multiple-output(MIMO).Moreover,OFDM will be one of the key techniques of the physical layer for the Fifth Generation(5G)mobile communications.However,OFDM signals suffer from large envelope fluctuation and serious spectrum leakage,which are the main drawbacks of OFDM systems.The large envelope fluctuation of OFDM signals lead to the degradation of the energy efficiency(EE),and the spectrum leakage results in the degradation of the spectral efficiency(SE).To improve the EE and SE of OFDM systems,we need to design OFDM signals with low envelope fluctuation and low spectrum leakage.This thesis focuses on the design of OFDM signals to improve the EE and SE of OFDM systems.The main contents are listed as follows:Firstly,this thesis proposes a signal cancellation(SC)method to improve the EE and SE of OFDM systems.For the SC method,several SC symbols are added on the OFDM subcarriers to generate the cancelation signals to mitigate the large envelope fluctuation and serious spectrum leakage of OFDM signals,which results in the improvement of the EE and SE.The SC method formulates a convex optimization problem,which minimizes the envelope fluctuation with constrained spectrum leakage.The SC method solves the optimization problem via the interior-point method,and the OFDM signal with low envelope fluctuation and low spectrum leakage can be obtained.Since the SC method has a little bit high complexity,we also proposed a low-complexity signal cancelation(SC-LC)method.The SC-LC method divides the OFDM subcarriers into two different categories,one is only for reducing the envelope fluctuation and the other one is only for reducing the spectrum leakage.The SC-LC method develops an iterative algorithm to reduce the envelope fluctuation and spectrum leakage of OFDM signals.Simulation results verify the effectiveness of the proposed SC method and SC-LC method.Secondly,this thesis proposes an active constellation modification(ACM)method to satisfy the constraints of the envelope fluctuation and spectrum leakage for OFDM systems.The key idea of the ACM method is that it modifies the constellation points of OFDM symbols to generate novel OFDM signals with low envelope fluctuation and spectrum leakage.In this way,the EE and SE of OFDM systems can be improved.However,the ACM method would introduce some nonlinear distortions into the OFDM systems,because it changes the constellations of OFDM symbols.To maintain the quality of services for wireless communications,the nonlinear distortion caused by the ACM method should be as small as possible.Thus,the ACM method formulates an optimization problem,which minimizes the nonlinear distortions with constrained envelope fluctuation and spectrum leakage.Then,the concave-convex procedure(CCCP)algorithm is applied to solve the optimization problem,and OFDM signals with low envelope fluctuation and spectrum leakage can be obtained.Simulation results verify that the ACM method achieves high EE and SE.Finally,this thesis considers the multi-antenna scenario,and studies the design of MIMO-OFDM signals with high EE and SE.As well known,MIMO-OFDM systems provide higher SE than single-antenna OFDM systems.However,MIMO-OFDM systems also suffer from the EE degradation due to the high envelope fluctuation.The partial transmit sequence(PTS)method can improve the EE of MIMO-OFDM systems by reducing the envelope fluctuation.However,the PTS method requires the transmission of side information,resulting in the SE degradation.In order to improve the EE of MIMO-OFDM systems without affecting the SE performance,we propose a phase off-set(POS)method.The POS method avoids sending side information by embedding the side information in the phase off-set information,thereby ensuring the high SE of the MIMO-OFDM systems.The POS method applies the maximum likelihood(ML)detector to obtain the phase off-set,and then,the side information can be obtained.Furthermore,two low-complexity ML-based detection algorithms have been proposed to reduce the complexity of the receiver.Simulation results show that the POS method can not only offer high EE and SE,but also achieve good bit error rate performance without sending side information.