Application Of Convex Optimization Algorithm In OFDM-IM System

Orthogonal frequency division multiplexing(OFDM)with index modulation(IM)is a new multi-carrier modulation where the sub-carriers have two states,active and idle.The basic idea is that the active subcarriers carry constellation symbol and the indices of idle sub carriy carrier information.OFDM-IM shows some advantages over OFDM in several special cases.For example,OFDM-IM shows stronger robustness to the inter-carrier interference than the conventional OFDM in the rapidly time-varying(RTV)channel.OFDM-IM has better bit error(BER)performance than OFDM in low and medium rate transmission.Owing to these advantages,OFDM-IM has gained wide attention and become a candidate key technology for 5G.In this paper,we first study the peak to average ratio(PAPR)problem of OFDM-IM.Previous studies have shown that OFDM-IM inherits the high PAPR problem from OFDM.We propose an efficient PAPR reduction method through utilizing the characteristics of the OFDM-IM signal.Concretely,a small dither in the idle sub-carrier is introduced to reduce the PAPR,and the design of the optimal dither signal can be solved by convex programming.Simulation results show that the proposed method can achieve a large PAPR reduction when a proper amplitude constraint on the dither signal is employed.The signal detection of OFDM-IM in the rapidly time-varying channel is studied.Due to the inter-carrier interference,the conventional block detectors based on the successive interference cancellation(SIC)strategy,e.g.,the signal power(SP)detector,suffer from severe error propagation especially in the case with large normalized Doppler frequency.To address this problem,a semidefinite relaxation(SDR)approach is first proposed.In the proposed SDR detector,the signal feature of OFDM-IM is presented properly as the convex constraints in the SDR programming problem.The SDR detector can avoid the error propagation effectively with a cost of higher polynomial complexity.To reduce the complexity,the group-based SIC-SDR detector is then proposed,where the subcarriers in one OFDM symbol is partitioned into multiple groups,and the SDR detection is performed over each group successively in conjunction with the ordered SIC strategy.However,similar to the SP detector,the SIC-SDR detector also suffers from the error propagation.To boost the performance,we propose to take the solution of the SIC-SDR detector as the initial estimation of a further local search algorithm(LSA).Concretely,after a proper definition of the nearest neighbors of an OFDM-IM signal vector,the proposed combined detector employing the likelihood ascent-search as the LSA is presented.Finally,the validity of the proposed detectors is justified by simulation results.