Blind Equalization Of Multi-Carrier Systems In Non-Cooperative Communications

Wireless signal transmission is affected by the complex channel environment.As a result,the received signal is interfered by ISI.It is necessary to recover the distorted signal by equalization technology after accurate synchronization.Only in this way can reliable transmission of wireless communication systems be guaranteed.As the third party,the non-cooperative communication cannot obtain the modulation and demodulation parameters in cooperative communication.In non-cooperative communication,estimated parameters are used for blind synchronization,which will cause synchronization offset.Therefore,the equalization in non-cooperative communication must have high synchronization offset robustness.Blind equalization doesn't require data assistance,and can rely on the statistical characteristics of the channel input and output to recover the distorted signal.Thus blind equalization is widely used in the communication field.In non-cooperative communication,blind equalization shows an irreplaceable status without the help of auxiliary information.In order to achieve blind equalization of OFDM signals with synchronization offset,the following work has been done in this paper:1.By analyzing the effects of SCO,STO and CFO on OFDM system,an input signal model of blind equalizer for non-cooperative communication system is proposed.This model equates the permissible offsets of SCO and STO as part of the channel response,preserving the effect of fractional CFO.In order to study the ICI caused by fractional CFO,a new concept,S_c(ICI average interference ratio),is defined to measure the impact of ICI in this paper.Through the simulation analysis of S_i-k(ICI coefficient)and S_c,four conclusions about ICI are proposed.And a phase rotation model of OFDM symbols under the influence of fractional CFO is established based on these four conclusions.2.Aiming at the equalization of OFDM signals with fractional CFO,Twice Equalization has proposed.Twice Equalizer consists of two blind equalizer groups.The CMA channel equalizers are used to restore the amplitude-frequency response and the amplitude attenuation caused by the decimal CFO.The MMA symbol equalizers are used to recover the phase-frequency response and the phase rotation between OFDM symbols caused by decimal CFO.Simulation results prove that Twice Equalization can meet the blind equalization needs of OFDM signals with small decimal CFO.3.CFO Feedback Equalization is proposed in thie paper.Using the new signal input method,the output signal of the CFO Feedback Equalizer has the value of constellation rotation estimation.By estimating the phase rotation of adjacent OFDM symbols,we can get the estimated fractional CFO.Feedback of the estimated value to the frequency offset corrector can effectively reduce the fractional CFO of the OFDM signal.However,CFO Feedback Equalization will cause the loss of effective information,which is of no practical significance when used alone.4.In order to solve the equalization problem of OFDM signals with large fractional CFO.We combine Twice Equalization and CFO Feedback Equalization together,and propose the Combined Blind Equalization.According to the traditional blind equalization optimization idea,the MMA+MMMA dual-mode blind equalizer is used to replace the MMA symbol equalizer in Twice Equalization,thereby an improved Combined Blind Equalization is proposed.The simulation results show that the improved Combined Blind Equalization has superior performance and shows a reliable equalization effect in a variety of channel environments in non-cooperative communication.