Key Technologies Of Cyclic Prefix Free OFDM Wireless Transmission

In a multipath channel,orthogonal frequency division multiplexing(OFDM)technique uses a cyclic prefix(CP)to resist inter symbol interference(ISI)and inter carrier interference(ICI).As long as the length of the CP is longer than or equal to delay spread,the linearly shifted OFDM symbols caused by multipath propagation can be converted to cyclically shifted OFDM symbols,and the data can be recovered by means of discrete Fourier transform.However,CP insertion will lead to a waste of the scarce spectrum resources.In order to increase the spectral efficiency,we suggest that a CP-free OFDM transmission will be a trend for future OFDM technique.In the CP-free OFDM system,multipath channel can bring severe ISI and ICI to an OFDM symbol.Owing to the difference between ISI and ICI,we use two algorithms to deal with them separately.To eliminate the ISI,we use the decision feedback equalization(DFE)to reconstruct the interference,and then input it into the detection interval to cancel the interference,leaving a “gap” in the detection interval.The “gap” is generated due to the linear shifts of OFDM symbols,and it will destroy the orthogonality between subcarriers,resulting in ICI.To solve this problem,we propose a cyclic prefix restoration(CPR)algorithm.CPR algorithm fills the “gap” with the estimated CP,and then the linearly shifted OFDM symbols will be converted to cyclically shifted OFDM symbols.In this thesis,the DFE combined with CPR algorithm is named as symbol cyclic shift equalization(SCSE)algorithm.To reduce the computational complexity of the CPR algorithm,the conjugate symmetric OFDM symbol is constructed using the time-frequency characteristic of pulse amplitude modulation,and thus the receiver can estimate the CP signal only by employment of a few samples instead of all samples.Therefore,the computational complexity of CPR algorithm can be reduced significantly.The simulation results show the spectral efficiency of SCSE-OFDM is higher than that of CP-OFDM.However,since the CPR algorithm brings estimation errors and DFE may cause the feedback errors,the bit error rate performance of SCSE-OFDM is worse than that of CP-OFDM.DFE requires the feedback signal be fed back to the detection interval very timely and any long feedback delay is not allowed.However,the feedback loop of the DFE used in SCSE is long and may lead to feedback delays.For the high transmission rate,if the hardware processing capability is not powerful enough,the feedback delay will lead to a detection outage,which will affect the robustness of SCSE algorithm.To address this issue,we present a CPR based successive multipath interference cancellation(CPRSMIC)algorithm.CPR-SMIC utilizes stored feedback equalization to eliminate ISI.The input of feedback loop needs not pass through the decision device,and compared with DFE,its loop length is much shorter,thus it can ensure the feedback signal be fed to the detection interval very timely.However,CPR will cause the estimation error due to noise,especially in the case that the first path to arrive is weaker than the subsequent paths,the estimation error will become significant.In order to avoid the estimation error caused by CPR,we construct a precoding matrix to weight the transmitted signal,and pre-eliminate the interference.In this thesis,the time domain precoding(TDP)algorithm is named as TDP-SMIC algorithm.The simulation results show that the capacity of CPR/TDP-SMIC OFDM is higher than that of CP-OFDM since there is no spectra wasted by CP.We demonstrate to eliminate the CP in an OFDM system can improve the performance of an OFDM system in terms of several aspects and even can extend its applications.In a long delay scenario,CP will introduce the unbearable loss on the spectral resource.Based on SCSE-OFDM and SMIC-OFDM,we investigate a CP-free OFDM wireless transmission method with long delay.This thesis gives the definition of the long delay scenario,that is,a special scenario whose delay spread is longer than the OFDM symbol,and model the channel under the long delay scenario.In the channel modeling,the channel matrix is divided into multiple channel sub-matrices,so that the dimension of each channel sub-matrix matches an OFDM symbol length.In the transmitter,the transmitted signal is precoded by a precoding matrix which is constructed by the channel sub-matrix;and at the receiver,the interferences between the multiple OFDM symbols are eliminated by a multi-block DFE.Considering feedback error and feedback delay caused by multi-block DFE,we extend the TDP-SMIC algorithm,and propose a TDP algorithm to construct the CP-free OFDM transmission with long delay.In TDP-OFDM,the interference is pre-eliminated at the transmitter,and after multipath propagation,the received signal will be protected from ISI and ICI.By the performance evaluation,we can know that the throughputs of the proposed CP-free OFDM transmission methods are significantly higher than that of CP-OFDM.