Research On Transmit Diversity In OQAM/FBMC Communication Systems

Compared with orthogonal frequency division multiplexing(OFDM),offset quadrature amplitude modulation based filter bank multicarrier(OQAM/FBMC)has lower spectral sidelobes due to the well-designed prototype filter.Moreover,OQAM/FBMC systems have higher spectral efficiency since they do not require cyclic prefix(CP).The above advantages have made OQAM/FBMC a promising candidate for future wireless communications systems.On the other hand,multiple-input multiple-output(MIMO)has attracted significant attention due to its great potential in boosting the performance of wireless communication systems.Hence,the integration of MIMO with OQAM/FBMC is expected to play a significant role in future wireless communication systems.This thesis mainly focuses on the transmit diversity techniques in MIMO-OQAM/FBMC systems,including Alamouti transmit diversity and delay diversity schemes.The main research contents are as follows:1.A block-wise Alamouti transmit diversity scheme is proposed for the OQAM/FBMC system.In OQAM/FBMC systems,the imaginary interference is random and influenced by the surrounding data.Through the analysis of the imaginary interference coefficients,we prove that the imaginary interference coefficients have special symmetry property.Based on this discovery,a block-wise Alamouti diversity scheme is proposed,which makes use of the symmetry property to reasonably group and encode the transmitted data.It is proved that the combined complex data after adding the imaginary part interference satisfy the orthogonal condition.Hence,the transmitted data can be successfully decoded at the receiver and the diversity gain is achieved.Moreover,the guard interval in the block wise scheme is further removed,and a simple equalization scheme is proposed to equalize the introduced interference.2.Alamouti coded OQAM/FBMC systems suffer from severe performance degradation in highly frequency selective channels.To address this problem,a novel receiver for the Alamouti coded OQAM/FBMC system based on frequency spreading is proposed.The introduction of CP simplifies the equalization of OFDM systems in highly frequency selective channels.However,OQAM/FBMC does not employ CP,which could only rely on the prototype filter to combat multipath fading.In the case of a highly frequency selective channel,the simple equalization processing is not enough.To improve the performance of OQAM/FBMC system in highly frequency selective channels,a novel receiver based on frequency spreading is proposed for the Alamouti scheme,where the Alamouti decoding module is moved to the front of the frequency despreading module.Unlike the traditional subcarrier-based Alamouti decoding scheme,the Alamouti decoding in the proposed receiver could be performed on each frequency point to better combat the frequency selectivity.Moreover,the full diversity is achieved on each frequency point by introducing a phase factor before Alamouti decoding.Simulation results show that the traditional Alamouti receiver has poor error performance under high order modulation in highly frequency selective channels,which can hardly work.The proposed scheme still has 4dB performance improvement compared with the traditional scheme under low SNR.3.The delay diversity scheme in the OQAM/FBMC system is studied.So far,research on delay diversity in the OQAM/FBMC system is still very scarce.To address this problem,two cyclic delay diversity schemes based on frequency spreading OQAM/FBMC are proposed.The equalization of the proposed schemes are performed on frequency frequency points before frequency despreading.However,the first scheme directly carries out cyclic delay processing for each symbol in the transmitter,which introduces interference at a large cyclic delay.To solve this problem,the second scheme is proposed.In the second scheme,the transmitted data of OQAM/FBMC systems are block-based by replacing linear convolution with circular convolution,and CP is inserted prior to each block.Then,the cyclic delay scheme is carried out on each data block,and the length of the cyclic delay is no longer limited.Since CP is inserted to each data block,the introduced overhead is smaller than that of the OFDM system.Simulation results show that the first scheme based on linear convolution is suitable for the flat fading channel.The second scheme based on cyclic convolution can still obtain diversity gain in frequency selective channels.In summary,the Alamouti transmit diversity scheme and delay diversity scheme in OQAM/FBMC systems are investigated.The proposed schemes can obtain high diversity gain in both flat fading and frequency selective fading channels,which guarantees the communication reliability.