Key Techniques For OFDMA/SC-FDMA Systems In High Speed Environment

In the high speed mobile environment, the large Doppler shift, the frequently cellhandover and the requirements of the wireless high data rate transmission make thedesign of the band limited next-generation mobile communications system be thechallenging one. To meet the requirements of wireless high data rate transmission in thehigh speed mobile scenario, the next-generation mobile communication systems, suchas the Long Term Evolution (LTE) system, uses orthogonal frequency divisionmultiplexing multiple address (OFDMA)/single-carrier frequency division multipleaccess (SC-FDMA) technique. However, the large Doppler shift makes the channelchange rapidly, and causes the destruction of the orthogonality between sub-carriers ofthe OFDMA system and the SC-FDMA system, which will seriously deteriorate theperformance of the systems. To solve these problems, we analyze the main factorsaffecting the performance of the systems in the high speed mobile environment, andresearch the Doppler frequency offset estimation technologies and the time-varyingchannel estimationin technologies in-depth for the OFDMA and the SC-FDMA systemsrespectively, and also investigate the co-signal interference suppression technology inthe dissertation. The main research works and innovation points are listed as follows:1. In the research of the high speed mobile transmission channel and the analyzationof the factors affecting the performance of the systems, the available channel models ofthe high speed train scenarios in domestic and international are investigated, and thenthe propositional channel models of the different scenarios for the domestic high speedtrain are given, according to the different effects of the doppler shift on the uplink anddownlink, the general expressions of the channels for the uplink and downlink arepresented. Based on these, the main fastors affecting the performance of theOFDMA/SC-FDMA systems are analyzed in the same channel model and the differentchannel models.2. In the research of the key technologies for the OFDMA system in the high speedmobile environment, the Doppler frequency offset estimation and the time-varyingchannel estimation methods are researched, i.e.,(1) In the Doppler frequency offset estimation, the Doppler frequency offsetestimation methods for the OFDMA system are investigated, according to thecharacteristics of multipath Rician received signal, the neval Doppler frequency offsetestimation method with the high accuracy is proposed. The proposed method greatly reduces the influence of the scattering component in the received signal on the accuracyof the frequency offset estimation, so it has high estimation accuracy, and satisfies therequirements of the frequency offset estimation range in the high speed mobileenvironment. Moreover, the proposed method has a low computational complexity.(2) In the time-varying channel estimation, to improve the accuracy of the pilotsubchannels estimation and the accuracy of the data subchannel estimation, atime-varing channel estimation method for the OFDMA system in the high speedmobile environment is proposed. In the proposed method, the noise reduction method isemployed to improve the accuracy of the pilot subchannel estimation, and the nonlinearinterpolation method is used to obtain the high-precision data subchannel estimation.Moreover, the proposed method has high estimation accuracy and a low computationalcomplexity.3. In the research of the key technologies for the SC-FDMA system in the high speedmobile environment, the Doppler frequency offset estimation and the time-varyingchannel estimation technologies are researched, i.e.,(1) In the Doppler frequency offset estimation, the Doppler frequency offsetestimation methods for the SC-FDMA system are investigated, according to therequirements of the high estimation accuracy and large estimation range in the highspeed mobile environment, a noval Doppler frequency offset estimation method isproposed. In the proposed method, the autoregressive (AR) model is employed betweenthe subframes to reduce the effect of the noise and interference on the accuracy of theDoppler frequency estimation. The proposed method solves the frequency offsetestimation problem for multi-user SC-FDMA system in the high-speed mobile scenario,and also has a low computational complexity.(2) In the time-varying channel estimation, due to the traditional method with thehigh estimation accuracy and the high complexity, a new fast time-varying channelestimation method is proposed for SC-FDMA system in the high speed mobileenvironment. In the new transform domain, polynomial basis expansion model (P-BEM)is employed to estimate the channel parameters of the data symbols, and an AR model isused to improve the estimation accuracy of the channel parameters of the data symbolsout of the two block pilots, and then channel estimation in the frequency domain isobtained according to the channel in the corresponding relation of the transform domainand frequency domain. Due to the proposed method performed in the new transformdomain, it has a low computational complexity. 4. In the co-signal interference suppression for the high speed mobile environment,the interference suppression scheme based on the array antenna and beamforming isproposed. In the proposed method, an array antenna is used to receive the signals fromthe multiple base stations or remote radio units (RRUs), and then the beamformingmethod is employed to separate the received signal with the multiple Doppler frequencyoffsets into the multiple signals, each of which is with a single Doppler frequency offset,and then the separated signals are compensated by the corresponding Doppler frequencyoffsets and equalized by the estimated channel responses respectively, then combinedinto a signal to be demodulated. By such processing, the co-signal interference in theneighboring base station or RRU is suppressed, which greatly improved the quality ofthe communication systems in the high speed mobile environment.