Research On Space-Time-Frequency Coding For MIMO-OFDM System

Multiple input multiple output (MIMO) is well known as a great breakthrough in smartantenna techniques of wireless mobile communication areas. It can exponentially improve thesystem capacity and spectral efficiency by transmiting the different information with differenttransmits with the same spectral band. The Orthogonal Frequency Division Multiplexing (OFDM)has the ability to overcome the multi-path interference and frequency selective fading. TheMIMO-OFDM system which combined MIMO and OFDM can future improve the transfer rates,enhanced communication reliability, and provide better performance. It is a most promisingcommunication system for the future wireless communication.In order to make full use of the diversity gain benefits in MIMO-OFDM system, theMIMO-OFDM system combining with space-time coding not only make full use of the MIMOsystem、OFDM system advantage respectively and but also get the diversity gain. whiletheoretically it is proved that the system can obtain the maximum diversity gain for the product oftransmit antennas, receive antenna number and multipath number, namely the full diversity gain.However, the existing MIMO-OFDM system can not ensure the full diversity gain under thespace-time coding ( STC ), therefore, to ensure the full diversity gain in the MIMO-OFDMsystem, space-time-frequency coding ( STFC ) in MIMO-OFDM system is put forward. In thispaper, space time frequency coding technology is studied detailly in the MIMO-OFDM system.First of all, the paper describes the research background and the significance, introduced thegeneral situation of the development of wireless communication, MIMO-OFDM system codingtechnology research and the development direction in the future.Secondly, the wireless communication channel characteristics are introduced and analyzed,introduced MIMO system, OFDM system, STBC principle, focuses on the analysis ofMIMO-OFDM-STBC system, MIMO-OFDM-STBC system performance, and the performanceof BER performance is analysed and compared.Then, focus on MIMO-OFDM system space-time-frequency coding ( STFC ): fromspace-time-frequency coding pairwise error probability, studied space-time-frequency coding( STFC ) design guidelines. In order to reduce the complexity of the space time frequency code( STFC ) and maintain the diversity gain and coding gain, space-time-frequency block coding( STFBC ) is studied based on subcarrier grouping, according to the design criteria, the systemcan obtain the maximum diversity gain and the maximum coding gain.when transmit antennanumber is larger than 2, In order to obtain a full rate in the space time frequency block code ( STFBC ) and maintain the full diversity gain, a kind of quasi orthogonal space time frequencyblock coding ( QOSTFBC ) method is proposed, the method can not only obtain the maximumdiversity gain and but also obtain full rate. The simulation resulus show that the performance ofthe proposed mothod is less than STFBC,but the mothod can obtain the full rate ,the efficiency offrequency spectrum can be increased. In order to further improve the system performance ofspace time frequency block coding ( STFBC ), combined with serial concatenated coding theory,this paper presents a serial concatenated space time frequency block coding ( SCSTFBC ) method.on the basis of maintaining the full diversity gain of STFBC,the coding gain of SCSTFBC isgreatly improved than STFBC.The simulation results show that, in the higher SNR, the proposedmethod is better than STFBC, and the performance of bit error rate and coding gain can beeffectively improved.Finally, this paper puts forward a new space-time-frequency coding method on basis ofspreading code, this method does not use the existing space-time codes to construct aspace-time-frequency coding and apply the concept of spreading code in the OFDM-CDMAsystem, the data symbols are spreaded to the time domain, spatial domain, frequency domain, andthen by setting appropriate parameters, the system can obtain full spatial diversity and fullfrequency diversity in frequency selective fading channel, because the receiver use MMSEdetection, the decoding complexity is not high. The simulation results show that, with the increaseof the number of ransmit antennas and receive antenna number, the proposed method is improvedcontinually ,and the BER performance of the proposed method is 4 dB than STFBC.