The Research On HARQ Techniques In IMT-Advanced System

Hybrid automatic repeat request(HARQ) technique,which is a combination of forward error control and automatic repeat request,is the main support technique for the high speed data transmission in 3GPP standard.It is indispensable for improving of data correctness,throughput,spectral efficiency,and quality of service.HARQ technology is one of the key technologies in IEEE 802.16 protocols.It is specified in details in IEEE802.16e.IMT-Advanced follow the instructions in IEEE802.16e,and add up new elements in it.Basing on the compatibility to IMT-Advanced,this thesis constructs a HARQ system.When the channel condition is bad,each packet is transmitted repeatedly, which lead to low reliability of the channel transmission.But the automatic repeat request could guarantee the correction of the packet,so it has high reliability.An improvement method of system throughput and packet error rate is put forward.Two new types HARQ scheme based on space-time codes are proposed and evaluated in this paper,and the main innovations and contributions are as follows:Firstly,the theorem of HARQ system is analyzed,and the research depth and diredction is expounded.Base on the regulations that are described in IMT-Advanced standard,three different types of HARQ system and their corresponding algorithms are compared.And different time-space codes in multiple-input multiple-output (MIMO) system are analyzed.Secondly,the HARQ system is analyzed in theory.The relation among maximum retransmission times,power in every transmission,modulation bits,coding rate in HARQ transmission mode and the system throughput is studied.A novel method to reduce packet error rate(PER) of MIMO-HARQ systems based on Space-Time Block Code(STBC) is proposed.The transmit antennas are selected based on the feedback from the receiver.The modulated symbols are encoded by STBC and obtained space time code blocks suitable for the number of selected antennas.The total PER is given by defining the decoding function and the best value of the maximum number of HARQ retransmissions is discussed.Theory and simulation results illustrate that this new HARQ scheme with antenna selection method has lower PER and the result is also satisfying.Finally,a new type HARQ scheme based on different space-time codes is proposed and evaluated.The new scheme is divided into three major steps which are V-BLAST processing and LDC processing also STBC processing.The orthogonal STBC can obtain all of the diversity gain of the MIMO system,the V-BLAST code can obtain all of the multiplexing gain of the MIMO system,and the LDC is the tradeoff between STBC and V-BLAST code.The traditional method simply retransfer signal,but the proposed method transfer the next line of data matrix and joint-decode them,so that can benefit from spatial diversity.At good condition of channel,the proposed method gets spatial multiplexing for retransfer hardly occur.The proposed method has the advantage of V-BLAST and LDC also STBC.It is a HARQ scheme that can improve transmit rate and improve resistance of interference at same time. Theory and simulation show the proposed method have higher throughput in most SNR regions and the gains are considerable.