Research On The Application Of Space-time Code In Precoding Based Downlink System

With the progress of society and the development of wireless communications,people's desire for high-speed information communications has become stronger than before,but due to limitation of spectrum resources,people have begun to seek more efficient signal processing methods to achieve high-speed data transmission.The MIMO technology and the massive MIMO technology are regarded as major discoveries in wireless communications theory because of their good multipath fading resistance and high spectrum utilization.Among them,precoding technology,as a key technology of the MIMO downlink system,can effectively enhance the desired signal while suppressing co-channel interference among users.For another,the space-time code technique is a method of obtaining the maximum diversity gain and coding gain by realizing the mapping relationship between the transmitted information stream and the antenna even when the transmitter has no the instantaneous channel information.In this thesis,the above two are combined together to obtain the diversity gain and multiplexing gain while improving system capacity and spectrum utilization.However,in a downlink system based on precoding,system performance largely depends on channel state information at the transmitter.In the time division duplex(TDD)system,it can be obtained through the channel reciprocity.But in frequency division duplex(FDD)system,it is usually required to obtain this based on the limited feedback technology.The implementation process of the finite-bit feedback system is to have the same codebook both sides,and the receiver selects the optimal codeword from the codebook by using the selection criterion according to the result of the channel estimation,then,the codeword's index is sent to the transmitter via a feedback link.This process involves the following three aspects:firstly,how to design the precoding codebook;secondly,how to select the codeword;finally,how to deal with situation where the channel state information is imperfect.Starting from the above problems and combining the space-time coding technology with precoding technology,this thesis focuses on the space-time code based transmission schemes in the MIMO downlink system.The main results include the following two aspects.On the one hand,assuming that the user fully knows the channel state information,by applying the Golden code into the finite-bit feedback system,a new codebook design and its related codeword selection method based on the determinant criterion are proposed.Then,a new double precoding matrix is reconstructed by using the selected codeword to implement signal transmission and a decoding method with low complexity,and for the traditional transmission scheme a joint ML and MMSE decoding method is proposed.On the other hand,for the MIMO downlink system with channel estimation error and feedback delay,new transmission schemes are proposed to compensate the system loss.Finally,the superiority of the proposed schemes are verified by simulations.