Research On Space-time Code And Correlative Techniques

Recently, wireless mobile communication has been the hot topic in the domain of communication. Nowadays, many techniques have been comprehensively considered as the major candidates of the future radio communication architecture. Among which includes OFDM and MIMO, the former can effectively combats multipath fading and improves transmission rate of data, the latter can effectively enhances system performance or notably improves system capacities using spatial diversity or spatial multiplexing techniques. Space-time code appears at the background of applications and immediately attracts much more attentions. Supported by the National High Technology Research and Development Program of China under Grant No. 2001AA123014 and National Science Foundation of China under Grant No. 60496310, our research work concentrates on the severe communication circumstances, for example, spatially correlative and time-frequency selective fading channel, and focuses on the design of codec for space-time architecture and the utilizing of linear precoding technique especially on the multiuser scenario trying to improve the system performance to combate various fading.Firstly, we provide the analytical expression of the joint space-time cross-correlation function and deduce the theoretical pairwise error probability of space-time block coded transmission scheme based on the existed spatially and temporally correlated MIMO channel model. Also, we investigate the impacts of antennas array spacing, angles, the spread of scatterings at the both ends of the link upon the system performance and further testify the validity of the general MIMO channel model. At the same time, the all existed MIMO channel models considered spatial correlation or temporal variation respectively, we present a hybrid model to model spatial-temporal correlation jointly. Finally, we arrive at a userful conclusion: high spatial correlation dominates the performance loss when the temporal variation is small, whereas large temporal variation contributes to the performance loss whatever degree spatial correlation is based on the two above channel models. Thus it can be seen various channel conditions, such as spatial correlation, time-frequency selective fading etc, have important influences on the space-time transmission scheme especially on the multiuser scenario.For the space-time block coded multiuser uplink, the design of conventional ZF and MMSE decoder assumed channels are quasi-static. When the channels are time-selective fading, these conventional decoders induce to the Inter-Symbol Interference (ISI) in order to obtain one user's signals, furthermore, the ISI becomes larger resulted from the increase of the channels time variations. So, we propose two kinds of new decoders for the space-time block coded multiuser uplink over time-selective fading channels. For the first kind of decoder, ISI is removed using jointly decoding based on the new received signal expressions over time-selective fading channels. For the second kind of decoder, we construct a transformed matrix using the ZF idea and act this matrix on the received signal vector to result in a diagonal matrix thereby eliminating the influence of ISI. The two above decoders are appropriate to two users system and the channels are flat time-selective fading. Next, we extend the above decoder to the existed space-time block coded multiuser architecture and substitute it for the iterative soft input soft output (SISO) multiuser dectection thus reducing the calculational complexities. Then, the above decoder is used at each flat fading subcarrier channel and is extended to the frequency-selective fading channels combining with OFDM technique. Compared with the conventional decoders, the new decoder obtains notable performance gains and does not exhibit error floors for the various system configurations.For the existed group layered space-time architecture, the conventional group detection technique is applied firstly to suppress the interferences from other groups and space-time decoding is then performed to obtain the desired group's signals. Dai et al proposed a decoding scheme but the essential ideas do not change, also, exploiting firstly group detection to suppress the other groups'interferences and ML is then used to get the desired group's signals. The all existed decoders assumed the channels are quasi-static. They are not appropriate once the channels are time-selective fading. So, we present a new decoder appropriate to group layered space-time architecture over time-selective fading channels. For the new decoder, the orthogonal pairs of matrixes are constructed firstly and employed to remove the inter-interference of all groups then use the low complexity matrix inverse algorithm to obtain the all groups'transmitted signals by parallel detecting. It is also proved that the old decoding scheme proposed by Zhuang et al is a special case of our decoder. Compared with the conventional decoders, the new decoder reduces greatly the calculational complexities.Finally, a precoding scheme is proposed for the multiuser downlink spatial multiplexing system. At the transmitter, the precoder is constructed as the function of all users'channel state information according to the performance criterion of minimizing average probability of vector symbol error. Since the multiuser interferecnce (MUI) is precancelled at the transmitter, each terminal user just utilizes the simple linear decoding to obtain the transmitted symbols. It is verified that our proposed precoding scheme arrives at almost the same performance as the single user precoding system. Next, we present a transmission scheme for the multiuser downlink over frequency-selective fading channels. At the transmitter, the multiuser MIMO system is transformed to parallel independent single user MIMO system through the perfect design of transmit preprocessing vectors at the subcarrier channel thus assuring the good elimination of MUI at each terminal user. At the receiver, use just simple linear decoder to achieve the maximum signal-to-noise ration of the decoded symbol thus minimizing the bit error rate of each user.