Research On Multi-antenna Codec Under Selective Fading Channels

The major driver for broadband wireless communications has been reliable high-data-rate services.This,together with the scarcity of bandwidth resources, motivate research toward developing efficient coding and modulation schemes that improve the quality and bandwidth efficiency of wireless systems. Multiple-input multiple-output systems,which deploy multiple antennas at both ends of the wireless link,explore the extra spatial dimension,to significantly increase the spectral efficiency,and to improve the link reliability relative to single antenna systems.It has been widely acknowledged that the space-time coding techniques can effectively exploit the spatial diversity created by multiple transmit antennas.However,these developments must cope with several performance limiting challenges that include mobile radio channel impairments, multiuser interference and size/power limitations at the mobile units.So,this paper is concentrated on the MIMO system codec over selective fading channels. The main contents and the contributions of this paper are as follows:Firstly,the impact of time variability over MIMO system as well as how to use Doppler diversity to improve the transmission performance are investigated. Relying on a basis expansion model for time-selective channels,the conventional codecs over time-selective channels are systematically summarized.On the basis of the conventional codecs,a systematic design of high-rate full-diversity space-time-Doppler(STD) codes is proposed for MIMO time-selective channels and the design criteria of full-diversity STD codes are shown.The proposed STD codes are constructed from a layered algebraic design,where each layer of algebraic coded symbols are parsed into different transmit antennas,time slots and symbol blocks without rate loss.It is shown that the proposed STD codes can achieve rate N_t and full-diversity N_tN_rN_b(Q+1),i.e.,the product of the number of transmit antennas N_t,receive antennas N_r,coding blocks N_b and channel complex exponential bases(Q+1).Simulation results have shown the proposed STD codes are efficient.Secondly,block-differential coding with multiple antennas for unknown doubly-selective channels is investigated in this paper.Coherent decoding requires reliable estimation of the underlying multi-channels at the receiver.This is a challenging and costly task,especially when the channel experiences high mobility induced fast-channel fading.Differential coding,which circumvents the need for channel estimation,is an attractive alternative in such environments. However,most differential coding schemes are designed only for time-selective channels or only for frequency-selective channels.With the scarcity of the differential coding schemes for doubly-selective channels,a new block-differential codec for systems that are equipped with an arbitrary number of transmit and receive antennas and operate in time and frequency selective(doubly selective) channels is proposed.It can achieve full diversity gains,while affording low-complexity maximum-likelihood decoding.Relying on the fact that the coefficients of complex exponential basis of BEM of each path are fixed for a block,the proposed scheme subdivide a block into multiple subblocks and performs differential encoding across the subblocks.By corresponding processing at the transmitter and receiver,signal detection does not need channel state information,and hence avoid estimation of the doubly-selective MIMO channels. Numerical results are presented to illustrate the performance of the proposed scheme over four styles of channel.Thirdly,the preprocessor for downlink of multi-user STBC system is investigated.Using the SVD decomposition theorem and the QR decomposition theorem,the precoder for downlink transmission is shown.Then,this paper proposes two precoding methods that use extra transmit antennas,beyond the minimum required,to provide additional degrees of diversity.Two proposed designs are:eigenmode selection and transmit selection.The principle of eigenmode selection is that every user signals on the best orthogonal basis, according to maximizing the Frobenius norm of the equivalent channel,which is the key parameter to determine the diversity performance of a single-user STBC system,and yet maintaing the zero interuser interference constraint.Multiuser antenna selection operates similarly to eigenmode selection with the additional constraint that only a subset of the available transmit antennas are employed. Simulations show that the proposed downlink precoder can effectively cancel the interference between mobiles,while still providing good diversity performance.In addition,the impacts of channel estimation error and channel correlation are researched and analyzed.Finally,considering the uplink of the communication system,a new air interface that combines STBC with SC-CDMA is proposed.DS-CDMA system suffers from inter-symbol interference(ISI) and multiuser interference(MUI) caused by multipath propogation,leading to a significant loss of performance.In order to enable the design of low complexity transceiver that can cope with multipath channels while still benefiting from the good properties of DS-CDMA and the space-multipath diversity,a new air interface that combines STBC with SC-CDMA is proposed.We get the expressions of input/output relationship.Three basic joint detection algorithms are shown:MF-BLE,ZF-BLE and MMSE-BLE. At the receiver,a joint multi-user detector and ST block decoder,optimized according to the MMSE criterion is designed.At last,simulations are done to corroborate our theoretical analysis.More work is needed to improve the transmission rate,channel capacity and link performance of MIMO system.