Research On Equalization Technology For Broadband Wireless Communiction System

To meet the explosive demand for high-rate wireless multimedia and interactive services, the common feature of many broadband wireless standards for high-rate communication system is the adoption of block transmissions. The primary challenge in wireless block transmission system is the time dispersive nature of the frequency-selective (multipath) fading channel, which can cause severe intersymbol interference (IS1), resulting in an inevitable system performance deterioration. Equalization techniques do not only compensate the linear distortion caused by channel frequency-selectivity, but they can make use of other anti-multipath fading measures with flexibility. Therefore, they can effectively improve the system performance, and are the key research hotspots in broadband wireless communication system.Under the support of National Science and Technology Major Project of China (2009ZX03002-009-01) and National Science Foundation of China (60672132,60872149), from the point of view of the reasonable tradeoff between performance and complexity, the dissertation mainly focuses on the equalization technology for broadband wireless communication system. The major work and contributions of the dissertation are summarized as follows.(1) For single-carrier frequency-division multiple access (SC-FDMA) uplink system, when user’s data blocks are passed through different multipath fading channel, they suffer from ISI and multiuser interference (MUI), resulting in system performance degradation. In order to deal with this limitation, the dissertation proposes a frequency domain iterative block decision feedback equalization (FD-IBDFE) algorithm. By fully exploiting the coefficient of input-decision symbol correlation, the proposed FD-IBDFE algorithm optimizes the tap coefficients of the feedforward and feedback filters. Therefore, in each iterative processing, maximized signal-to-interference-plus-noise ratio (SINR) is achieved, and the reliability of the detected symbols is gradually increased. Furthermore, the proposed FD-IBDFE algorithm can fully utilize diversity combining technique to accelerate the convergence. Extensive simulation results are presented to show that, the proposed algorithm can effectively mitigate the interference and provides better performance.(2) For single-carrier block transmission (SCBT) system with high-level modulation and/or large channel delay spread, the conventional turbo equalization schemes become prohibitively complex. Therefore, the design of low complexity soft-input soft-output turbo equalization algorithm has attracted considerable attention. To further reduce complexity, the dissertation proposes a low complexity frequency domain turbo equalization scheme. Based on the minimum mean square error (MMSE) criterion, a SI SO frequency domain block DFE (FD-BDFE) is also provided. In each iteration, the proposed FD-BDFE algorithm can effectively exchange soft extrinsic information with SISO channel decoder. With the help of the soft-decision symbols, the proposed algorithm can adaptively update the feedforward and feedback tap coefficients, thus, it can mitigate the detrimental effect of ISI and reduces error propagation phenomena. The performance and convergence property of the proposed algorithm is analyzed through extrinsic information transfer (EXIT). Both analytical and simulation results show that, the performance of the proposed algorithm is similar to that of the conventional high complexity time domain exact MMSE linear equalization (LE) turbo scheme, but with a much lower complexity and faster convergence.(3) Conventional space-time block codings (STBCs) for single-carrier block transmission, all assume that the wireless channel is quasi-static over two consecutive transmission time slots. However, in some high-mobility environments which induce Doppler shift, this hypothesis is not usually satisfied. As a result, system performance will deteriorate. Based on the discrete Fourier transform (DFT) extended properties, an one time slot time-reversal STBC (TR-STBC) scheme for SCBT is proposed. To collect the potential frequency diversity embedded in multipath fading channel at high signal-to-noise (SNR), a low complexity suboptimal MMSE-based frequency domain DFE (FD-DFE) is also proposed. The diversity order analysis and simulation results demonstrate that the proposed scheme with MMSE FD-LE has almost the similar performance as that of the conventional scheme, but with more robust against Doppler shift caused by high-mobility. Moreover, if FD-DFE is further incorporated into the proposed scheme, a significant performance improvement is achieved.(4) To obtain the performance improvement, conventional distributed STBCs (DSTBCs) for amplify-and-forward (AF) relay-assisted SCBT use redundancy zero-padding (ZP) precoding method at the transmitter. However, its spectral efficiency is reduced when channel delay spreads reach up to dozens of symbols. Following the similar principle as one time slot TR-STBC scheme, through the cyclic prefix (CP) assisted instead of the ZP, a novel two time slots distributed TR-STBC scheme over frequency-selective fading channel is proposed, where two time slots are used to transmit one data block. The proposed scheme has the same spatial diversity and data transmission rate as those of the conventional DSTBC schemes, where four time slots are used to transmit two data blocks. Theory analysis and simulation results show that the proposed scheme with MMSE FD-LE has the similar performance as DSTBC scheme without sacrificing additional spectral efficiency, and can provide a reasonable tradeoff between performance and complexity.