Coded Modulation Faster-than-Nyquist Transmission with Precoder and Channel Shortening Optimization

Faster-than-Nyquist(FTN)signaling can improve the spectrum efficiency(SE)of the transmission system.In this paper,we propose a coded modulation FTN(CM-FTN)transmission scheme with precoder and channel shortening(CS)optimization to improve bit error rate(BER)performance and reduce the complexity of FTN equalizer.In our proposal,the information rate(IR)or spectral efficiency(SE)is employed and verified as a better performance metric for CM-FTN than the minimum Euclidian distance(MED).The precoder of CM-FTN is optimized for maximizing the IR criterion using the bare-bones particle swarm optimization(BB-PSO)algorithm.Further,a three-carrier CM-FTN system model is used to capture the broadening effect of precoder.Also targeting for the IR maximization,the inter-symbol interference(ISI)length for CS is optimized to reduce the receiver complexity without performance loss.Simulation results demonstrate that our method has a 0.6dB precoding gain compared with the nonprecoding scheme and a maximum of 87.5%of the complexity of FTN equalizer is reduced without BER loss.     

MLSE Equalizers for Frequency Discrimination Receiver of MSK Optical Transmission System

We propose maximum-likelihood sequence estimator (MLSE) equalizers based on either Viterbi algorithm or template matching temple matching (TM) for the equalization of impairments imposed on the minimum shift keying (MSK) modulation formats in long haul transmission without optical dispersion compensation. The TM-MLSE equalizer is proposed as a simplified alternative for the Viterbi-MLSE equalizer. It is verified that the Viterbi-MLSE equalizer can operate optimally when noise approaches a Gaussian distribution. Simulation results of the performances of the two MLSE equalizers for optical frequency discrimination receiver-based optical MSK systems are described. The transmission performance is evaluated in terms of: (1) the chromatic dispersion (CD) tolerance for both Viterbi-MLSE and TM-MLSE equalizers; (2) transmission distance limits of Viterbi-MLSE equalizers with various number of states; (3)the robustness to fiber polarization mode dispersion (PMD) of Viterbi-MLSE equalizers; and (4) performance improvements for Viterbi-MLSE equalizers when utilizing sampling schemes with two and four samples per bit over the conventional single sample per bit. With a small number of states (64 states), the non-compensating optical link can equivalently reach up to approximately 928 km SSMF for 10 Gb/s transmission or 58 km SSMF for 40 Gb/s. The performance of 16-state Viterbi-MLSE equalizers for optical frequency discrimination receiver (OFDR)-based optical MSK transmission systems for PMD mitigation is also numerically investigated. The performance of Viterbi-MLSE equalizers can be further improved by using the sampling schemes with multiple samples per bit compared to the conventional single sample bit. The equalizer also offers high robustness to fiber PMD impairment.     

OFDM系统频率选择性信道功率分配方案

自适应分配技术根据子信道的瞬时估计值动态地分配传输比特数和发送功率,可以优化正交频分复用（OFDM,Orthogonal Frequency Division Multiplex）系统的整体性能。这里讨论了基于容量优化的自适应比特分配算法,基于误比特率优化的最佳功率分配算法和次佳功率分配算法。仿真结果表明,对不同信道环境下三种算法的特点和性能进行了分析和比较。仿真结果表明,自适应分配技术可以优化系统的容量和误比特率。     

SC-FDE系统中频域均衡算法的改进

分析了单载波频域均衡系统中基于线性均衡技术的最小均方误差线性均衡算法,在此基础上提出了一种对白噪声能量的估计算法,并依此对SC-FDE系统中的均衡技术进行改进。在MATLAB下对提出的算法进行仿真验证,仿真结果表明,采用改进的频域均衡算法所得到的误码率和理论计算值非常吻合,并且使得星座图得到了很好的收敛,提高了通信系统的性能。     

基于BMST编码的超Nyquist传输系统性能研究

超奈奎斯特(FTN)传输技术是一种能够有效提高频谱效率的非正交传输技术。非等功率分配的分层FTN结构可以通过调节各层的码率或功率进行多级译码,从而实现干扰抵消,进而有效降低码间串扰(ISI)所带来的译码复杂度。分组马尔可夫叠加传输(BMST)编码是一种码率灵活可变的长卷积码构造方法,且能够获得额外的编码增益。介绍和讨论了FTN和BMST的原理,提出了一种基于BMST编码的FTN传输方案。仿真结果表明,该传输系统能够显著提高频谱效率。     

映射信道信息的编码空时级联系统

传统的空时检测和卷积码级联译码方案将空时检测输出的信息仅作为编码的先验信息,这导致了信道译码过程中信息不能充分利用。通过将空时检测输出的先验信息映射为对应编码的信道信息,这使得信道译码器在译码过程中能够充分利用空时检测输出的所有信息,从而提高了译码性能。分析和仿真结果表明,所提的译码方案同传统方案相比,有效地降低了误比特率,使性能得到了较大的提高。     

Turbo-Coded OFDM Transmission Over a Nonlinear Channel

In this paper, deliberate level clipping and turbo coding are combined to achieve an orthogonal frequency division multiplexing (OFDM) transmission system with a low peak-to-average power ratio (PAR) and a good performance. Using the linear approximation technique based on the minimum mean square error (MMSE) criterion, we first modify the metric computation for the turbo decoding in order to consider the distortion effects of the nonlinearity, caused by the Cartesian clipper. Also, this paper introduces a modified turbo decoder which simultaneously performs the data estimation and signal reconstruction. In other words, the turbo decoder iteratively recovers the clipped signal by using the estimated data, and then improves the data estimation by using the newly recovered signal. Numerical results are presented showing an improvement in the performance of the OFDM transmission system over the nonlinear channel, an increase in the efficiency of the high power amplifier (HPA), and/or an expansion of the transmitter coverage area.     

Redundant Paraunitary FIR Transceivers for Single-Carrier Transmission Over Frequency Selective Channels With Colored Noise

Recently, the use of redundant memoryless single-carrier transmitters has been reported as an efficient choice to reduce distortions introduced by finite-impulse response (FIR) channels. In this work, redundant FIR transceivers are proposed to address not only channel frequency selectivity but additive colored noise, which strongly degrades the performance of memoryless transceivers. The transmitter is shown to be paraunitary, resulting in a simple receiver. The proposed system is optimized like a modulated filter bank. Channel shortening and post-combiner equalizers are used to improve system performance. Comparisons with recent proposed schemes are presented, illustrating the efficiency of the new structure for selective channels with colored noise.     

SC—FDE系统中频域均衡算法研究

单载波频域均衡技术以其能够有效避免单载波时域均衡复杂度过大,以及正交频分复用技术峰均比较高的不足,而成为宽带无线接入系统物理层的重要组成技术.本文研究了SC-FDE系统中的最小均方误差判决反馈均衡(MMSE-DFE)和基于LMS准则的自适应均衡算法,MATLAB仿真表明,由于LMS自适应均衡算法能有效的跟踪信道变化并更新均衡器的抽头系数,其性能略优于最小均方误差判决反馈均衡算法.     

Comparison of Performance of 16-ary QASK and MSK Over a Frequency Selective Rician Fading Channel

The probability of error of both a 16-ary QASK system and an MSK system, each operating over a Rician fading channel, is considered. The channel model has two key features: short term variations are modeled by making the channel frequency selective, and long term variations are accounted for by allowing the specular component of the received signal to fade according to a Rayleigh density. The improvement gained by using maximal-ratio diversity combining is derived, as well as the degrading effects of an adjacent channel. Finally, a comparison between MSK and 16-ary QASK operating over the same channel is presented, and it is shown that MSK results in a smaller probability of error.     

On Signal Detection for OSTBC-Based Amplify-and-Forward Cooperative Relaying System Over Time Selective Channel

Wireless Personal Communications - The orthogonal space-time block code (OSTBC) and cooperative relaying schemes are the two major spatial diversity techniques to achieve high link reliability in...     

频率选择性衰落信道下长扩频序列CDMA 系统的盲波束成形

本文提出一种适用于频率选择性衰落信道下长扩频序列CDMA系统,基于DMMSE准则的盲波束成形方案。该方案反频率选择性衰落信道分解成若干个非频率选择性衰落子信道,不需要训练符号开销,回避跟踪衰落信道参数,以所需用户的扩频序列作为永久性训练序列。仿真结果验证了其抵抗远近效应、抑制多址干扰的能力。     

瑞利衰落下Turbo-TCM与发射分集级联系统的性能

为了比较Turbo码与不同发射分集技术的结合在对抗多径衰落时的作用,本文比较了Turbo-TCM方案分别与Alamouti提出的编码分集技术(ACD)和传统的正交发射分集技术(OTD)的级联系统在瑞利衰落信道下的性能.针对级联系统的译码,本文给出了一种具有低译码时延的次优译码算法,该算法的特点是各模块独立译码,先算比特对数似然比再进行二进制Turbo码译码.同时进行了仿真.     

Simulation of TV Transmission Over the Communications Satellite Channel

Time-domain simulation is used to evaluate NTSC and multiplexed analog component (MAC) television transmission over the Communications satellite channel using analog frequency modulation. The simulation program is used to measure transmission impairments caused by thermal noise, distortion resulting from spectral truncation by the band-limited channel, AM-to-PM conversion in the satellite TWTA, and the onset of impulse noise in the FM demodulator as the receiver approaches threshold. Although subjective testing is necessary before final comparisons can be made, the simulation results give insight into the sources of transmission impairments and the tradeoffs among certain transmission parameters.     

Cross-Layer Power Allocation for Packet Transmission Over Fading Channel

In this paper, for packet transmission over flat fading channel in single-input-single-output system, we consider the power control problem in a cross-layer design where adaptive modulation is adopted at physical layer to improve spectral efficiency and the queues are modeled as of finite length at data link layer. The goal is to identify the optimal queuing-aware power allocation algorithm to minimize the overall system packet error rate under the constraint of long-term transmit power. One crucial step which we call `inner?? problem is to find the optimal power vector at a given target packet error rate at physical layer. Rather than attack the multi-dimensional optimization problem directly using conventional methods, we first observe that the `inner?? problem is closely related to an average reward Markov decision process problem, and relax the former to the latter so as to take advantage of its equivalence with linear program which allows efficient solution. Since randomness in the associated Markov decision process is only slight, at most mild, we propose an approximately deterministic policy as suboptimal solution to the `inner?? problem with insignificant performance degradation. We also propose two-parameter power allocation functions to achieve suboptimal results with low complexity. The impacts of system parameters on the overall system performance are also evaluated. The accuracy of the numerical result is verified by Monte Carlo simulations.     

Frequency-Domain Channel Estimation for SC-FDE in UWB Communications

Recently, single-carrier block transmission with frequency-domain equalization (SC-FDE) has been shown to be a promising candidate for ultra-wideband (UWB) communications. In this paper, we address the channel estimation problem for SC-FDE transmission over UWB channels. A mean-square error (MSE) lower bound for the frequency-domain linear minimum mean-squared error (LMMSE) channel estimator is derived, and the optimal pilot sequence that achieves this lower bound is obtained. Further simplification leads to a frequency-domain channel estimator with reduced computational complexity. The performance of the simplified estimator for SC-FDE over UWB channels is evaluated and compared with that with perfect channel state information. The effects of nonoptimal and optimal pilot symbols are also investigated. Our results show that the proposed frequency-domain channel estimator performs well over UWB channels with only small performance degradation, compared with that with perfect channel estimation     

Time and Frequency Synchronization with Channel Estimation for SC-FDMA Systems Over Time-Varying Channels

The constant amplitude zero autocorrelation sequence based synchronization and its usage in the block-type channel estimation for the single carrier frequency division multiple access (SC-FDMA) systems are vulnerable to the time-varying channels. Therefore, the channel estimation errors limit the performance of SC-FDMA systems in fast fading environments and result an irreducible error floor. In this paper, cyclic prefix based maximum likelihood estimator of time/frequency synchronization with comb-type channel estimation for the SC-FDMA systems are proposed to track the channel variations. Moreover, the residual time/frequency offsets calculations are derived for SC-FDMA systems. Simulation results confirm and illustrate that the proposed receiver is capable of tracking fast fading channel parameters and improving the overall performance as compared with the conventional receiver.     

Quaternary Transmission Over Satellite Channels with Cascaded Nonlinear Elements and Adjacent Channel Interference

The performance of coherent QPSK, offset QPSK, and MSK modulation systems is investigated via computer simulation for TDMA transmission over satellite channels with cascaded nonlinear elements, up- and downlink fades, intersymbol interference, adjacent channel interference, and thermal noise. Three satellite transponder concepts are considered: TWTA transponders nominally operated at their saturation power under clear sky conditions, TWTA transponders nominally operated at input overdrive, and hard-limiting transponders. The performance of these modulation techniques in terms of bit error rate versus energy per information bit over thermal noise power densityE_{b}/N_{0}has been evaluated for the three system concepts as a function of various system parameters. For transponderBTproducts in the range of 1.5-2.0, modem filter characteristics and waveform shaping have been carefully and nearly "optimally" selected to combat adjacent channel interference, which is the major source of impairments, especially under uplink fades. It has been found that, for certain system environments, MSK and OQPSK outperform QPSK. As a byproduct of this investigation, various means to minimize the effects of adjacent channel are proposed.     

一种旋绕型频率选择表面的传输特性分析

利用基于矩量法的电磁分析软件Ansoft Designer研究了一种新的旋绕型双方环频率选择表面(new convoluted double square loop frequency selective surface,简称NCDSL FSS)的传输性能.结果表明,该结构对入射电磁波的入射角、极化方式等不灵敏,比经典CDSL FSS受栅瓣影响小;通过调节其凹槽深度,两频带间隔可围绕单元尺寸、阵列周期均相同的传统同心双方环频率选择表面(double square loop frequency selective surface,简称DSL FSS)的频带间隔双向调节,这是CDSL FSS无法做到的;而且NCDSL FSS高频谐振点处的-0.5 dB相对反射带宽也总是比CDSL FSS和DSL FSS的大.因此该结构是一种简单且性能较优的频率选择器件.     

频率选择性衰落MIMO信道容量分析

随着传输速率越来越高,信号所占用的带宽越来越宽,实际信道呈现出频率选择性衰落特性.根据频率选择性衰落信道特性,建立了频率选择性衰落MIMO信道模型,采用频域和时域的方法研究了在频率选择性衰落下,发射端已知和未知信道状态信息的MIMO信道容量,并给出了相关的仿真结果.研究结果表明:在相同发射功率和传输带宽下,MIMO系统可以不牺牲信号带宽而显著提高信道的容量,并且天线数量和输入信噪比的大小对信道容量具有不同的影响.研究结果为如何提高频率选择性衰落环境下MIMO信道容量提供了依据.