相关衰落信道下的一种层间联合自适应SR-ARQ传输机制

传统自适应传输机制在相关慢衰落信道下有较差的系统性能.针对相关衰落信道,提出了一种层间联合自适应传输机制.利用有限状态马尔可夫链来描述相关衰落信道,同时通过构建一个多状态马尔可夫系统模型来分析系统吞吐性能,并对调制方式和传输分组大小进行优化.另外,推导了系统平均吞吐性能的表达式.仿真结果显示所提出的自适应传输机制在相关衰落信道上能获得更好的系统性能.     

空时编码在卫星通信系统中的应用

无线数据传输发展迅速,人们对数据服务的要求也越来越广泛。卫星传输信道存在着多径衰落,而且频谱带宽有限,而空时编码(STC)作为对抗多径衰落的有效办法,同时能够趋近多输入多输出(MIMO)信道理论上的容量极限,日益引起人们的重视。探讨了空时编码(STC)在卫星通信系统中的应用,并进行了性能比较,表明空时编码(STC)能够带来更好的效果。     

MFSK水声远程通信技术及试验研究

海洋水声信道是多途、时变和频散的信道。论文针对浅海远程80～100km,设计出基于MFSK高数据率水声远程信息传输最佳系统。本系统经湖试验证表明:该技术在远程通信恶劣条件下具有较强抗多径干扰和信道衰落能力,对同步精度要求低,可实现80～100km,40bit/s,10-4量级通信精度的信息可靠传输,性能稳健,易于工程实现。     

LMSS中的遮蔽衰落模式分析

本文对ＬＭＳＳ遮蔽环境中的衰落分布进行了分析，利用现有实验数据和ＥＲＳ模式的基本形式，重新确定了经验模式参数在不同遮蔽条件下与仰角和频率的关系；     

A Combined Interleaving Technique for Trellis Coded MPSK Systems in Rayleigh Fading Channels

Freedom of mobility is the latesttrend in the evolution of digital communicationsystems. In these systems bandwidth and powerconstraints limit the mobility of the users. TCM(trellis coded modulation) is a combined coding andmodulation scheme which increases the productivity ofa digital communication system without increasing thetransmitted power or the required bandwidth. A mobilecommunication channel is affected from noise andfading because of the multipath propagation. On aRayleigh fading channel, coding should be used withtechniques which uncorrelate the received energy ofeach consecutive coded symbol. Interleaving is one ofthese techniques and used to make burst errors intorandom errors which can be corrected by errorcorrecting codes. In this paper, the trellis codingsystem which combines a regular convolutional encoder,bit interleaver, coordinate interleaver, and anappropriate decoder is considered. The analytical biterror probability upper bounds are derived for the8-PSK TCM system whose diversity is increased by usinga regular 2/3 rate convolutional code followed bythree bit interleavers and a 8-PSK signal mapperfollowed by two coordinate interleavers. Theanalytical and simulation results show that toincrease the diversity of the trellis coded M-PSKsystems in Rayleigh fading channels is the primary keyfor reliable transmission of high quality voice anddigital data.     

无线局域网信道特性的研究

无线信道是整个现代数字无线移动通信研究的基础,从电磁波传播方式、无线信道对传输信号的影响以及等效低通信道的时变冲激响应等几个方面对无线信道的特性进行了研究。结果表明,信号通过无线信道传播时,会产生各种形式的衰落,衰落对无线移动通信系统传输性能的影响是很大的,在实际中应设法消除。     

Adaptive Power and Bit Allocation in Multicarrier Systems

We present two adaptive power and bit allocation algorithms for multicarrier systems in a frequency selective fading environment. One algorithm allocstes bit based on maximizing the channel capacity, another allocates bit based on minimizing the bit-error-rate （BER）. Two algorithms allocate power based on minimizing the BER. Results show that the proposed algorithms are more effective than Fischer＇s algorithm at low average signal-to-noise ration （SNR）. This indicates that our algorithms can achieve high spectral efficiency and high communication reliability during bad channel state. Results also denote the bit and power allocation of each algorithm and effects of the number of subcarriers on the BER performance.     

A class of generalized models for shadowed fading channels

The commonly used models for shadowed fading channels arise by compounding the Nakagami distribution with others such as the log-normal and gamma distributions. A distribution that is more flexible than the Nakagami distribution is the well known Weibull distribution. In this paper, we derive a collection of generalized models for shadowed fading channels by compounding the Weibull distribution with other distributions belong to some fifteen flexible families. We also illustrate the superior performance of these models over the standard Nakagami models.     

BER Analysis of Equalized OFDM Systems in Nakagami, <Emphasis Type="Italic">m</Emphasis> < 1 Fading

The orthogonal frequency division multiplexing (OFDM) transmission scheme is currently experiencing increased popularity due to advances in very large scale integration technology. It is used for a variety of broadband systems such as asymmetric digital subscriber lines, very-high-speed digital subscriber lines, digital video, and audio broadcasting, and wide local area network standards such as IEEE 802.11a, IEEE 802.11 g, and ETSI Hiperlan/2. However, propagation impairments can cause severe degradation in bit error rates (BER) for coherent detection. We derive a semi-analytical method to evaluate BER of a quadrature phase shift keying (QPSK)-OFDM system in Nakagami, m < 1 fading and additive noise where pilot-assisted linear channel estimation and channel equalization are used. This allows modeling of more severe fading environments than can be depicted by a Rayleigh distribution. Numerical simulations are used to validate the proposed methods. The techniques developed can be applied to other channel estimation techniques, modulation schemes and the performance evaluation of equalized single carrier narrowband systems.     

Performance Analysis of Multiuser Detectors for Uplink Quasi-synchronous MC-CDMA Systems

This paper analyses and compares the average bit error rate (BER) of different multiuser detectors (MUD) in the uplink of a multicarrier code- division multiple access (MC-CDMA) system. In particular, maximum likelihood, zero-forcing, minimum mean-square error and interference cancellation-based multiuser detectors have been analysed for the special case of uncorrelated subcarriers. The derived BER expressions are based upon previous results on diversity combining and also on recent findings on multiple input multiple output (MIMO) architectures. The subcarrier correlation is considered in the context of physical parameters currently under discussion for future wireless systems to give an indication up to what extent the assumption of uncorrelated subcarrier fading is plausible.