Cramer-Rao Lower Bounds for Semi-Blind ML Channel Estimation in UWB Systems

This paper derives a unified representation of the Cramer-Rao lower bounds (CRLBs) for semi-blind maximum-likelihood (ML) channel estimation in time-hopping (TH) ultra-wideband systems with pulse amplitude and position modulation, pulse amplitude modulation and pulse position modulation (PPM). These bounds subsume the existing results for both pilot-based and blind ML channel estimation in TH-PPM as special cases. A numerical example is used to illustrate the effects of different data modulation formats and system parameters on the CRLBs derived     

Blind adaptive receiver based on RLS-CMA for TH-UWB system

A novel receiver is proposed in this paper that uses the blind adaptive multi-user detection(MUD) technique to deal with the presence of multiple-access interference(MAI) and inter-symbol interference(ISI) in terms of high speed time-hopping ultra-wideband(TH-UWB) system.The receiver firstly adopts constrained recursive least squares constant modulus algorithm(RLS-CMA) to conduct adaptive decorrelation for eliminating MAI and ISI,and then uses projection approximation subspace tracking with deflation(PASTd)...     

Multiuser detection of time-hopping PPM UWB system in the presence of multipath fading

Ultra-wideband (UWB) impulse radio (IR) systems are currently being considered for several applications due to their attractive features that include low-power carrierless and ample multipath diversity. Among the various modulation and multiple-access schemes, time-hopping (TH) pulse position modulation (PPM) is a popular technique in application. Most past works rely on strict power control and perform single-user detection (matched filtering) on the desired signal. This paper aims to apply multiuser detection techniques in binary PPM (BPPM) UWB IR multiple-access systems. Moreover, we consider frequency-selective multipath fading channels to account for the wireless cellular environment. A class of linear multiuser detectors (LMDs) is applied to extract the information bits while eliminating multiuser interference (MUI) in the presence of multipath fading. Simulation results are provided to compare the performance of different LMDs.     

超宽带跳时PPM信号在多径环境下的误码率性能优化

针对超宽带跳时PPM信号在不当参数选择时在确定性多径信道可能出现的不同数据符号的误码率不平衡性问题，提出解决TH-PPM调制信号可能存在误码率不平衡这一问题的两种方法，一种是Rake接收机选择动态的判决门限，另一种是改进TH-PPM调制方式，即对待发送的数据符号进行平衡编码。通过理论分析和计算机仿真表明，两种方法都能改善跳时PPM信号在多径环境下存在的误码率不平衡问题，比较而言，平衡编码方法具有实现的简单性以及在对信道变化、Rake接收机信道估计以及合并方式的稳健性。     

高斯脉冲的超宽带系统抗时间抖动性能分析

由于收发信机之间的不同步、定时追踪及收发间的相对速率等因素所引起的时间抖动在超宽带通信中是不可避免的.详细推导出在单链路及多用户环境中时间抖动对高斯脉冲的跳时脉位调制超宽带系统的性能影响的数学表达式,并作了相应的仿真分析.结果表明,系统的抗时间抖动性能主要取决于单脉冲信号的自相关特性及引入时间抖动的严重程度.     

Multi-access Interference Cancellation Receiver for Time-Hopping Ultra Wideband Communication

In this paper, a low complexity coherent joint maximum-likelihood detection receiver is developed for canceling multi-access interference in impulse radio (IR) based ultra wideband (UWB) wireless communication systems. Unlike previous joint demodulation receivers, where the complexity is enormous and the other users’ codes are required, the proposed approach does not need to know the time hopping (TH) codes of the other users which makes it a very attractive solution. The performance of joint demodulation receiver depends on the ability to estimate the channel coefficients of desired and interfering users. A novel and practical algorithm for estimating multiple users’ channel responses is developed. The performance of the proposed algorithms are tested through computer simulations and the results are compared with the performance of the conventional single user UWB receiver. It is observed that the proposed joint demodulation receiver provides significant performance gains with respect to conventional single user receiver. This paper is presented in part in IEEE International Conference on Communications (ICC) 2004, Paris, France.     

Fixed-point blind adaptive multiuser detection algorithm for IR UWB systems in multipath channel

Using the hypothesis that data transmitted by different users are statistically independent of each other, this paper proposes a fixed-point blind adaptive multiuser detection algorithm for Time-Hopping (TH) Impulse Radio (IR) Ultra Wide Band (UWB) systems in multipath channel, which is based on Inde-pendent Component Analysis (ICA) idea. The proposed algorithm employs maximizing negentropy criterion to separate the data packets of different users. Then the user characteristic sequences are utilized to identify the data packet order of the desired user. This algorithm only needs the desired user’s characteristic se-quence instead of channel information, power information and time-hoping code of any user. Due to using hypothesis of statistical independence among users, the proposed algorithm has the outstanding Bit Error Rate (BER) performance and the excellent ability of near-far resistance. Simulation results demonstrate that this algorithm has the performance close to that of Maximum-Likelihood (ML) algorithm and is a subopti-mum blind adaptive multiuser detection algorithm of excellent near-far resistance and low complexity.     

On the asymptotic distribution of the correlation receiver output for time-hopped UWB signals

In ultra-wideband (UWB) communications based on time-hopping (TH) impulse radio, one of the most frequently studied receivers is the correlation receiver. The multiuser interference (MUI) at the output of this receiver is sometimes modeled as a Gaussian random variable. In order to justify this assumption, the conditions of validity of the Central Limit Theorem (CLT) have to be studied in an asymptotic regime where the number of interferers and the processing gain grow toward infinity at the same rate, with the channel degree being kept constant. An asymptotic study is made in this paper based on the so-called Lindeberg's condition for the CLT for martingales. Nonsynchronized users sending their signals over independent multipath channels are considered. These users may also have different powers. It is shown that when the frame length grows and the repetition factor is kept constant, then the MUI does not converge in distribution toward a Gaussian random variable. On the other hand, this convergence can be established if the repetition factor grows at the rate of the frame length. In this last situation, closed-form expressions for the signal-to-interference-plus-noise ratio (SINR) are given for TH pulse amplitude modulation (PAM) and pulse position modulation (PPM) UWB transmissions.     

Joint-detection and interference cancellation based burst-by-burst adaptive CDMA schemes

Spread adaptive quadrature amplitude modulated (AQAM) code-division multiple access (CDMA) is proposed as a powerful means of exploiting the time-variant channel capacity fluctuations of wireless channels. It is studied in comparison to variable spreading factor (VSF)-based techniques. These adaptive-rate transmission methods are compared in the context of joint detection and interference cancellation assisted adaptive CDMA (ACDMA) systems. More explicitly, we exploit the time-variant channel quality of mobile channels by switching either the modulation mode (AQAM) or the spreading factor (VSF) on a burst-by-burst basis. The most appropriate modulation mode or spreading factor is chosen based on the instantaneous channel quality estimated. The chosen modem mode or spreading factor is communicated to the remote communicator either through explicit signalling or extracted at the receiver using blind detection techniques. The multiuser joint detector (JD) and the successive interference cancellation (SIC) receiver are compared in the context of these adaptive schemes, with the conclusion that the JD outperformed the SIC receiver in the ACDMA schemes at the cost of increased complexity. Finally, the performance of the uncoded AQAM JD-CDMA scheme is also compared to that of adaptive trellis coded modulation (TCM) assisted AQAM JD-CDMA, which allows us to incorporate adaptive channel coding without any bandwidth expansion. We also show that in the particular scenario studied, adaptive TCM outperformed adaptive turbo TCM since the system was designed for maintaining a low turbo-interleaver delay.     

A Decorrelating-MRC Based Space–Time Multiuser Receiver for Time-Hopping UWB System

In this paper, we propose a two-stage linear multiuser detector (LMD) for ultra wideband (UWB) multiple single-input multi-output (M-SIMO) system and multipath fading environment. Time-hopping (TH) and antipodal pulse amplitude modulation (PAM) are employed for the multiple access system. The decorrelating detector is first employed at the front end of each receive antenna to eliminate the multi-user interference (MUI), then a set of maximum-ratio-combiners (MRC) are proceeded to maximize the signal-to-noise power ratio (SNR) for each user. Since the channel information is crucial for the Decorrelating-MRC (D-MRC) receiver, we develop a subspace-based blind M-SIMO channel estimation method. The effect of channel estimation error on system performance is extensively evaluated. It is also verified from the analytical and numerical results that by exploiting both spatial and temporal diversities, the D-MRC receiver dramatically improves system performance even without additional coding. Moreover, we demonstrate that both the D-MRC receiver and subspace-based blind channel estimator are computationally feasible and near-far resistant.

Blind, Adaptive Channel Shortening Equalizer Algorithm Which Can Provide Shortened Channel State Information (BACS-SI)

Channel shortening equalization plays an important role in multicarrier modulation (MCM) systems. In this paper, we propose a blind channel shortening equalizer structure named blind, adaptive channel shortening equalizer which can provide the shortened channel state information (BACS-SI). The algorithm depends on the minimization of a cost function defined as the sum-squared difference of the autocorrelations of the shortened channel impulse response (CIR) and a target impulse response. The surface is proven to be multimodal; however, minima are shown to be related to each other in a certain way. A two-phase approach is proposed. In the first phase, the cost function is minimized by a stochastic gradient descent algorithm in order to find an arbitrary minimum. In the second phase using the relation between minima, genetic algorithms are employed to find the best minimum according to a fitness function. The algorithm can both successfully shorten the channel and also explicitly provides shortened CIR which is a necessary information for the proper operation of a MCM receiver, in contrast to many other algorithms proposed in the literature which cannot directly provide this information.      

Time-hopping and frequency-hopping multiple-access packetcommunications

Time-hopping and frequency-hopping multiple-access (TH/FHMA) packet communication systems are proposed and investigated. In TH/FHMA communication systems, a message packet is encoded into several subpackets via a Reed-Solomon error correcting code. The subpackets are transmitted over the channel using time-hopping and frequency-hopping patterns. It is assumed that the channel is noiseless and the side information is perfect so that all subpacket collisions can be correctly detected. Slot-synchronous and totally asynchronous TH/FHMA systems are analyzed in detail, and they are shown to have excellent throughputs at small packet erasure rates. Various time-hopping techniques which significantly reduce the multiple-access interference are developed     

基于自适应步长ICA的CO-OFDM系统的偏振效应均衡研究

提出一种基于自适应步长独立成分分析(ICA)法的 信道均衡算法,用于相干光正交频分复用 (CO-OFDM)系统偏振效应的补偿。基于50Gbit/s 16QAM CO-OFDM系统,偏振效应严重影响其系统传输性 能。接收端在完成系统公共相位误差(CPE)补偿后,用本文所提 算法对系统偏振效应进行 盲均衡,根据每次迭代中信道频响分离矩阵的变化量自适应调整迭代步长,然后进行 盲相互干扰(ICI)相位噪声 补偿。仿真结果表明,本文算法的系统性能补偿效果接近于差分群延迟(DGD) 为零时 的系统性能,从而证明本文算法可以有效补偿偏振效应对高速光CO-OFDM系统 性能的 劣化。与固定步长ICA算法相比,本文算法能够极大地提高了迭代算法的收敛速度,极大 地降低了算法复杂度。     

Ultra-wide bandwidth time-hopping spread-spectrum impulse radio forwireless multiple-access communications

Attractive features of time-hopping spread-spectrum multiple-access systems employing impulse signal technology are outlined, and emerging design issues are described. Performance of such communications systems in terms of achievable transmission rate and multiple-access capability are estimated for both analog and digital data modulation formats under ideal multiple-access channel conditions     

A Blind Adaptive Technique for Cancelling ICI in OFDM-WPAN Systems

This paper proposes a hybrid OFDM/TDMA (Orthogonal Frequency Division Multiplexing/Time Division Multiple Access) scheduling method to obtain fine bit-rate granularity in wireless personal area networks (WPANs). A new blind adaptive receiver structure that combats the effects of inter-carrier interference (ICI) caused by users’ frequency offsets is also proposed to improve the system throughput. The proposed blind adaptive receiver is based on constant modulus (CM) criterion and requires no more knowledge of the frequency offsets of interferers’ signals than the conventional receiver. The advantage of our countermeasure is that frequency-guard or redundant data is not needed, thus the throughput performance will not degrade, and estimation of the frequency offset is also no longer necessary. In order to make the adaptive receiver realizable, we develop a soft-constraint-satisfaction (SCS) based algorithm to achieve faster convergence. Simulation results show that our proposal is capable of significantly reducing the influence of ICI.     

Performance of UWB SSMA using orthogonal PPM-TH over dense multipath channels

In this work we study ultra wideband (UWB) communications over dense multipath channels using orthogonal pulse position modulation (PPM) for data modulation and time-hopping (TH) for code modulation. We consider the effects of the multiple access interference (MUI) in asynchronous spread spectrum multiple access (SSMA) based on random TH codes. We use a realistic multipath channel to analyze the effects of the transmission rate in the number of users for different bit error rate (BER) values.     

基于Hermite函数的超宽带脉冲波形设计与性能分析

根据联邦通信委员会关于超宽带辐射功率谱的限制以及适用于多址方式的窄脉冲要求,设计了一组基于Hermite函数的超宽带脉冲波形。所设计的脉冲波形具有正交性和不随阶数变化的恒定脉宽两个重要特性,从而提供了良好的防时间抖动性能并有效降低符号间干扰;为了评估相关接收机的性能,对设计脉冲信号的自相关和互相关函数的闭合表达式进行了推导;分析了跳时脉位调制超宽带系统中脉冲波形在加性高斯白噪声、多址干扰、多径衰落信道等多种环境下的误码率性能,探讨了时间抖动对系统性能的影响,并将仿真结果与高斯单周期脉冲进行比较。分析结果表明该脉冲设计方案提供了一种灵活而系统的超宽带脉冲设计方法,所设计脉冲既满足辐射功率谱限制又提供良好的系统误码性能。     

The effect of NBI on UWB time-hopping systems

This letter presents an analysis of the effect of narrowband interference (NBI) on ultrawideband (UWB) time-hopping (TH) systems in the presence of multipath fading using both analytical derivations and simulations. Our analysis demonstrates that NBI may be an issue in some instances. In addition, we suggest three NBI suppression schemes for combating NBI in UWB TH systems. Single-link performance of these schemes in conjunction with a Rake-type receiver structure is estimated for both the ideal all-Rake receiver and the simpler partial-Rake receiver in an indoor environment. Two UWB pulse shapes that meet the Federal Communications Commission rules for UWB communications are considered in the investigation.     

The impact of impulse postfix length on the BER performance of IP‐OFDM systems

The impulse postfix OFDM (IP‐OFDM) system exploits the IP, which consists of a high power impulse sample and several zero samples at the end of a zero padded‐OFDM symbol block, to estimate channel impulse response (CIR) in time domain. In this paper, the impact of IP length on the BER performance of the IP‐OFDM system is analyzed. According to the analytic results, the BER performance can be significantly degraded with both a shorter length of IP as well as a longer length of IP than that of the CIR. Thus, an adaptive IP scheme, which adjusts the length of IP adaptively depending on the length of CIR, is proposed to enhance the BER performance of IP‐OFDM systems and its effectiveness is demonstrated by computer simulations. The BER performance of the IP‐OFDM systems with the proposed adaptive scheme is compared with that of the conventional IP‐OFDM system over various modulation schemes. Simulation results show that the IP‐OFDM with the proposed scheme can achieve about 2 dB performance enhancement compared with that of conventional systems at BER=10^?2. Copyright © 2010 John Wiley & Sons, Ltd.     

Ultra-Wideband Communications using Hybrid Matched Filter Correlation Receivers

Transmitted-reference (TR) schemes for time-hopping impulse radio (TH-IR) ultra-wideband (UWB) communications allow the use of simple receiver structures that are able to combine energy from different multipath components without channel estimation. A conventional TR receiver consists of a simple delay-and-multiply operation combined with an integrator. On the downside, it shows a performance loss due to non-linear operations on noise terms (generation of noise-noise cross-terms) when forming the decision variable. This paper describes a hybrid receiver structure for UWB communications that reduces these noise-noise cross-terms by first performing a "matched filtering" operation matched to the time-hopping sequence of pulses. The receiver retains most of the simplicity of the conventional TR receiver, but requires an analog correlator for the time-hopping sequence of pulses. The performance the proposed receiver is analyzed in both AWGN and multipath channels. For the AWGN case, the exact expression for the bit error probability is obtained, which takes into account the nonGaussian nature of the noise-noise cross-terms arising in the correlators. For the multipath case, both inter-frame interference and multipath interference from the reference pulse to the data pulse are considered, and approximate closed-form expressions are derived based on the assumption of a large integration interval. Also approximate criteria for optimal integration interval are obtained for the best receiver performance. Simulation studies are presented to analyze the performance of the proposed receiver structure and to confirm the theoretical analysis