Smart UWB System Based on STC and TR Techniques for BER Performance Enhancement and Interference Rejection

In this research, a novel smart UWB system is introduced. The proposed system is based on using an adaptive maximum ratio combining (MRC) Rake receiver. The proposed adaptive Rake receiver uses Genetic algorithm (GA) to adaptively select the delays of the fingers of the Rake receiver depending on the channel impulse response. It adaptively selects the delays that will allow the Rake receiver to capture most of the energy in the multipath components with minimum complexity. This adaptive Rake receiver is referred to as a GA Rake. The adaptive GA Rake is applied to a single-input single-output and space time coding (STC) multi-input single-output UWB systems. The performance of those systems using a GA Rake is compared to their performance when using a conventional MRC-Rake receiver and showed a great enhancement in performance with less receiver complexity. Also, in this paper, the smart UWB system using STC is modified by using the time reversal (TR) pre-coding technique. The modified system is referred to as a TR smart UWB system. This modification leads to more enhancements in performance and more reduction in receiver complexity over the smart UWB system. Moreover, this paper also shows the ability a TR smart UWB system in combating interference from other UWB systems.     

Channel characterization of time reversal UWB communication systems

An ultra wideband (UWB) communications system that applies time reversal to transmit the desired signal is investigated. Exact expressions for the first- and second-order moments, cross-correlation, intersymbol interference metric, and correlation coefficient of time reversal (TR) UWB equivalent channel are derived in terms of the physical channel parameters such as delay spread and mean excess delay. These expressions are verified by simulated and experimental results. It is shown that TR-UWB excess delay is very smaller than UWB and its delay spread decreases as signaling bandwidth increases. Semi-analytical results show that the time reversal UWB delay spread is approximately the same as UWB. Furthermore, an ISI metric is derived for TR-UWB channel based on transmitted signal and UWB channel parameters. Moreover, correlation coefficient of two TR-UWB received signals with different power delay profile is computed analytically. Simulation and analytical results show that for τ?>?0.3T _w correlation coefficient is below 0.25 and for τ?>?T _w correlation coefficient is zero, where T _w is the transmitted pulse width. Finally, theoretical performance of a receiver with one tap matched filter is computed and compared with measured and simulated result.     

Energy Detector in Ultra Wideband Systems Using Phase Compensation Technique

Energy detectors have the advantage of simple structure and inexpensive price. Due to the low signal to noise ratio (SNR) of the received signal in ultra-wideBand (UWB) system, these desirable advantages can be achieved at the expense of non-trivial performance degradation. This paper presents a phase compensation (PC) technique to improve the performance of energy detector in UWB systems. In PC-UWB, the frequency dependent phase of the system response at the transmitter is extracted and its opposite spectral phase is used as prefilter. Because of Low complexity, cost and energy consumption of energy detectors, PC techniques has extensive potential for future of UWB communication systems. Measurement results show that the use of PC-UWB leads to signal power concentration at the receiver, which reduces the number of RAKE fingers required in coherent detection as well as achieves a higher data rate with less intersymbol interference. However time reversal UWB can achieve secure data transmission, but its performance is worse than PC-UWB. Simulation results show that phase compensation reduces the inter symbol interference impacts. Therefore it is possible to use a simple receiver with insignificant performance degradation. It is also shown that PC-UWB considerably outperforms TR-UWB and has satisfying performance in SNR greater than 13 dB.     

Interference Cancellation with Space Diversity for Downlink MC-CDMA Systems

Modern wireless communications require an efficient spectrum usage and high channel capacity and throughput. Multiple-input and multiple-output (MIMO), Linear equalizers, multi-user detection and multicarrier code-division multiple access (MC-CDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate multiple access interference (MAI), eliminate Inter symbol interference (ISI) and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Multiple-output (SIMO) downlink MC-CDMA systems. It employs an interference cancellation scheme to suppress the interference caused by the multipath fading channel. Also, the proposed scheme is developed for MIMO MC-CDMA system. The performance analysis of Downlink MIMO MC-CDMA systems with V-BLAST over frequency selective fading channel is investigated under various number of transmit and receive antennas. The simulation results show proposed SIMO equalization with parallel interference cancellation scheme is effective in reducing the ISI and the MAI. It improves the performance significantly and the simulation results show that MIMO MC-CDMA with V-BLAST multi-user detection provides high data rate and the BER significant improvement.     

A novel self-pilot-based transmit-receive architecture for multipath-impaired UWB systems

In the last few years, ultra-wideband (UWB) systems became an appealing technology for wireless communication applications. Unfortunately, when the transmission channel is affected by intersymbol interference (ISI), system performance of UWB systems equipped with receivers based on conventional matched filters presents error-floor phenomena. Aimed by these considerations, in this letter, we present a novel transmit-receive scheme allowing blind channel estimation and minimum mean-square error linear channel equalization. Essentially, the proposed scheme exploits a very short duration of the UWB pulse for achieving reliable blind deconvolution of the received signal. A nice feature of the resulting system is that blind deconvolution of the received signal is achieved without power and throughput losses. Simulation results support the effectiveness of the proposed scheme, and show that it is able to gain about 8 dB over current UWB receivers based on matched filtering on several test channels impaired by ISI.     

针对超宽带发射参考接收机中符号间 干扰的Turbo均衡

本文对超宽带发射参考技术在时分多址方式和传统脉冲对结构下对高数据传输速率造成的符号间干扰模型进行了分析,证明其属于二阶符号间干扰系统,系数由波形相关矩阵决定.在此模型的基础上利用超宽带室内信道慢衰落的特点,给出了一种无重叠导频进行模型系数的估计方法.引入Turbo原理,采用修正的Turbo均衡器对此非线性符号间干扰系统进行均衡.IEEE802.15.3a室内多径信道仿真表明,Turbo均衡在非线性符号间干扰系统下仍然有效.     

Equivalent system model and equalization of differential impulse radio UWB systems

A discrete-time equivalent system model is derived for differential and transmitted reference (TR) ultra-wideband (UWB) impulse radio (IR) systems, operating under heavy intersymbol-interference (ISI) caused by multipath propagation. In the systems discussed, data is transmitted using differential modulation on a frame-level, i.e., among UWB pulses. Multiple pulses (frames) are used to convey a single bit. Time hopping and amplitude codes are applied for multi user communications, employing a receiver front-end that consists of a bank of pulse-pair correlators. It is shown that these UWB systems are accurately modeled by second-order discrete-time Volterra systems. This proposed nonlinear equivalent system model is the basis for developing optimal and suboptimal receivers for differential UWB communications systems under ISI. As an example, we describe a maximum likelihood sequence detector with decision feedback, to be applied at the output of the receiver front-end sampled at symbol rate, and an adaptive inverse modeling equalizer. Both methods significantly increase the robustness in presence of multipath interference at tractable complexity.     

应用于窄脉冲超宽带通信的均衡算法及其实现

由于高传输速率及多径效应的影响,在窄脉冲超宽带(IR-UWB)通信系统中,基于传统横向滤波器的时域均衡技术已无法对抗严重的码间干扰。通过收发两端联合设计,提出了一种以数据块为基础的基于码元的频域均衡技术,同时介绍了其应用前提和算法实现。仿真结果表明,所提方法能有效对抗超宽带通信系统中存在的码间干扰,是窄脉冲超宽带通信系统中更为现实的技术选择。     

Application of convolutional error correcting codes in ultrawideband M-ary PPM signaling

The interference issues related to ultrawideband (UWB) radio pose tight restrictions on the maximum data rate of UWB radio telecommunication systems. A possible solution is to reduce the required signal to interference ratio (SIR) that gives satisfactory performance to the UWB system. In this letter, we propose coded M-ary UWB radio communication systems. Two classes of convolutional codes, namely, low-rate superorthogonal codes and high-rate punctured codes are considered for this purpose. Simulation results on the bit error rate of the proposed system indicates that the system is capable to work in lower SIR's and therefore supports higher data transmission rates in a real interference environment compared to the previously proposed UWB communication systems.     

M-ary orthogonal coded/balanced ultra-wideband transmitted-reference systems in multipath

A new M-ary orthogonal coded signaling is introduced to avoid the inter-frame interference that is especially detrimental to realizing high rate ultra-wideband (UWB) transmitted-reference (TR) systems. To further increase the information rate, the inter-pulse interference by an overlap of multipath-delayed pulses is controlled by integrating the signaling and a pair of balanced matched Alters in a joint manner, so as to permit a shorter time delay between the reference and data pulses in TR systems. To evaluate an achievable information rate increase relative to conventional TR, the symbol error probability (SEP) is theoretically derived for the proposed M-ary orthogonal coded/balanced TR system, considering the realistic IEEE standard UWB channel models. In addition, we consider the issue of receiver complexity and present two alternative low- complexity receiver implementations for the proposed TR system.     

基于反转镜技术的超宽带通信模型分析

由于超宽带系统的发射功率受到了严格的限制,因此这种系统的中心问题是通过多径传播有效地接收发射机发射的信号.对点到点无线通信来说,应把注意力放在能够有效地消除由于频率选择性衰落带来的码间干扰(ISI).根据通信中的时间反转镜技术运用在多入单出(MISO)系统中可明显地减少超宽带系统中的码间干扰 (ISI)的事实,并基于一种利用时间反转镜技术的多入单出系统,通过对信躁比的离散化分析,表明此系统的性能优于聚束系统.     

On the Use of Uniform Circular Arrays for Characterizing UWB Time Reversal

Time reversal (TR) is a promising technique for the improvement of future telecommunication systems. A better understanding of this phenomenon requires deeper investigation of real propagation channels. This paper presents a new scheme to characterize the space-time focusing characteristics of time reversal. The originality of this technique relies on the use of a virtual Uniform Circular Array, which allows to simply derive the space-time focusing characteristics of the channel. An extensive ultrawideband (UWB) propagation experiment is reported, and the method is applied to the experimental data. Different focusing parameters are computed in different configurations. In particular, the paper shows that the delay spread parameter is inappropriate to study time focusing, and a new parameter characterizing the maximum symbol rate is introduced. In a UWB single-input single-output (SISO) configuration, time reversal increases the power of the strongest path by up to 10 dB and the total received power by up to 5 dB. Our analysis shows that when the received signal is appropriately focused in time, the data rate can be increased by a factor of three. The received signal is also focused in space, and the average size of the focus area is approximately $3lambda$ , where $lambda$ is the wavelength of the central frequency. We observed that the focusing spot is larger than the ideal case and is often directed inline with the transmitter direction. At $9lambda$ away from the target, the received signal is attenuated by up to 10 dB in a non line-of-sight environment, which demonstrates the high focusing capabilities of the TR-UWB scheme.      

空时编码在单天线UWB通信系统中的应用

超宽带(UWB)适用于基带多用户通信、战场无线通信和高数据率多媒体业务等通信系统,其数据传输速率高、功耗低、多径分辨能力强。但超宽带脉冲信号时域支撑区极窄,信道为密集多径,将空时编码技术引入超宽带通信系统,能够提升无线通信系统的信道容量与抗误比特率性能。在对UWB空时分组编码系统模型性能理论分析的基础上,对空时分组码在单天线UWB系统应用方案与UWB空时分层码方案进进行了简要介绍,利用Matlab对IEEEUWB信道模型进行仿真,提出了空时编码在UWB通讯技术中应用后提升短距高速率无线通信的性能的结论 。     

An Analysis of Interference Mitigation Capability of Low Duty-Cycle UWB Communications in the Presence of Wideband OFDM System

Low duty-cycle (LDC) algorithm is interference mitigation technique, which can reduce the average interference to the existing radio systems by lowering pulse repetition interval or pulse occupation time. In this paper, the coexistence environment between low data rate ultra wideband (UWB) communication system such as wireless sensor network and the existing wideband system using orthogonal frequency division multiplexing (OFDM) such as 4th generation mobile cellular system (4G), worldwide interoperability for microwave access (WiMAX), and field pickup unit (FPU) is considered. In order to analyze the interference mitigation capability of LDC algorithm with impulse based UWB (LDC-UWB) system, the frame error rate (FER) of wideband OFDM system is examined for two types of LDC-UWB system: the signal with random polarity such as binary pole signals and without random polarity such as mono pole signals. We present that LDC algorithm is an efficient interference mitigation technique for low data rate UWB communication via computer simulations regardless of definitions of transmitted energy of UWB communication system, and also that the signal with random polarity is suitable for LDC-UWB system to mitigate interference to the other radio systems. We further investigate the adequate duty-cycle of LDC-UWB system for each definition of transmitted power of UWB communication.     

Coherence and intersymbol interference in digital fiber optic communication systems

Because of the quadratic relation between optical power and optical field, the transmission of optical power through an optical fiber is, in principle, nonlinear. Under usual conditions, random fluctuations of the incoherent source field and mode mixing tend to linearize the system. Previous assessments of the effect of intersymbol interference (ISI) on the performance of digital fiber optical communication systems are based on the assumption of linearity. This paper examines the effect of ISI on error rates in the other limit of complete coherence where fields are additive but powers are not. It is found that in this nonlinear regime and for weak dispersion, ISI may reduce the error rates slightly, whereas strong dispersion augments it considerably.     

基于TR-RASK的多输入多输出UWB方案

本文提出了一种基于时间反演接收天线移位键控的多输入多输出超宽带（time reversal receive antenna shift keying multiple input multiple output ultra wide band,TR-RASK-UWB）方案,较好地解决了MIMO-UWB系统中多天线干扰和多径干扰的问题。首先,该方案利用RASK调制将发射天线阵列形成空间波束,同时使用TR技术聚焦发射信号到达目标接收天线;其次,通过最大接收功率检测算法理论推导了该方案在UWB多径信道下的误比特率（bit error rate,BER）表达式,并通过仿真得到验证;最后结果表明,在发射天线和接收天线相同的情况下,所提方案的性能相比于MIMO-UWB有较大提升。      

Multiple Access Performance of Balanced UWB Transmitted-Reference Systems in Multipath

Recently, a novel balanced transmitted-reference (TR) system has been proposed for UWB communications. This TR system is capable of properly eliminating the inter-pulse interference (IPI) between the reference and data pulses in a single user multipath environment. In this paper, we investigate its multiple access (MA) performance by evaluating the second- order moments of noise and interference terms. The analytical framework developed here can also be used to accurately evaluate the MA performance of conventional TR system. It is shown that the proposed balanced TR system can achieve comparable MA capacity as conventional TR, while operating at higher information rate. On the other hand, given a target information rate, the balanced TR system in conjunction with M-ary signaling offers higher MA capacity than conventional TR system. The performance results suggest there exists a trade-off between the inter-pulse distance, thus the achievable information rate, and the MA capacity.     

Modulated Coded OFDM Systems with Special Precoder

Orthogonal frequency-division multiplexing (OFDM) has been considered as a strong candidate for next-generation wireless communication systems to achieve high rate data transmission in a mobile environment. However, the performance of OFDM systems may be degraded when inter-symbol interference (ISI) channels have spectral nulls, and the data rate overhead due to the insertion of cyclic prefix is high when ISI channels have many taps. Recently, the precoded OFDM systems and vector OFDM (VOFDM) systems were proposed to combat these two problems, respectively. We propose a novel modulated coded OFDM system with special precoder that is robust to spectral nulls and with reduced cyclic prefix length. The precoder can be easily formulated by channel coding and digital modulation. This precoding scheme uses the redundancy information introduced by channel coding (such as zeros inserted in precoded OFDM system). It is able to remove the spectral nulls of an ISI channel without knowing the ISI. Simulation results show that our proposed OFDM system performance is better than precoded OFDM and VOFDM system.     

Mitigation of channel estimation error in TR–UWB system based on a novel MMSE equalizer

The time reversal (TR) technique combined with the ultra-wideband (UWB) system offers a new potential for decreasing the cost and complexity of the UWB receivers. In spite of TR–UWB's good performance in perfect channel state information (CSI), it is very sensitive to the channel estimation error. The effect of channel imperfection on the TR–UWB system is considered in this paper. At first, based on a minimum mean square error (MMSE) equalizer receiver, a prefilter is calculated in closed form to improve the performance of the TR–UWB system in an imperfect CSI scenario. Furthermore, for comparison purposes, a similar calculation for prefilter is carried out based on a simple matched filter (MF) receiver. Then, in order to improve the MF receiver performance, a two-stage iteration-based algorithm is developed. The initial value for this iteration-based improved algorithm is considered to be a prefilter which is calculated in the TR–UWB system with MMSE equalizer. This optimized algorithm causes the channel estimation error in the TR–UWB system to become zero in some steps. Finally, exhaustive simulations are done to demonstrate the performance advantage attained by the improved algorithm.     

基于矩阵逆幂法的SIMO信道模型盲均衡方法

码间干扰是实现高速数据传输的主要限制因素,如何减小码间干扰始终是通信信号处理的研究方向之一。文章根据SIMO（Single Input Multiple Output）系统模型的互关系,利用输入信源和观测噪声的统计特性,给出了一种基于矩阵逆幂法的信道盲均衡算法。计算机仿真结果表明,该算法自适应递推性有效,并在低信噪比的情况下较传统的CR（Cross Relations）方法有更好的均衡效果。