基于天线分集的一种超宽带Rake接收机结构

提出了一种新的跳时超宽带无线通信系统的接收机结构,这种接收机将天线分集和Rake接收相结合, 从而提高了系统对多用户干扰和多径衰落的抑制能力。理论分析和仿真结果表明,这种接收机的性能优于常规的Rake接收机。     

Adaptive Rake receiver based on the nonlinear ACM technique

Ultra‐wideband (UWB) system is one of the possible solutions to future short‐range indoor data communications with large frequency bandwidth. However, it must coexist with other narrowband wireless systems that may cause interference to each other, and furthermore a large bandwidth will inevitably result in multi‐path fading. The Rake receiver is applicable to combat multi‐path fading but its performance degrades greatly when the narrowband interference (NBI) is present. Although some optimized Rake receivers were proposed to suppress the NBI, such as the minimum mean square error (MMSE) one, their computational complexities are usually too high to be practically implemented. In this paper, we present a new adaptive Rake receiver which can effectively suppress the NBI, based on the nonlinear Masreliez‐type approximate conditional mean (ACM) technique. Simulation results show that it outperforms the previous schemes and even it achieves almost the same performance as that of a MMSE Rake receiver but with much lower complexity. Copyright © 2010 John Wiley & Sons, Ltd.     

Digital multi-carrier spread spectrum versus direct sequence spreadspectrum for resistance to jamming and multipath

We compare single user digital multi-carrier spread spectrum (MC-SS) modulation with direct sequence (DS) SS (with a modified implementation) in the presence of narrowband interference (NBI) and multipath fading. We derive closed-form expressions for the symbol error probability for both the linear MMSE receiver as well as the conventional matched-filter receiver under different scenarios: additive white Gaussian noise (AWGN) channel with NBI, multipath channel with or without NBI. We show that DS-SS can achieve the same performance as MC-SS if the spreading code is carefully designed to have perfect periodic autocorrelation function (PACF). On the other hand, MC-SS is more robust to narrowband interference and multipath fading than is DS-SS with the widely used spreading codes that do not possess perfect PACE. Our analysis reveals that the performance improvement of MC-SS is precisely due to the implicit construction of an equivalent spreading code having nonconstant amplitude but possessing perfect periodic autocorrelation     

Performance of Multicarrier CDMA Rake System over Rayleigh Fading Channel

Based on the theory of multicarrier (MC) technique and the Rake receiver, a multicarrier DS-CDMA Rake system is proposed, where a data sequence multiplied by a spreading sequence modulates multiple carriers. The receiver provides a Rake for each subcarrier, and the outputs of the Rakes are combined by a maximal-ratio combiner. The average probability of error of the system is derived from an uncorrelated subcarrier and frequency-selective fading channel model. The system performances are evaluated over Rayleigh fading channel with an exponential multipath intensity profile(MIP) and with a rectangular MIP, respectively, when multipath interference is present. It is found that this kind of model has larger superiority in an exponential MIP than in a rectangular MIP.     

Channel shortening equalizer for multi-access TH-UWB in the presence of multipath and multiuser interference

In this paper a novel channel shortening equalizer (CSE) for time hopping ultra wideband (TH-UWB) multiple access system with pulse position modulation (PPM) is presented. As UWB channels have very long impulse responses as compared to the narrow pulse width, CSE can reduce the number of correlators. In UWB systems, due to the received pulse that is very similar to the channel impulse response (CIR), the proposed algorithm maximizes the shortening signal to inter-symbol and multiuser interferences ratio (SSINR), defined as the ratio of the received signal energy inside the desired window to the energy in the wall and multiuser interference. The existence of the proposed CSE before correlation receiver decreases the complexity of the receiver architecture by significantly reducing the number of effective channel taps. Further we extend our method to derive general expression for the bit error rate (BER) performance in the presence of inter-symbol and multiuser interferences. Computer simulation results are provided to compare the performance of the proposed method with a MSSNR CSE, lower bound, also known as All-Rake, Partial-Rake, and Selective-Rake in terms of Rake operational temporal windows and BER.     

Performance of an SIMO FM-DCSK Communication System

A single-input multiple-output (SIMO) architecture of the frequency-modulated (FM) differential code-shift keying (DCSK) modulation technique is proposed. The new scheme employs orthogonal Walsh functions at the transmitter, with parallel substreams transmitted with a single antenna to help achieve a significant increase of the data rate. Multiple antennas are used at the receiver end to form an SIMO structure so as to obtain a diversity gain. Simulation results demonstrate that at a higher signal-to-noise ratio, the proposed SIMO FM-DCSK architecture has an outstanding bit error rate performance, in contrast to the direct-sequence (DS) vertical Bell Labs layered space-time (VBLAST) scheme that uses a complicated Rake receiver and minimum mean-square error detection, at the same data rate over multipath fading channels. In particular, the new scheme does not require any prior knowledge of the channel states, exact synchronization, and the complex Rake receiver, making the proposed algorithm simpler and yet more efficient than the DS-VBLAST scheme.     

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.     

Hermite脉冲超宽带系统在多径信道的性能分析

研究了一种基于Hermite脉冲的超宽带系统在多径信道下的性能。考虑到定时抖动对Rake相关接收的影响,通过Hermite脉冲的相关函数可以计算Rake捕获的信号能量,进而得到接收机的输出信干比。在假设符号间干扰为高斯噪声下,得出系统在多径信道下的理论误比特性能。结果表明,符号间干扰能量对Rake接收机合并路径数目的变化并不敏感;增加合并路径数目可以提高系统的误比特性能;大的定时抖动范围会造成系统性能下降。     

一种改进的基于二维扩频的WCDMA非Rake接收机

针对WCDMA发射的扩频信号,不同于传统的Rake接收算法,本文采用变换域处理技术,提出了一种充分利用多径能量的二维扩频接收技术。分析了这种充分利用多径能量的二维扩频接收机的算法复杂度。通过计算机仿真,比较了传统的Rake接收算法、充分利用多径能量的二维扩频接收算法的误码率性能。结果表明,在Rake接收机难以精确分辨信道中的每条多径的情况下,使用充分利用多径能量的二维扩频接收机具有更好的误码率性能。     

60 GHz芯片间无线互连系统中的Rake接收性能

在60 GHz芯片间无线互连信道中存在着多径干扰问题,采用Rake接收是提高系统性能的重要手段。针对脉冲超宽带( IR-UWB)的芯片间无线互连系统,分析了多径信道下Rake接收机的误码性能。在IEEE 802.15.3 c信道模型基础上,对不同分支数以及不同合并方案下的选择Rake ( S-Rake)和部分Rake(P-Rake)接收机误码性能进行了研究。仿真结果表明采用支路数为2的P-Rake在数据速率为10 Gb/s时仍具有良好的抗多径性能,这为芯片间无线互连系统的Rake接收方案提供了技术参考。     

一种频率选择性衰落信道下的盲自适应去相关Rake接收机

多用户检测是DS－CDMA系统中的一项关键技术,而Rake接收是解决多径效应的一种有效方法,本文将基于Kalman滤波的多用户检测器与Rake接收相结合,提出了一种频率选择性衰落信道下的盲自适应去相关Rake接收机,研究结果表明,这种接收机具有较强的抑制多址干扰和克服“远－近”效应能力,并且能快速收敛。     

DS—CDMA通信中的一种2D-Rake接收机

Rake接收机在3G系统中已被广泛使用,实际应用证明它能够有效对抗移动无线信道中的多径衰落。传统Rake接收机有其时域一维处理的局限性,为获得更好的系统性能,本文在分析空时信道模型的基础上提出一种新的空时2D-Rake接收机算法,它将波束形成技术与Rake相结合。这种波束形成算法能够在干扰方向上完全陷零,其权重仅与阵列天线的导向矢量有关,与接收信号无关。最后简要分析了所提出的2D-Rake接收机的性能。仿真结果表明,这种空时2D-Rake接收机与传统Rake接收机相比,具有较好的性能,而且该算法对噪声具有鲁棒性。     

Pilot-Channel-Assisted Log-Likelihood-Ratio Selective Rake Combining for Low-Rate Ultra-Wideband Communications

For ultra-wideband (UWB) communications with signal energy dispersed by a large number of multipath components, the design of a Rake receiver that can provide a desirable output signal-to-noise ratio (SNR) using only a moderate number of fingers becomes an important issue. In this paper, we propose a pilot-channel-assisted log-likelihood-ratio selective combining (PCA-LLR-SC) scheme for UWB Rake receivers to be used in long-range low-rate UWB communications envisioned by the IEEE 802.15.4a PHY specification. The pilot and data channels are constructed using quadrature sinusoidal bursts that have the same Gaussian envelope. The system parameters are optimized through jointly minimizing the channel estimation mean square error and maximizing the receiver output SNR. Extensive simulations confirm that the proposed PCA-LLR-SC scheme is capable of providing robust low-rate UWB communications in fast-fading multipath channels and in the presence of multi-user interference.     

超宽带系统中基于认知的Rake接收机设计

直接序列扩频超宽带（DS-UWB,directed．spread．spectrum ultra wide band）系统采用窄脉冲和扩频技术进行通信,由于带宽很宽,有可能与同频段窄带系统共存。当存在窄带干扰时,最优接收机为采用MMSE准则的Rake接收机,而不存在窄带干扰时,MRC合并算法Rake接收机为最优接收机。由...     

Performance of space-frequency combining at the antenna array of a MC-CDMA system

In this paper, a multicarrier code-division multiple-access (MC-CDMA) system employing antenna array at base station with quaternary phase shift keying (QPSK) modulation is proposed. This receiver adopts a space-frequency two-dimensional (2-D) Rake receiver structure. Based on the detailed analysis of the interference characteristics of the proposed system, the bit error rate (BER) performance of the system is provided. With regard to spatial domain combining, the optimum and suboptimum combining weight is derived, while the suboptimum set of weights is simplified on sense of that only the knowledge of array vector of desired user alone is sufficient for the combining. Simulation results verify the analysis and show that better performance is achieved for the proposed antenna array MC-CDMA system than the corresponding single antenna approach.     

P-order metric UWB receiver structures with superior performance

The generalized Gaussian probability density function is shown to better approximate the probability density function of the multiple access interference in ultra-wide bandwidth systems than the Gaussian approximation and the Laplacian density approximation. Two ultra-wide bandwidth receiver structures based on this new approximation using a p-order metric receiver decision statistic are investigated for the detection of time-hopping ultra-wide bandwidth wireless signals in multiple access interference channels. The first receiver outperforms both the conventional matched filter ultra-wide bandwidth receiver and the soft-limiting ultra-wide bandwidth receiver when only multiple access interference is present in UWB channels. The second new receiver with adaptive limiting threshold outperforms the conventional matched filter ultra-wide bandwidth receiver, the soft-limiting ultra-wide bandwidth receiver, and the adaptive threshold soft limiting ultra-wide bandwidth receiver in all multiple access interference-plus-noise environments. In multipath channels, a new Rake receiver based on the p-order metric receiver is proposed for signal detection. Mathematical analysis and numerical results show that this new Rake receiver can achieve larger signal-to-interference-plus-noise ratio than the standard matched filter Rake receiver when multipath components are resolvable in UWB channels.     

Enhancing Data Rates of TH-UWB Systems Using M-ary OPPM-BPSM Modulation Scheme: A System Perspective

This paper proposes and studies a combined modulation scheme for time hopping ultra wideband (TH-UWB) radio system based on orthogonal pulse position modulation (OPPM) and biorthogonal pulse shape modulation (BPSM). The proposed scheme offers a high data rate with satisfactory system performance by using high-level multi-dimensional modulation schemes. It allows one to increase the number of orthogonal pulses in each pulse position and, consequently, be able to adopt shorter pulse repetition intervals than those used in conventional M-ary OPPM or M-ary biorthogonal PPM (BPPM) modulation schemes. The performance is analyzed by using characteristic function and Gaussian approximation in additive white Gaussian and multipath scenarios, respectively. The proposed scheme reduces multiple access interference considerably in multi-user environments. Finally, simulation results are provided to validate the theoretical results.     

Multiple access capacity of UWB m-ary impulse radio systems with antenna array

In this paper, the multiple access capacity of an Mary pulse position modulation (PPM) impulse radio (IR) system with antenna array is analyzed in dense multipath environments. An antenna array with Rake receivers is used to capture the signal energy from multipaths. Multiple access performance of the system is evaluated in terms of number of supported users for a given bit error rate and bit transmission rate with different number of antenna elements and selected paths. Numerical results show that the multiple access capacity of an M-ary IR system can be improved significantly by increasing the number of antenna elements and/or by adding more paths coherently at the receiver.     

低信噪比下短波数传系统Rake接收机设计

在信噪比较低的条件下,短波数据传输系统必须具备抵抗多径衰落的能力。Rake接收技术是抗多径衰落的有效手段之一。这里在直扩-八进制移相键控（DS-8PSK）短波数传系统的基础上,设计了一种改进型Rake接收机,通过在接收端对信号进行相关搜索、多径分离和多径合并,实现了有用信号的分集接收。仿真结果表明,设计的Rake接收机能有效解决短波信道中多径衰落的难题,保证低信噪比条件下短波数据传输的可靠性。     

超宽带Rake接收机的性能分析

超宽带信号经多径信道传播会产生严重的时间弥散，采用Rake接收是提高超宽带接收机性能的重要手段。文中首先介绍了加性高斯白噪声信道模型和IEEE802．15．3a标准信道模型。通过比较分析几种典型调制的超宽带冲激无线电信号的最佳接收机，推广到Rake接收机的性能分析。重点分析并仿真了不同信道、不同调制方式、不同判决方式、不同合并方式、不同速率的情况下Rake接收机的性能。