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.     

高速发射参考超宽带系统中的接收分集及均衡方案

在高传输速率发射参考超宽带(TR-UWB)系统中,采用传统自相关接收机将引入2阶符号间干扰(ISI)。该文证明,通过多天线接收及等增益合并方案可获得显著的接收分集增益,并且通过多天线合并降低ISI模型的非线性程度,消除误码率地板效应。合并后的ISI模型可采用线性模型进行近似,采用数据辅助自适应算法对合并信道进行估计,大大降低原非线性模型中信道估计器和均衡器的复杂度,均衡后的性能与完整2阶ISI模型相比也随天线数增加而趋于相同。     

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.     

带有循环前缀的单载波通信系统中干扰抵消辅助的频域均衡技术

在单载波通信系统中,循环前缀(简称CP)的存在不仅能减轻由于多径信道造成的传输数据块间信号干扰(简称IBI),而且可以采用频域均衡(简称FDE)技术来补偿由多径信道引起的频率选择性信号衰落.本文主要分析了带有循环前缀的单载波通信系统(简称CP-SC)中广泛应用的基于线性最小均方误差(简称LMMSE)准则的FDE技术中的残余信号间干扰(RISI)问题,提出了一种干扰抵消辅助的LMMSE-FDE接收机结构,它能很好的消除RISI,改善系统性能.     

SC-FDE系统的一种新型判决反馈均衡器

针对单载波频域均衡系统MMSE均衡器存在残留码间干扰的缺点,提出MMSE-RISIC判决反馈均衡器消除残留码间干扰.MMSE-RISIC均衡器采用传统MMSE均衡后的判决数据,对残留码间干扰进行估计并消除.残留码间干扰的估计主要采用FFT和IFFT运算,与其他方法相比计算量较小.对该均衡器在不同信道下进行了计算机仿真,结果表明,在频率选择性衰落信道条件下,系统性能有了较为明显的提高.     

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.     

Equalization for DS-UWB Systems—Part I: BPSK Modulation

Ultra-wideband wireless transmission has attracted considerable attention both in academia and industry. For high-rate and short-range transmission, direct sequence based ultra-wideband (DS-UWB) systems are a strong contender for consumer market applications. Due to the large transmission bandwidth, the UWB channel is characterized by a long root-mean-square delay spread and the RAKE receiver cannot always overcome the resulting intersymbol interference. We therefore study equalization for DS-UWB systems. This paper is comprised of two parts. In this first part, we consider DS-UWB with binary phase-shift keying (BPSK) modulation, which is the mandatory transmission mode for DS-UWB systems promoted by the UWB Forum industry alliance. We derive matched filter bounds for optimum equalization taking into account practical constraints like receiver filtering, sampling, and the number of RAKE fingers when RAKE preprocessing is applied at the receiver. Our results show that chip-rate sampling is sufficient for close-to-optimum performance. For analysis of suboptimum equalization strategies we further study the distribution of the zeros of the channel transfer function including RAKE combining. Our findings suggest that linear equalization is well suited for the lower data rate modes of DS-UWB systems, whereas nonlinear equalization is preferable for high-data rate modes. Moreover, we devise equalization schemes with widely linear processing, which improve performance while not increasing equalizer complexity. Simulation and numerical results confirm the significance of our analysis and equalizer designs and show that low-complexity (widely) linear and nonlinear equalizers perform close to the pertinent matched filter bound limit.     

Minimum BER Block-Based Precoder Design for Zero-Forcing Equalization: An Oblique Projection Framework

This work devises a minimum bit error rate (BER) block-based precoder used in block transmission systems with the proposed cascaded zero-forcing (ZF) equalizer. The study framework is developed as follows. For a block-based precoder, a received signal model is formulated for the two redundancy schemes, viz., trailing-zeros (TZ) and cyclic-prefix (CP). By exploiting the property of oblique projection, a cascaded equalizer for block transmission systems is proposed and implemented with a scheme, in which the inter-block interference (IBI) is completely eliminated by the oblique projection and followed by a matrix degree-of-freedom for inter-symbol interference (ISI) equalization. With the available channel state information at the transmitter side, the matrix for ISI equalization of the cascaded equalizer is utilized to design an optimum block-based precoder, such that the BER is minimized, subject to the ISI-free and the transmission power constraints. Accordingly, the cascaded equalizer with the ISI-free constraint yields a cascaded ZF equalizer. Theoretical derivations and simulation results confirm that the proposed framework not only retains identical BER performance to previous works for cases with sufficient redundancy, but also allows their results to be extended to the cases of insufficient redundancy.     

Space–Time Turbo Equalization With Successive Interference Cancellation for Frequency-Selective MIMO Channels

This paper addresses the issue of iterative space–time equalization for multiple-input–multiple-output (MIMO) frequency-selective fading channels. A new soft equalization concept based on successive interference cancellation (SIC) is introduced for a space–time bit-interleaved coded modulation (STBICM) transmission. The proposed equalizer allows us to separate intersymbol interference (ISI) and multiantenna interference (MAI) functions. Soft ISI is successively suppressed using a low-complexity suboptimum minimum mean square error (MMSE) criterion. The decoupling of ISI and MAI offers more flexibility in the design of the whole space–time equalizer. Different multiantenna detection criteria can be considered, ranging from simple detectors to the optimal maximum a posteriori (MAP) criterion. In particular, we introduce two soft equalizers, which are called SIC/SIC and SIC/MAP, and we show that they can provide a good performance-to-complexity tradeoff for many system configurations, as compared with other turbo equalization schemes. This paper also introduces an MMSE-based iterative channel state information (CSI) estimation algorithm and shows that attractive performance can be achieved when the proposed soft SIC space–time equalizer iterates with the MMSE-based CSI estimator.      

Matching pursuit-based tap selection technique for UWB channel equalization

In this letter, matching pursuit (MP) based tap selection technique is proposed and applied to ultra wideband (UWB) indoor channel equalization in the presence of inter symbol interference (ISI) and multiple access interference (MAI). Given the limited training sample support, quadratic constraint is incorporated into MP algorithm to insure the robustness for tap selection. The proposed method is shown to outperform the conventional minimum mean square error (MMSE) equalizer significantly given the same amount of training symbols.     

A highly sustainable multi-band orthogonal wavelet code division multiplexing UWB communication system for underground mine channel

The underground mine channel is highly hostile for communication. A sustainable communication system is required to be integrated as a multipurpose system capable of transferring all types of information such as data, voice and video on identical infrastructure. With its large bandwidth, the Ultra Wide Band (UWB) provides a promising solution to satisfy these requirements. The Multi-Band Orthogonal Frequency Division Multiplexing (MBOFDM) UWB system provides high resilience to Inter-Symbol Interference (ISI) caused by multipath fading channels. This study compares the performance of various existing multi-band UWB systems in the IEEE UWB channel and underground mine channel: MBOFDM, Multi-Band Orthogonal Frequency Code Division Multiplexing (MBOFCDM) and Multi-Band Orthogonal Wavelet Division Multiplexing (MBOWDM). Further, the application of 2D spreading to the MBOWDM UWB system is attempted, and as a result, the Multi-Band Orthogonal Wavelet Code Division Multiplexing (MBOWCDM) UWB system is proposed. The performance of the MBOWCDM system in the IEEE UWB channel and underground mine channel is investigated. The performance of the MBOWCDM UWB system is observed to be superior to those of other multi-band UWB systems. Moreover, the MBOWCDM UWB system supports a higher data rate with low complexity and lower overheads.     

Channel precoding for indoor radio communications using dimensionpartitioning

A new phase precoding technique is developed to combat the intersymbol interference (ISI) resulting from a frequency-selective slowly fading channel in a personal communication system using quadrature phase-shift keying (QPSK). Based on a new dimension partitioning technique, the precoder predistorts only the phase of the transmitted signal to keep a constant transmitted signal amplitude and, therefore, to ensure the stability of the precoder even in equalizing a nonminimum-phase channel. Under the constraint of the constant amplitude, the dimension partitioning method is developed to guarantee the possibility of correct detection for all transmitted information symbols and to further improve the transmission accuracy by increasing the size of the decision regions. Analytical and simulation results demonstrate that over frequency selective Rayleigh and Rician fading channels, the system using the proposed channel precoder can achieve a bit error rate (BER) comparable with that using a conventional decision feedback equalizer (DFE). The precoder can outperform the DFE in an indoor environment where there is a strong direct propagation path. The main advantage of using the precoder is that the impairment of ISI due to multipath propagation on the transmission performance can be mitigated without increasing the complexity of the portable unit receiver. The proposed technique is especially useful for personal communications, where ISI due to multipath fading channels can severely deteriorate the BER transmission performance and where the simplicity of portable units is a vital characteristic of the system     

Adaptive Decision Feedback Space–Time Equalization With Generalized Sidelobe Cancellation

In wireless communications, cochannel interference (CCI) and intersymbol interference (ISI) are two main factors that limit system performance. Conventionally, a beamformer is used to reduce CCI, whereas an equalizer is used to compensate for ISI. These two devices can be combined into one as space–time equalizer (STE). A training sequence is usually required to train the STE prior to its use. In some applications, however, spatial information corresponding to a desired user is available, but the training sequence is not. In this paper, we propose an adaptive decision feedback STE to cope with this problem. Our scheme consists of an adaptive decision feedback generalized sidelobe canceller (DFGSC), a blind decision feedback equalizer (DFE), and a channel estimator. Due to the feedback operation, the proposed DFGSC is not only superior to the conventional generalized sidelobe canceller but also robust to multipath channel propagation and spatial signature error. Theoretical results are derived for optimum solutions, convergence behavior, and robustness properties. With the special channel-aided architecture, the proposed blind DFE can reduce the error propagation effect and be more stable than the conventional blind DFE. Simulation results show that the proposed STE is effective in mitigating both CCI and ISI, even in severe channel environments.      

Mitigating multiple access interference and intersymbol interference in uncoded CDMA systems with chip-level interleaving

The presence of intersymbol interference (ISI), in addition to multiple access interference, severely hampers the performance of a code-division multiple-access (CDMA) communication system. In such a situation, channel coding can be used to obtain better performance, but at the cost of a reduction in rate of flow of information. In this paper, it is shown with the help of simulation results that the chip-interleaved CDMA system effectively combats ISI without requiring additional channel coding. The system differs from the conventional CDMA system in the sense that, the chip sequence resulting from the pseudo noise (PN) sequence modulation is interleaved before transmission. Two receivers are proposed, one based on the turbo equalization method which employs a maximum a posteriori equalizer of exponential complexity and the other based on minimum-mean square error-optimized iterative interference cancellation principles which is of linear complexity. Simulation results are provided which show that error rates close to the no-ISI single-user case can be obtained. The shortcomings of the coded CDMA with turbo detection system in the presence of ISI are also discussed.     

最小均方误差均衡器的Matlab仿真设计

随着集成电路与计算机技术的发展,数字通信以其特有的优越性已得到广泛应用。就数字通信而言,误码率和频谱效率是两个主要的系统衡量指标。由于常用信道传输特性不是理想的,数字信号经过传输后会产生严重的码闽干扰,这对于接收机的正确判决非常不利,从而增加了通信的误码率,因此码间干扰的消除对提高通信系统的传输质量有重要意义。介绍基于最小均方误差准则的线性均衡器原理,利用Matlab对最小均方误差均衡器进行仿真设计,并对其性能进行分析。结果表明,最小均方误差均衡器能够有效消除码间干扰,增强基带传输系统的传输特性,是一种有效的均衡接收技术。     

A 5-6.4-Gb/s 12-channel transceiver with pre-emphasis and equalization

A 5-6.4 Gb/s transceiver, consisting of a parallel 12-channel transmitter (Tx), 12-channel receiver (Rx), clock generators based on LC-VCO phase-locked loops (PLLs), and a clock recovery unit, was developed. The Tx has a five-tap pre-emphasis filter, and the Rx has an equalizer with an intersymbol interference (ISI) monitor. Monitoring the ISI enables fine adjustment of loss compensation. The pre-emphasis filter in the Tx and the equalizer in the Rx compensate for transmission losses of up to 20 dB at 6.4 Gb/s, respectively. Both the Tx and Rx channels, including the PLLs, are 3.92 mm/sup 2/ in area. The transmitter dissipates 150 mW/channel at 6.4 Gb/s when compensating for a loss of 20 dB, and the receiver 90 mW/channel when compensating for the same loss.     

RLS Algorithm with Variable Fogetting Factor for Decision Feedback Equalizer over Time-Variant Fading Channels

In a high-rate indoor wireless personal communication system, the delay spread due to multipath propagation results in intersymbol interference (ISI) which can significantly increase the transmission bit error rate (BER). Decision feedback equalizer (DFE) is an efficient approach to combating the ISI. Recursive least squares (RLS) algorithm with a constant forgetting factor is often used to update the tap-coefficient vector of the DFE for ISI-free transmission. However, using a constant forgetting factor may not yield the optimal performance in a nonstationary environment. In this paper, an adaptive algorithm is developed to obtain a time-varying forgetting factor. The forgetting factor is used with the RLS algorithm in a DFE for calculating the tap-coefficient vector in order to minimize the squared equalization error due to input noise and due to channel dynamics. The algorithm is derived based on the argument that, for optimal filtering, the equalization errors should be uncorrelated. The adaptive forgetting factor can be obtained based on on-line equalization error measurements. Computer simulation results demonstrate that better transmission performance can be achieved by using the RLS algorithm with the adaptive forgetting factor than that with a constant forgetting factor previously proposed for optimal steady-state performance or a variable forgetting factor for a near deterministic system.     

MIMO系统中的非常数模半盲均衡方案

在多输入多输出(Multiple-Input Multiple-Output, MIMO)无线通信中,信道衰落、随机噪声、空间多径效应和共信道干扰等因素会产生符号间干扰,造成接收端信号严重失真。为提高MIMO系统传输性能,有效减少信道中相位失真,提出了一种改进的非常数模半盲均衡方案(Non-constant Modulus Algorithm, NCMA)。该方案结合了MIMO信道先验知识对半盲均衡器的初始值进行预设,采用变步长迭代更新均衡器权系数,寻找到最小目标函数值。仿真结果表明,改进后的算法能通过控制变步长减小剩余稳态误差,加快收敛速度,改善信道均衡效果。     

超宽带系统在窄带干扰条件下的性能研究

超宽带（Ultra Wide-Band，UWB）系统发射信号的带宽在一个非常大的频段范围内，易与已存在的窄带无线通信系统的带宽形成重叠。因此，有必要研究UWB系统在频段重合范围内的抗干扰能力。文中首先分析了直接扩频超宽带系统在最小均方误差准则检测方式下，RAKE接收机的比特误码率（Bit Error Rate，BERl，然后研究了普通窄带系统的功率谱密度，最后做出了仿真分析。结果表明，在CM1信道传播下，窄带干扰对UWB系统不会造成很大影响，而在CM2信道传播下会照成一定影响，必须通过其他通信手段如信道编码来降低BER，实现通信的可靠性。     

Decision-Feedback Equalization for Pulse-Position Modulation

In this paper, we propose a minimum mean squared error (MMSE) decision feedback equalizer (DFE) for pulse position modulated (PPM) signals in the presence of intersymbol interference (ISI). While traditional uses of PPM may not have had ISI, PPM is increasingly being considered for use in situations where ISI is an issue, such as high-performance optical communication systems and ultrawideband communications. First, we review previous work on the subject which used the zero-forcing criterion under strict assumptions about the channel and equalizer lengths. Then, we derive a computationally efficient MMSE equalizer which removes these restrictions, and is suitable for use with training-based stochastic gradient-descent algorithms. Finally, we demonstrate the performance of the proposed equalizer with simulations.