Performance analysis of non-coherent UWB receivers at different synchronization levels

Non-coherent ultra-wideband (UWB) receivers require no channel state information for demodulating the received signal. The primary non-coherent receiver in the UWB literature is the autocorrelation receiver, which autocorrelates the received signal at specific time lags, circumventing problems of template signal design and multipath energy combining. A unique advantage of the UWB autocorrelation receiver is its robustness to synchronization errors, which has not been explored yet to date. This paper investigates two major UWB schemes employing autocorrelation receivers: the transmitted reference (TR) scheme (R. Hoctor and H. Tomlinson, 2002) and the differential (DF) scheme (M. Ho et al., 2002). Performance is analyzed for TR and DF receivers at different synchronization accuracy levels, their robustness to synchronization errors is shown, and the existence of a tradeoff between performance and synchronization complexity for non-coherent UWB receivers is revealed. As a result of our analysis, comparisons of TR and DF schemes are also made in the presence of synchronization errors, which have not been addressed before. Simulations corroborate our findings.     

Transmit-reference UWB system based on m-sequence codes

In this article,a novel transmit-reference(TR) signaling scheme is proposed for ultra-wideband(UWB) system,where by invoking m-sequence codes the reference and data pulses can be transmitted side by side to increase the data rate.This structure enables demodulation with a simple and practical autocorrelation receiver despite existence of severe inter-pulse interference(IPI).To evaluate detection performance of the new designs,closed-form expression for bit error probability(BRP) is theoretically derived and...     

Chaos-Based transmitted-reference ultra-wideband communications

Transmitted-reference (TR) ultra-wideband (UWB) communication systems have gained increasing popularity for the usage in the low data rate application, due to its non-coherent receiver structure. In conventional TR system, non-coherency at the receiver is achieved by sending reference pulses prior to the data-bearing pulses. Then, at the receiver side, reference pulses are used as template signals for correlation with data-bearing pulses. Therefore, the orthogonality between reference and data pulses is obtained in time division multiple access (TDMA) fashion. However, the implementation of a wideband delay line is very difficult in the current low power integrated circuits. In this paper, a TR method called Chaos-Based TR (CB-TR) is proposed. In the proposed method, chaotic sequences are used to separate the reference and data pulses. Such approach exploits the benefits of chaotic signals, such as non-periodicity, easy-to-generate, impulse-like autocorrelation value and low cross-correlation value. Furthermore, in order to decrease the influence of some negative properties of conventional chaotic maps, a modified chaotic generator (MCS) is proposed. Simulation results over the IEEE 802.15.4a channel model show comparable bit error rate performance to other TR methods.     

TR-UWB系统性能分析与仿真

分析比较了3种基于自相关接收原理的超宽带发送参考（TR）冲击无线电系统,即简单TR（STR）系统、平均TR（ATR）系统和差分TR（DTR）系统在多径信道中的误码率（BER）性能、实现复杂度和传输速率,并对这3种系统的性能在CM1和CM2信道中进行了计算机仿真。分析和仿真结果表明,ATR和DTR的性能恒优于STR,但ATR和DTR的性能的优劣则取决于具体的参数取值,在一定的参数条件和复杂度约束下,DTR系统是一种在性能、复杂度和传输速率方面都能兼顾的比较折中的传输方案。     

Performance of ultra-wideband communications with suboptimal receivers in multipath channels

The performance of a single-user ultra-wideband (UWB) communication system employing binary block-coded pulse-position modulation (PPM) and suboptimal receivers in multipath channels is considered. The receivers examined include a RAKE receiver with various diversity combining schemes and an autocorrelation receiver, which is used in conjunction with transmitted reference (TR) signaling. A general framework is provided for deriving the performance of these receivers in multipath channels corrupted by additive white Gaussian noise (AWGN). By employing previous measurements of indoor UWB channels, we obtain numerical results for several cases which illustrate the tradeoff between performance and receiver complexity.     

Slightly Frequency-Shifted Reference Ultra-Wideband (UWB) Radio

A promising ultra-wideband (UWB) radio technique being widely considered for low-data-rate applications, such as those often encountered in sensor networks, is the transmitted reference (TR) UWB scheme. However, the standard TR-UWB scheme, while often motivated by the simplicity of its receiver, is still dogged by implementation concerns. In particular, the receiver requires an extremely wideband delay element, which is difficult to incorporate into low-power integrated systems. In this paper, a TR scheme is proposed in which the separation between the data and reference signals, rather than being a time delay, is a slow rotation over the symbol interval. This provides a (slightly) frequency-shifted reference that, while orthogonal to the data-bearing pulse, still goes through a nearly equal channel. A detailed analysis of the proposed scheme is provided. Simulation results demonstrate the expected result that frequency shifting of the reference in the proposed manner is not effective for high-data-rate systems that experience appreciable intersymbol interference. However, for the targeted low-to-moderate data-rate applications, numerical results demonstrate that the proposed system not only achieves the primary goal of providing a much simpler receiver architecture, but also that it outperforms the standard TR-UWB system     

Code-multiplexed UWB transmitted-reference radio

In traditional transmitted reference (TR) ultrawideband systems the reference component is time-shifted and orthogonal relative to the data-bearing signal. This paves the way to a correlation receiver in which the local template is derived from the incoming waveform using a delay line. As analog delay lines are difficult to implement with current technology, an alternative TR system has recently been proposed in which reference and data components are made orthogonal by a frequency shift rather than a time shift. The resulting receiver has no delay lines and has better performance compared to the traditional scheme. In the present paper we discuss a third way to achieve orthogonality, i.e., by modulating reference and data components with two distinct code sequences. Even in this case the receiver has no delay lines. However, it is simpler to implement and has better performance than the frequency-shift based receiver.     

基于判决自相关超宽带接收机研究

首先介绍了基于判决自相关(简称DDA)超宽带接收机的接收原理和结构,理论分析了接收误码率特点;比较了基于判决自相关接收机与Rake接收机和基本TR接收机的基本参数要求,并在标准信道模型下仿真对比了它们的接收性能;考察了窗口宽度因子N对基于判决自相关接收机接收性能和结构的影响;通过提高判决自相关系统脉冲功率可在相同能量效率条件下获得与平均TR系统相近的接收性能,且具有更高传输率的优点。     

Unified Analysis of UWB Transmitted-Reference Schemes in the Presence of Narrowband Interference

Transmitted-Reference (TR) signaling, in conjunction with an autocorrelation receiver (AcR), offers a low- complexity alternative to Rake reception in ultrawide bandwidth systems. This paper provides a unified performance analysis of various TR schemes by developing an analytical framework based on the sampling expansion approach. Specifically, we derive the uncoded bit error probability (BEP) of different TR signaling schemes, including TR and differential TR (DTR) signaling valid for a broad class of fading channels. We consider both AcRs and modified AcRs with noise averaging. We further develop a quasi-analytical method as well as an approximate analytical method to extend the BEP analysis to include the effect of narrowband interference (NBI). We show that the approximate analytical method is particularly useful in obtaining BEP expressions that provide insight into the effect of NBI. We quantify the effects of NBI and channel power dispersion profile on the optimum integration interval of an AcR. Finally, we compare TR and DTR signaling in terms of their sensitivity to NBI.     

超宽带传输参考接收机的性能研究

分析了超宽带传输参考(TR)接收机的原理和优势,针对UWB通信应用在多个比特传输中信道基本不变的特点,将TR信号集合扩展为一帧几个参考信号和多个数据信号,提出了一种改进TR接收机。文中对基本TR接收机和改进TR接收机的性能进行了理论分析,并采用IEEE的CM1、CM2多径信道模型进行了性能仿真,结果表明,综合考虑性能、传输效率和成本,改进TR接收机优于基本TR接收机。     

Optimal and suboptimal receivers for code‐multiplexed transmitted‐reference ultra‐wideband systems

In this study, optimal and suboptimal receivers are investigated for code‐multiplexed transmitted‐reference (CM‐TR) ultra‐wideband systems. First, a single‐user scenario is considered, and a CM‐TR system is modeled as a generalized noncoherent pulse‐position modulated system. Based on that model, the optimal receiver that minimizes the bit error probability is derived. Then, it is shown that the conventional CM‐TR receiver converges to the optimal receiver under certain conditions and achieves close‐to‐optimal performance in practical cases. Next, multi‐user systems are considered, and the conventional receiver, blinking receiver, and chip discriminator are investigated. Also, the linear minimum mean‐squared error (MMSE) receiver is derived for the downlink of a multi‐user CM‐TR system. In addition, the maximum likelihood receiver is obtained as a performance benchmark. The practicality and the computational complexity of the receivers are discussed, and their performance is evaluated via simulations. The linear MMSE receiver is observed to provide the best trade‐off between performance and complexity/practicality. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Analysis of UWB transmitted-reference communication systems in dense multipath channels

Transmitted-reference (TR) signaling, in conjunction with an autocorrelation receiver (AcR), offers a low-complexity alternative to Rake reception. Due to its simplicity, there is renewed interest in TR signaling for ultrawide bandwidth (UWB) systems. To assess the performance of these systems, we develop an analytical framework based on the sampling expansion approach. In particular, we derive closed-form expression for the bit-error probability (BEP) of TR signaling with AcR that can be used to exploit multipath diversity inherent in wideband channels. We further extend our analysis to the BEP derivation of modified AcR with noise averaging. Our methodology does not require the Gaussian approximation and is applicable for any fading scenario, provided that the correlator output signal-to-noise ratio (SNR) can be characterized in terms of a characteristic function. We show that the validity of the conventional Gaussian approximation depends on the time-bandwidth product and the number of transmitted pulses per symbol. Our results enable the derivation of a computationally simple lower bound on the BEP of TR signaling with AcR. This lower bound allows us to obtain the SNR penalty associated with an AcR, as compared with All-Rake and Partial-Rake receivers.     

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     

Orthogonally-spread block transmissions for ultra-wideband impulse radios

Differential, transmitted reference (TR) and energy detection (ED) based ultra-wideband impulse radios (UWBIR) can collect the rich multipath energy offered by UWB channels with a low-complexity receiver. However, they perform satisfactorily only when the channel induced inter-pulse interference (IPI) is negligible. This can be achieved by appending a guard interval with duration greater than or equal to the channel's delay spread to each frame ? an operation limiting the maximum achievable data rate. As a remedy, this Letter advocates block transmissions in conjunction with orthogonal spreading sequences to remove the introduced IPI. The resultant scheme requires no channel knowledge besides timing offset and incurs slightly more complexity than non-block alternatives, while it increases the data rate at no cost in error performance. Given a fixed data rate of 25Mbps, the novel block scheme exhibits about 1.8 dB gain relative to its non-block counterpart in single-user simulated tests.     

Performance of UWB Receivers with Partial CSI Using a Simple Body Area Network Channel Model

Ultra wideband (UWB) communication is a very promising candidate for the use in wireless body area networks (BAN). The high UWB peak data rate allows for medium average data rates in combination with a very low duty cycle, which is the key for a very low power consumption. Devices in a wireless BAN require low complexity. Hence, mainly non-coherent receivers such as energy detector and transmitted-reference receiver are suited. In this paper, the symbol-wise maximum-likelihood (ML) detectors for pulse position modulation (PPM) and transmitted reference pulse amplitude modulation (TR PAM) are derived assuming partial channel state information (CSI) at the receiver. Additionally, also the ML detectors for a combination of PPM and TR PAM are presented. The performance of the derived receiver structures is evaluated using a novel BAN channel model not distinguishing line-of-sight and non line-of-sight situations. This simple channel model is based on 1100 channel measurements in the frequency range between 2 and 8 GHz, which were measured in an anechoic chamber. Using the BAN channel model, performance of the derived receiver structures is evaluated showing that the knowledge of the average power delay profile (APDP) at the receiver improves performance substantially. Requiring only slightly more complexity such receivers are a well suited alternative to non-coherent receivers for the use in a BAN.     

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.     

Performance of transmitted reference pulse cluster ultra‐wideband systems with forward error correction

Recently, an improved transmitted reference (TR) signaling scheme, referred to as transmitted reference pulse cluster (TRPC), was proposed for low‐rate ultra‐wideband (UWB) communications. Compared with conventional TR, TRPC has a more compact and uniform spacing for the reference and data pulses and therefore addresses the implementation problems posed by the long delay line requirement, as well as provides better bit error rate (BER) performance. In this paper, a TRPC‐UWB system, which includes practical forward error correction (FEC) coding such as that specified in the IEEE 802.15.4a standard, as well as more powerful convolutional codes, is developed. A performance analysis, which highlights the importance of selecting appropriate FEC codes, is presented. Results show that with a suitable FEC code, the TRPC‐UWB system is a promising candidate for low‐rate wireless personal area networks. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Improved autocorrelation demodulation receivers based on multiple-symbol detection for UWB communications

The autocorrelation demodulation (ACD) has potential for use in medium-range low-cost UWB communications, however, its weak performance is a bottle neck. To improve ACD's performance, it is proposed to combine the multiple-symbol detection with ACD to form a multiple-symbol-based ACD receiver. In contrast to ordinary ACD scheme that uses symbol-by-symbol decision, the proposed structure uses block-based multiple-symbol joint decision. Performance of the new scheme is evaluated analytically for arbitrary block sizes employing Gaussian approximation approach. The accuracy of the method is verified by computer simulation. The evaluation shows that the proposed scheme offers several dB improvement in performances with only a moderate increase in complexity.