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.     

Performance analysis of an over-sampling multi-channel equalization for a multi-band uwb system

This paper proposes an over-sampling multi-channel equalizer per sub-band for multi-band ultra-wideband (UWB) system and compares its performance with conventional RAKE receiver when operating in practical UWB channel models. Three transmission modes have been considered, and inter-symbol interference (ISI) is found to be inherent to certain transmission modes due to the large UWB channel delay spreads. Through detailed analytical and simulation studies, the proposed over-sampled minimum mean square error (MMSE) equalizer is shown to be able to handle ISI under any channel conditions or transmission modes, with an acceptable BER. In addition, the rich multipath diversity of the UWB channels is harnessed by the over-sampling scheme, for output SNR improvement. Over-sampling is done in the expense of an increase in system complexity     

Conditions and Performance of Ideal RAKE Reception for Ultra-Wideband Signals in Lognormal Fading Channels

In this paper, the conditions and performance of ideal RAKE reception for time hopping Ultra-Wideband (UWB) investigated. Owing to the complex propagation phenomena and specific structure of UWB signals, new problems relevant to the operation of RAKE receivers arise. This motivates us to reconsider the conditions under which a RAKE receiver can work effectively with negligible interference between fingers. Key findings are that the conditions not only relate to the property of time hopping codes, but also modulation methods. An analytical technique is introduced to derive explicit expressions of RAKE performance for various combining methods for a lognormal fading channel. Numerical results show that RAKE reception can largely improve the performance, and equal gain combining has comparable performance to maximum ratio combining.     

A research of UWB rake receiver based on novel RLS adaptive algorithm

A modified RAKE receiver based on novel Recursive Least Squares （RLS） adaptive algorithm is proposed. The receiver uses L-fingered correlators, which are composed of RLS adaptive filters, to enhance the performance of multipath receiving. It can also track the amplitude of the received signal to form a real-time amplitude estimation which is correlated with the power of excess delay bin. The simulation results based on the IEEE UltraWide Band （UWB） channel models （CMI to CM4） show that the novel RLS algorithm can alter the attenuation estimation with the finger＇s power delay profile, and RAKE receiver with few fingers can be employed to get high performance.     

Characterization of ultra-wide bandwidth wireless indoor channels: a communication-theoretic view

An ultra-wide bandwidth (UWB) signal propagation experiment is performed in a typical modern laboratory/office building. The bandwidth of the signal used in this experiment is in excess of 1 GHz, which results in a differential path delay resolution of less than a nanosecond, without special processing. Based on the experimental results, a characterization of the propagation channel from a communications theoretic view point is described, and its implications for the design of a UWB radio receiver are presented. Robustness of the UWB signal to multipath fading is quantified through histograms and cumulative distributions. The all RAKE (ARAKE) receiver and maximum-energy-capture selective RAKE (SRAKE) receiver are introduced. The ARAKE receiver serves as the best case (bench mark) for RAKE receiver design and lower bounds the performance degradation caused by multipath. Multipath components of measured waveforms are detected using a maximum-likelihood detector. Energy capture as a function of the number of single-path signal correlators used in UWB SRAKE receiver provides a complexity versus performance tradeoff. Bit-error-probability performance of a UWB SRAKE receiver, based on measured channels, is given as a function of the signal-to-noise ratio and the number of correlators implemented in the receiver.     

Performance of Super-Orthogonal Convolutional Coding for Ultra-Wideband Systems in Multipath and Multiuser Channels

This paper investigates the application of super-orthogonal convolutional codes (SOCC) to a direct-sequence based ultra- wideband (DS-UWB) system under a realistic environment including inter-symbol interference (ISI) and multiuser interference (MUI). The effect of MUI and ISI on the performance of SOCC is analyzed by using both maximum ratio combining (MRC) RAKE receiver and minimum mean-squared error (MMSE) RAKE receiver followed by matched filter receiver. The analysis shows that in the case of employing simple MRC-RAKE receiver, the performance of SOCC is ffected by MUI and ISI because of the short length of the spreading sequence. In order to combat MUI and ISI, a code-hopping scheme is proposed in conjunction with SOCC. The results show that SOCC scheme outperforms the higher-rate conventional convolutional coded scheme for multipath and multiple access channels. Furthermore, the use of MMSE-RAKE receiver to suppress interference is analyzed and the subsequent increase of the system capacity is observed.     

A RAKE Receiver With an ICI/ISI Equalizer for a CCK Modem

In this paper, we first derive the theoretical performance of a complementary code keying (CCK) code on an additive white Gaussian noise (AWGN) channel and over a multipath channel. To derive the error performance, we use the weight and cross-correlation distributions of the CCK code for optimal and suboptimal decoding, respectively, based on union bound. In addition, we propose a RAKE receiver for a CCK modem, which is suitable for a multipath environment with a large delay spread. The RAKE receiver principle is acceptable for modest multipath because it can coherently combine multipath components to provide signal-to-noise ratio (SNR) enhancement. However, as the delay spread is larger and the data rate of systems goes higher, intersymbol interference (ISI) generated due to multipath environments are increased. To handle the increasing ISI, the CCK modem needs an equalization technique to remove the ISI, together with RAKE processing. Thus, our proposed system is based on a channel matched filter (CMF) with a decision feedback equalizer (DFE). The CMF is applied for RAKE processing, whereas the DFE structure is used for ISI cancellation. In our system, ISI is calculated and removed by using a decoded CCK codeword.     

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。     

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.     

基于IEEE UWB标准信道模型的RAKE接收机性能分析

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。介绍了Intel的UWB多径信道模型。对采用TH-PPM调制的超宽带系统在Intel多径信道模型下A-RAKE，S-RAKE和P-RAKE的误码率性能和对系统复杂性的要求进行了分析。在实际的RAKE接收机设计中，S-RAKE和P-RAKE的性能与支路数有关，S-RAKE要优于P-RAKE。S-RAKE和P-RAKE的性能随着支路的增加而减小。     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

An Ordered Successive Interference Cancellation Scheme in UWB MIMO Systems

In this letter, an ordered successive interference cancellation (OSIC) scheme is applied for multiple‐input multiple‐output (MIMO) detection in ultra‐wideband (UWB) communication systems. The error rate expression of an OSIC receiver on a log‐normal multipath fading channel is theoretically derived in a closed form solution. Its bit error rate performance is analytically compared with that of a zero forcing receiver in the UWB MIMO detection scheme followed by RAKE combining.     

Analysis of a direct-sequence spread-spectrum cellular radio system

The uplink and downlink performance of a digital cellular radio system that uses direct sequence code division multiple access is evaluated. Approximate expressions are derived for the area averaged bit error probability while accounting for the effects of path loss, log-normal shadowing, multipath-fading, multiple-access interference, and background noise. Three differentially coherent receivers are considered: a multipath rejection receiver, a RAKE receiver with predetection selective diversity combining, and a RAKE receiver with postdetection equal gain combining. The RAKE receivers are shown to improve the performance significantly, except when the channel consists of a single faded path. Error correction coding is also shown to substantially improve the performance, except for slowly fading channels     

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.     

A Single Input-Multiple Output Time Reversal UWB Communication System

Time reversal is a promising technique for the improvement of UWB communication systems. Intersymbol interference (ISI) limits the system performance in such wireless systems. This paper presents a general ISI analysis for time reversal UWB communication systems. The time reversal UWB system gives good performance for rates below the coherence bandwidth but at higher data rates the performance of the system is limited by intersymbol interference and bit error rate saturates even for high signal-to-noise ratio. To mitigate the ISI effects, a single input/multiple output (SIMO) time reversal UWB system is used and its performance is analyzed. It is shown that by using a SIMO TR transceiver, ISI reduces and the system capacity increases. Transmitted signal power at SIMO time reversal decreases, therefore in low data rate SISO performance is better than SIMO, But in high rate scenario, SIMO TR suppresses the ISI better than the SISO TR and its performance is better than SISO TR. It is possible to compensate the reduced power by using a receiver with more sensitivity.     

基于MIMO-DFE的BLAST接收机的差错传递控制的研究

近期对于BLAST(Bell Laboratories Layered space-time)的研究从平坦衰落信道转向频率选择性衰落信道,基于MIMO-DFE(Multiple-Input-Multiple-Output Decision Feedback Equalizer)的接收机结构因此受到了广泛的关注.然而由于这类接收机本身的反馈结构,性能可能会由于差错传递而大大下降.本文将传统判决反馈均衡器的差错传递控制方法推广到多入多出(MIMO)系统中,仿真结果表明,本文提出的带差错传递控制的接收机能够有效控制差错传递.     

A statistical approach to the analysis of DS/CDMA cellular systemsemploying RAKE receivers and sectorized antennas

RAKE receivers and sectorized antennas are used in direct-sequence/code-division multiple-access (DS/CDMA) cellular systems to improve the system performance. This paper presents a statistical method for analyzing the performance of DS/CDMA cellular radio systems employing RAKE receivers and sectorized antennas. Average bit error rates in the system are estimated considering the multipath fading effects of the environment. (The fast fading is assumed to be Rayleigh distributed, and the distance-dependent means of the multipath components have an exponential power delay profile.) The analysis of RAKE receivers quantifies the performance improvement that could be achieved by increasing the number of RAKE fingers. Sectorized antennas improve the system performance by reducing the interference at the receiver. In a perfectly sectorized system, assuming three sectors per cell, the capacity of the system can be improved by a factor of three. However, due to the imperfection in practical antennas, it is not possible to achieve this improvement. The performance of systems employing practical sectorized antennas (with finite front-to-back ratios and overlapping sectors) is compared with the performance of perfectly sectorized systems. The analysis shows that the incremental performance improvement diminishes with each incremental increase in the number of RAKE fingers. Performance degradation due to finite front-to-back ratio is shown to be insignificant for practical values of the front-to-back ratio of sectorized antennas. However, the reliability of mobile reception can be degraded significantly in areas where adjacent sectors overlap     

A continuous-time IR-UWB RAKE receiver for coherent symbol detection

The ultra-wide bandwidth released for unlicensed use by FCC a decade ago has initiated significant research efforts. The large ultra-wide bandwidth is attractive not only for increased data transfer speed but may also be exploited for added functionality like high-precision ranging in wireless sensor networks. RAKE based receivers are preferred for ultra-wideband (UWB) technology due to wide bandwidth. However, designing RAKE based correlating receivers remains quite challenging. Correlating receivers are also power consuming due to high-speed DSPs, ADC and matched filter. Timing synchronization is another issue associated with correlating receivers. In this paper a impulse radio ultra-wideband (IR-UWB) RAKE receiver is presented utilizing a continuous-time binary value coding scheme for power-efficiency and coherent symbol detection without the need for synchronization to achieve precise ranging using time-of-flight technique. A working prototype of the IR ranging transceiver which uses the IR-UWB RAKE receiver is presented with measured high-precision ranging towards 1.4 cm.     

FPGA-based architecture and implementation techniques of a low-complexity hybrid RAKE receiver for a DS-UWB communication system

In this paper, the architecture of a low-complexity Direct Sequence Ultra-Wideband (DS-UWB) receiver subsystem which incorporates a Channel Estimator (CE) and a novel hybrid Partial/Selective (HPS) maximal ratio combining (MRC) RAKE receiver is presented. Three different design techniques followed by FPGA implementation are investigated and compared and system performance results are provided. The proposed architectures combine the benefits of both partial and selective RAKE receiver algorithms and the obtained results demonstrate the trade-off between energy capture, performance and receiver complexity. All design approaches focus on a highly parallel, modular and optimized for high performance system which is necessary for demanding and low-cost applications of UWB communications.     

A Survey on Various Coherent and Non-coherent IR-UWB Receivers

The recent developments in radio technologies, paves its way to impulse radio (IR) ultra-wideband (UWB) communication, which is used for low power, short range and high bandwidth communication, thereby exploiting a large portion of radio spectrum. In this paper, a brief review of the work done by various researchers on coherent and non-coherent IR-UWB receivers has been analysed, based on their bit error rate (BER) performances, as well as pros and cons of using these receivers. An in depth study on the receivers concludes that, non-coherent IR-UWB receiver is preferred over its counterpart coherent IR-UWB receiver even though it comes at the expense of poor BER performance. The simulation results prove that, though the performances are same, the low complexity of energy detector (ED) receivers gives an edge over the autocorrelation receivers. Further, ED receiver suffers from noise, which paves way to using weighted ED (WED) receiver. The superiority of WED receivers over all the other non-coherent UWB receivers is further confirmed by the simulation performed in AWGN and IEEE 802.15.4a UWB channels. It can also be concluded from the review that, some special receivers such as generalized likelihood ratio test, multi-symbol differential detector and decision feedback differential transmitted reference, when clubbed with UWB systems, lead to further improvement in BER performance.