Pilot-based estimation of time-varying multipath channels forcoherent CDMA receivers

Reliable coherent wireless communication requires accurate estimation of the time-varying multipath channel. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) systems: (i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission and (ii) impact of channel estimation errors on coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of filters is derived. A key channel characteristic controlling system performance is the normalized coherence time, which is approximately the number of symbols over which the channel remains strongly correlated. It is shown that the estimator performance is characterized by an effective signal-to-noise ratio (SNR)-the product of the pilot SNR and the normalized coherence time. A simple uniform averaging estimator is also proposed that is easy to implement and delivers near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits joint multipath-Doppler diversity. It is shown that the overall receiver performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite effects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can significantly mitigate the error probability floors that plague conventional CDMA receivers under fast fading due to errors in channel estimation     

Low Complexity Rake Receivers in Ultra-Wideband Channels

One of the major issues for the design of ultra-wideband (UWB) receivers is the need to recover the signal energy dispersed over many multipath components, while keeping the receiver complexity low. To this aim we consider two schemes for reduced-complexity UWB Rake receivers, both of which combine a subset of the available resolved multipath components. The first method, called partial Rake (PRake), combines theirs/ arriving multipath components. The second is known as selective Rake (SRake) and combines the instantaneously strongest multipath components. We evaluate and compare the link performance of these Rake receivers in different UWB channels, whose models are based on extensive propagation measurements. We quantify the effect of the channel characteristics on the receiver performance, analyzing in particular the influence of small-scale fading statistics. We find that for dense channels the performance of the simpler PRake receiver is almost as good as that of the SRake receiver, even for a small number of fingers. In sparse channels, however, the SRake outperforms the PRake significantly. We also show that for a fixed transmitted energy there is an optimum transmission bandwidth     

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.     

Adaptive Receiver Structures for Fiber Communication Systems Employing Polarization-Division Multiplexing

Polarization-division multiplexing (PDM) is emerging as a promising technique for increasing data rates without increasing symbol rates. However, the distortion effects of the fiber transmission medium poses severe barriers for the implementation of this technological alternative. Especially, due to the fiber-induced polarization fluctuation orthogonally transmitted PDM signals are mixed at the receiver input. Therefore, a receiver compensation structure needs to be implemented to recover the original orthogonal transmitted components from their mixtures at the end of the fiber channel. This is in fact the focus of this article where a receiver algorithm is based on a recently proposed blind source separation scheme exploiting magnitude boundedness of digital communication signals. Through the use of this scheme, new receiver algorithms for recovering the original polarization signals in an adaptive manner are proposed. The key feature of these algorithms is that they can achieve high separation performance while maintaining the algorithmic complexity in a fairly low level that is suitable for implementation in optical fiber communication receivers. The performance of these algorithms are illustrated through some simulation examples.     

Performance comparisons of channel estimation techniques in multipath fading CDMA

The problem of pilot-symbol-aided estimation of multipath fading channels in up-link code-division multiple-access (CDMA) systems is considered. The transmitted symbol streams of each user are divided into time-slots; and each time-slot contains a number of pilot-symbols followed by information data symbols. Channel estimation is based on interpolation of the channel values corresponding to the pilot symbols in adjacent time-slots. Existing channel estimation techniques, including the weighted multislot average method and the wavelet expansion method, are studied. Two new channel estimation methods, namely, the robust channel interpolator, and the polynomial channel interpolator, are developed and are compared with these techniques. It is seen that the two new channel estimation methods significantly outperform the existing methods in multipath fading CDMA systems, for a wide range of Doppler values, and under various receiver schemes (with single or multiple receive antennas), such as the RAKE receiver, the interference cancellation receiver, and a receiver which performs iterative channel estimation and interference cancellation.     

水声多径信道中的标识延迟空时扩展发射分集

水声信道存在严重的传播时延和多径时延,该文提出了一种带有信道标识的延迟空时扩展(LDSTS)发射分集方案,通过信道探测、延时发射和Rake接收来消除传播时延和多径时延的影响,且顺序延时发射保证了方案的实用性。文中给出了采用频移键控和相移键控调制的LDSTS方案的信号模型、误码率分析和比特误码率的仿真。仿真表明,在多径水声信道中,LDSTS可以更好地实现完全发射分集。     

Diversity-assisted channel estimation and multiuser detection for downlink CDMA with long spreading codes

In downlink communication of a direct-sequence (DS) code-division multiple-access (CDMA) system, each user's short spreading codes are superimposed by base station's common long codes. This situation creates much difficulty in blind signal detection when multipath propagation occurs. However, when spatial/temporal diversity is available at the receiver, it is shown in this paper that subspace technique can be directly applied to estimate the common downlink multipath channel. Then, typical linear receivers, such as zero-forcing (ZF), minimum mean-square-error (MMSE) and RAKE receivers can be designed to detect the desired signal. Since the data covariance matrix is used but estimated from finite data samples, performance of both channel estimator and receivers gets perturbed. It is thus thoroughly and jointly analyzed by perturbation analysis. Justification of analysis and comparison of different receivers are also made through simulations.     

Multipath combining scheme in single-carrier transmission systems

In this letter, we introduce and investigate the RAKE combining receiver which is widely used in the code-division multiple access (CDMA) systems to the non-spectrum-spreading single-carrier transmission system. The initial estimate of the transmitted data is obtained by linear single-carrier equalizers, and then all the multipath signals are constructed from this initial solution and channel impulse response. By interference cancellation (IC) technique, we can acquire every multipath component in the received signal after cancelling the sum of all the other multipath signals constructed. Finally, all the components are combined together using selection combining (SC), equal gain combining (EGC) or maximal ratio combining (MRC), so that temporal diversity gain from the combined output can be obtained. Simulation results show that bit error rate (BER) performance of the new combining receiver based on zero forcing (ZF) and minimum mean square error (MMSE) equalizers can achieve the SNR gain dramatically in the SUI-5 wireless communication link.     

An integrated framework for MC-CDMA reception in the presence of frequency offsets, phase noise, and fast fading

The combination of code-division multiple-access and multicarrier (MC) modulation has been proposed to develop high data-rate wireless communication systems. Due to the longer symbol duration in comparison with single-carrier systems, MC systems are more sensitive to various imperfections, including phase noise and frequency offsets due to local oscillators and Doppler spreading due to motion that results in temporal channel variations. The performance of current systems is significantly limited by these imperfections because they disperse the transmitted power in a particular subcarrier into adjacent subcarriers, thereby causing interference between the subcarriers at the receiver. We consider a single-user communication system and use a canonical model for the received signal that efficiently captures the effects of all impairments. The model uses Fourier basis functions that are fixed for all imperfections while the expansion coefficients depend on imperfections. Using the model, we introduce a receiver structure that implements the matched filter (MF), and hence, optimal. The MF is implemented through a Rake receiver in the frequency domain. The receiver fully compensates for frequency offsets as well as phase noise, thereby eliminating the performance loss due to these factors. Furthermore, in contrast to existing designs, it delivers improved performance under fast fading by exploiting Doppler diversity. Finally, the same integrated receiver structure works for all imperfections eliminating the need for devising a separate correction technique for each.     

异步协同系统频选信道下基于线性预处理的DSTC结构设计

在协同中继系统中,应用分布式空时码(Distributed Space Time Coding, DSTC),可以在有效提高系统效率的同时获得全协同分集。但是,各中继节点的异步传输和节点间的多径衰落会破坏空时码字的结构,使之不能获得全分集。本文针对两中继的异步协同系统,提出了一种频率选择性信道下的基于线性预处理的DSTC传输结构。在此传输结构中,源节点对发送数据块进行预处理后发送给中继节点,中继节点对接收信号进行简单的共轭重排等处理,使得在目的节点形成DSTC的结构。其中,为抵抗异步传输和多径衰落引入的符号间干扰(Inter-symbol Interference, ISI),在源节点处和中继节点处均加入循环前缀(Cyclic Prefix, CP)。于是目的节点对接收到的信号进行DFT处理后,可以运用ML算法对数据信息进行检测。理论分析和仿真表明,当存在定时误差和节点间为频率选择性信道时,目的节点运用ML检测算法该传输结构可获得全空间分集和全多径分集。然后,本文考虑了信道各径延迟为整数倍符号周期的情况,并且证明了该传输结构的分集增益只与节点间信道的有效信道长度有关。      

Study on multiple receiver systems in non-line-of-sight ultraviolet communication systems

In this article, multiple receiver effects in a non-line-of-sight （NLOS） ultraviolet （UV） communication system is studied. The idea of using multiple receivers for diversity reception is known as a practical, effective and widely applied technique in wireless communications. The current approach is to use multiple antennas at the receiver in order to improve the quality of the received signal. A method of modeling and simulation is proposed to depict the principle and feasibility of the multiple receiver adopted in UV communication. The study provides an insight to the channel characteristics and achievable capabilities of ultraviolet communication systems with multiple receivers. It provides guidelines for practical system design with discussions on trade off between the receiver gain and the additional cost.     

Understanding the Time and Frequency Varying Characteristics of a Multipath Wireless Channel

Designing a spectrally efficient wireless channel requires a comprehensive understanding of its time and frequency varying characteristics, making it a stochastic medium of communication. These characteristics become more challenging to cater at the receiving terminal in a multipath transmission. This is because of the fading effect and Doppler shift of the transmitted frequency, specifically in cellular mobile radio systems and vehicle to vehicle communications. This paper presents the modeling, simulation, and then characterization of a cellular mobile radio multipath channel over its time and frequency varying fading gain. For this purpose, a discrete-time Finite Impulse Response (FIR) type filter of such a channel is designed, modeled, and simulated using time and frequency varying characteristics of the received signal. The simulated channel response is further analyzed in terms of coherence bandwidth, coherence time, delay spread, Doppler spread, and symbol time.

     

Deconvolution in the presence of Doppler with application tospecular multipath parameter estimation

High-resolution multipath parameter estimates can be obtained through various deconvolution procedures, all of which-in the limit-rely on some form of inverse filtering. Although deconvolution in a multipath environment free from Doppler is well understood and well documented, this is not true for the case when motion of the multipath components relative to the receiver imposes a Doppler shift on the transmitted probing signal. This paper describes the effect of Doppler on a broad class of deconvolution methods by studying the effect of Doppler on the output of an inverse filter. It is shown that in the presence of Doppler, the deconvolution outputs are comprised chiefly of two signal-related functions, one of which may be designed in such a way as to be free from the range-Doppler coupling effects inherent in correlation processing. Knowledge of these two functions provides insight into the signal design issues relevant to deconvolution-based multipath parameter estimation systems and is useful in designing appropriate constraints and post-processing algorithms that may lead to an accurate extraction of the Doppler and delay parameters of the multipath channel. These results are applied to two known deconvolution methods: the method of projection onto convex sets (POCS) and the method of least squares (LS)     

Adaptive communication receivers

This paper describes the results of an investigation of some communication receivers whose response to an input signal changes in a manner determined by the input signal. The problem considered is the design of a communication receiver to receive a message which is coded intoMfixed unknown signal waveforms and transmitted through a noisy channel. An optimal (minimum probability of error at each time interval) receiver is derived which has an exponentially growing structure. It requires(M - 1)M^{n-1}subsystems to receive thenth message symbol. The derivation suggests forms of adaptive receivers which need a more practical amount of equipment to implement, which we call the gremlin and the decision-directed adaptive receiver. The gremlin receiver is a taught-learning machine since, after it makes a decision, a gremlin tells it what the correct decision was. The decision-directed receiver is a self-taught learning machine, using its own output instead of a gremlin's. It is shown that the gremlin receiver converges to a matched filter for the unknown signal and that, in any practical case, the decision-directed receiver performs almost as well. Finally, some results of an experimental simulation of the decision-directed receiver are presented. A plot of the relative frequency of error vs. time is given for a number of different signal-to-noise ratio's (SNR's).     

失真自适应接收机技术及信号形式选择

对流层散射信道是一种典型的多径衰落信道,多径传播造成了信号的深衰落和多径扩散。而多径扩散现象的存在,会使速率高、码元窄、频谱宽的信号遭受频率选择性衰落的影响,造成传输中的信号码元之间产生码间干扰,严重影响信息传输质量。在此提出了一种基于失真自适应接收技术和3/4占空半余弦信号形式的数字接收机方案,能够自适应地跟踪失真信号的变化而进行最佳相干接收,使系统具有稳定的通信性能。     

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.     

A Multicarrier CDMA System for Joint Time-Frequency Diversity Using Divided Spreading Sequence

One of the principal disadvantages of multicarrier modulation technique is the sensitivity to the frequency offset introduced by Doppler shift. This frequency offset introduces inter-carrier interference (ICI) among the multiplicity of carriers in the multicarrier modulated signal. However, Doppler spread induced by temporal channel variations can provides another means for diversity. In this paper, we propose a modified multicarrier code division multiple access (CDMA) system to exploit Doppler diversity as well as multipath diversity. The key work of our framework is a canonical time-frequency-based decomposition of the mobile wireless channel into series of independent fading channel. The decomposition naturally leads to a time-frequency generalization of the Rake receiver that exploits both multipath and Doppler diversity.     

Multichannel signal processing for data communications in thepresence of crosstalk

Transceiver designs for multiple coupled channels typically treat the crosstalk between adjacent twisted pairs as random noise uncorrelated with the transmitted signal. The authors propose a transmitter/receiver pair that compensates for crosstalk by treating an entire bundle of twisted pairs as a single multi-input/multi-output channel with a (slowly varying) matrix transfer function. The proposed transceiver uses multichannel adaptive FIR filters to cancel near- and far-end crosstalk, and to pre- and postprocess the input/output of the channel. Linear pre- and postprocessors that minimize mean squared error between the received and transmitted signal in the presence of both near- and far-end crosstalk are derived. The performance of an adaptive near-end crosstalk canceller using the stochastic gradient (least-mean-square) transversal algorithm is illustrated by numerical simulation. Plots of mean squared error versus time and eye diagrams are presented, assuming a standard transmission line model for the channel. A signal design algorithm that maps a vector input bit stream to a stream of channel symbol vectors is also presented and illustrated explicitly for s simple model of two coupled channels     

Communication over fading dispersive channels,optimal receivers and signals

We study the problem of designing the optimal receiver for a dispersive channel and the optimal signals to be transmitted over a given dispersive channel. The assumed transmission channel is WSSUS (Wide Sense Stationary Uncorrelated Scatterers), randomly time variant, so it is characterized by its scattering function. We study first the effective realization of the optimal receiver, given a scattering function, and we point out that the structure of the optimal receiver varies according to the discrete or continuous structure of the scattering function over the time frequency plane. We give some precise results in the case of multipath transmission and in the case of modulation-transmission.In a second part, we are interested in the signal design problem. Some new results are given which permit defining—in a way more direct than usual—the class of optimal signals for a given dispersive communication.     

Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d²TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.