Optimum soft decision demodulation for ISI channels

Two different methods of soft-decision demodulation for channels with finite intersymbol interference (ISI) in the presence of additive white Gaussian noise (AWGN) are analyzed. In both schemes, the cutoff rate R₀ of the discrete channel created by the demodulator output quantization is chosen as the design criterion. Expressions for the optimal thresholds of the quantizer associated with the demodulation of binary signals are derived. Results for the channel with memory equal to one symbol duration are presented. As a special case, the (1-D) channel with soft decision demodulation is analyzed. Closed-form solutions show that a 4-b quantizer improves performance substantially in this case     

Signal set design for band-limited memoryless multiple-access channels with soft decision demodulation

Signal sets are identified that maximize the cutoff rate region for a multiple-access channel with an additive white Gaussian noise, in which the demodulator output alphabet is allowed to be infinite ("infinitely soft decisions"). The optimizing designs consist of a simplex signal set for each sender, such that each sender's set is orthogonal to those of the other senders. For "second moment" and for "fractional out-of-band-energy" bandwidth constraints on the signals of each sender, conditions are derived under which mutually orthogonal simplex sets are still optimal. For the second moment constraint, simplex sets derived from sinusoidal functions yield an optimal design and, for the out-of-band energy constraint, simplex Sets derived from prolate spheroidal wave functions are optimal. Choices of signal sets that maximize the cutoff rate region for an additive shot-noise limited multiple-access optical channel, subject to average energy and peak amplitude constraints, are also identified.     

Decision feedback-based demodulation of doubly differential PSKsignals

A decision feedback-based differential demodulator of double differential phase-shift keying (DDPSK) signals is presented, It is comprised of two consecutively connected differential detectors, which are based on certain reference signals obtained from previously detected data and received signals. Computer simulations demonstrate that the bit error rate performance of the proposed demodulator is comparable to that of the coherent receiver of DDPSK signals     

Optimal demodulation of PAM signals

Kalman filtering theory is applied to yield an optimal causal demodulator for pulse-amplitude-modulated (PAM) signals in the presence of white Gaussian noise. The discrete-time data (or sampled continuous-time data) are assumed to be either a stationary or non-stationary Gaussian stochastic process, in general nonwhite. Optimal demodulation with delay is also achieved by application of Kalman filtering theory. The resulting demodulators (fixed-lag smoothers) are readily constructed and their performance represents in many cases a significant improvement over that for the optimal demodulator without delay. The fixed-lag smoothing results are in contrast to those for amplitude-modulated signals (AM) where only approximate fixed-lag smoothing is possible, and this with considerable design effort. The performance of the optimal PAM demodulator is shown to be equivalent to that of an optimal discrete filter for the discrete data.     

基于MMSE准则的最佳相干FSK解调器

 本文提出用MMSE准则 设计最佳相干FSK解调器.在信道噪声分布未知条件下,导出了该解调器的结构.并给出了分析和计算机模拟实验结果,证明了该解调器在AWGN信道条件下与普通FSK解调器误码性能相同,在信号各个频率振幅受到的衰减不一致,并且为ACGN信道条件下误码性能优于普通FSK解调器,与文献[2]基于MAP准则的改进型FSK解调器的误码性能十分接近;在信道中的噪声为其它分布(如均匀分布)情况下,也有类似上述的结果.     

Domains of attraction of Shalvi-Weinstein receivers

Domains of attraction (DoA) of Shalvi-Weinstein (1990) receivers are analyzed. It is shown that there is a one-to-one correspondence between DoA in the receiver parameter space and those in the global (or combined channel-receiver) parameter space. For general noiseless channels, DoA of SW receivers in the global response space are the minimum distance decision regions on a unit sphere. In the presence of noise and for the class of orthogonal channels, DoA of SW receivers for independent and identically distributed (i.i.d.) input signals are the minimum distance decision regions on an ellipsoid determined by the channel coefficients and the noise variance. The DoA in the receiver parameter space are also characterized for the general nonuniformly distributed sources. The size of the DoA is shown to be affected by the signal power, the signal constellation, the noise level, and the channel condition. It is also demonstrated that although the optima of the Shalvi-Weinstein algorithm and those of the constant modulus algorithm are one-to-one correspondent, their DoA are different in general     

Likelihood-Based Demodulation in Multi-User Chaos Shift Keying Communication

This paper deals with the theory of multi-user chaos shift keying communication systems in the coherent case and in particular with bit error performance under near optimal demodulation. The new approximated likelihood and near optimal demodulator is based on the statistical likelihood of the received signal and is a theoretical generalization of the conventional correlation demodulator involving autocorrelations of the spreading. Near optimal theoretical results beginning from an exact analysis are derived for the bit error rate of a typical user; there is emphasis on their structure in regard to separating the effects of channel noise and interference from other users, and in comparison with single-user results. The new demodulator is shown under particular assumptions to be a form of rake demodulator in the sense that it involves autocorrelation-like terms at various lags. The results are illustrated by theoretical calculations and numerical simulations which point out the enhanced performance of the near optimal demodulator relative to the correlation demodulator.     

CFAR detection for multistatic radar

In this paper a multistatic radar system with n transmitters and one receiver is considered and several constant false alarm rate (CFAR) algorithms for detection are introduced. The decision statistics of the proposed detectors are the sum of the n largest returning signals in an array of N+n range cells. It is shown that the proposed decision statistic satisfies the CFAR condition and it is justified that the sum of the largest returning signals is the optimal statistic. The proposed CFAR detectors are simulated both in homogenous and non-homogenous backgrounds and their performances are compared with the performance of a monostatic radar of higher power. It is shown that a multistatic radar outperforms a monostatic radar under equal transmit power condition.     

Decision feedback postprocessor for zero-crossing digital FMdemodulator

A baseband digital narrow-band FM receiver, called zero-intermediate frequency zero-crossing demodulator (ZIFZCD), has been developed. This demodulator may offer low complexity and simple implementation. However, the bit error rate (BER) performance of the ZIFZCD is inferior to that of the limiter-discriminator-integrate-and-dump (LDI) demodulator. In this paper, a simple decision feedback postprocessor (DFP) is proposed to improve the performance of the ZIFZCD. Analysis and simulation BER results of the ZIFZCD with the DFP are presented for minimum-shift keying (MSK) and Gaussian MSK (GMSK) signals under additive white Gaussian noise (AWGN) and mobile fading environments     

Optimum orthogonal quantization of signal space (Corresp.)

Orthogonal quantization is a partition of signal space that is achieved by independent quantization of each of itsMorthogonal axes. A closed form expression is derived for the quantized channel cutoff rate and for the optimum orthogonal quantization similar to the one that has been derived for binary signaling. While orthogonal quantization is natural for communication systems in which the transmitted signals are themselves orthogonal, it can also be profitably applied to other signals, e.g., a simplex set in a lower dimensional space. Though orthogonal quantization is inferior to optimal quantization, it is essentially simpler and does not incur great loss in performance. A numerical example illustrates the relative merits of optimal and orthogonal quantization for the simplex set in the plane.     

Performance analysis of a double-dwell serial search technique forcellular CDMA networks in the case of multiple pilot signals

The performance of a double-dwell serial search technique is analyzed for cellular code-division multiple-access (CDMA) networks in the case of multiple pilot signals. A general expression for the mean acquisition time is obtained by considering the multiple H₁ regions formed by the pilot signals coming from different base stations. The statistics of the demodulator output in code acquisition systems are discussed in detail for synchronous cellular CDMA networks with a focus on the nonstationarity of the interference in the forward link. The probabilities of detection and false alarm are then derived for frequency-selective Rayleigh fading environments in the case where the interference can be approximated by a stationary Gaussian noise process. Numerical evaluations are performed to examine the effects of decision thresholds, postdetection integration, fading rate, and so on, with emphasis on the case where the mobile station is located around the cell boundary     

Blind equalization of MIMO-FIR channels driven by white but higherorder colored source signals

The goal of the article is to find a minimal amount of statistical information (or a much weaker condition) about source signals for which blind equalization is possible for multiple-input multiple-output finite-impulse response (MIMO-FIR) channels. First, a sufficiently broad framework is set up within which such a theoretical problem is well posed. Within this framework, it is shown that second-order statistics (SOS) alone are not sufficient for blind equalization when the source signals are white. Additional higher order statistics (HOS) are needed. Then we show that the only additional higher order statistical information needed is spatial fourth-order cumulants. Though it has not yet been proved to be minimal, it is interesting to note that this is the same as the weakest known condition on the source signals even for an instantaneous mixture. We then show a necessary and sufficient condition for blind equalization when the source signals are white and spatially fourth-order uncorrelated. Based on this condition, criterion (A) for blind equalization of MIMO-FIR channels is developed by exploiting the temporal fourth-order statistics. Finally, based on this criterion, a new necessary and sufficient condition in terms of cumulants for the blind equalization of MIMO-FIR channels is obtained     

Partitioning capabilities of two-layer neural networks

It has been observed that feedforward neural nets with a single hidden layer are capable of forming either convex decision regions or nonconvex but connected decision regions in the input space. In this correspondence, it is shown that two-layer nets with a single hidden layer are capable of forming disconnected decision regions as well. In addition to giving examples of the phenomenon, it is explained why and how disconnected decision regions are formed. Through the hypothesization of the existence of additional virtual cells formed by the first layer, it is shown how the decision regions formed by the second layer can indeed be disconnected. It is shown that the number of such disconnected regions can be very large. Using a recent theoretical result about the sufficiency of two layers to approximate arbitrary decision regions in a finite portion of the space, an example is given of how that is possible with the use of virtual cells     

Acquisition Behavior of the Cross-Coupled Phase-Locked Loop FM Demodulator

In this work, the acquisition behavior of a cross-coupled phase-locked loop (CCPLL) FM demodulator capable of suppressing cochannel and adjacent channel interferers is examined. This novel detector consists of two phase-locked loop (PLL) demodulators interconnected in such a manner as to permit one PLL to lock onto and track the stronger received signal, while the other loop tracks and demodulates the weaker of two received signals. The demodulator has two separate outputs, namely, the outputs of each PLL, and thus possesses the capability of demodulating both the stronger as well as the weaker received signals even though they are cochannel and share the same frequency band. The transient response of the CCPLL system is obtained using computer-aided analysis of the defining coupled nonlinear differential equations. From these results, steady-state "stability regions" are derived which reveal that range of loop parameters where successful separation and demodulation of the received cochannel signals is assured. The CCPLL receiver has numerous technological applications in suppressing unintentional or intentional CW, AM, FM, and AM/FM interferers. Experimental results demonstrating such potential are included.     

A competitive Neyman-Pearson approach to universal hypothesis testing with applications

The problem of hypothesis testing for parametric information sources whose parameters are not explicitly known is considered. A new, modified version of the Neyman-Pearson criterion of optimality, where the uniform constraint on exponential rate of the false-alarm probability is replaced by one that depends on unknown values of the parameters, is proposed. An optimal universal decision rule, based on Kullback-Leibler divergence, is developed and shown to be efficient in the sense of achieving exponential decay of both misdetection and false-alarm probabilities for all values of unknown parameters, whenever such an efficient decision rule exists at all. Furthermore, necessary and sufficient conditions for the existence of such efficient universal tests are established and the best universally achievable error exponents are presented. Finally, the proposed approach is applied to several important problems in signal processing and communications and compared to the generalized likelihood ratio test (LRT).     

π/4-DQPSK信号的多普勒频移快捕和跟踪

提出了一种适用于数字卫星移动突发通信系统的多普勒频移快捕和跟踪方法。该方法根据突发通信时帧结构的特点,利用独特码的调制输出进行多普勒频移的快捕,并根据π/4-DQPSK信号的特点,采用判决反馈PLL环跟踪多普勒频移的变化。仿真结果表明,采用该方法的解调器误比特率Pb与成形滤波器的滚降系数α无关。多普勒频移在-Rs/2~Rs/2范围内变化时,对解调器的误比特率Pb没有影响。     

频率选择性衰落信道下FH/ST-FSK系统抗干扰性能分析

空时频移键控(ST-FSK)具有良好的抗衰落特性、较低的译码复杂性,最近正受到广泛的重视。虽然跳频通信(FH)在军事通信领域得到了广泛的应用,但是易受人为干扰却是跳频通信的顽疾,因此结合FH／ST-FSK实现扩频通信具有重大意义。把ST-FSK信号与受同一PN序列控制的频率合成器输出进行混频,接收端分别用多根天线完成对已跳信号的解跳和解调。这样,ST-FSK进行空间分集所获得的增益能有效地补偿频率选择性衰落对信号的影响。而通过适当地调整FSK波形基的频率间隔,又可以有效地对抗部分频带噪声干扰。理论分析证明,当频率间隔满足一定条件时注入ST-FSK解调器的干扰功率达到最小。最后,MATLAB仿真结果证明了结论。     

1.6-Gb/s 16-Level Superposed APSK Modem with Baseband Signal-Processing Coherent Demodulator

A 1.6-Gb/s 16-level superposed APSK modem using a newly developed baseband signal-processing coherent demodulator is described. ln the modulator, the superposing modulation method is adopted in which two QPSK carriers having different levels are superposed so that the combined signal forms a 16-level APSK carrier. Sixteen signal points are arranged in lattice form on the signal space. In the demodulator, a newly developed carrier recovery loop is adopted in which quantizing error signals are detected in the process of data regeneration; they are used for generating the phase error signal to control a VCO. The principles of operation and theoretical analysis of this carrier recovery loop is described, through which it is shown that the recovered carrier has inherently low-pattern jitter power. Furthermore, it features a very simple circuit configuration. The circuit configurations of an experimental modem and some important test results are deseribed in detail. It is shown that the modem has improved performance characteristics in such a high-speed region as 1.6 Gb/s while its circuit configuration is much simplified.     

主动声纳的混沌波形设计和解调方法

该文为解决主动声纳隐蔽性差的问题,提出了一种新的波形设计方法:将特定编码调制到混沌信号中,作为主动声纳的发射信号。文中全面考虑了水声信道滤波和加性噪声对回波的影响,采用自适应滤波器解调回波,通过识别编码检测回波信号。提出了改进的相平面Lyapunov自适应滤波器作为解调算法。仿真结果表明:该解调算法在加性白噪声和滤波情况均有较好的解调效果,并能满足实时性的要求。与现有的主动声纳发射信号相比,提高了主动声纳的隐蔽性。     

On Spatial Aliasing in Microphone Arrays

Microphone arrays sample the sound field in both space and time with the major objective being the extraction of the signal propagating from a desired direction-of-arrival (DOA). In order to reconstruct a spatial sinusoid from a set of discrete samples, the spatial sampling must occur at a rate greater than a half of the wavelength of the sinusoid. This principle has long been adapted to the microphone array context: in order to form an unambiguous beampattern, the spacing between elements in a microphone array needs to conform to this spatial Nyquist criterion. The implicit assumption behind the narrowband beampattern is that one may use linearity and Fourier analysis to describe the response of the array to an arbitrary wideband plane wave. In this paper, this assumption is analyzed. A formula for the broadband beampattern is derived. It is shown that in order to quantify the spatial filtering abilities of a broadband array, the incoming signal's bifrequency spectrum must be taken into account, particularly for nonstationary signals such as speech. Multi-dimensional Fourier analysis is then employed to derive the broadband spatial transform, which is shown to be the limiting case of the broadband beampattern as the number of sensors tends to infinity. The conditions for aliasing in broadband arrays are then determined by analyzing the effect of computing the broadband spatial transform with a discrete spatial aperture. It is revealed that the spatial Nyquist criterion has little importance for microphone arrays. Finally, simulation results show that the well-known steered response power (SRP) method is formulated with respect to stationary signals, and that modifications are necessary to properly form steered beams in nonstationary signal environments.