Chair: C. R. Johnson Jr., Cornell University (USA)
Urbashi Mitra, Ohio State University
H. Vincent Poor, Princeton University (USA)
Motivated by a previous study of adaptive multi-user demodulators for Direct-Sequence Spread-Spectrum Multiple Access, detectors for spread-spectrum signals are investigated. Due to the prohibitive complexity of the locally optimum detector for such a stochastic multi-variate signal in impulsive channel noise, moderate complexity, distribution-free detectors are pursued. In particular, the differential SNR's (processing gain) of correlator based structures are determined. This performance measure is apt given the relatively low signal strength of spread digital signals. The numerical results (in the context of the prior investigation of adaptive multi-user demodulators) are consistent with previous findings and impel the development of a hybrid detector which is composed of linear and non-linear structures. The asymptotic normality of the test statistics under study is also examined.
Jian Zhan, Portland State University
Jin S. Zhang, Portland State University
Fu Li, Portland State University
Zhigang Fan, Xerox Corporation (USA)
It was proved in that feedforward schemes are universally capable of approximating any measurable function to any desired degree of accuracy. In this paper, we propose a new realization of such feedforward scheme to the channel equalization problem -- Parallel Feedforward Equalization (PFE). An important feature of the new approach is the decomposition of any equalization into linear and nonlinear components. the new approach chooses Fj(.) (j=1, 2, ...) from a family of nonlinear functions to approximate the nonlinear component decomposed from the desired mapping f(.). The other new idea proposed in this paper is a measure called Nonlinearity Distribution which characterizes the nonlineality in multipath fading channels. The architecture of the new equalization consists of parallel feedforward nonlinear filters, each of them has a specifically tailored nonlinear function Fj(.).
J. Ilow, University of Toronto (CANADA)
D. Hatzinakos, University of Toronto (CANADA)
A.N. Venetsanopoulos, University of Toronto (CANADA)
This paper considers the performance of FH SS radio networks in a Poisson field of interfering terminals using the same modulation and power. Assuming 1/rm attenuation of signal strength over distance, the interference in the network is modeled as a mixture of Gaussian and circularly symmetric a-stable noise. The problem is relevant to mobile communication systems where the mobility of users requires random modeling of transmitter positions in the network. In this paper, we derive formulas for probability distributions of the interference in FH SS radio networks. We generalize the results of Ref. [1], where attention was limited to Cauchy RV's, a special subclass of stable distributions, and where the effect of a background (Gaussian) noise was neglected. Based on the formulas derived, we calculate the probability of symbol error for radio links in environments varying from urban settings to office buildings. The results obtained allow the prediction of the performance of wireless systems under a wide range of conditions.
Michael Reuter, NCCOSC (USA)
Richard North, NCCOSC (USA)
James Zeidler, NCCOSC (USA)
Multichannel adaptive equalization combines the concepts of spatial diversity and temporal equalization to improve receiver performance in a line-of-sight communication environment. In this paper, we derive the optimal Wiener filter for conditions common to this environment. We specifically analyze the effect of a CW interference on this solution and demonstrate the dependence of receiver performance on the MAEQ spatial-temporal beampattern.
Mehmet Zeytinoglu, Ryerson Polytechnic University (CANADA)
Yen Chuan Hsu, University of Westminster (UK)
In this paper, we investigate variable-bit rate coding of wideband stereo audio signals for transmission over asynchronous transfer mode networks. We develop a layered source coding algorithm which decomposes the stereo signal into subband signals. The subband signals are then analyzed with respect to their relative energy and are assigned to high and low priority data streams. The output data rate of the audio coder has to be characterized and its statistics should be provided at the time when the initial connection to the network is established. We derive a probability distribution model for the output data rate expressed in terms of the asymptotic distribution of the DFT coefficients and the PSD function of the audio signal.
Ilan Sharfer, University of Michigan (USA)
Alfred O. Hero, University of Michigan (USA)
Maximum Likelihood (ML) estimation method for simultaneous amplitude, time delay, and data demodulation in direct sequence spread spectrum communication is proposed. The likelihood function is analytically intractable, so we consider a recursive estimation algorithm. The Expectation-Maximization (EM) algorithm has found increasing use in similar problems, however, for this case it is analytically intractable. Recently, a variant of the EM algorithm, called Space Alternating Generalized EM (SAGE), has been derived. In this work we apply the SAGE algorithm to the sequence estimation problem in a way which allows for simple sequential updates of the parameters. The resulting algorithm maximizes the penalized likelihood function, where the penalty is chosen to ensure good synchronization performance. Simulation results show that the algorithm has fast convergence, and essentially achieves optimal performance.
P. C. Sapiano, University of Bath (UK)
J.D. Martin, University of Bath (UK)
R.J. Holbeche, University of Bath (UK)
A method is presented for classifying multi- level PSK signals in the presence of additive white Gaussian noise (AGWN). The technique is based on the Discrete Fourier Transform (DFT) of a phase histogram. The probability of correct classification is given and it is found that the technique performs well at low SNR. The benefits of this technique are that it is simple to implement and requires no prior knowledge of the SNR of the signal for the classification.
Pierre Vila, Thomson CSF
Francois Pipon, Thomson CSF
Didier Pirez, Thomson CSF
Luc Fety, CNAM (FRANCE)
For digital radio communications systems operating on a jammed frequency-selective fading channel, the receiver performance can be improved by using the joint antenna diversity and equalization techniques to combat both time- and frequency-selective fades and jammers effects. The optimum, in the sense of minimum mean-squared error (MMSE), structure of the linear equalizer (LE) and the decision feedback equalizer (DFE) for coherent receiver antenna diversity has been derived for an unjammed environment [1]. In the present paper, the structure of the MMSE antenna diversity equalizer is determined for a jammed multipath channel. We show that is composed of the multidimensional matched filter (MMF) followed by a symbol-rate sampler and the MMSE LE or DFE equalizer for single antenna receivers.
Javier R. Fonollosa, Universitat Politecnica de Catalunya (SPAIN)
Jose A. R. Fonollosa, Universitat Politecnica de Catalunya (SPAIN)
Zoran Zvonar, Analog Devices (USA)
Josep Vidal, Universitat Politecnica de Catalunya (SPAIN)
Multiuser detection in Code Division Multiple Access systems usually requires either knowledge of the transmitted signature sequences and channel state information or use of a known training sequence for adaptation. In this paper we develop a scheme that can be employed for the joint adaptive blind multiuser identification and detection in asynchronous CDMA systems. This scheme relies on a multiuser Viterbi algorithm that incorporates an adaptive estimation of the overall channel impulse responses, given by the convolution of the signature sequences of the users and corresponding physical channels impulse responses. Once the overall channel responses are estimated, the blind multiuser detection algorithm performs like the maximum-likelihood sequence estimator. Results are provided to illustrate the convergence of the blind multiuser approach, near-far resistance and sensitivity to the algorithm initialization.