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Ernest R. Adams, Cranfield University RMCS (U.K.)
Peter C.J. Hill, Cranfield University RMCS (U.K.)
Direct-sequence spread spectrum signals are undetectable by conventional intercept receivers. However, the properties of the m-sequences used to generate such signals give rise to characteristic patterns of triple correlation peaks. These patterns remain discernable for short signal intercepts in the presence of data modulation, other m-sequences and noise, allowing the detection of individual m-sequences.
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Bernd-Peter Paris, George Mason University (U.S.A.)
Geoffrey C. Orsak, George Mason University (U.S.A.)
Hongda Chen, George Mason University (U.S.A.)
Nirmal Warke, George Mason University (U.S.A.)
The problem of distinguishing reliably between signaling formats in the presence of noise, interference, unknown dispersive channel conditions, as well as timing and frequency mismatches is addressed. Methods based on a combination of blind equalization and universal classification are presented and their performance is assessed through simulations.
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Konstantinos I. Diamantaras, Aristotle University Thessaloniki (Greece)
Michael G. Strintzis, Aristotle University Thessaloniki (Greece)
Noisy Principal Component Analysis (NPCA) was introduced recently as an extension of PCA in the assumption that the linear features are unreliable. The level of noise in the representation variables is found to have effects in the rank of the optimal solution resembling the water-filling analogy in information theory. The NPCA problem needs to be coupled with certain constraints so that it permits a finite solution. We present the solution of the NPCA problem under different constraints which can be useful in applications involving bandwidth limitations. One of these applications is the design of optimal subband coders incorporating quantization noise. In addition to the NPCA-optimality another advantage of the new design approach is that it works entirely in the time domain and thus the costly and difficult transformations to and from the Z-domain can be avoided.
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Sergio Barbarossa, University of Rome ``La Sapienza'' (Italy)
Anna Scaglione, University of Rome ``La Sapienza'' (Italy)
Sergio Spalletta, University of Rome ``La Sapienza'' (Italy)
Stefano Votini, University of Rome ``La Sapienza'' (Italy)
The aim of this paper is to propose an adaptive method for suppressing wideband interferences in spread spectrum (SS) communications. The proposed method is based on the time-frequency representation of the received signal, from which the parameters of an adaptive time-varying interference excision filter are estimated. The approach is based on the generalized Wigner-Hough Transform as an effective way to estimate the instantaneous frequency of parametric signals embedded in noise. The performance of the proposed approach is evaluated in terms of SNR improvement factor and bit error rate, in the presence of linear FM interferences plus white Gaussian noise.
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Meritxell Lamarca, Univ. Politècnica de Catalunya (Spain)
Gregori Vázquez, Univ. Politècnica de Catalunya (Spain)
This paper addresses the use of multichannel receivers for blind equalization in TDMA under frequency selective channels and OFDM systems in frequency flat fading channels. A new criteria is proposed for blind equalization of finite length mobile channels.
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Catharina Carlemalm, KTH, Stockholm (Sweden)
Andrew Logothetis, University of Melbourne (Australia)
This paper addresses the problem of detection and discrimination of the two phenomena double talk and echo path change in a telephone channel. It is of uttermost importance to quickly detect a change in the echo path while not confusing it with double talk, since the echo canceller should react differently whether an echo path change or double talk has occurred. In this paper, novel algorithms of low complexity are proposed. The system is described with a Markov modulated finite impulse response (FIR) filter. Depending on whether double talk or an echo path change occurs different parameters in the channel model change abruptly. Based on model assumptions, maximum likelihood (ML) parameter estimates of the communication channel are obtained via recursive (off--line) or iterative (on--line) methods using the expectation maximization (EM) algorithm. This enables us to use a Hidden Markov Model (HMM) state estimator to yield the minimum probability of error in identifying the state of the communication channel, i.~e.~the possible presence of double talk and/or echo path change. The proposed algorithms are experimentally verified using a real speech signal and impulse responses created from measured impulse responses from real hybrids.
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Kutluyl Dogançay, University of Melbourne (Australia)
Vikram Krishnamurthy, University of Melbourne (Australia)
Equalisation errors result from discrepancies between transmitted symbols and their estimates at the channel equaliser output in a digital communication system. This paper presents an on-line test to detect the occurrence of equalisation errors without direct access to the channel input. The test draws on the observation that for linear time-invariant (LTI) channels the relationship between transmitted symbol estimates generated by the equaliser and the noisy channel output can be represented by an underlying linear time-invariant model if and only if no equalisation errors are present in the sequence of transmitted symbol estimates. The presence of equalisation errors renders this relationship time-varying, of which the occurrence is detected by the proposed on-line test using the recursive least squares (RLS) algorithm. Simulation studies corroborate the good detection performance of the test.
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Hakan A. Cirpan, Stevens Inst. of Technology (U.S.A.)
Michail K. Tsatsanis, Stevens Inst. of Technology (U.S.A.)
Direct sequence Code Division Multiple Access (CDMA) systems are considered in this paper, in a multipath environment. We address the problem of estimating each user's signature waveform without requiring training sequences. We show that the problem is considerably simplified if a novel transmission strategy is adopted, which combines spreading and interleaving. In this setup, chip sequences corresponding to successive bits are interleaved before transmission. Novel channel estimation algorithms are developed in this chip interleaving framework and their performance is analyzed. Adaptive implementations are derived and some illustrative simulations are presented.
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Timothy A. Thomas, Purdue University (U.S.A.)
Michael D. Zoltowski, Purdue University (U.S.A.)
A space-time processing scheme is proposed for TDMA based cellular/PCS communications that cancels co-channel interference while simultaneously equalizing the desired signal. This is achieved through processing of a training signal designed from a PN code. At the receiver, the symbol waveform is built up above the noise and interference through correlation with a sampled waveform designed to exploit the PN code. From this built up signal, interference canceling beams are formed along with estimates of the initial equalizer coefficients. Then a decision directed algorithm is applied to track variations in the multipath channel. Simulations involving three-ray multipath are presented demonstrating the efficacy of the method.
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Jiunn-Tsair Chen, Stanford University (U.S.A.)
Arogyaswami Paulraj, Stanford University (U.S.A.)
In this paper, we propose a novel algorithm for the MLSE equalizer for the GSM system. Specifically, we use a parametric model for the channel, to obtain a modified Viterbi Equalizer which we refer to as the Parametric Channel-Viterbi Equalizer (PC-VE). In contrast to the conventional Viterbi Equalizer with a FIR channel description, the PC-VE avoids the linear approximation error and has a lower computational complexity. The proposed algorithm is applicable to both single and multi-antenna receivers. Some simulation results that illustrate the performance of the proposed algorithm are presented.
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Carles Antón-Haro, Univ. Politècnica de Catalunya (Spain)
José A.R. Fonollosa, Univ. Politècnica de Catalunya (Spain)
Zoran Zvonar, Analog Devices (U.S.A.)
Javier R. Fonollosa, Univ. Politècnica de Catalunya (Spain)
In this paper, two probabilistic adaptive algorithms for jointly detecting active users in a DS-CDMA system are compared. The first one, which is based on the theory of Hidden Markov Models (HMM) is proposed within the CDMA scenario and compared with the previously developed Viterbi-based algorithm. Both techniques are completely blind in the sense that no knowledge of the signature sequences, channel state information or training sequences is required for any user. After convergence, an estimate of the signature of each user convolved with its physical channel impulse response (CIR), and estimated data sequences are provided. This CIR estimate can then be used to switch to any decision-directed (DD) adaptation scheme. Performance of the algorithms is verified with simulations as well as with experimental data from an Underwater Acoustics (UWA) environment. In both cases, performance is found to be highly satisfactory, showing the near-far resistance of the analyzed algorithms.
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Chong-Meng Samson See, DSO (Singapore)
Colin F.N. Cowan, Queen's University of Belfast (Northern Ireland)
We propose new methods for fast blind identification and equalization of communication channels based on channel outputs sampled at symbol rate. These methods exploit the FA property of the transmitted sequence as well as the algebraic structures and relationships of the symbol and channel parameter matrices to achieve blind equalization. The block based methods proposed herein are fast converging and data efficient. Simulated studies have illustrated the proposed methods capable of achieving reliable blind equalization with sequence length as short as $20$ samples over a wide-range of SNR.
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Ahmad S. Bahai, Algo Tel Inc. (U.S.A.)
Mohsen Saraf, Algo Tel Inc. (U.S.A.)
In this paper a new parameter estimation technique based on channel impulse response estimation is introduced which is desirable in randomly time-varying systems such as burst transmission mobile systems, since it is robust in environments with considerable delay spread, thus desirable in burst transmission systems. The shortcomings of some of the existing algorithms are also discussed here.
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