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Abstract - COMM3

COMM3.1	Branch-Hopped Wavelet Packet Division Multiplexing T. Davidson, A. Schott, K. Wong (McMaster University, Canada) Wavelet Packet Division Multiplexing (WPDM) is a high-capacity, flexible and robust orthogonal multiplexing technique in which wavelet packet basis functions are chosen as the coding waveforms. By analogy with frequency-hopped communication schemes, incorporation of time variation into the WPDM scheme offers the potential for further performance improvements, especially in frequency-selective fading channels. We consider a `Branch-Hopped' WPDM scheme which employs an efficient modular switched transmultiplexer structure to induce the time variation. We determine classes of `slow' and `fast' hopping schemes analogous to their frequency-hopped counterparts, and evaluate several switching strategies for the transmultiplexer. For a given switching strategy we then design the filters within the transmultiplexer modules to provide further robustness to frequency-selective fading channels.
COMM3.2	Joint Multipath-Doppler Diversity in Fast Fading Channels A. Sayeed (University of Wisconsin-Madison, USA); B. Aazhang (Rice University, USA) We present a new framework for code-division multiple access (CDMA) communication over fast fading mobile wireless channels. The performance of the RAKE receiver, which is at the heart of existing CDMA systems, degrades substantially under fast fading encountered in many mobile scenarios. Due to the time-varying nature of the fast fading channel, we employ joint time-frequency processing, which is a powerful approach to time-varying signal processing. Whereas the RAKE receiver exploits multipath diversity to combat fading, our framework is based on joint multipath-Doppler diversity facilitated by a fundamental time-frequency decomposition of the channel into independent flat fading channels. Diversity processing is achieved by a time-frequency generalization of the RAKE receiver which can be leveraged into several important aspects of system design. Performance analysis shows that CDMA systems based on the time-frequency RAKE receiver, due to their inherently higher level of diversity, can potentially deliver significant performance gains over existing systems.
COMM3.3	A Blind Joint Estimator for Multipath Diversity and PN Timing Error in Direct-Sequence Spread-Spectrum Receivers J. Lin (National Taiwan University, Taiwan, ROC) A blind joint estimator for multipath diversity and PN code timing error is proposed in this paper for direct-sequence spread-spectrum signaling on a frequency-selective fading channel. In the multipath diversity combiner, a modified known modulus adaptive (KMA) algorithm is used to cope with time-varying multipath effects and to perform multipath diversity combining in the blind mode. In the code timing recovery, the timing error signal is extracted from each propagation path independently and also combined in the same fashion as the multipath diversity combining process. By taking advantage of the inherent diversity based on a known modulus adaptive (KMA) algorithm, this modified code timing recovery can avoid the problem due to the drift or flutter effects of the error signals, and provide better performance on frequency selective fading channels. Extensive computer simulation results have verified the analysis and indicated very attractive behavior of the proposed joint estimator for multipath diversity and PN code timing error.
COMM3.4	A Receiver Diversity Based Code-Timing Estimator for Asynchronous DS-CDMA Systems Z. Liu, J. Li, S. Miller (University of Florida, USA) We propose a receiver diversity based code-timing estimator for DS-CDMA systems. The systems are assumed to work in a flat fading and near-far environment, where an arbitrary antenna array is used at the receiver of the system to achieve the spatial diversity. We show that by utilizing the information collected via multiple antenna sensors, the length of the training sequences can be greatly reduced. We also show that the algorithm is an asymptotic maximum likelihood estimator. As a result, the mean-squared error of the code-timing estimates obtained by the algorithm approaches the Cramer-Rao lower bound as the length of the training sequence increases. Moreover, the algorithm does not require the search over a parameter space and the code-timing is obtained by rooting a second-order polynomial, which is computationally very efficient.
COMM3.5	Fixed Point Error Analysis of Multiuser Detection and Synchronization Algorithms for CDMA Communication Systems C. Sengupta, S. Das, J. Cavallaro, B. Aazhang (Rice University, USA) Conventional correlation based single-user techniques for Direct Sequence Code Division Multiple Access (DS-CDMA) wireless communication systems is susceptible to performance degradation due to interference from other users. Recent research has focused on development of several multiuser techniques where information about multiple users is used to improve performance for each individual user. Due to performance benefits of these methods, they are attractive candidates for implementation in future cellular systems. In this paper we present an error analysis of fixed point implementation of some of these techniques.
COMM3.6	Real Time Implementation of a Symbol Timing Recovery Algorithm for a Narrowband Wireless Modem J. Grimm, R. Kumar, J. Kusuma, J. Krogmeier (Purdue University, USA) This paper examines some of the complexity issues arising in the real time implementation of the symbol timing subsystem of a narrowband wireless modem. The modem prototype has been designed for 4 kHz channels in the 220-222 MHz land mobile band. In an effort to achieve high bandwidth efficiency, the modem architecture employs transmitter diversity, pilot symbol assisted modulation, and trellis coded modulations. An experimental, non-real time system has been implemented and extensively field tested demonstrating bandwidth efficiencies in excess of 3 bits per second per Hz. The work on this project is currently focused on the real-time implementation of the baseband receiver functions using the Texas Instruments C54x fixed point digital signal processor. Here we report on some of the performance/complexity tradeoffs present in the design of a DSP implementation of the symbol timing recovery. Symbol timing is the first function performed in the baseband receiver.
COMM3.7	Broadband Nonstationary Interference Excision for Spread Spectrum Communications Using Time-Frequency Synthesis S. Lach, M. Amin (Villanova University, USA); A. Lindsey (Rome Laboratory, USA) A new method is introduced for interference excision in spread spectrum communications. Time-frequency synthesis techniques are used to synthesize the nonstationary jammer from the time-frequency domain using least-squares methods. The synthesized jammer is then subtracted from the incoming data in the time domain, leading to increased signal to interference ratio at the input of the correlator. The paper focuses on jammers with constant modulus where the jamming signal is a polynomial phase. With this apriori knowledge, the jammer signal amplitude is restored by projecting each sample of the synthesized signal to a circle representing its constant modulus. With the phase matching provided by the least-squares synthesis method and amplitude matching underlying the projection operation, the paper shows a significant improvement in receiver performance/bit error rates over the case where no projection is performed.
COMM3.8	A New Non-Linear System for Estimating and Suppressing Narrowband Interference in PN Spread Spectrum Modulation A. Perez-Neira, J. Roca, M. Lagunas (Universitat Politecnica de Catalunya, Spain) This work develops a novel dynamic fuzzy logic system that, based on a fuzzy basis function expansion, successfully solves the non-linear problem of narrowband interference prediction and rejection in DS-SS. A fuzzy basis function representation provides a natural framework for combining both numerical and linguistic information in a uniform fashion. The result is a low complexity non-linear adaptive line enhancer, which offers a faster convergence rate and an overall better performance over other well-known non-linear line enhancers.
COMM3.9	A Novel Time-Frequency Exciser in Spread Spectrum Communications for Chirp-Like Interference A. Bultan, A. Akansu (New Jersey Institute of Technology, USA) A novel time-frequency exciser is developed for the removal of chirp-like interferences in direct sequence spread spectrum (DSSS) communications. The chirplet decomposition iteratively expands signals in terms of time-frequency localized waveforms. The interference signal components are highly correlated with chirplets, and are represented by a few of the highest energy components of the decomposition. These components are excised from the received signal, and an excised signal goes through a detector for a decision. The proposed time-frequency exciser outperforms the conventional Fourier transform based excisers for chirp-like interference classes.
COMM3.10	A Comparative Performance Evaluation of DMT (OFDM) and DWMT(DSBMT) Based DSL Communications Systems for Single and Multitone Interference A. Akansu, X. Lin (New Jersey Institute of Technology, USA) Multicarrier modulation techniques have been proposed in the digital subscriber line (DSL) applications. In this paper, the performance of DMT (OFDM) and DWMT (DSBMT) techniques for single and multitone interference are investigated. It is shown that a DMT system is sensitive to the location of narrow band interference. DMT technique needs additional narrow band interference canceller before forward FFT transform for performance improvements. In DSBMT technique, due to a limited spectral overlap between its subcarriers, single (multi)- tone interference could effect only a few subchannels which correspond to these interferences. DSBMT has a superior performance than DMT.

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