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Abstract - COMM5

COMM5.1	A Fast Blind Source Separation for Digital Wireless Applications M. Torlak, L. Hansen, G. Xu (University of Texas, Austin, USA) The problem of blindly estimating multiple digital co-channel communication signals using an antenna array is studied in the presence of multipath fading. We develop a fast sequential-estimation algorithm for separating multi-user signals based on the geometric observation made by Hansen and Xu. When the signals are constrained to a finite alphabet, it is possible to visualize the geometric properties of the problem, which can be exploited to sequentially extract the digital co-channel communication signals. We will present simulation results comparing speed and BER performance with different methods.
COMM5.2	Two Stage Code Reference Beamformer in Mobile Communications X. Mestre, M. Nájar, M. Lagunas (UPC, Spain) This paper addresses a new architecture for blind adaptive beamforming when dealing with Frequency Hopping (FH) modulation in cellular mobile communications systems. The proposed Code Reference Beamformer (CRB) takes advantage of the inherent frequency diversity to estimate beforehand the noise plus interference correlation matrix, which is employed as the first part of the framework. Then, a second stage is adaptively obtained without any a priori knowledge of either the direction of arrival or the array manifold. Using this information, the first stage is in turn readjusted and, as a result, the scheme is able to track non-stationary scenarios following the channel variations with no previous references.
COMM5.3	Downlink Beamforming Avoiding DOA Estimation for Cellular Mobile Communications T. Asté, P. Forster, L. Féty (Laboratoire d'Electronique CNAM, France); S. Mayrargue (CNET, France) A new technique to overcome induced difficulties of FDD for the design of a forward link beamformer for cellular mobile communications is presented. It uses the array topology at the basestation in order to transpose second order statistics of the propagation channel from uplink frequency to downlink frequency, thus enabling to optimize directly any beamforming criterion based on these statistics at downlink frequency, without feedback nor DOA (Direction Of Arrival) estimation. It can be applyied whatever criterion is used to design the beamformer is. Effectiveness is verified by the mean of simulation results.
COMM5.4	Maximum Likelihood Multichannel Estimation under Reduced Rank Constraint P. Forster, T. Asté (CNAM, France) This paper deals with the maximum likelihood estimation of the multichannel impulse response in a mobile communication system whose base stations are equipped with antennas arrays. The following problem is solved: using the training sequence, find the maximum likelihood multichannel impulse response from one mobile to the base station under a reduced rank constraint in the presence of gaussian noise and jammers with unknown covariance matrix. Our results find applications in equalization (the reduced rank channel estimate can be used in a Viterbi algorithm), and in the estimation of the directions of arrival (DOA) of the paths from the mobile to the base station. In this last application, a MUSIC like algorithm is developped using the estimated channel subspace.
COMM5.5	Performance Enhancement of the Decorrelating Detector Using Antenna Arrays K. Kim, S. Park, H. Kim, Y. Kim, I. Song (Korea Advanced Institute of Science and Technology, Korea) In this paper, a vector channel model is proposed and some statistical properties of the asymptotic efficiency of the decorrelating detector with base-station antenna arrays are investigated. It is shown that we can get gain over the conventional decorrelator by employing antenna arrays and the gain increases as the number of antennas increases and the angle dispersion decreases. It is also shown that we can increase the asymptotic efficiency of the decorrelating detector with base-station antenna arrays up to unity if we use infinite number of antennas when the channel is angle-nondispersive: we cannot, however, increase the asymptotic efficiency of the decorrelating detector by employing base-station antenna arrays if the dispersion is infinite.
COMM5.6	Vector-Sensor Array Processing for Estimating Angles and Times of Arrival of Multipath Communication Signals P. Chua, C. See (DSO National Laboratories, Singapore); A. Nehorai (University of Illinois, Chicago, USA) We develop vector-sensor array processing to estimate the angles-of-arrival (AOAs) and time delays of multipath channels in the space-time-polarization domain. A MUSIC-type algorithm for joint angle and delay estimation with a vector-sensor array is derived. Potential applications include multipath channel estimation and mobile localization. Simulation results show that the space-time-polarization parameterization of the multipath channels results in improved accuracy and resolution performance.
COMM5.7	Decoupled Direction Finding: Detection P. Pelin (Chalmers University of Technology, Sweden) Antenna arrays are likely to be an important feature of future mobile communication systems. With an antenna array, mobile users can be separated by a spatial filtering procedure allowing several users on the same carrier frequency. The uplink part (mobile to base) not only can, but is better solved without using any spatial knowledge in terms of direction of arrival (DOA). However, DOA estimation remains an important issue in the overall system, both for downlink beamforming, as well as channel allocation. Previous results have shown that DOA-estimation is best performed in a post-detection manner, i.e., using the estimated symbol sequence for DOA estimation. In this way, the estimation problem can be decoupled to treat individual users separately. To estimate the number of propagation paths from a specific user, a detection scheme is derived based on the DOA estimation criterion function.
COMM5.8	The Effects of Local Scattering on Direction of Arrival Estimation with MUSIC and ESPRIT D. Asztely, B. Ottersten (Royal Institute of Technology, Sweden) In wireless communication scenarios, multipath propagation from local scatterers in the vicinity of mobile sources may cause angular spreading as seen from a base station antenna array. This paper studies the effects of such local scattering on direction of arrival (DOA) estimation with the MUSIC and ESPRIT algorithms. Previous work has considered rapidly time-varying scenarios, and concluded that local scattering has a minor effect on DOA estimation in such scenarios. This work considers the case in which the channel is time-invariant during the observation period. The distribution of the DOA estimates is derived, and the results show that local scattering has significant impact on DOA estimation in the time-invariant case. In addition, numerical examples are included to illustrate the analysis, and to demonstrate that the results may be used to formulate simple estimators of angular spread.
COMM5.9	Optimal Downlink Power Assignment for Smart Antenna Systems W. Yang, G. Xu (The University of Texas, Austin, USA) Smart antenna systems have the potential to substantially increase the range of base stations and boost the SINRs of signals. In this paper, we study several criteria in downlink weighting vector design which is key to exploit the full potential of smart antenna systems, and give the optimal power assignment when the orientations of the weighting vectors are known. Simulation results have shown significant improvement offered by the proposed optimal power assignment method. In particular, we can equalize each user's downlink performance by significantly reducing the output power. Since the power amplifiers at the base station are the most expensive subsystems, this approach can lead to significant cost reduction for a base station.
COMM5.10	Optimal Array Combiner for Sequence Detectors M. Lagunas, A. Perez-Neira, J. Vidal (UPC, Barcelona, Spain) The use of spatial diversity at the receiver front-end together with a sequence detector implies a joint design problem of the spatial combiner and the sequence detector impulse response. This joint design is usually faced under the constraint that the impulse response of the sequence detector is matched to the channel plus combiner response. This procedure maximizes the signal to noise ratio at the input of the detector but does not guarantee that the so-called effective signal to noise ratio is maximized. This work presents a procedure that, starting from the matched criteria, faces directly the maximization of the effective signal to noise ratio, yet preserving all the features of the spatial processor in terms of co-channel and high order intersymbol interference rejection.

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