Beamforming Based Mobile User Tracking P. T. Karttunen 1 , T. I. Laakso 1 and J. Lilleberg 2 1 Helsinki University of Technology Laboratory of Telecommunications Technology Otakaari 5A, FIN-02015 Espoo, Finland E-mail: Petri.Karttunen@hut.fi, Timo.Laakso@hut.fi 2 Nokia Mobile Phones P.O. Box 50, FIN-90571 Oulu, Finland E-mail: Jorma.Lilleberg@nmp.nokia.com ABSTRACT - One way to increase system capacity in the future telecommunication systems is to employ adaptive antennas at the base station. They enable reception and transmission utilizing narrow beams which can dramatically reduce interference for other users. In this paper we propose a fast multi-user tracking system based on the spatial domain beamforming concept. The simulation results show that the proposed tracking system reacts fast and gives small tracking errors. 1. INTRODUCTION The target tracking problem arises in numerous applications, e.g., mobile communications where for each moving user appropriate beamforming based connections have to be established and maintained. Target tracking methods in this context enable continuous locating of mobile terminals as they move around in the cell. For this aim, efficient and robust mobile user tracking system is needed. The location and tracking problem of multiple moving targets could be solved by utilizing numerous different beamforming methods like MUltiple SIgnal Classification (MUSIC) or Maximum Likelihood (ML) methods [1]. However, continual application of these kinds of algorithms is prohibited from the computational complexity point of view. Furthermore, they also introduce the data association problem, i.e., the beamformer has no way to associate location estimates to different mobile users. In this paper we employ the method which can track multiple moving sources efficiently by using a conventional beamforming strategy without any greater performance losses in the case of Direction-of-Arrival (DOA) pointing errors [2]. The tracking system is enhanced by introducing an adaptive control strategy [3]. 2. TRACKING MODEL The tracking model for the antenna array used in the simulations will be developed in this section. The adaptive antenna configuration is illustrated in Figure 1, where the antenna array receiver with M elements at the base station and N surrounding mobile users are shown. The communication signals S n (n=1, …, N) are crude modeled as a zero-mean Gaussian distributed processes [4]. The additive noise process is also drawn from a Gaussian distribution. Furthermore, it is assumed that samples from the signal and noise process do not correlate with each other. The beamforming concept is based on the antenna steering vector which represents the azimuth θ n response of the antenna array for each source. The beamforming operation forms the beams for each source and extracts the desired communication signals. The tracking problem becomes to that of continuous location estimation as the mobile users move around the base station. Figure 2 shows the components of the tracking system. The communication signals impinged on the antenna array are downconverted and digitized in the receiver frontend. The baseband user signals are estimated in the beamforming unit by using the conventional block beamforming strategy. In the tracking unit the steering vectors are updated based on the received sample block which reflect new locations for mobile users. The updating rule can be derived by minimizing all the interference signal and noise components orthogonal to desired user signal component [5]. In the model fitting unit the new azimuth tracking angles for each user are determined by projecting the steering vector θ n 4 1 Reference element 2 3 M d S 1 S 2 S N Broadside direction Endside direction Figure 1 Adaptive antenna array receiver. Beamformer output Beamforming unit Tracking unit Adaptive step size control unit Dynamical model unit Model fitting unit z -1 R e c e i v e r Figure 2 Tracking system.