Multiuser Channel Estimation for Ultra-Wideband Systems Using up to the Second-Order Statistics
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Multiuser Channel Estimation for Ultra-Wideband Systems Using up to the Second-Order Statistics Zhengyuan Xu Department of Electrical Engineering, University of California, Riverside, CA 92521, USA Email: [email protected]
Jin Tang Department of Electrical Engineering, University of California, Riverside, CA 92521, USA Email: [email protected]
Ping Liu Department of Electrical Engineering, Arkansas Tech University, Russellville, AR 72801, USA Email: [email protected] Received 27 September 2003; Revised 11 March 2004 In a pulse-position modulation-based ultra-wideband (UWB) communication system, multiple access is enabled by assigning unique time-hopping sequences to different users. Each user’s data information is carried by positions of short pulses which are directly transmitted through an unknown and possibly dense multipath channel. Single-user channel estimation methods have been proposed by maximum likelihood optimization that treats multiple access interference as Gaussian noise. In this paper, multiuser channel estimation methods are proposed based on a pulse-rate discrete-time system model and up to the second-order statistics of the channel outputs. The model can be regarded in a trilinear structure and also resembles a code-division multipleaccess (CDMA) system with newly defined hopping-code dependent matrices and inputs for each user. Considering that either the mean or covariance of received signals contains sufficient information for all unknown channels, least squares and covariance matching ideas are successfully applied to estimate all channels blindly. Accordingly, closed-form solutions are derived. Those channel estimates can be used to design typical linear receivers. Performance of each proposed estimator is analyzed and also verified by computer simulations. Corresponding receivers’ performance is also studied numerically. Keywords and phrases: ultra-wideband, channel estimation, covariance matching, least squares.
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INTRODUCTION
Ultra-wideband (UWB) technology is originated from works in the time-domain electromagnetics early in the 1960s [1, 2]. It is based on a widely recognized fact that electromagnetic signals for radio transmission and radar do not need to have an approximately sinusoidal time variation, as discussed in detail in [3] which shows that waves with arbitrary time variation can be radiated. With a novel antenna design technique [4], the generated UWB signals are able to communicate by baseband short pulses. Thereafter, the technique was immediately applied to radar, communications, automobile collision avoidance, positioning systems, and liquid-level sensing [5]. The term “ultrawideband” was not applied until late 1980s by the US Department of Defense when a covert property with low probability of interception and detection (LPI/LPD) was realized.
With a recent release of the spectral mask from the Federal Communications Commission (FCC) [6], there emerges an increasing interest in UWB techniques for both commercial and military applications [7, 8]. In general, a UWB signa
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