A Subspace Approach to Blind Multiuser Detection for Ultra-Wideband Communication Systems
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A Subspace Approach to Blind Multiuser Detection for Ultra-Wideband Communication Systems Zhengyuan Xu 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]
Jin Tang Department of Electrical Engineering, University of California, Riverside, CA 92521, USA Email: [email protected] Received 26 September 2003; Revised 29 January 2004 Impulse radio-based ultra-wideband (UWB) communication systems allow multiple users to access channels simultaneously by assigning unique time-hopping codes to individual users, while each user’s information stream is modulated by pulse-position modulation (PPM). However, transmitted signals undergo fading from a number of propagation paths in a dense multipath environment and meanwhile suffer from multiuser interference (MUI). Although RAKE receiver can be employed to maximally exploit path diversity, it is a single-user receiver. Multiuser receiver can significantly improve detection performance. Each of these receivers requires channel parameters. Existing maximum likelihood channel estimators treat MUI as Gaussian noise. In this paper, we derive a blind subspace channel estimator first and then design linear receivers. Following a channel input/output model that transforms a PPM signal into a sum of seemingly pulse-amplitude modulated signals, a structure similar to a code-division multiple-access (CDMA) system is observed. Code matrices for each user are identified. After considering unique statistical properties of new inputs such as mean and covariance, the model is further transformed to ensure that all signature waveforms lie in the signal subspace and are orthogonal to the noise subspace. Consequently, a subspace technique is applicable to estimate each channel. Then minimum mean square error receivers of two different versions are designed, suitable for both uplink and downlink. Asymptotic performance of both the channel estimator and receivers is studied. Closed-form bit error rate is also derived. Keywords and phrases: ultra-wideband, subspace decomposition, multiuser detection, asymptotic performance.
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INTRODUCTION
Research in impulse radio (IR) ultra-wideband (UWB) systems has lasted for several decades whose interest remains growing. Beginning with a focus on radar applications in military networks [1], the topic has spanned over a wide range of spectrum, such as those in commercial and other government applications [2, 3, 4]. With recent release of spectral mask from the Federal Communications Commission (FCC) [5], communication society has witnessed an increasing interest in recent years [6]. A conventional IR system transmits trains of time-hopping (TH) short-duration pulses with a low duty cycle and uses pulse-position modulation (PPM). Therefore, multipath down-to-path delay differentials in nanosecond is resolvable at the receiver, significantly mitigating multipath distortion and p
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