Comparison between Coherent and Noncoherent Receivers for UWB Communications
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Comparison between Coherent and Noncoherent Receivers for UWB Communications Giuseppe Durisi Istituto Superiore Mario Boella, 10138 Torino, Italy Email: [email protected]
Sergio Benedetto Center for Multimedia Radio Communications (CERCOM), Politecnico di Torino, 10129 Torino, Italy Email: [email protected] Received 1 October 2003; Revised 1 March 2004 We present a comparison between coherent and noncoherent UWB receivers, under a realistic propagation environment, that takes into account also the effect of path-dependent pulse distortion. As far as coherent receivers are concerned, both maximal ratio combining (MRC) and equal gain combining (EGC) techniques are analyzed, considering a limited number of estimated paths. Furthermore, two classical noncoherent schemes, a differential detector, and a transmitted-reference receiver, together with two iterative solutions, recently proposed in the literature, are considered. Finally, we extend the multisymbol approach to the UWB case and we propose a decision-feedback receiver that reduces the complexity of the previous strategy, thus still maintaining good performance. While traditional noncoherent receivers exhibit performance loss, if compared to coherent detectors, the iterative and the decision-feedback ones are able to guarantee error probability close to the one obtained employing an ideal RAKE, without requiring channel estimation, in the presence of static indoor channel and limited multiuser interference. Keywords and phrases: ultra-wideband communications, noncoherent detection, multisymbol differential detection.
1. INTRODUCTION Ultra-wideband (UWB) systems are based on the transmission of subnanosecond pulses, typically obtained by directly driving an antenna with short electrical pulses. According to the FCC regulation of February 2002, signals belonging to this category are required to possess a −10 dB bandwidth which exceeds 500 MHz or 20% of its fractional bandwidth [1]. Recently, this technology has been considered for both adhoc [2] and indoor wireless personal area networks (IEEE 802.15.3a). UWB characteristics are claimed to meet the requirements of these applications, in particular, low complexity, low cost, low power consumption, and high data rate connectivity [3]. Furthermore, the fine delay resolution, guaranteed by the large signal bandwidth, provides a high robustness in dense multipath environments [4]. On the other hand, to fully exploit the channel diversity, a conventional coherent RAKE receiver must be able to capture and track the energy associated with a high number of multipath replicas. In [5], it is shown that the number of paths to be considered to reach the 85% of the overall energy can sometimes exceed 100. In addition, the radiation and propagation processes can act on the transmitted pulse as a filter
whose characteristics vary from path to path. Therefore, the received signal can be seen as a train of distorted waveforms that often show little resemblance with the transmitted pulse [6, 7]. Due to complexity constraints, only a sma
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