A MULTICODE APPROACH FOR HIGH DATA RATE UWB SYSTEM DESIGN

INTRODUCTION

UWB technology is a serious candidate for the new emerging Wireless Personal Area  Network (WPAN). In this context UWB is expected to provide very high data rates ≥ 500Mbps for short range communication. Many physical layer schemes have been proposed to address such rates on the wireless medium. Historically pulse based systems have been first proposed as the main modulation technique for UWB. The pulse based modulations inherently occupies a very large bandwidth (several GHz), which is required due to the very low emitting power. This technique is currently considered for low data rate in the sensor network context, but seems to be discarded for high data rate transmissions. Another physical layer, Multi Band Orthogonal Frequency Division Modulation (MB-OFDM), has been proposed within the IEEE802.15.3a task group. This technique is based on the well known OFDM scheme which allows good immunity towards multipath channels. In this proposal, only a 500MHz band is used at a given time, and a frequency hopping technique spreads the signal over several GHz. Finally, Direct Sequence Spreading (DSS) has also been proposed in the UWB framework (DSUWB). In this scheme, the data are spread over few GHz of bandwidth by means of very high rate sequences. In the existing proposal, the difficulty comes with the increase of the data rate, which imposes to shorten the spreading sequences. As a consequence there is an increase of the interference which results either in performance degradation or in receiver complexity increase.

UWB Spectrum

Ultra Wide Band is defined as any system which operates with a bandwidth greater than one fourth the central frequency or larger than 500MHz. In UWB some parameters like propagation attenuation and penetration through obstacles are not constant on the whole bandwidth. This makes the channel much different from the narrow band case, mostly due to the fact that the central frequency in UWB signals cannot be considered as the carrier frequency. This increases the channel response length, and largely impacts the overall system performance. From a regulatory point of view since UWB systems overlap with existing ones, they must be limited by a spectral mask which defines the maximum power density spectrum of the transmitted UWB signal. More specifically, in the USA, the FCC limits the spectrum of UWB signals as described in the so called ‘part 15 rule’ document which is concerned with intentional radiators. The main allocated bandwidth lies between 3.1GHz and 10.5GHz, with an upper power spectral limit of −41dBm/MHz. Other frequency ranges are allowed in FCC part 15 but are not considered in our design.

UWB Channels

It is well known that wideband systems are impaired by inter symbol interference due to multipaths. In such systems (typically a bandwidth of 5-20MHz), the receiver distinguishes some groups of paths, each group being described by an attenuation coefficient ck(t). The channel is thus identified by a finite length vector [c0 . . . cN−1]. These paths (or group of paths) correspond to spatial details which size is about few meters.