Lower bounds for ranging accuracy with multi band OFDM and direct sequence UWB signals

The definition of ultra wide band (UWB) signals set by the Federal Communications Commissions (FCC) opened the way to both impulse and non-impulse UWB signal formats. This is reflected within the IEEE 802.15.3a TG, aiming at the definition of a standard for UWB-based high bit rate WPANs. The two main proposals considered in this group are in fact a multi band OFDM approach, based on the transmission of non-impulse OFDM signals combined with frequency hopping (FH), and the direct-sequence (DS) UWB approach, based on impulse radio transmission of UWB DS-coded pulses. In this paper we analyze the ranging capabilities of the two proposals by first determining the Cramer-Rao lower bound (CRLB) for the error in distance estimation using both an ideal channel model and a real channel model taking into account the effect of multipath. Next, we investigate the impact of receiver structure and synchronization sequences adopted in the two proposals on the ranging accuracy. Results show that synchronization sequences play a key role in determining the ranging accuracy. Although the DS-UWB signal is in general best suited for ranging, thanks to its larger bandwidth and its higher frequencies of operation, specific synchronization sequences may in fact lead to better ranging accuracy for the MB-OFDM signal.