System Architectures for High-rate Ultra-wideband Communication Systems: A Review of Recent Developments

Ultra-Wideband (UWB) was approved by the Federal Communications Commission (FCC) in Mar. 2002 for unlicensed operation in the 3.1-10.6 GHz band subject to modified Part 15 rules. The rule limits the emitted power spectral density (p.s.d) from a UWB source measured in a 1 MHz bandwidth at the output of an isotropic transmit antenna at a reference distance to that shown in Figure 1. Further, the transmitted signal must instantaneously occupy either i) a fractional bandwidth in excess of 20% of the center frequency or ii) in excess of 500 MHz of absolute bandwidth to be classified as a UWB signal. The maximum allowable p.s.d for UWB transmission of 41.3 dBm/MHz corresponds to approximately 0.5 mW of average transmit power when the entire 3.1-10.6 GHz band is used, effectively limiting UWB links to short ranges. Nevertheless, the potential for exploiting such low power UWB links for high data rate wireless PAN connectivity (in excess of 100 Mbps) at ranges up to 10 m particularly for in-home networking applications has led to considerable recent interest in this technology. The combination of the ultra-wide bandwidths and low average transmitted power presents unique challenges for UWB radio design. Such large bandwidths imply highly frequency selective multipath channels, with a large number (order of hundreds) of resolved multipath components . Due to stringent limitations on average and peak transmit power, power efficient modulation schemes such as binary pulse (amplitude and/or position) modulation are typically used for UWB signaling; to increase data rate therefore requires scaling of the PRF which in turn leads to increased inter-symbol interference in a strong multipath environment that necessitates costly equalization and/or coding to meet performance specifications. An alternative for higher data rates is to employ power efficient M-ary orthogonal signaling schemes although they require higher bit SNR at the receiver input.