Section 9.2. UWB Physical Layer Issues

9.2. UWB Physical Layer Issues

The physical layer transmits data bits to one or more receivers and receives data bits from a transmitter using appropriate modulation and error coding schemes to maximize throughput and minimize the bit or symbol error rate. Because a UWB signal occupies a large amount of bandwidth, the data rates in a UWB system can be very highin excess of 100 megabits per second (Mbps). In an Impulse UWB (I-UWB) [6] or a Direct Sequence UWB (DS-UWB) [7] system the pulse is so narrow, less than 10 ns and closer to 1 ns in some schemes, that synchronous transmission is essential to ensure good reception. In addition, because the bandwidth used by a UWB system is so large (several GHzs of bandwidth) the U.S. Federal Communications Commission (FCC) has imposed strict regulations on transmission power for UWB transmitters. This restriction limits the maximum effective radio range for most UWB devices to be about 10 meters for high data rates. Synchronous transmission, high data rates, and short radio range are unique characteristics of the physical layer that influence the design of higher layer protocols, especially the data link layer.

Several modulation schemes can be applied in a UWB physical layer. For a single band UWB system, there are many modulation schemes to choose from, such as impulse modulated or direct-sequence phase coded. I-UWB uses time hopping codes for signal modulation, whereas DS-UWB uses direct sequences to spread a symbol over several UWB pulses. For multichannel (MC) UWB systems, Orthogonal Frequency Division Multiplexing (OFDM) is a possible modulation scheme. The data link design depends on the particular modulation scheme used in a UWB system; thus, the design of the data link layer should differ with the modulation scheme, as discussed in Section 9.3.