1.8. Single Band vs. Multiband
The ability of UWB technology to provide very high data rates for short ranges (less than 10 meters) has made it an excellent candidate for the physical layer of the IEEE 802.15.3a standard for wireless personal area networks (WPANs). However, two opposing groups of UWB developers are battling over the IEEE standard. The two competing technologies are single band and multiband. The single-band technique, backed by Motorola/XtremeSpectrum, supports the idea of impulse radio that is the original approach to UWB by using narrow pulses that occupy a large portion of the spectrum. The multiband approach divides the available UWB frequency spectrum (3.1 GHz to 10.6 GHz) into multiple smaller and nonoverlapping bands with bandwidths greater than 500 MHz to obey the FCC's definition of UWB signals. The multiband approach is supported by several companies, including Staccato Communications, Intel, Texas Instruments, General Atomics, and Time Domain Corporation.
To date, several proposals from both groups have been submitted to the IEEE 802.15.3a working group, and the decision is yet to be made because both technologies are impressive and have technical credibility. The following subsections discuss the two leading candidates for the 802.15.3a standard: direct-sequence UWB (DS-UWB) and multiband orthogonal frequency division multiplexing (OFDM).
1.8.1. Direct-Sequence UWB
Direct-sequence UWB is a single-band approach that uses narrow UWB pulses and time-domain signal processing combined with well-understood DSSS techniques to transmit and receive information. Figure 1-13 illustrates this approach.
Figure 1-13. DS-UWB transmits a single pulse over a huge swath of spectrum to represent data. Courtesy of Staccato Communications Inc.
Data representation in this approach is based on simple bi-phase shift keying (BPSK) modulation, and rake receivers are used to capture the signal energy from multiple paths in a multipath channel. According to the proposals sent to the IEEE 802.15.3a standardization committee by the proponents of this technology, the DS-UWB technique is scalable and can achieve data rates in excess of 1 Gbps. The technical reason behind using DS-UWB is the propagation benefits of ultra-wideband pulses, which experience no Rayleigh fading. In contrast, narrowband transmissions degrade significantly due to fading.
1.8.2. Multiband OFDM
The multiband UWB approach uses the 7500 MHz of the RF spectrum available to UWB communications in a way that differs from traditional UWB techniques. The UWB frequency band is divided into multiple smaller bands with bandwidths greater than 500 MHz. Figure 1-14 depicts the result.
Figure 1-14. The multiband approach divides the available UWB spectrum into several nonoverlapping smaller bands. Courtesy of Staccato Communications Inc.
This approach is similar to the narrowband frequency-hopping technique. Dividing the UWB spectrum into multiple frequency bands offers the advantage of avoiding transmission over certain bands, such as 802.11a at 5 GHz, to prevent potential interference. In the multiband approach, UWB pulses are not as narrow as in traditional UWB techniques; therefore, synchronization requirements are more relaxed. A variety of modulation techniques have been proposed by industry leaders for the multiband approach; however, OFDM, which was initially proposed by Texas Instruments, offers improved performance for high-data-rate applications.
As explained briefly, both technologies are technically valid and impressive. Supporters of DS-UWB criticize the multiband OFDM systems for their complexity, which results from using complex Fast Fourier Transforms (FFTs). On the other side, advocates of multiband OFDM believe that their technique offers better coexistence with other radio services, and they disapprove of DS-UWB because of possible interference concerns. The debate will likely continue until the IEEE 802.15.3a standardization committee reaches a decision.