Section 10.1. Specialized Applications for UWB Signals

10.1. Specialized Applications for UWB Signals

With a low duty cycle and wide bandwidth, UWB is naturally suitable to radar applications, which were among the first applications studied for UWB [1]. As the bandwidth of a pulse increases, it provides finer resolution of radar images for improved target identification. UWB also provides low probability of intercept and detection because the wide bandwidth-to-information ratio results in low energy across the spectrum. Thus, UWB was also initially considered for secure communications in military applications. More recently, the high-speed communications capability of UWB has been exploited for wireless personal area networks (WPANs) such as IEEE 802.15.3a (refer to Appendix B), which provides a high data rate physical layer (PHY) for personal area multimedia networking. Furthermore, UWB permits coexistence with both narrowband systems and other UWB systems. Regulatory bodies limit UWB devices to low average power in order to minimize interference with narrowband systems. Thus, UWB provides a method to reuse large amounts of existing spectrum without disturbing existing users, and it should be available worldwide in the near future.

Another important property of UWB is its precision position location capability [25], which is explained in more detail in Appendix A. The Cramer-Rao Lower Bound (CRLB) provides a limit on the accuracy of the delay estimate (which reduces to the ranging accuracy) that increases with the bandwidth and the signal-to-noise ratio (SNR) of the received signal [6]. See Appendix A for details on the CRLB as related to ranging. Even at low SNR, UWB can resolve distances with subcentimeter accuracy using simple signal processing. When nodes share distance estimates, they can cooperatively calculate location information, which is an important feature for many sensor network applications and network protocols [7, 8]. Because UWB RF circuitry is shared for communications and ranging purposes, UWB may provide low-cost, high-resolution ranging; and unlike GPS, UWB's location capabilities can function in indoor environments. UWB's capability to provide both location awareness and communications has made it an ideal candidate for the upcoming 802.15.4a standard for low data rate, low power, and location aware applications.

As shown in Figure 10.1, UWB provides a fundamental trade-off between data rate and link distance. The performance of five modulation schemes is presented. The bottom trace represents a long-distance communications system that uses a simple 2-PAM or PPM modulation, transmitting 100 pulses for every data bit (N = 100). Immediately above is a trace representing a system with the same modulation scheme, but now only transmitting 10 pulses for every data bit. At the top of the graph is a trace representing a system which uses 256-ary modulation, or 8 data bits per UWB pulse. Note that systems transmiting multiple pulses per data bit are able to maintain a specific data rate for a longer link distance.

Figure 10.1. Trade-Off Between Data Rate and Distance for UWB.

SOURCE: P. Rouzet, "PULSERS Presentation to IEEE 802.15.4," IEEE P802.15 Working Group for Wireless Personal Area Networks Publications, [9]. © IEEE, 2003. Used by permission.

Further, the baseband nature of UWB systems allows simple hardware implementation with no intermediate mixers or downconversion stages. UWB devices may have a nearly all-digital implementation in CMOS with minimal analog RF electronics. This simple architecture can translate to low power dissipation and low cost, which opens a variety of possible mobile applications.

Finally, UWB is relatively immune to multipath induced fading effects in both indoor and outdoor environments. For an application such as maritime asset tracking, the multipath environment may be particularly harsh, with several layers of densely packed containers stacked inside a ship's metal hold. With an appropriate receiver, a UWB system may even harvest energy from the resolvable multipath signals to improve data rate or BER.