The nature of the short-duration pulses used in UWB technology offers several advantages over narrowband communications systems. In this section, we discuss some of the key benefits that UWB brings to wireless communications.
1.5.1. Ability to Share the Frequency Spectrum
The FCC's power requirement of 41.3 dBm/MHz, equal to 75 nanowatts/MHz for UWB systems, puts them in the category of unintentional radiators, such as TVs and computer monitors. Such power restriction allows UWB systems to reside below the noise floor of a typical narrowband receiver and enables UWB signals to coexist with current radio services with minimal or no interference. However, this all depends on the type of modulation used for data transfer in a UWB system.
As we discuss in Chapter 3, some modulation schemes generate undesirable discrete spectral lines in their PSD, which can both increase the chance of interference to other systems and increase the vulnerability of the UWB system to interference from other radio services. In Chapter 4, we present a detailed discussion on interference from UWB on narrowband and wideband radio systems. Figure 1-6 illustrates the general idea of UWB's coexistence with narrowband and wideband technologies.
Figure 1-6. Coexistence of UWB signals with narrowband and wideband signals in the RF spectrum
1.5.2. Large Channel Capacity
One of the major advantages of the large bandwidth for UWB pulses is improved channel capacity. Channel capacity, or data rate, is defined as the maximum amount of data that can be transmitted per second over a communications channel. The large channel capacity of UWB communications systems is evident from Hartley-Shannon's capacity formula:
where C represents the maximum channel capacity, B is the bandwidth, and SNR is the signal-to-noise power ratio. As shown in Equation 1-5, channel capacity C linearly increases with bandwidth B. Therefore, having several gigahertz of bandwidth available for UWB signals, a data rate of gigabits per second (Gbps) can be expected. However, due to the FCC's current power limitation on UWB transmissions, such a high data rate is available only for short ranges, up to 10 meters. This makes UWB systems perfect candidates for short-range, high-data-rate wireless applications such as wireless personal area networks (WPANs). The trade-off between the range and the data rate makes UWB technology ideal for a wide array of applications in military, civil, and commercial sectors. We explore such applications later in this chapter and in Chapter 5.
1.5.3. Ability to Work with Low Signal-to-Noise Ratios
The Hartley-Shannon formula for maximum capacity (Equation 1-5) also indicates that the channel capacity is only logarithmically dependent on signal-to-noise ratio (SNR). Therefore, UWB communications systems are capable of working in harsh communication channels with low SNRs and still offer a large channel capacity as a result of their large bandwidth.
1.5.4. Low Probability of Intercept and Detection
Because of their low average transmission power, as discussed in previous sections, UWB communications systems have an inherent immunity to detection and intercept. With such low transmission power, the eavesdropper has to be very close to the transmitter (about 1 meter) to be able to detect the transmitted information. In addition, UWB pulses are time modulated with codes unique to each transmitter/receiver pair. The time modulation of extremely narrow pulses adds more security to UWB transmission, because detecting picosecond pulses without knowing when they will arrive is next to impossible. Therefore, UWB systems hold significant promise of achieving highly secure, low probability of intercept and detection (LPI/D) communications that is a critical need for military operations.
1.5.5. Resistance to Jamming
Unlike the well-defined narrowband frequency spectrum, the UWB spectrum covers a vast range of frequencies from near DC to several gigahertz and offers high processing gain for UWB signals. Processing gain (PG) is a measure of a radio system's resistance to jamming and is defined as the ratio of the RF bandwidth to the information bandwidth of a signal:
The frequency diversity caused by high processing gain makes UWB signals relatively resistant to intentional and unintentional jamming, because no jammer can jam every frequency in the UWB spectrum at once. Therefore, if some of the frequencies are jammed, there is still a large range of frequencies that remains untouched. However, this resistance to jamming is only in comparison to narrowband and wideband systems. Hence, the performance of a UWB communications system can still be degraded, depending on its modulation scheme, by strong narrowband interference from traditional radio transmitters coexisting in the UWB receiver's frequency band [2, 4, 5].
1.5.6. High Performance in Multipath Channels
The phenomenon known as multipath is unavoidable in wireless communications channels. It is caused by multiple reflections of the transmitted signal from various surfaces such as buildings, trees, and people. The straight line between a transmitter and a receiver is the line of sight (LOS); the reflected signals from surfaces are non-line of sight (NLOS). Figure 1-7 represents the multipath phenomenon in narrowband and UWB signals.
Figure 1-7. (a) The multipath phenomenon in wireless links. (b) Multipath's effects on narrowband signals. (c) Multipath's effects on ultra-wideband pulses.
As shown in Figure 1-7, the effect of multipath is rather severe for narrowband signals; it can cause signal degradation up to 40 dB due to the out-of-phase addition of LOS and NLOS continuous waveforms. On the other hand, the very short duration of UWB pulses makes them less sensitive to the multipath effect. Because the transmission duration of a UWB pulse is shorter than a nanosecond in most cases, the reflected pulse has an extremely short window of opportunity to collide with the LOS pulse and cause signal degradation.
Although the short duration of UWB pulses makes them less sensitive to multipath effects compared to narrowband signals, it doesn't mean that UWB communications is totally immune to multipath distortion. Research on UWB channel modeling has shown that depending on the UWB modulation scheme used, low-powered UWB pulses can become significantly distorted in indoor channels where a large number of objects and scatterers are closely spaced.
For a comprehensive discussion on various UWB modulation techniques and their performance in multipath channels, refer to Chapter 3.
1.5.7. Superior Penetration Properties
Unlike narrowband technology, UWB systems can penetrate effectively through different materials. The low frequencies included in the broad range of the UWB frequency spectrum have long wavelengths, which allows UWB signals to penetrate a variety of materials, including walls. This property makes UWB technology viable for through-the-wall communications and ground-penetrating radars. However, the material penetration capability of UWB signals is useful only when they are allowed to occupy the low-frequency portion of the radio spectrum.
1.5.8. Simple Transceiver Architecture
As mentioned earlier in this chapter, UWB transmission is carrierless, meaning that data is not modulated on a continous waveform with a specific carrier frequency, as in narrowband and wideband technologies. Carrierless transmission requires fewer RF components than carrier-based transmission. For this reason UWB transceiver architecture is significantly simpler and thus cheaper to build. Figure 1-8 compares the block diagrams of typical narrowband and UWB transceivers.
Figure 1-8. (a) A typical narrowband transceiver architecture. (b) An example of a UWB transceiver architecture.
As shown in Figure 1-8, the UWB transceiver architecture is considerably less complicated than that of the narrowband transceiver. The transmission of low-powered pulses eliminates the need for a power amplifier (PA) in UWB transmitters. Also, because UWB transmission is carrierless, there is no need for mixers and local oscillators to translate the carrier frequency to the required frequency band; consequently there is no need for a carrier recovery stage at the receiver end. In general, the analog front end of a UWB transceiver is noticeably less complicated than that of a narrowband transceiver. This simplicity makes an all-CMOS (short for complementary metal-oxide semiconductors) implementation of UWB transceivers possible, which translates to smaller form factors and lower production costs.
Table 1-1 shows the main advantages and benefits of UWB systems over narrowband wireless technologies.