UWB technology for communications is not all about advantages. In fact, there are many challenges involved in using nanosecond-duration pulses for communications. Some of the main difficulties of UWB communications are discussed in the following subsections.
1.6.1. Pulse-Shape Distortion
The transmission characteristics of UWB pulses are more complicated than those of continuous narrowband sinusoids. A narrowband signal remains sinusoidal throughout the transmission channel. However, the weak and low-powered UWB pulses can be distorted significantly by the transmission link. We can show this distortion mathematically with the widely used Friis transmission formula:
where Pr and Pt are the received and transmitted signal power, respectively; Gt and Gr are the transmitter and receiver antenna gains, respectively; c is the speed of light; d is the distance between the transmitter and the receiver; and f is the signal frequency.
This formula shows that the received signal power will decrease quadratically with the increase in frequency. In narrowband signals with a very narrow frequency band, the change in frequency only minimally changes the received power and hence can be overlooked. However, due to the wide range of frequencies that is covered by the UWB spectrum, the received power drastically changes and thus distorts the pulse shape. This will limit the performance of UWB receivers that correlate the received pulses with a predefined template such as classical matched filters. We discuss UWB receivers and their performance in detail in Chapter 3.
1.6.2. Channel Estimation
Channel estimation is a core issue for receiver design in wireless communications systems. Because it is not possible to measure every wireless channel in the field, it is important to use training sequences to estimate channel parameters, such as attenuations and delays of the propagation path. Given that most UWB receivers correlate the received signal with a predefined template signal, prior knowledge of the wireless channel parameters is necessary to predict the shape of the template signal that matches the received signal. However, as a result of the wide bandwidth and reduced signal energy, UWB pulses undergo severe pulse distortion; thus, channel estimation in UWB communications systems becomes very complicated .
1.6.3. High-Frequency Synchronization
Time synchronization is a major challenge and a rich area of study in UWB communications systems. As with any other wireless communications system, time synchronization between the receiver and the transmitter is a must for UWB transmitter/receiver pairs. However, sampling and synchronizing nanosecond pulses place a major limitation on the design of UWB systems. In order to sample these narrow pulses, very fast (on the order of gigahertz) analog-to-digital converters (ADCs) are needed. Moreover, the strict power limitations and short pulse duration make the performance of UWB systems highly sensitive to timing errors such as jitter and drift. This can become a major issue in the success of pulse-position modulation (PPM) receivers, which rely on detecting the exact position of the received signal. For a thorough discussion on UWB PPM receivers, refer to Chapter 3.
1.6.4. Multiple-Access Interference
In a multiuser or a multiple-access communications system, different users or devices send information independently and concurrently over a shared transmission medium (such as the air interface in wireless communications). At the receiving end, one or more receivers should be able to separate users and detect information from the user of interest. Interference from other users with the user of interest is called multiple-access interference (MAI), which is a limiting factor to channel capacity and the performance of such receivers. The addition of MAI to the unavoidable channel noise and narrowband interference discussed earlier can significantly degrade the low-powered UWB pulses and make the detection process very difficult. Figure 1-9 represents a UWB multiple-access channel.
Figure 1-9. A UWB multiple-access channel
As shown in Figure 1-9, separating each user's information from the combination of heavily distorted and low-powered UWB signals from all users is a very challenging task. A comprehensive study of multiple-access techniques in UWB systems appears in Chapter 3.
Table 1-2 summarizes the challenges and problems that narrow pulses can bring to UWB communications systems.