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.

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.

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 carrierbased 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.
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.