1.7 Differences Between UWB and Spread Spectrum
Although UWB is a growing technology and more information on its concepts and capabilities becomes available every day, many misconceptions are associated with its name. A good number of people confuse UWB communications with wideband spread-spectrum techniques. Even though both UWB and spread-spectrum techniques have their origins in military secure communications, we need to clarify a fundamental difference between the two. For this reason, it is important to briefly review two commonly used spread-spectrum techniques—DSSS and FHSS.
1.7.1 Direct-Sequence Spread Spectrum
In direct-sequence spread spectrum (DSSS), a pseudorandom code is used to spread each data bit with a large number of chips, where a chip interval is much smaller than a bit interval, as shown in Figure 1-10. These code words spread the data to a larger bandwidth than required to transmit information. In Figure 1-10, the data bit 1 is represented by a four-bit code (1010) and the data bit 0 is represented by another four-bit code (1100).
Figure 1-10 A data sequence and a spreading code using DSSS
Spreading the data to shorter duration chips in time results in a spread of energy in the frequency domain to slightly above a typical narrowband receiver's noise floor. In order to transmit data, each of the chips is modulated with conventional narrowband techniques.
1.7.2 Frequency-Hopping Spread Spectrum
The frequency-hopping spread-spectrum (FHSS) technique was invented by actress Hedy Lamarr and patented in 1942 as "Secret Communication System" . FHSS in concept is exactly like DSSS in terms of spreading the signal energy in the frequency domain and offering the advantages of wideband communications. However, the wide bandwidth does not result from spreading the data, as in the DSSS technique. Instead, FHSS hops the frequencies used for transmission and reception according to a pseudorandom code, and the combination of those frequencies generates a wide bandwidth. The change in frequencies that represent the data bits happens so fast that detection becomes very difficult for unauthorized parties. As shown in Figure 1-11, the signal hops from one frequency to another at each instance in time.
Figure 1-11 Frequency hopping in the FHSS technique
1.7.3 The Essential Differences
Both the DSSS and the FHSS techniques offer a spread in the frequency domain and provide advantages over narrowband communications such as lower power spectral density, covertness, frequency diversity for better performance in multipath channels, and resistance to intentional and unintentional jamming. So the natural question is "What is the difference between UWB and spread spectrum?"
Although UWB and spread-spectrum techniques share the same advantage of expanded bandwidth, the method of achieving the large bandwidth is the main distinction between the two technologies. In conventional spread-spectrum techniques, the signals are continuous-wave sinusoids that are modulated with a fixed carrier frequency. In UWB communications, on the other hand, there is no carrier frequency; the short duration of UWB pulses directly generates an extremely wide bandwidth (as we saw in Equation 1-2). Another distinguishing factor in UWB is the very large bandwidth. Spread-spectrum techniques can offer megahertz of bandwidth, while UWB pulses provide several gigahertz of bandwidth. Figure 1-12 shows the time and frequency domain representation of narrowband, wideband, and UWB signals.
Figure 1-12 The transition from narrowband to wideband and ultra-wideband in the time and frequency domains
As shown in Figure 1-12, in narrowband technologies, CW signals occupy a well-defined and narrow range in the frequency spectrum; however, in wideband technologies, the frequency range of the CW signals is spread to slightly above the noise floor due to the use of spreading sequences. In UWB, the short duration of a pulse automatically creates the very large bandwidth of several gigahertz without using the spreading codes. Also, notice that narrowband signals are always present, so their duty cycle is 100 percent, while UWB pulses are present only for a very short time, with a duty cycle of less than 0.5 percent. As explained in Section 1.3, the low duty cycle offers very low transmission power and extra covertness compared to spread-spectrum techniques. However, the low transmission power could be a disadvantage for UWB systems, because the information can travel only short distances. Therefore, for long-range applications, spread-spectrum techniques are still more appropriate.