There are two types of spread spectrum technologies: direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). This section shows the basic principles of these methods as well as a comparison between them.
5.1.1 Direct Sequence Spread Spectrum
DSSS is the technology that employs a pseudo random noise code (PN sequence), which is independent of the signal to be transmitted, and it uses it to modulate or spread the narrow band signal into a signal with much wider bandwidth. Figure 1 and Figure 2 illustrate this process in the time and frequency domains. At the receiver side, the same PN sequence is used to de-spread the wideband signal back to the original narrowband signal (Meel, 1999).
Figure 1. The spread spectrum process in the time domain (Meel, 1999)
Figure 2. The spread spectrum process in the frequency domain (Chipcenter, 2006)
Power density is the distribution of power over frequency. Although the spread signal has the same amount of energy and carries the same information as the narrow band signal, it has much lower power density due to its much wider bandwidth. This makes it more difficult to detect the existence of the spread spectrum signal. Processing gain is the power density ratio between narrowband signal and spread spectrum signal, which in this case is usually higher than 10 (Chipcenter, 2006).
On the other hand, if the spread signal is contaminated by narrow band jam during transmission the energy of the jam signal will spread over a much wider bandwidth when de-spread, resulting a relatively high signal to noise ratio (SNR) at the receiver which leads to easy detection of the de-spread signal as illustrated in Figure 3. This is the reason for the high robustness of the spread signal against a narrow band jammer.
Figure 3. De-spread signal and jammer (Chipcenter, 2006)