Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 36

An example of frequency hopping spread spectrum is illustrated in Figure 2.12, where three hops can occur during the modulation duration time, T_s, of a single symbol. In reality, defense signals tend to create fast hopping such that T_s is as small as possible.

Figure 2.12 Fast hopping where three hops occur during the modulation of one symbol.

With frequency hopping spread spectrum, both the hopping pattern and the chip code vectors are not known to an external spectrum sensor. External commercial spectrum sensors that sense a military signal should rely on spectrum sensing techniques that do not utilize frequency hopping and spread spectrum detection (i.e., not attempt to find the spreading code and the hopping pattern of the military signal). A benign method could be simple energy detection. In defense applications, a spectrum sensor can detect the presence of a malicious signal that attempts to jam the used defense signal through different means, including jamming a subset of the frequency slots (f₁ − f₈ in Figure 2.12) continually.¹⁴ This type of spectrum sensing is performed by the military system to overcome malicious jammers when the defense application signal is not a secondary user.