Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 35

Spread spectrum is implemented differently in commercial signals than in military signals. Commercial signals tend to use direct sequence spread spectrum, which is easy to sense, while military signals tend to use frequency hopping spread spectrum, which is intentionally made hard to detect.

Figure 2.10 shows the commercial case where a chip code C_n is module‐2 added to the signal binary stream X_n before modulation. The resulting binary stream m_n is a pseudo random sequence. The modulated signal s(t) is a spread spectrum signal.

Figure 2.10 Direct sequence spread spectrum modulation.

Figure 2.11 shows a frequency hopping spread spectrum, which divides the available spectrum into slots. The carrier frequency then hops among these slots at a rate that is known as the frequency hopping rate. The hopping range of W Hertz is divided it into N slots with each slot size being Δf. The carrier frequency is always decided by the frequency synthesizer at each moment. The synthesizer is driven by a pseudorandom generator which generates unique Ninput vectors to the frequency synthesizer.

Figure 2.11 Frequency‐hopping spread spectrum signal modulation.

With Figure 2.11, a pseudorandom generator, clocked at the hopping rate R_c, feeds the frequency synthesizer. The frequency synthesizer generates the hopping carrier frequency for the modulator expressed as cos(ω_it + θ). Notice that the binary stream X_nis modulated over the carrier with frequency ω_i, which corresponds to the ith slot of the N slots available for hopping. The hopping rate is determined by R_c.

When autocorrelation detection is used and the presence of the targeted signal is hypothesized to exist, the spectrum sensor can use a chip code generator to generate all the chip codes the targeted signal is known to use. One of the chip codes that is used by the targeted signal will result in the highest correlation with the sensed signal. The spectrum sensor may be able to help the local DSA decision fusion agent pinpoint which chip code is not used in a certain vicinity at a certain time, allowing for the opportunistic use¹³ of the sensed frequency band only with an unused chip code.