Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 38

This spectrum sensing approach relies on pre‐knowledge of the signal to be sensed. Some commercial wireless signals use known synchronization patterns to align the receiving node processing to the received signal. These patterns can be exploited by the spectrum sensor to hypothesize the presence of the sensed signal. Signal synchronization patterns can include preambles, mid‐ambles, regularly transmitted pilot patterns, spreading sequences,¹⁵ etc.¹⁶ These patterns allow the spectrum sensor to correlate the received signal with a known copy of itself (it is essentially a form of coherent detection). This correlation process leads to a spectrum sensing result that outperforms energy detector based sensing. The reliability of the correlation process increases when the known signal length increases. Waveform‐based detection is used with known signals such as IEEE 802.11 signals.

While the autocorrelation based signal detection explained in Section 2.4.2 can be influenced by noise and the time lag between the samples, waveform based spectrum sensing is only affected by the presence of noise as the signal patterns align with the correlation process. Chapter 3 shows how the decision‐making process of waveform based spectrum sensing may differ from that of simple energy detection spectrum sensing.