Dynamic Spectrum Access Decisions. George F. Elmasry

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Figure 2.6 Frequency domain energy detection.

      As with time domain energy detection, frequency domain energy detection has to consider the presence of noise. The method used to estimate the noise power spectral density can rely on discrete Fourier transformation (DFT) where the digitized data is divided into segments and a sliding window is used to estimate the average noise spectral density. One reason to choose frequency domain energy detection over time domain energy detection in augmented sensors is the higher accuracy of noise estimation but the price for that is the need for more computational power.

      Notice how with the three energy detection techniques covered so far, the outcome is simple:

      It is important to note that the hypothesizing and decision‐making processes covered in Chapter 3 can be tricky under certain circumstances, such as fading channels. While frequency domain energy detection can implement good techniques such as the sliding window explained above, distributed and centralized DSA techniques can have a view of spectrum sensing that is more comprehensive than a local node. Distributed and centralized DSA techniques can analyze spectrum sensing information per RF neighbor and further overcome the uncertainty that can result from fading channels.

While Section 2.3.1 introduced same‐channel in‐band sensing, Sections 2.3.2 and 3.3.3 introduced the most common forms of augmented sensing where the augmented sensor can be configured to sense a frequency band defined by the carrier frequency f₀ and bandwidth W as illustrated in Figures 2.4 and 2.5. Augmented sensors can be built to sense a wide band of frequencies and sense multiple sub‐bands each defined by a carrier frequency f₀ and a bandwidth W simultaneously. This simultaneous sensing utilizes parallel paths where each path starts with a bandpass filter configured for the carrier frequency f₀ and bandwidth W for one of sub‐bands being sensed.

2.4 Signal Characteristics Spectrum Sensing

      There are different signal characteristics that a spectrum sensor can detect. Here, we go beyond simple energy detection with no prior knowledge of the signal being sensed to having some prior knowledge of the signal and the ability to synthesize the detected signal to extract more information.

      2.4.1 Matched Filter Based Spectrum Sensing

      This technique requires pre‐knowledge of many aspects of the sensed signal such as bandwidth, operating frequency, modulation type and order, pulse shaping, and frame format. The spectrum sensor can quickly detect the presence of the sensed signal with high accuracy. This technique can be used before discovering more detailed signal characteristics such as spreading code and hopping pattern.

      The matched filter will accentuate the targeted signal S(t) and will suppress other signals and noise. Notice that signals other than the targeted signal S(t) are essentially noise with respect to S(t). The impact of the suppressed signals and noise are referred to as W(t). The design of this matched filter includes:

      Notice that with wireless communications systems where we decode symbols, minimizing the probability of symbol error also uses signal and noise energy. However, the probability of error in spectrum sensing has two folds. With spectrum sensing, we have a probability of false alarm where the matched filter decides that S(t) is detected but S(t) was absent and the probability of misdetection where the matched filter decides that S(t) is absent but S(t) was present.⁹

Figure 2.7 shows the use of a matched filter to detect the presence of a signal S(t). Notice that a spectrum sensor can sense more than one signal type using a bank of matched filters. Once the spectrum sensor decides the signal type, other signal synthesizing techniques can be used to discover characteristics such as spreading code or orthogonality.

Figure 2.7 Signal detection using matched filters.

Some of the disadvantages of the matched filter sensing technique include the following:

       The implementation complexity may not be practical to implement for a large set of signals. Consider the detection of all types of commercial cellular signals and other known commercial but not cellular signals.

       Large power consumption is needed to execute the various receiver algorithms.

      2.4.2 Autocorrelation Based Spectrum Sensing

This spectrum sensing approach has more advantages than the matched filter approach described in the previous section when attempting to detect the presence of a narrow band signal. Autocorrelation estimation can result in a constant false alarm rate (CFAR) by employing techniques that reduce the dependency on noise power. With this sensing technique, the sensor can have a stored representation of the signal as samples S(t) = (s₀, s₁, …, s_{N − 1}) or has a signal generator that can generate a copy of the sensed signal and sample it. The sensed signal S^*(t) is sampled to produce. The result of the correlation process¹⁰ between S(t) and S^*(t) is compared to threshold to decide if the sensed signal matches S(t) or not, as shown in Figure 2.8.

Figure 2.8 Signal СКАЧАТЬ