Dynamic Spectrum Access Decisions. George F. Elmasry
Notice the difference between time‐domain cyclic marks and frequency domain cyclic marks. OFDM signals use a frequency‐domain cyclic prefix, which protects the OFDM signals12 from inter‐symbol interference. This cyclic prefix can be utilized in a frequency‐domain based correlation technique to affirm the presence of the targeted signal. If the targeted signal uses a preamble of symbols, this preamble can be utilized to affirm the presence of the signal in time‐domain correlation. Both cyclic prefix and time‐domain preamble can help the autocorrelation capable sensor decrease the probability of misdetection and the probability of false alarm when making a decision regarding the presence of the targeted signal.
2.4.3 Spreading Code Spectrum Sensing
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 Cn is module‐2 added to the signal binary stream Xn before modulation. The resulting binary stream mn 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 Rc, feeds the frequency synthesizer. The frequency synthesizer generates the hopping carrier frequency for the modulator expressed as cos(ωit + θ). Notice that the binary stream Xnis modulated over the carrier with frequency ωi, which corresponds to the ith slot of the N slots available for hopping. The hopping rate is determined by Rc.
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.
2.4.4 Frequency Hopping Spectrum Sensing
An example of frequency hopping spread spectrum is illustrated in Figure 2.12, where three hops can occur during the modulation duration time, Ts, of a single symbol. In reality, defense signals tend to create fast hopping such that Ts 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.
2.4.5 Orthogonality Based Spectrum Sensing
This type of spectrum sensing is common among cooperative spectrum users of the same signal. With orthogonal spectrum sensing, the decision‐making entity can be distributed, centralized or hybrid, as introduced in Chapter 1. The centralized decision‐making entity is sometimes referred to as the decision fusion center (DFC). The decision‐making process attempts to exploit signal orthogonality for cooperative spectrum use while mitigating the effect of fading, shadowing, out‐of‐range, and other factors that can increase the probability of false alarm and the probability of misdetection.
Orthogonal cooperative spectrum sensing communications systems have to take into consideration the use of multiple‐input multiple‐output (MIMO) antennas where multipath fading is a critical factor. Some implementations of this spectrum sensing technique use a hybrid approach between a local, distributed, and centralized decision‐making processes. In a typical system, referred to as the standard centralized fusion model, each node transmits its local decision outcome to a centralized DFC or a peer node. Spectrum sensing traffic, which include this reporting of node decision, can use a standalone channel known as parallel access channel (PAC) or can use one of the orthogonal signal multiple access channels (MACs). Reporting to peer nodes can use a separate channel from reporting to a centralized DFC, which is often referred to as the cooperative channel. Figure 2.13 shows three types of channels: (i) the sensed communications channel indicated by the thick lines; (ii) the reporting channel to the centralized DFC indicated by the thin lines; and (iii) the cooperative channel for peer‐to‐peer reporting of spectrum sensing decisions indicated by the dashed lines.
Figure 2.13 Cooperative spectrum sensing with MIMO DFC.
Notice in Figure 2.13 that all the nodes can have MIMO antennas (not just the DFC centralized entity). This model can work with a single‐input single‐output antenna for each node or a MIMO for each node. In either case, the centralized location must have a massive MIMO antenna in order to account for multipath fading.
With this cooperative mode, the ROC decision‐making process explained in the next chapter is altered to a complementary receiving operating characteristics