Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 37

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 (CROC) decision‐making process, which takes into consideration the difference in spectral efficiency.