using hybrid design that include cooperative distributed decision fusion. The 5Gs take on cooperative spectrum sensing has some different and unique angles such as:
energy conservation of end‐user devices
leveraging the fact that 5G networks can be dense
avoiding information redundancy
optimizing resource utilization with heterogeneous networks overlay
the consideration that end‐use devices can connect to each other with device‐to‐device (DTD) communications protocols.
One of the core concepts in 5G is the use of a spectrum agent (SA) in order to offload some of the burdens of spectrum sensing and fusion away from the 5G end‐user devices. Notice that 5G can use both licensed and unlicensed spectrum, making it important to detect what frequency bands other communication systems operating in the same geographical location are using.
The dense deployment of 5G small cells is leveraged to turn each cell into a SA and turn inactive15 small cells into SAs. Figure 6.11 illustrates the compound overlay of active and inactive entities in a 5G deployment in an urban area where we can have a macrocell with the largest area of coverage shown in the figure with the large tower at its center, multiple picocells as illustrated by the small towers, and many femtocells as illustrated with the femtocell icons. This illustration uses inactive femtocells as SAs where an end‐user device can obtain spectrum awareness information from an SA even though the device has not made the decision to select this SA as an access point or to use other access points.
Figure 6.11 The compound overlay of 5G entities in a 5G deployment.
Notice in Figure 6.11 that a macrocell user can be utilizing a licensed spectrum band while an end user obtaining spectrum awareness information from a SA can end up using unlicensed spectrum band using the femtocell as the access point. With this approach, the end user does not have to perform the spectrum sensing functionality or the computationally extensive spectrum sensing information fusion. Instead, the end‐user device obtains processed information with decision fusion results from the SA. One can see from this compound overlay that if and when an SA acts as a femtocell, it may have to resort to unlicensed spectrum because the macrocell is using the licensed spectrum bands. As illustrated be the dotted lines in Figure 6.11, an end user would send the SA a spectrum service request and the SA can then do any of the following:
1 Periodically detect spectrum utilization in the area and decide if an available licensed band can be used or if an unlicensed band must be used. This decision is based on the SA periodically detecting and analyzing spectrum utilization in the area. Here, the SA is acting as a spectrum fusion center, deciding spectrum availability and broadcasting it to the end users in the area.
2 Not perform fusion and rely on the macrocell as the fusion center where all the SAs, femtocells and picocells send the macrocell their own spectrum sensing information and the macrocell performs the fusion and send the fusion results to the SAs, femtocells, and picocells.16
In any case, the SA would respond to the spectrum access service request from the end user with a frequency band it can use regardless of whether that band is licensed or unlicensed, and regardless of whether the decision is made by the SA or the macrocell.
Note that 5G standards give the service provider flexibility in how to implement spectrum sensing, how to fuse sensing data, and how to create decision. A different hybrid form of local, cooperative distributed, and centralized17 spectrum sensing technique can be used by the different 5G services providers and the utilized technique can always be enhanced with time. The 5G service provider would have prerequisites such as removing the burden of spectrum sensing from the end user, and the goal of achieving spectrum utilization efficiency. The use of a hybrid spectrum sensing approach where the macrocell has a bird's eye view of spectrum use in this example and the ability to turn idle femtocells into SAs can always improve spectrum efficiency. The macrocell would use computationally complex spectrum arbitration techniques that can better optimize spectrum resources use dynamically from a global perspective and these techniques can be enhanced with time with new software releases.18
6.4.1 The Macrocell as the Main Fusion Center
Figure 6.12 shows a 5G spectrum fusion case where the macrocell is the only fusion center for spectrum sensing information. With this case, the following events can occur (notice that these events are continually occurring):
1 The SA continually senses spectrum and collects spectrum sensing information (without any fusion). It consolidates spectrum sensing information for all frequency bands.
2 The SA sends the consolidated spectrum sensing information to the macrocell.
3 Picocells and femtocells also send their consolidated spectrum sensing information to the macrocell.
4 The macrocell fusion center processes spectrum sensing information received from all SAs, all picocells, and all femtocells.
5 The macrocell fusion center creates fusion results and sends them to each SA, femtocell, and picocell.
6 An end user makes a spectrum service request from the SA.
7 The SA uses the fusion results from the macrocell and replies to the end user with a recommendation of which spectrum band(s) to use.
Figure 6.12 5G use of the macrocell as the fusion center.
This case can simply be described as a hybrid of distributed sensing and centralized fusion. Notice that this case can be morphed to the case where the SA, femtocells, and picocells perform a form of spectrum sensing information fusion and send the macrocell fusion center the results of its own local fusion. In either case, the macrocell fusion results are broadcasted to all lower hierarchy entities and the SA can't rely on its own local fusion results to respond to frequency band service requests from end users. Notice that even with the macrocell being the only place for fusion, the SAs, femtocells, and picocells perform a form of cooperative spectrum sensing.
If some SAs have limited energy and must conserve energy, the macrocell can select a subset of the SAs to perform spectrum sensing and collect sensing information. The macrocell algorithm has to consider the hidden node challenge discussed earlier in this book and select SAs that are geographically distributed to optimize sensing impact.
6.4.2 Spectrum Agents Operate Autonomously
With this case, SAs can share spectrum sensing information in a distributed manner along with their geolocation
