Dynamic Spectrum Access Decisions. George F. Elmasry

networks can be deployed together and these networks will morph to the unique scenario needs while the waveform cognitive engines manage the configuration of links internal to the waveforms network and the master engines manager global routes.

       Making global dynamic route tables independent of route tables local to a network makes it possible for heterogeneous networks to be connected and optimized seamlessly.

      There are also disadvantages of this heterogeneous distributed cooperative DSA approach:

       The amount of DSA control traffic can grow exponentially as the number of nodes in a network increases and as the number of heterogeneous networks increases.

       Some nodes can go out‐of‐synch or temporarily lose connectivity and some information exchange control traffic can be lost. Retransmission of control traffic can further cause an already large volume of DSA control traffic to grow larger and can delay the adaptive route creation decisions.

       There are limits to global views of a large‐scale set of heterogeneous networks through distributed protocols. The master DSA agent in each gateway node will create a spectrum map that is less accurate than the spectrum map that can be created by a centralized arbitrator.

      To make different types of IP networks work seamlessly, Internet routing protocols faced similar challenges to what is described here for DSA challenges. This led to the creation of routing areas and gateway protocols such as the border gateway protocol (BGP). In some IP network design, certain routing configurations had to be left for a centralized network manager to decide. Similarly, the reliance on a distributed cooperative DSA for large‐scale heterogeneous networks will always show some limitations and lead us to consider the need for a centralized spectrum arbitrator to curb control traffic volume and arbitrate fairly between heterogeneous networks in certain cases while allowing a human (network manager) to monitor the cognitive network's health and set policies and rule sets. The network manager can change policies when needed.

One simple way to curb control traffic volume is to reduce its rate. Figure 4.7 illustrates a conceptual view of how control traffic volume can depend on the updating intervals. As updating interval periods decrease (more frequent exchange of DSA messages occur), control traffic volume increases. On the other hand, as the updating interval periods increase (less frequent exchange of messages occur), control traffic volume decreases. There is a desired cutoff limit of the amount of DSA control traffic depending on the heterogeneous systems under design. The limitation of the amount of DSA control traffic can lead to curbing message exchange rate and the impact of this limitation can make resource allocation and dynamic routing tables less dynamic and less reactive to the deployment needs. This can be harmful, especially for fast‐moving nodes in very dynamic MANETs.

Figure 4.7 Conceptual view of how control traffic volume depends on updating rate.

Notice that within the heterogeneous MANET designs presented in this section, the distributed cooperative DSA¹³ has a mix of local DSA decisions made by the waveform DSA engine and distributed DSA decisions made between the master DSA agents. Although these systems are referred to as distributed cooperative DSA systems, in essence they are hybrid between local DSA and distributed DSA decision systems with two different hierarchies of distributed cooperative decision fusion. Similarly, adding a centralized arbitrator to this construct creates a different type of hybrid DSA design that has more hierarchical DSA entities.

4.5 Using a Centralized DSA Arbitrator

Figure 4.8 illustrates an example of using a centralized DSA arbitrator with a simplified scenario of three heterogeneous MANETs. The large dashed circles represent a MANET current area of coverage. The small gray circles represent nongateway nodes while the small black circles represent gateway nodes. Gateway nodes are platforms with more than one waveform and are able to be nodes of more than one MANET. The black arrows represent the centralized DSA control plane where the centralized DSA arbitrator sends and receives control traffic to and from the gateway nodes. With this approach, the gateway nodes proxy their respective networks to the centralized DSA arbitrator such that local fusion in the gateway nodes is more comprehensive than fusion in the other network nodes.¹⁴ The idea here is to have the gateway nodes send a network‐based spectrum utilization and interference picture to the centralized DSA arbitrator. Although the centralized arbitrator would decide spectrum allocation dynamically between the networks, this allocation is a hybrid allocation that takes into consideration the role of distributed cooperative DSA decisions within each network.

Figure 4.8 The use of a centralized DSA arbitrator in a hybrid DSA system of heterogeneous MANETs.

The centralized DSA arbitrator would remove some of the heterogeneous spectrum allocation tasks from a distributed cooperative manner to a centralized manner. This will result in adding a centralized DSA control plane, as illustrated in Figure 4.8, but can drastically reduce the control traffic volume. Here, a gateway node is tasked to obtain the spectrum utilization map of its own network through its own fusion technique, send it to the centralized arbitrator, and proxy the centralized arbitrator's decision only when it cannot reach it. Note that messages sent over the centralized DSA control plane can be multicast messages that reaches the centralized arbitrator and a subset of peer gateway nodes with a single over‐the‐air transmission. This will allow the gateway nodes to rely on the centralized arbitrator for global decision fusion in most cases. In rare cases, when the centralized arbitrator is not reachable for a decision, the gateway node will rely on the distributed cooperative protocol mentioned in the previous section 15 to act as a proxy for the centralized arbitrator.

      Notice that the way a gateway can proxy a network spectrum utilization can depend on the gateway capabilities. This proxy approach can range from merely consolidating the network sensors' data into one message sent to the centralized arbitrator without fusion at the gateway node to performing extensive fusion at the gateway node and sending the centralized arbitrator a fused spectrum map. Factors such as SWaP limitations at the gateway node can guide the design of the gateway node fusion capabilities.

      With this type of hybrid DSA system, we have a mix of centralized spectrum allocation and distributed cooperative spectrum allocation within each network. Each network is assigned a pool of frequency bands to use based on the network's own distributed cooperative protocols while the centralized arbitrator changes the allocation of this pool of frequencies (redistributes frequency pools between all the networks) when its own spectrum map shows the need for such change.

      While the distributed cooperative hybrid system can work with a certain number of heterogeneous MANETs, as the number of networks in a large‐scale deployment increases, the need for a centralized arbitrator will increase. Notice that if the design elects that a gateway node would proxy a network spectrum map using extensive fusion, this could reduce the amount of DSA control traffic drastically. The gateway node can abstract the network spectrum map, fusing spectrum sensing information into very compressed messages sent to the centralized arbitrator.

The global spectrum map within the centralized arbitrator has its own advantages. In addition to reducing DSA control traffic volume, the use of a centralized arbitrator can also further optimize spectrum resources allocation.