Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 82

This is an upward thread that can occur in the gateway DSA cognitive engine. With this thread, the gateway can be receiving spectrum sensing information from the following sources:

1 Local, if the gateway has augmented spectrum sensors or the gateway can be sensing the frequencies in use as explained in the previous chapters.

2 From peer gateway nodes. As explained earlier in this chapter, gateway nodes can be sharing their fused spectrum sensing information with peer gateway nodes.

3 From the network nodes. The gateway node can be a member node of one or more networks. These networks can have augmented sensors configured to sense (probe) certain frequency bands and can be forwarding spectrum sensing information of the used frequencies to the gateway.

Figure 5.5 shows the progress of this thread to the fusion part. Collected spectrum sensing information from the different sources are fused to produce actions such as the following:

1 Spectrum awareness map. This map shows what frequency bands in the area of operation of this network suffer from interference, what probed frequency bands can be used, and what probed frequency bands should be avoided because they are occupied. As explained earlier, this spectrum awareness map at the gateway is more accurate than the spectrum awareness map at the network nodes and the spectrum awareness map can be more accurate if peer gateways share spectrum sensing information with each other in a distributed cooperative manner.

2 Reprioritizing of backup frequencies. The probing of unused frequency within the network can lead to changes in the backup frequency order of use. Also for a network that uses cooperative distributed DSA internal to the net, spatial use of the assigned frequency pool can be altered with time.

Figure 5.5 Gateway DSA engine upward propagation of spectrum sensing information.

As Figure 5.5 shows, fusion will result in updates in the gateway DSA engine information repository and this can trigger upward updates to the following entities:

1 The central arbitrator. Now the gateway DSA engine sees a different spectrum awareness map than the last time the gateway DSA engine updated the central arbitrator DSA engine. The gateway updates the central arbitrator with the new spectrum map.

2 The updates to the central DSA engine, which are sent over the same control plane, will reach the peer gateways.

Notice the asynchronous aspects of using cognitive engines where each cognitive engine works based on what information it collects and decisions occur as a result of fusion. Although these engines are not timely synchronized with each other, they work collectively, and sometimes using heuristics⁶ algorithms, to optimize the use of spectrum resources. In the meantime, DSA is offered as a set of cloud services at any point of the heterogeneous networks hierarchies regardless of the status of the control plane. If some spectrum awareness propagation messages are lost, DSA services will always be available when a service is requested.

The thread in Figure 5.5 shows one possible gateway cognitive DSA engine flow. Fusion can always trigger an action at any entity and this action can trigger a message flow that leads to other cognitive DSA engine fusions and triggers. The DSA design of large‐scale systems has to consider the amount of control traffic that can be generated from all of this information propagation upward, downward, and to peers. The design has to consider the tradeoffs mentioned in this book, which include the thresholds that trigger information dissemination. These thresholds must be selected and updated dynamically based on bandwidth availability.