Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 81

One critical trigger that can reach the DSA cognitive engine in the gateway node is the receipt of a frequency plan from the central arbitrator. As mentioned earlier, the gateway DSA engine can defer frequency change in its network to the central arbitrator or the central arbitrator's own fusion can trigger a frequency change (frequency plan) that affects one or more networks. This frequency plan can contain frequency change for a network that the gateway is a node of, a list of frequencies to sense within the network identifying which sensor would sense which frequency, and a list of backup frequencies for the network to consider when network formation using the operational frequency fails. This is one of many possible paths that can go through the gateway DSA cognitive engine.

As Figure 5.4 shows, once the frequency plan is received by the gateway cognitive engine, it will result in an update to the gateway information repository and this update should trigger the decision maker to perform the following steps:

1 Disseminate frequency change to all nodes in the network with a frequency change time that is dependent on the waveform characteristics. For example, if the waveform uses TDMA with EPOCH time, the frequency change time defined by the gateway DSA engine can be at the start of the next EPOCH or it can be after N EPOCHs from the current time. It is critical to ensure that the frequency change message will reach all nodes before the actual change of frequency can occur and that all nodes synchronously switch to the new frequency.5

2 Schedule for a frequency change locally to the gateway node so that the gateway and all the nodes in the network can continue the same network formation with the new frequency.

3 If the frequency change message is for a network that uses cooperative distributed DSA techniques, the frequency change message will contain a new pool of frequency bands for the network. The gateway DSA engine will have more work to do as it will have to disseminate the new pool and continue to perform cooperative distributed DSA within the network so that all the nodes in the network collectively try to optimize the use of the new pool of frequencies.

Figure 5.4 Gateway DSA cognitive engine thread for a frequency change.

Note that Figure 5.4 is one possible thread the gateway DSA cognitive engine may execute. This is an example of a thread that propagates down the hierarchy. The next section gives an example of a thread that propagates up the hierarchy.