Dynamic Spectrum Access Decisions. George F. Elmasry

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Figure 4.4 The interface between a cognitive routing engine and a cognitive DSA engine within a node.

      Notice here the presence of another distributed cognitive engine for routing parallel to the DSA distributed decision fusion engine.⁵ Notice also that each engine may impact the other engine directly through information exchange and indirectly through the implemented decisions. For example, routing decision made by the cognitive routing engine will impact the spectrum map estimation created by the DSA decision fusion engine and vice versa. Notice also that the implementation of such an engine can use heuristic approaches that make it hard to predict the impact of creating reactive decisions. These cognitive engines may also have to coordinate before executing a decision such that each engine can predict the impact of other engines' decisions. This coordination will create an integrated system that avoids bad decisions.⁶

      Figure 4.4 illustrates examples of information exchange and indirect impacts between these two cognitive engines. The routing engine may send the DSA engine a set of frequency slots it wants to consider for transmission with each frequency associated with the direction of transmission (based on knowing the geolocation of the receiving node) and the considered transmission power. The routing engine may expect the DSA engine to send it a reply to this query with the estimated interference level per each frequency slot. Based on this estimated interference level, the routing engine may decide which slot to use for the targeted destination node. When the over‐the‐air links are established, the used frequencies will have an impact on the spectrum map estimated by the DSA engine.

      One important aspect of the routing engine that impacts the spectrum map estimated by the DSA engine is the antennas' directional model used by the routing engine. This antennas' directional model directly affects the decision if a frequency slot can be used or not. Let us consider the use of an antenna directional model that shows the involvement of the routing engine with a spectrum map and how the routing engine influences the DSA engine. The transmitting node DSA engine would need to estimate the interference power at the receiving node per each candidate frequency slot so that the routing engine would have enough information to decide which frequency slot to use. Notice that the DSA engine estimation of interference power has to be comprehensive considering all the nodes in the geographical proximity that are using the same frequency, the directions this frequency is being used, and the transmission power level of each use in order to ensure that the estimated interference level is accurate enough for the routing engine. Assuming a free space path loss, the following formula can be used by the DSA engine to estimate the interference level had a candidate slot been used by the routing engine.

      (4.1)

      where:

      Notice that the DSA engine has to have knowledge of the geographical location of nodes in close vicinity and which frequencies are used at which direction for each node. This allows the DSA engine to estimate the interference power at the receiving node had the local node used the candidate frequency slot. Based on this interference level, the local node routing engine will determine if the interference level exceeds a given threshold or not before selecting the frequency slot to transmit at. Notice how such design can lead to relying more on distributed cooperative DSA decisions to coordinate between nodes.

The DSA engine has to estimate the pointing direction between two nodes in an accurate way. Figure 4.5 illustrates the different factors the DSA engine has to consider. The DSA engine may use an absolute north as a reference to how the different angles in this estimation are related. Figure 4.5 shows how “north” is used as a reference and how the angles are related.