Dynamic Spectrum Access Decisions - George F. Elmasry

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4.3.

      A designer of a hybrid DSA system has to consider the trade space taking into consideration all the factors that can affect the final solution to include bandwidth limitations, hardware limitation such as SWaP and processing power, speed and accuracy of decision making, and the system's requirements. One important factor that can affect hybrid design is the role of other cognitive processes in the cognitive wireless network nodes. These cognitive processes can influence the design of DSA decision fusion as detailed in the next section. Keep in mind that DSA capabilities are one of many aspects of wireless cognitive networking capabilities.

      Let us illustrate this critical factor with the case of a distributed cooperative directional MANET that is designed to route around jammed areas. The design of such a system has to feed the results of the DSA decision fusion to a cognitive routing process/engine and the cognitive routing engine has to use DSA information to create reactive directional routes relying on directional antennas. The cognitive routing engine has different objectives, including:

      1 creating directional routes that allow communications around compromised areas

      2 increasing spectrum reuse

      3 reducing interference between the directional MANET nodes.

       The cognitive routing engine may need to control transmitting power based on the receiving node location. This is needed to reduce the impact of the spectrum utilized by the directional MANET on the overall spectrum map. Power level is selected by the routing engine to be the minimum power needed to reach the next‐hop node given the current interference level.

       Both routing and DSA engines need to know the antenna gain pattern associated with the utilized antenna.

       Nodes have to continually register with each other and disseminate their locations to each other using protocols such as terrestrial geolocation protocol (TGP). This is needed so that beams are formed optimally. TGP information would need to be shared between both the routing engine and the DSA engine.

       In the case of the routing engine considering a new route, this route can have multiple over‐the‐air hops. Each hop (the next‐hop) is identified by source and destination nodes geolocations, a frequency band, direction of the formed link, and power level. This will require the cognitive routing engine to feed the DSA engine with its own potential links (spectrum use) such that the DSA engine draws a potential spectrum map that includes what spectrum would have been emitted had the new route considered by the routing engine been implemented. The estimated interference level affecting the new route can be calculated by the DSA engine and the interference levels can be sent to the routing engine.

 Schematic illustration of 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)equation

      where:

       I is the receiving node and PI is the estimated interference

       Pj is transmit power from node j (all nodes j in geographical proximity are using this frequency slot at the same time)

       GR is receiver antenna gain

       δR is the absolute value of the difference between the pointing angle of the receive antenna and the bearing to the transmit node

       GT is transmitter antenna gain

       δT is the absolute value of the difference between the pointing angle of the transmit antenna and the bearing to the receive node

       R is distance between receiver and transmitter

       α is (C/4πf)2

       link closure is a function of (δR, δT).

      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.

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