Читать книгу Dynamic Spectrum Access Decisions - George F. Elmasry - Страница 41

When spectrum sensing information is propagated with location information defined by the sensor's latitude, longitude, and elevation, the fused information can create more spectrum use opportunities. The fusion process can show how at any given time, spectrum opportunity can be available in some parts of the area of operation while being fully occupied in other parts. The geographical space dimension helps the fusion process estimate propagation loss (path loss) in space to further ensure that spectrum reuse will not interfere with the sensed signals. Figure 2.14 shows an example of geographical separation creating opportunistic spectrum use in the case of a secondary user opportunistically using a primary user's spectrum. Figure 2.15 illustrates the case of a set of heterogeneous MANETs where DSA allows them to cooperatively and dynamically share a set of frequency bands (f₁, f₂, and f₃) and an area of operation indicated by the dashed rectangle while avoiding interference. Notice that the illustration in Figure 2.15 differs from dynamic use of frequency slots within a single network. With Figure 2.15, a centralized entity may be utilized to fuse spectrum sensing information collected from sensors dispersed geographically through the area of operation to dynamically use a set of predefined frequency bands.

Figure 2.14 Geographical separation creating opportunistic spectrum use for a secondary user.

Figure 2.15 Geographical separation creating the DSA of a limited set of frequency bands for a set of heterogeneous MANETs.

Figure 2.16 illustrates the advantage of having a centralized entity to arbitrate DSA. If the spectrum reuse decision is made at the local node, there remains the potential of interfering with a hidden primary user. This issue can occur if there is severe multipath fading or shadowing of the RF signal as it is emitted from the primary user while the sensor targeting the primary user's transmissions fails to sense the primary user signal. With Figure 2.16, the circles on the right‐hand side represent the AOC of the primary user. One can see how the secondary user AOC depicted by the circle on the left‐hand side causes unintended interference with the primary user's receiver since the transmitting signal from the primary user could not be detected. One can see how distributed cooperative sensing or the use of a centralized arbitrator can better address the hidden primary user problems. In order to reduce the chance of encountering a hidden node problem, one can rely on:

1 geographically dispersed spectrum sensors

2 a centralized DSA decision process, which can have a bird's eye view of the area of operation

3 the centralized DSA decision‐making process having an algorithm that can estimate the AOC of the primary user based on multiple sensor input.

Figure 2.16 The hidden node problem.