Opportunistic radio systems aim to exploit 'spectrum holes' by finding bands and transmission characteristics that will not cause harmful interference to the primary users of that band. This paper explores whether it is harder/easier to peacefully coexist with primary systems that operate at different scales in terms of their coverage area and transmission power. When sensing a large scale primary, a small scale secondary user can make its own decision about transmission based on the sensing results from its neighborhood. This assumption fails when the scale of the primary is comparable to the scale of the secondary user. In this scenario, we need to decouple sensing from admission control - a sensor network is required to perform the sensing. For small primaries, the environment over which the sensing results are valid is small which imposes certain minimum density requirements for sensor nodes. Collective sensing is used to localize the primary while a separate admission control algorithm decides on which secondaries can safely transmit. Location information of the primary and secondary users is key for such an admission control algorithm to operate successfully. In the case of a large primary, location uncertainty did not impact results significantly since decisions are made about a primary that is very far as compared to the inter-sensor distances. This is no longer valid for a small primary and hence more exact location information is paramount. With location uncertainty of primary and secondary users, the effective primary user footprint can increase significantly. We focus our discussion around a toy model of the Part 74 'wireless microphone users' that are a concern to the IEEE 802.22 Working Group.
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