A new opportunistic spectrum access overlay/underlay paradigm for enabling heterogeneous wireless networks of primary and secondary, or multiple "secondary" users to coexist on the same frequency in the same geographic location is proposed and evaluated. In this new paradigm of cognitive coexistence, underlay nodes operate without overt coordination with overlay nodes or other underlay bases. Instead of overt coordination, the underlay nodes monitor their local environment and their effect upon it to determine acceptable channels for coexistence. Moreover, with the employment of multiuser detection in some or all of the underlay nodes (and in none of the overlay nodes), high rate underlay links are possible in the presence of even the most severe overlay interference. The result is a backward compatible coexistence of overlay and underlay nodes with high throughput for the underlay network while maintaining acceptable data rates for the overlay network. This paper describes what we define as the cognitive coexistence strategy (CCS) that enables nodes to determine among all occupied bands which band is most likely to result in a successful coexistence between its link and the already existing link on that band. The CCS takes advantage of the adaptive nature of the overlay nodes and the unpredictable, but often present asymmetric channel due to the different path losses between a transmitter and its intended and unintended receivers. This paper presents a preliminary simulation-based investigation of the opportunity that exists for a pair of cognitive coexistence radios (CCR) operating as a small cell base and mobile in an urban wireless cellular environment, showing sufficient opportunity for high throughput coexistence that can be advantaged by the cognitive capability of the CCR nodes.

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A data collection in the 2.4GHz ISM band was performed in order to characterize the coupling between a colocated transmitter and receiver in a time-varying, frequency-selective, wideband outdoor channel. Once least-squares channel estimates are obtained, channel length, RMS delay spread, coherence time, and coherence bandwidth are calculated. The values help assess the effects of line-of-sight (LOS) versus non-line-of-sight (NLOS) and traffic conditions, and are being used to inform the design of an advanced adaptive communications system.

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