An Opportunistic Spectrum Access Scheme for Multicarrier Cognitive Sensor Networks

In this paper, we consider a broadband cognitive radio wireless sensor network operating in a crowded spectrum scenario, where a sensor transmitter (i.e., a sensor node that needs to transmit information to another sensor node or to a sink) opportunistically uses the channel of a primary system adopting an underlay paradigm. Both the primary and sensor systems employ an orthogonal frequency-division multiplexing (OFDM) modulation scheme. The proposed approach is reminiscent of additive superposition coding: specifically, the sensor transmitter behaves as an amplify-and-forward relay for the primary system, by sending a linearly weighted combination of the received primary signal and its own block of symbols. Such a superposition is performed by the sensor transmitter on the fly, in a single OFDM symbol of the primary system. Moreover, the proposed scheme does not require spectrum sensing at the sensor nodes and does not demand any cooperation between primary and sensor systems, thereby accounting for the inherent resource constraints of sensor nodes. With reference to both primary and sensor communications, we address the problem of power allocation by considering different amounts of channel state information and develop a worst case analysis of the achievable information rates. The developed algorithms and the derived formulas, which are corroborated by numerical Monte Carlo simulations, allow one to recognize and discuss interesting tradeoffs between the main parameters of the cognitive radio sensor transmission.