In cognitive radio networks (CRNs), spectrum sensing is critical for guaranteeing that the opportunistic spectrum access by secondary users (SUs) will not interrupt legitimate primary users (PUs). The application of full-duplex radio to spectrum sensing enables SU to carry out sensing and transmission simultaneously, improving both spectrum awareness and CRN throughput. However, the issue of spectrum sensing with full-duplex radios deployed in heterogeneous environments, where SUs may observe different spectrum activities, has not been addressed. In this paper, we give a first look into this problem and develop a light-weight cooperative sensing framework called PaCoSIF, which involves only a pairwise SU transmitter (SU-Tx) and its receiver (SU-Rx) in cooperation. A dedicated control channel is not required for pairwise cooperative sensing with instantaneous feedback (PaCoSIF) because sensing results are collected and fused via the reverse channel provided by full-duplex radios. We present a detailed protocol description to illustrate how PaCoSIF works. However, it is a challenge to optimize the sensing performance of PaCoSIF since the two sensors suffer from spectrum heterogeneity and different kinds of interference. Our goal is to minimize the false alarm rate of PaCoSIF given the bound on the missed detection rate by adaptively adjusting the detection threshold of each sensor. We derive an expression for the optimal threshold using the Lagrange method and propose a fast binary-searching algorithm to solve it numerically. Simulations show that, with perfect signal-to-interference-and-noise-ratio (SINR) information, PaCoSIF could decrease the false alarm rate and boost CRN throughput significantly against conventional cooperative sensing when SUs are deployed in spectrum-heterogeneous environments. Finally, the impact of SINR error upon the performance of PaCoSIF is evaluated via extensive simulations.

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In this paper, we propose a novel approach to measuring ground surface displacement at high accuracy by radio interferometric ranging (RIR) with extremely low-cost devices. A major challenge to RIR in accurate measurement is multipath in mountainous environments. We propose a novel RIR scheme called Downslope Interferometric Phase (DIP) to decouple the effect of reflective path from that of line-of-sight path. With a two-path channel model, we find that the displacement errors in DIP fluctuate periodically with respect to measurement frequencies. Based on such a revelation, we propose to employ multiple carrier frequencies from large enough bandwidth to mitigate multipath. Experiments both in open spaces and dense multipath environments show that millimeter level displacement can be detected by DIP within several minutes. With better performance and much lower cost than carrier phase GPS receivers, DIP provides a promising method to build landslide monitoring and early warning systems covering vast areas at reasonable cost.

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