In this work, we provide the implementation and analysis of a cognitive transceiver for opportunistic networks. We\nfocus on a previously introduced dynamic spectrum access (DSA) - cognitive radio (CR) solution for primary-secondary\ncoexistence in opportunistic orthogonal frequency division multiplexing (OFDM) networks, called cognitive\ninterference alignment (CIA). The implementation is based on software-defined radio (SDR) and uses GNU Radio and\nthe universal software radio peripheral (USRP) as the implementation toolkit. The proposed flexible transceiver\narchitecture allows efficient on-the-fly reconfigurations of the physical layer into OFDM, CIA or a combination of both.\nRemarkably, its responsiveness is such that the uplink and downlink channel reciprocity from the medium\nperspective, inherent to time division duplex (TDD) communications, can be effectively verified and exploited. We\nshow that CIA provides approximately 10 dB of interference isolation towards the OFDM receiver with respect to a\nfully random precoder. This result is obtained under suboptimal conditions, which indicates that further gains are\npossible with a better optimization of the system. Our findings point towards the usefulness of a practical CIA\nimplementation, as it yields a non-negligible performance for the secondary system, while providing interference\nshielding to the primary receiver.
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