The existing and new wireless technologies, such as smart phones, tablets, and IoT apps are rapidly consuming radio spectrum. The traditional regulation of spectrum requires a fundamental reform in order to allow for more efficient and creative use of the precious airwave resources. Cognitive radio (CR) has been widely recognized as a promising technique to increase the efficiency of spectrum utilization. It allows the unlicensed secondary users (SUs) to coexist with the primary users (PUs) in licensed bands. The SUs are allowed to utilize only the unoccupied spectrum resource and leave it whenever the incumbent PUs are ready to transmit. Thus, reliable identification of the spectral holes in particular licensed frequency bands is required.
Current cognitive communication systems deploy the half-duplex (HD) radios to transmit and receive the signals by orthogonal resources. The SU communication is usually realized by the popular “Listen-before-Talk” (LBT) protocol, in which the SUs sense the target channel before transmission. Though the LBT protocol has been proved effective, it actually dissipates the precious resources by employing time-division duplexing, and thus, unavoidably suffers from two major problems: 1) transmit time decrease due to sensing, and 2) sensing accuracy impairment due to data transmission.
It would be desirable if the SUs can continuously sense the spectrum and meanwhile transmit when a spectrum hole is detected. This, however, seems impossible with the conventional half-duplex systems. A full-duplex (FD) system, where a node can send and receive the signals with the same time and frequency resources, offers the potential to achieve simultaneous sensing and transmission in CR systems. Specifically, SU can sense the target spectrum band in each time slot, judge if the band is occupied, and make decisions on whether to transmit data in the adjacent slot on the basis of the sensing result and access mechanism. As the FD technology enables to explore another dimension of the network resources in CR systems, it thus requires new designs of the network protocols, signal processing and resource allocation algorithms.
For example, one of the major challenges faced by FD-CR is how to deal with the residual self-interference issue in sensing process, beneath which lies a secondary transmit power optimization problem to maximize the system throughput. Another challenge is how to manage the resources in space, frequency, and device dimensions to improve the spectrum efficiency for the secondary network.
Further applications of FD-CR comprise many important scenarios, such as FD cognitive MIMO, FD cognitive relay, and FD cognitive access point, etc. All these present a new design paradigm for enhancing the spectrum usage for future wireless communications and networks.