The security of communication over shared wireless spectrum is becoming increasingly important as spectrum becomes scarce.
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Spectrum scarcity and transmission security are two of the largest challenges facing modern wireless communications. One solution to spectrum scarcity is to establish a ‘cognitive radio’ network that allows licensed ‘primary’ users, such as mobile carriers and television broadcasting companies, to share the idle spectrum with unlicensed ‘secondary’ users, such as private users. This approach ensures that no wireless spectrum goes to waste.
Information theory is an extremely useful tool for studying the transmission rates of different communication channels in a given network. It is also a powerful tool for the study of security issues because it can calculate the capacities of legitimate and illegitimate (eavesdropping) communication channels. Ying-Chang Liang at the A*STAR Institute for Infocomm Research and co-workers at Nanyang Technological University have now used information theory to calculate the maximum rate at which information can be securely transmitted between primary and secondary users. Their findings demonstrate the most secure way to operate a cognitive radio network.
The researchers began by mathematically formulating the problem of finding the ‘secrecy capacity’—the maximum secure rate of transmission of a secondary transmitter broadcasting to a secondary receiver over spectrum that is shared with a primary user, while at the same time being spied on by an eavesdropper. This is a complex optimization problem that needs to account for transmission power and interference strengths. By developing a novel set of mathematical transformations, Liang and his co-workers were able to reduce this problem to a sequence of simpler problems that are performed routinely on unsecured cognitive radio networks.
As a result, the team was able to prove that beamforming is the optimal transmission strategy for a secure cognitive radio channel. Beamforming involves the use of multiple transmission antennas, each carrying a scaled version of the transmitted signal, to shape the transmission beam, and is used in unsecured cognitive radio networks to avoid interference with the primary user. Liang and his co-workers have shown that beamforming is also the best way to maximize the secrecy capacity of a secure cognitive radio network.
The researchers also introduce three beamforming approaches that produce sub-optimal secrecy capacities, but that have the benefit of reduced calculational complexity. Their study of cognitive radio security can be applied broadly, says Liang. “These results are relevant both for existing cellular wireless networks and for new technologies like femtocells, which broadcast cellular signals to indoor users over spectrum that is shared with outdoor users.”
The A*STAR-affiliated researchers contributing to this research are from the Institute for Infocomm Research.
