4D Arrays as Enabling Technology for Cognitive Radio Systems

Time-modulation (TM) in four-dimensional (4-D) arrays is implemented by using a set of radio-frequency switches in the beam forming network to modulate, by means of periodic pulse sequences, the static excitations and thus control the antenna radiation features. The on-off reconfiguration of the switches, that can be easily implemented via software, unavoidably generates harmonic radiations that can be suitably exploited for multiple channel communication purposes. As a matter of fact, harmonic beams can be synthesized having different spatial distribution and shapes in order to receive signals arriving on the antenna from different directions. Similarly, the capability to generate a field having different frequency and spatial distribution implies that the signal transmitted by time-modulated 4-D arrays is direction-dependent. Accordingly, such a feature is also exploited to implement a secure communication scheme directly at the physical layer. Thanks to the easy software-based reconfigurability, the multiple harmonic beamforming, and the security capability, 4-D arrays can be considered as an enabling technology for future cognitive radio systems. In this paper, these potentialities of time-modulated 4-D arrays are presented and their effectiveness is supported by a set of representative numerical simulation results.