Space-Time Spectral White Spaces in Cognitive Radio: Theory, Algorithms, and Circuits

Space-time spectral white spaces in a cognitive radio environment are defined based on multidimensional spatio-temporal spectral properties of radio waves received by a planar array of antennas. Spectral occupancy of a given carrier frequency pertaining to a particular direction in space is expressed by the volume of a semi-cone shaped geometrical region in the 3-D spatio-temporal frequency space ω. A combined approach employing low complexity array processing and conventional time-frequency spectrum sensing is proposed towards the detection of space-time white spaces in ω. The detection scheme employs four subsystems; antenna array, front-end processing, 3-D spatio-temporal array processing, and 1-D spectrum sensing. Key components in the antenna array and front-end processing subsystems are described including an example of a broadband Vivaldi antenna simulated in the frequency range 1.25-2 GHz. The array processing subsystem employs 3-D infinite impulse response digital beam filters, as a low complexity alternative to conventional phased arrays. One potential realization of the 1-D spectrum sensing subsystem is described by using a tunable bandpass filter followed by an energy detector. Simulation examples are provided by considering different directions of arrival, effect of multi-path replicas, signal to noise ratio changes and both narrow band and wideband signals in the normalized temporal frequency range (0,π).