Cognitive radio technology proposes the utilisation of under-utilised spectrum resources which may include time, frequency, geographical location, direction, polarisation et cetera. Frequency is the conventional spectrum resource, considered to be exploited for cognitive radio, especially in the field of antenna design. We address the unconventional directional resource for cognitive radio, from antenna design perspective. The design concept of a multi-band compact array, capable of providing separate and simultaneous access to frequency and directional resources, is presented. The initial explorations are carried out for three frequency resources (bands) and three directional resources, providing nine degrees-of-freedom altogether. Laboratory version of the proposed antenna system is then used to gain proof-of-principle through line-of-sight measurements in an over-the-air test-bed, followed by static outdoor measurements in a multipath scenario. At the end, simulations are performed for arbitrary arrays in heterogeneous propagation scenarios to study the influence of antenna radiation pattern on the availability of directional opportunity. Recommendations are made for possible antenna design based on the simulation results.
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