A method of spectrum access for wireless communications by a secondary unlicensed unit in a CRN environment comprising one or more secondary unlicensed mobile units and one or more primary licensed units includes receiving information indicative of a location and movement of one or more secondary unlicensed units and a channel usage pattern and spatial distribution of one or more primary licensed units is accessed and a guard distance to shield the one or more primary licensed units from interference is calculated. The channel availability to the one or more secondary unlicensed units is then calculated according to a two-state Markov model including the guard distance as a constraint and based on the information indicative of the location and movement of the one or more secondary unlicensed units and the channel usage pattern and spatial distribution of the one or more primary licensed units.