Techniques and systems for formalizing discontiguous spectrum frequency planning maximize system capacity while minimizing the potential for adjacent channel interference. Within the context of an overall system frequency plan, spatial separation can be used to offset overlap resulting from channelization mapping when additional frequencies are not available. Site by site modification and/or design can be used to prevent intermodulation products and/or adjacent channel interference effects. A variety of factors can be taken into account including, for example, control channel position and separation in a new spectrum, probability block utilization, threshold levels for adjacent channel interference, symmetric vs. non-symmetric adjacent channel interference, frequency plan type, Antenna Near Part (ANP) separation, intermodulation effects, local topography, frequency environment conditions, and site availability. These techniques are especially useful for (although not limited to) establishing a downbanded cellular (DBC) radio communication system by overlaying 30 kHz channelization on portions of the Specialized Mobile Radio (SMR) band having 25 kHz channelization.