Optimizing the spectral placement of future radio services

Several candidate frequency bands are currently being considered for allocation to future air/ground (A/G) communications systems. This paper presents a methodology for quantitatively comparing the inherent suitability of candidate bands. Attainable link capacity, which varies with carrier frequency and directly affects the overall spectral efficiency of a system, is used as suitability metric. Capacity bounds are derived from fundamental frequency-dependent propagation characteristics and design constraints imposed by operational system requirements. "Acceptable" and "optimal" frequency ranges (in which attainable link capacity exceeds 10% and 80% of maximum, respectively) are identified for various system implementations involving different combinations of antenna directionality, link direction, and link bandwidth. It is shown that, depending on the implementation considered, the optimal frequencies may fall within a range as narrow as 80-160 MHz or as wide as 40-4500 MHz (although, of course, only a small fraction of the "optimal" range could realistically be made available to any new aeronautical system). The methodology could serve as an analytical basis for evaluating future proposals for adjusting band allocations, so that new systems can be placed in bands where they will operate with maximal effectiveness and spectral efficiency.