Clearing a Frequency Subband for Enhanced Aeronautical Communications

The Federal Aviation Administration (FAA) is exploring the feasibility of clearing part of the 117.975-137 MHz air/ground (A/G) radio band for use by a VHF digital aeronautical data communications network beginning in the 2010-2015 time frame. The cleared subband would be taken from the 13.4 MHz of spectrum currently allocated to air traffic services (ATS) that use the present-day analog A/G voice radio system. The growing spectral congestion in the band makes it a challenge to clear any part of it for the future aeronautical data network without adversely affecting the existing A/G voice system. In this paper we analyze key issues that would arise when clearing a subband in conjunction with a transition to a new A/G voice system architecture, in which many users would operate in 8.33-kHz channels rather than the 25-kHz channels used today. Our analysis examines the tradeoffs among the amount of spectrum cleared, the effort involved in clearing it, and the impact on the remaining capacity of the A/G voice ATS system. We establish bounds on the amounts of ATS spectrum likely to be clearable in a wide variety of scenarios, and recommend measures for approaching those bounds while controlling implementation complexity and preserving enough voice spectrum to accommodate anticipated future growth of ATS demand.