Shared Spectrum, Sunspots, and the Birthday Paradox: Lessons Learned from Sharing of the 5.4-5.7 GHz (UNII-2) Frequency Band
5 Pages Posted: 2 Apr 2012 Last revised: 16 Aug 2012
Date Written: March 31, 2012
Pursuant to a recommendation by NTIA, the FCC has required that equipment operating in the UNII-2 band (5.4-5.7 GHz) conform to technical requirements which purport to limit interference with government radar systems and ensure uniform loading of the spectrum. DFS, or dynamic frequency selection, requires equipment to test frequencies for an extended period before transmitting, and to relocate to a different frequency -- generally with a substantial interruption of service -- if "radar" is detected. The FCC also requires that equipment select the channels on which it operates at random, excluding only those channels in which the DFS algorithm detects a radar signature. The explicit intent of this requirement was to create uniform loading of all available channels.
Unfortunately, under real world conditions, these requirements preclude the use of the spectrum for mission critical broadband distribution. During the solar storm of March 2012, solar flares were misinterpreted as "radar," interrupting broadband service. Likewise, the random channel selection requirement -- due to a nonintuitive statistical phenomenon known as the "Birthday Paradox" -- has proven to create less uniform loading of spectrum than would either cognitive radios capable of sensing energy levels or manual selection of channels by technically skilled operators. The requirement also complicates use of the spectrum by, in many instances, precluding co-location of multiple transmitters. This, in turn, necessitates the use of larger sectors and/or omnidirectional antennas, which experience and generate more interference than would more narrowly focused ones. This paper discusses lessons learned in the field by direct experience with the DFS and random channel selection requirements and explores ways in which the development of spectrum sharing policy could be refined to prevent untoward results.
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