FCC: New Spectrum for Wireless Backhaul, Relaxed ACM Rules

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On 9 August 2011, the FCC announced several changes to the rules (Part 101) that govern the use of microwave communications in the Fixed Service bands in the U.S. These changes are great news for operators and will be encouraging increased adoption of microwave technology as a wireless transmission alternative to fiber for next generation mobile networks and fixed/private networks.

New Frequency Band for Fixed Services

The FCC opened 650 MHz of new spectrum for Fixed Service (FS) operators in the 6875-7125 MHz and 12700-13100 MHz bands, which will be shared with the incumbent Fixed and Mobile Broadcast Auxiliary Service (BAS) and Cable TV Relay Service (CARS). These bands will primarily be used as an alternative to the 6 and 11 GHz “Common Carrier” bands in rural areas, where the band is not currently licensed to TV mobile pickup stations used in newsgathering operations.

Frequency allocations in these new bands should commence later this year and will be based upon the existing 25MHz channelization. To facilitate adoption, the FCC is also allowing the use of 5, 8.33 and 12.5 MHz channels, as well as 50 MHz channel operation in the 12700-13100 MHz band using two adjacent 25MHz channels.

Allowing Adaptive Modulation

Adaptive Modulation, or AM—or ACM when used with Adaptive Coding—is a relatively recent innovation in microwave technology that allows the radio to dynamically adapt to path conditions to allow a much higher degree of spectrum efficiency, increased wireless link throughput, use of smaller antennas or a combination of all three benefits.

Up until now, the use of AM was restricted by the requirement to comply with FCC spectrum efficiency rules, which dictate a minimum data rate for certain bands. For example in the 6 GHz band a minimum capacity of 130 Mbit/s, or 3xDS3, must be maintained at all times within a 30 MHz channel assignment, using 64QAM modulation. The FCC now allows AM operation where the capacity of the link may drop below the minimum data rate, as long as the operators “design their paths to be available at modulations compliant with the minimum payload capacity at least 99.95 percent of the time,” or in other words, operators will have to “design their paths to operate in full compliance with the capacity and loading requirements for all but 4.38 hours out of the year.”

Aviat Networks, through our membership of the Fixed Wireless Communications Coalition (FWCC), supported rule changes to permit ACM, and the FCC included in its Rulemaking (Clause 48) our analysis on the benefits of ACM in terms of reducing the costs associated with tower leasing:

By way of hypothetical, consider a single link in the 6 GHz band that would require 10-foot antennas with a 99.999 percent standard instead of 6-foot antennas under the 99.95 percent standard. The total cost increase over a 10-year period in this hypothetical example could exceed $100,000.

The smaller antennas offer a number of advantages over larger ones, including more TCO savings over those 10 years.

Still Under Consideration by the FCC

Of all the new proposals being considered, the FCC also announced a Further Notice of Proposed Rulemaking (FNPRM) to further investigate the following proposals:

  • Allowing Smaller Antennas in Certain Part 101 Antenna Standards without materially increasing interference
  • Exempting Licensees in Non-Congested Areas from Efficiency Standards to allow operators to increase link length in rural areas
  • Allowing Wider Channels, including 60 MHz in the 6 GHz band, and 80 MHz in the 11 GHz bands
  • Revising Waiver Standard for Microwave Stations Near the Geostationary Arc to align with ITU regulations
  • Updating Definition of Payload Capacity rules in Part 101 rules to account for Internet Protocol radio systems

Aviat Networks continues to work on these issues, via the FWCC, which we believe will assist operators in lowering their total wireless network operational costs by taking advantage of the newest innovations that are now available in microwave technology.

With these new rules, along with the potential for further changes under consideration, microwave solutions provide an even more compelling case to enable mobile operators in the U.S. to keep pace with the IP mobile backhaul capacity demand driven by the introduction of new 4G wireless/LTE wireless networks.

Ian Marshall
Regulatory Manager, Aviat Networks


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