- August 23, 2011
- Aviat Networks, backhaul, Dick Laine, Frequency range, Internet Protocol, microwave, Microwave transmission, Principal Engineer, Rain Attenuation, Rain Fade, Rain Fading, Telecommunication, Time-division multiplexing, wireless
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Rain fading (also referred to as rain attenuation) at the higher microwave frequencies (“millimeter wave” bands) has been under study for more than 60 years. Much is known about the qualitative aspects, but the problems faced by microwave transmission engineers—who must make quantitative estimates of the probability distribution of the rainfall attenuation for a given frequency band as a function of path length and geographic area—remains a most interesting challenge, albeit now greatly assisted by computer rain models.
A surprising piece of the puzzle is that the total annual rainfall in an area has almost no correlation to the rain attenuation for that area. A day with one inch of rainfall may have a path outage due to a short period of extremely high localized rain cell intensity, while another day of rain may experience little or no path attenuation because rain is spread over a long period of time, or the high intensity rain cell could miss the microwave hop completely.
Over the years, we have learned a lot about deploying millimeter wave microwave hops for our customers:
- Rain outage approximately doubles in each higher millimeter wave band, e.g. 18 to 23 GHz
- Rain outage is directly proportional to path length—assuming a constant fade margin for each hop
- Rain outage in tandem-connected short hops is the same as for a single long hop—if they have the same fade margin
- Multipath fading in optimally aligned millimeter wave hops does not occur during periods of heavy rainfall, so the entire path fade margin is available to combat rain attenuation fades
More information about assessing rain-induced attenuation is available in our white paper, Rain Fading in Microwave Networks.
- August 12, 2011
- ACM, Adaptive Coding, Adaptive Coding and Modulation, Adaptive Modulation, AM, Antennas, Aviat Networks, BAS, Broadcast Auxiliary Service, Cable TV Relay Service, CARS, Clause 48, Common Carrier, FCC, Federal Communications Commission, Fixed Services, Fixed Wireless Communications Coalition, FNPRM, Frequency range, Further Notice of Proposed Rulemaking, FWCC, Ian Marshall, Internet Protocol, Part 101, Regulatory Manager, Rulemaking, Telecommunication, wireless
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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.
Regulatory Manager, Aviat Networks
- June 24, 2011
- ACM, Adaptive Modulation, Business, ComReg, Frequency range, Ian Marshall, Ireland, microwave, Regulatory Manager, Spectrum, Spectrum Management, Telecommunications, wireless
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The Irish communications regulator, ComReg, recently issued a consultation on its spectrum management strategy for 2011-2013. This was a wide-ranging consultation covering all aspects of spectrum management. However, in terms of interest to the microwave fixed point to point business were the following items:
A stated intention to open new bands for fixed point to point microwave wireless backhaul at 26GHz, 28GHz and 31GHz in line with the relevant ECC recommendations. In addition, ComReg requested comments on the following proposals regarding the use of Adaptive Modulation (ACM) and Cross Polarization (XPIC).
“Given the benefits identified from the use of Adaptive Coding & Modulation (ACM) in terrestrial Fixed Links, ComReg is proposing to make the deployment of ACM mandatory for all new fixed link applications across all fixed link frequency bands from 01 June 2012,” the consultation reads.
“With a view to encouraging spectrum efficiency in congested frequency bands, ComReg is proposing to make dual polarization mandatory for all new fixed link applications, where more than one link is required on the same path in the same frequency band, from 1 June 2012.”
The above two proposals demonstrate ComReg’s forward vision in embracing new wireless technology to increase the viability of using microwave solutions for critical traffic. Compared with some other regulators around the world, this is a welcome and refreshing approach.
Also ComReg indicated its intention to explore the possibility of using alternative licensing schemes, e.g. light licensing or link registration, in bands above 50GHz that are under consideration for opening in Ireland. Let me know your thoughts.
Regulatory Manager, Aviat Networks