LTE Backhaul: The View from Africa

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Telecom Tower, Johannesburg, South Africa. Photo credit: Marc_Smith / Foter / CC BY

LTE has been moving more and more to the forefront in mobile cellular networks around the world. Africa, and particularly the Republic of South Africa, is the latest hotbed of LTE rollouts, with the leading country operators of Vodacom, MTN and Cell C coming online since late in 2012. In conjunction with these LTE access rollouts, our technical marketing manager in the region, Mr. Siphiwe Nelwamondo, has been authoring a series of columns on enabling LTE in a leading regional technology media Internet site, ITWeb Africa.

Naturally, his focus has been on backhaul. In the first installment of his series, Mr. Nelwamondo looked closely at the backhaul requirements of LTE. Chief among these requirements are speed, Quality of Service (QoS) and capacity. He concluded that it is too early to close the book on the requisite parameters for supporting LTE backhaul. Part two of the features, he examined the basis on which microwave provides the technical underpinnings for LTE backhaul—especially as related to capacity. More spectrum, better spectral efficiency and more effective throughput were Mr. Nelwamondo’s subpoints to increasing capacity.

Having more spectrum for microwave backhaul is always nice, but it’s a finite resource and other RF-based equipment from satellites to garage door openers is in competition for it. Bettering spectral efficiency may be accomplished by traditional methods such as ACM and might be increased through unproven-in-microwave techniques like MIMO. Throughput improvement has wide claims from the plausible low single digit percentage increases to the more speculative of upping capacity by nearly half-again. Data compression and suppression are discussed. The truth is LTE, while data-intensive, probably will not require drastic measures for backhaul capacity until at least the next stage of LTE-Advanced.

If indeed capacity increases are necessary in the LTE backhaul, number three and the most current piece of Mr. Nelwamondo’s contains additional information. Nothing is better than having something bigger than normal or having many of the standard model. As the analogy applies to LTE microwave backhaul, bigger or wider channels will increase capacity, of course. A larger hose sprays more water. Or if you have two or three or more hoses pumping in parallel that will also support comparatively more water volume. The same is true of multiple microwave channels.

However, the most truly and cost effective capacity hiking approach is proper network planning. Mr. Nelwamondo points out that in Africa—more than some places—mobile operators are involved in transitioning from TDM planning to IP planning. While TDM planning was dependent on finding the peak traffic requirement per link, IP planning allows the flexibility to anticipate a normalized rate of traffic with contingencies to “borrow” capacity from elsewhere in a backhaul ring network that is not currently being utilized. Along with several other IP-related features, this makes determining the capacity a lot more of a gray area. Some operators solve this by simply “over-dimensioning” by providing too much bandwidth for the actual data throughput needed, but most cannot afford to do this.

The fourth and final entry in Mr. Nelwamondo’s series will appear soon on other LTE backhaul considerations of which you may not have thought. Sign up below to be notified when it is available. [contact-form-7 404 "Not Found"]

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Wind Farm Interference on Microwave Links: Is it a Real Problem?

Wind Turbine

Microwave links can traverse orbits of large industrial wind turbines. New studies are needed to examine the effect of turbines on modern microwave link technology. Photo credit: mcdlttx (D Turner) via Flickr.

Since the 1980s, there has been concern about the potential interference that wind turbine farms can cause to wireless communication equipment. The focus has been on TV, civilian and military radar and point-to-point microwave systems. This led to studies to evaluate the degradation effects that wind turbine farms have on these systems. They concluded that physical propagation effects such as dispersion and diffraction of electromagnetic signals propagating through wind turbine farms produce low-level, long-delay, multipath distortion on telecommunications equipment [1] [2].

In recent years, these conclusions have been used to recommend an overzealous approach to the design of digital microwave paths that go through or over wind turbine farms. Overzealous recommendations with little supporting evidence have made it standard operating procedure to establish “exclusion zones” around wind turbine farms [3].

In the case of microwave links, the technology has made great improvements since the 1980s. In the 1980s, microwave links were analog and more vulnerable to interference and multipath distortion created by wind turbine blades. New digital microwave radios with Forward Error Correction (FEC), Adaptive Coding and Modulation (ACM) and high dispersive fade margin are better equipped to deal with interference and multipath distortion produced by wind turbine blades. These technological advances were not available in the 1980s and 1990s when the most rigorous studies about wind farm interference were completed.

Besides the improvements in digital microwave technology, wind turbines have also changed. In the 1980s they were smaller (compared to contemporary units) and mostly made of metal; today wind turbines are bigger, and the blades are mainly made of reinforced fiberglass, which is transparent to microwaves. Although obstruction due to the wind turbine pole and generator case will create path loss and possible diffraction of the signal, poles and casings are very thin (compared to the Fresnel zone radius) and would have to be in the direct line of sight of the link to produce significant penetration loss.

Although a conservative approach to microwave path planning is always recommended, and detailed planning and path surveying for each path are necessary, an overzealous design based on outdated studies can lead to unnecessary CapEx and OpEx. More rigorous studies based on detailed field measurements with high performance digital microwave wireless communication equipment must be undertaken to establish wind turbine clearance criteria based on current technology and field measurements.

A good starting point can be to check current links that are intentionally or accidentally traversing the orbits of a wind turbine blade. Aviat Networks would be like to hear from users that have this situation (leave a comment for this blog). Further analysis of a link in this condition can prove or disprove the hypothesis that modern microwave radios are relatively unaffected by modern fiberglass wind turbine blades and thin wind turbine pole structures. If this hypothesis is confirmed, current guidelines can be relaxed to avoid passive repeaters and bigger towers that represent additional costs.

Eduardo Sanchez
Marketing Engineering Specialist
Aviat Networks

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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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Ireland Issues Spectrum Consultation on Wireless Communications

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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.

Ian Marshall
Regulatory Manager, Aviat Networks

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