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Small cell will enable mobile usage in dense urban environments but will need a backhaul solution to make it possible. Photo credit: Ed Yourdon / Foter / CC BY-SA
The Case for Small Cell Backhaul
As the search for frequency bands with suitable capacity for small-cell backhaul continues, frequency bands above 50GHz start to appear attractive because they offer both high-bandwidth availability and short range owing to their inherent propagation characteristics. The white paper available at the bottom of this blog examines spectrum in the 57-64GHz range to see whether it can be of use for small cell backhaul.
In many countries, the frequency range 57-66GHz is split into a number of discrete bands with differing requirements and conditions of use and/or licensing. These differences will be highlighted where applicable.
From a global point of view, the use of this spectrum by Fixed Services (FS) is being addressed by the ITU-R in its draft report on Fixed Service use trends in WP5C, which is currently under development and states:
57 GHz to 64 GHz
The radio-frequency channel and block arrangements of these bands for FS are defined in Recommendation ITU-R F.1497.
In 2011, around 700 links were in use in this band in a few administrations. The majority of the links are used for fixed and mobile infrastructure.
The air absorption around 60 GHz is over 10 dB/km. This condition restricts the hop length; on the other hand, the spectrum reuse efficiency is high. This feature makes the band suitable for small cell mobile backhaul.
Clearly, a global reported usage of 700 links would suggest a great deal of underutilization, although with unlicensed use in many countries it is difficult to know whether these figures are accurate or not. Regardless, there are reasons as to why this could be the case, while noting that the ITU-R believes this band has potential for small cell backhaul.
One factor is that this spectrum is not allocated solely to the Fixed Service. In fact, in many countries the Fixed Services have no access to this spectrum at all. A more detailed country-by-country breakdown follows. Please sign up below to receive the entire white paper.
Over the course of the last 18 months, a new application has grown by leaps and bounds for microwave networks: low latency. Low-latency microwave networks find most of their applicability in financial transactions, such as for executing trading instructions between major stock exchanges and trading houses in other cities.
Typically, low-latency microwave is used to “replace” traditional-fiber based networks linking financial centers. The business driver for microwave-instead-of-fiber in low latency is the time it takes to transmit trading instructions. With microwave, latency is reduced by a few milliseconds as compared to fiber. Nevertheless, those few milliseconds can translate into a trading edge over rival investors, which means big bucks. Low latency investors will pay a premium for this edge resulting in increased revenue for low-latency microwave network operators.
However, as with most financial functions, low latency is subject to a set of stringent regulations. The scenario is doubly difficult when low-latency microwave networks transmit across international boundaries. This compares to linking financial centers within a single country, which is relatively straightforward from a regulatory perspective because there is only one set of rules. The fact is when connecting financial centers in different nations and the operator’s network has to traverse other countries’ borders, the process becomes orders of magnitude more complex. Download the complete article for a fuller examination of some of these issues and why there should be widespread support for greater international harmonization of microwave regulation.
Because of need for higher capacities, the trend toward shorter link distances for mobile backhaul and declining product costs, 70/80GHz (i.e., E-band) solutions are gathering significant interest for mobile backhaul and enterprise access applications. However, because these frequencies are new to most people, there is little understanding of costs and other issues related to licensing the 70-80GHz spectrum.
The ECC held a meeting in March to further consider updating regulations to allow the use of asymmetrical links in microwave backhaul (Photo credit: blese via flickr)
Last autumn we wrote about potential plans from a microwave competitor regarding using asymmetric band plans for point to point microwave communication links. To update this topic, we have put 10 things in parentheses that you should know about the current status of asymmetrical links in wireless backhaul. Last month at an Electronic Communications Committee SE19 (Spectrum Engineering) meeting this microwave technology subject was discussed again. (1) The proposal under consideration has been reduced in scope and (2) the regulators present still wish to see more evidence regarding the need for change before agreeing to such significant amendments.
Asymmetric Band Plan Altered A quick reminder of what was originally requested back in the autumn of 2011; a move from channel sizes of 7, 14, 28 and 56MHz to channel sizes of 7, 14, 21, 28, 35, 42, 49 and 56MHz in order to support different granularities of channel widths in all bands from L6GHz to 42GHz. However in March these proposals were altered to reflect channel sizes of 7, 14, 28 and 56MHz (i.e., no change to existing channel sizes) and asymmetric only in the 18GHz band and above.
The national regulatory authorities stated that even the (3) revised proposal cannot be accommodated with existing planning tools so they cannot imagine asymmetric links being deployed alongside existing links in their countries. A few stated that in block allocated spectrum the owner of the spectrum may be able to implement this channelization, but Aviat Networks believes that (4) the complexity of coordinating links even in block allocated spectrum should not be underestimated.
Traditionally, links are planned on an equal bandwidth basis, e.g., 28MHz + 28MHz, with a constant T/R spacing throughout the band in question. This new proposal would see links of 28MHz + 7MHz and furthermore makes the claim that spectrum would be saved. Numerically speaking this arrangement would save 21MHz for each pair, but (5) saved spectrum is only of value if it is reused. In many cases the “saved” spectrum would be orphaned due to difficulties coordinating it into usable pairs.
Asymmetric Channel Plan Limits Future
In our last blog on this topic we reflected on the fact that while there is some level of asymmetry today, (6) this trend may well be balanced in the near future by cloud services and other services that involve the user uploading content. We believe that (7) committing to an asymmetric channel plan now limits the future. (8) Symmetric channel planning allows networks to dynamically adjust to changing demands. A related concern is the fact that (9) spectrum once reallocated may not be easily clawed back to create symmetric pairs in the future. While some applications are experiencing asymmetry in traffic presently, we should not forget that some traffic patterns are still symmetric and where asymmetry is a feature, (10) the scale of this phenomenon may be overstated. Indeed, a major European operator present at the SE19 meeting voiced skepticism about the need for asymmetric support.
What do you think? Will mobile traffic remain or increasingly become asymmetric? Are asymmetric microwave links needed or can they be practically deployed in existing bands? Answer our poll below and tell us. Select all answers that apply.
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.
Smartphones such as the HTC Mogul are driving the demand for more wireless spectrum to be released.
To help relieve wireless network congestion, the Obama Administration made a commitment to release up to 500 MHz of spectrum for reuse in commercial wireless solutions. In April 2011, the NTIA updated the progress toward this commitment in its first interim report. This 500 MHz of spectrum—comprising 280 MHz of underused commercial spectrum and 220 MHz of federally owned radio spectrum now administered by the NTIA—would help ease the growing shortage of spectrum as demands on the wireless network rise. This demand is primarily fueled by the explosive adoption rate of smartphones and other mobile broadband devices and the corresponding infrastructure—both access and mobile backhaul—required to support their use.
The timescales and conditions for the availability of this spectrum is in the hands of the FCC and is expected to take about five years as the first part of its 10 year plan. However, the first four blocks of spectrum have recently been identified for release by the NTIA at 1675-1710 MHz, 1755-1780 MHz, 3500-3650 MHz, 4200-4220 MHz and 4380-4400 MHz.
It is estimated that an auction of 500 MHz of spectrum could raise more than $20 billion for the U.S Treasury.
Many wireless technology industry commentators expect the lower bands to be taken up for wireless access. But the higher three bands could be allocated for mobile backhaul use to begin the process of easing congestion in the current 6GHz bands.
The microwave backhaul industry welcomes this first step. We look forward to follow through on further spectrum releases—especially in the 4 to 8GHz range—which is suitable for high-capacity trunking backhaul.
In response to the recent FCC docket 10-153, many stakeholders proposed relaxing antennas requirements so as to allow the use of smaller antennas in certain circumstances. This is an increasingly important issue as tower rental costs can be as high as 62 percent of the total cost of ownership for a microwave solutions link. As these costs are directly related to antenna size, reducing antenna size leads to a significant reduction in the cost of ownership for microwave equipment links.
The Fixed Wireless Communications Coalition (FWCC), of which Aviat Networks is a major contributor, proposed a possible compromise that would leave Category A standards unchanged while relaxing Category B standards. The latter are less demanding than Category A, and after some further easing, might allow significantly smaller antennas. The rules should permit the use of these smaller antennas where congestion is not a problem, and require upgrades to better antennas where necessary.
A further detailed proposal from Comsearch proposed a new antenna category known as B2, which would lead to a reduction in antenna size of up to 50 percent in some frequency bands. This would be a significant cost saving for link operators.
At the present time, the industry is waiting for the FCC to deliberate on the responses to its 10-153 docket, including those on reducing antenna size.
See the briefing paper below for more information.
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.
Now, a number of the CEPT recommendations for the new point to point bands over 40GHz contain provisions for TDD operation. TDD is accommodated either as an alternative band plan or a mixed TDD/FDD band plan, in addition to the more common FDD band plan. However, CEPT recommendations are only just that—recommendations. How these bands will be implemented in each country will be determined by the individual national regulatory authority.
Recently, we asked a number of European national regulators about if and how they would introduce TDD operation in these new bands. The general response was that they were not opposed to the introduction of TDD in principle, and that such operation would have to be worked into existing or revised band plans. One complication raised was that spectrum would have to be reserved for guard bands between TDD and FDD segments within the same band. Regulators usually try to avoid having to waste valuable spectrum in this way. Also, once a band plan is established and the spectrum allocated to users, efforts to introduce TDD operation at a later date is extremely difficult.
Some regulators have already issued new national band plans at 42GHz and above, and to date none of these allow for TDD operation. Furthermore, for countries that have allocated new bands through spectrum auction, there we see the usual FDD style symmetric band approach.
Despite the appeal of TDD operation from a cost perspective, early indications are that although provision for TDD operation is being made in these higher bands, practical complications and concerns over maximizing the use of new bands may prevent its widespread introduction.
What are your thoughts on using TDD more in national band plans? Leave a comment, if you’d please.
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