3 Truths about High Fade Margin in Point-to-Point Path Design

Microwave Transmission ImageCurrently, there are no known ITU or North American error performance standards that address outage probability on all-packet point-to-point microwave radios. According to both the Vigants and ITU-R P.530 models, the probability of outage (i.e., Severely Errored Second Ratio) is inversely proportional to fade margin.

Truth or Myth: Higher Fade Margins Equal Better Performance?
This brings us to consider the following myth: Do higher fade margins improve error performance? Even though it makes sense intuitively, the concept of improving performance with high fade margins is not applicable to critical links—long links in low-lying, flat and humid regions. For this reason, a cautionary note needs to be disseminated among the global RF planning community.

Fade Margin and its Meaning in Point-to-Point Design
During the days of analog radios, high fade margins were required because noise was additive on a per hop basis, and any disturbance affected performance. It is important to recognize that annual or monthly outage time, not path fade margin, is the error performance objective for all-packet microwave radios. An all-packet radio will perform essentially error-free just a few dBs above threshold.

Truth 1: Critical Link C or k-Factors Reduce Fade Margin, Increase Outage Time
For long (40km+/25-miles+) and flat paths deployed in low elevations (200m/656-feet and lower) and humid areas, the geo-climatic model will yield a high geo-climatic factor (C or k-factor) that will reduce fade margin and consequently increase outage time from 300 sec/year (99.999% availability) to perhaps ~1500 sec/year (99.9952% availability). The logic is that to reduce the outage time, large (>3m/9.8-foot) antennas would be required.

Truth 2: Large Antennas Have Narrow Beamwidth, Decouple at Night
However, large antennas have a narrow beamwidth that would render the path unusable due to antenna decoupling because of dramatic changes of the k-factor at night.

Truth 3: High Output Power Does Not Accommodate High Nocturnal k-Factors
On the other hand, high output power would not accommodate very high nocturnal k-factor values and as a consequence a high fade margin would be useless—not to mention expensive to implement!

Four Principles of “Critical” Region Path Engineering
During our 54 years of existence in Silicon Valley, Aviat Networks has accumulated vast experience in the understanding of microwave radio propagation and performance in divergent geo-climatic conditions around the globe. Consequently, Aviat Networks recognizes the need to observe four path engineering commandments when implementing links in critical (i.e., low elevation, high humidity, ducting) regions as opposed to just concentrating on fade margin:

1. Adequate path clearance above suspected atmospheric boundary layers
2. Optimized antenna spacing
3. Proper antenna sizes and exacting alignments
4. Fade margin

In critical regions, wide radio channels (i.e., 28 MHz; 56 MHz) are dramatically affected by divergent tropospheric dielectric boundaries, which cannot be mitigated by high RF power or very large antennas. For these designs, sound path engineering is crucial, not necessarily high fade margin.

For additional information on high fade margins in wireless path design see our video “Check List for a Successful Microwave Link,” presented by noted microwave transmission expert Dick Laine, principal engineer for network engineering support at Aviat Networks.

Ivan Zambrano
Senior Network Engineer
Aviat Networks

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What to Expect from IWCE in 2012

As you know, IWCE (International Wireless Communications Expo) is just around the corner (Feb 20-24 Las Vegas) and is the premier event for government, public safety, utilities and transportation.  We are excited to be exhibiting once again at this event.

We can expect to hear about 2 key themes:

1. Public Safety migration to LTE
The introduction of LTE technology into public safety networks is happening now and represents a huge change for state/local agencies.  LTE is a brand new technology for this market and represents a new way of thinking for many folks.  LTE brings new services and applications, different network planning and design assumptions, more capacity requirements, and more IP traffic.  Understanding how to build microwave networks that best support the cost, capacity and mission critical requirements of public safety LTE will be key to building mission critical LTE data networks.  Aviat has unique solutions to solve these complex challenges.

2. Security of critical infrastructure
The current and ongoing migration of public safety networks toward IP/LTE is increasing the opportunities and motivations malicious activity. As the amount of critical data rises in the broadband public safety network, security has become of greater concern.  This will be a key topic at the show.  Again, Aviat has a unique strong security solution which we’ll be talking extensively about at IWCE.  In fact, in addition to the exhibition, we will be speaking on a panel at the IWCE show regarding cyber threats to the public safety network infrastructure on February 23rd at 3:30 – 4:45pm which we would like to invite you to attend.

Please check back after the show for an update on how things went!!

Gary Croke
Product Marketing
Aviat Networks

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The Wireless Transmission Blog is #One

Today is the 1st anniversary of this blog!

It has come a long way in its first year and it is still the only blog that specializes in wireless transmission!

In the past year the blog has received more than 45,000 visitors. Topics have ranged from “Small Cell Mobile Backhaul: The LTE Capacity Shortfall” to “Just How Recyclable is my Radio?” and it has the luxury of a wide range of contributors.

The post that received the most hits was “What is Asymmetrical Link Operation?” and the post that received the most comments was “The World’s Longest All-IP Microwave Link”?
Thanks to the contributors and the people who took the time to comment.

We are looking forward to more insightful posts in 2012 including a daily commentary from Mobile World Congress in February. Stay tuned.

Aviat Networks

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The Impact of Streaming Video on Wireless Network Services

Video call between Sweden and Singapore, on So...

Sustained video streaming, such as a video call over a mobile network, strains the stat mux paradigm of oversubscribing Ethernet microwave backhaul. However, proper management can ensure a consistent, high-quality user experience can be maintained. Image via Wikipedia (author: Kalleboo)

Mobile backhaul networks today support Ethernet microwave transport for 3G and 4G wireless technology services alongside legacy 2G and 3G TDM-based microwave equipment. However, as late as 2009 these wireless network services were solely TDM transport. One of the primary benefits of moving to Ethernet microwave transport has been the inherent statistical multiplexing (stat mux) gains. Stat mux relies on the fact that not everyone is “talking” at the same time and when they do, their IP radio packet sizes are variable, whereas networks based on TDM have to be provisioned statically for peak rates to individual wireless microwave sites.

With the advent of Ethernet, the typical practice is to oversubscribe all the wireless network services (based on individual peak rates) knowing that there is a statistical improbability of hitting the peak rate across all your wireless communication towers at the same exact moment.

Now enter video streaming where data is “streamed” between two wireless communication points over a sustained period (e.g., 30-second YouTube video clips, Skype HD Video Conferencing, Netflix movies). The sustained aspect of these video streams begins to strain the overall stat mux paradigm. Not only does video remain sustained but also it uses large-size IP radio packets that do not vary greatly. VoIP does the same thing, but the effect is much less significant as the overall bandwidth utilization is much lower.

Oversubscription becomes more challenging the more active video streaming is at any given moment. Imagine a scenario where the latest cat-playing-a-piano video gets posted online and everyone starts viewing it at virtually the same time. For a large swath of bandwidth, stat mux will reach zero for approximately four minutes. The upside is that you can add more bandwidth and/or offer differentiated wireless network services levels that guarantee certain bandwidth or application performance. Even so, video streaming does not totally negate the benefits of an Ethernet microwave transport, it just needs to be properly understood and managed to ensure a consistent user experience across all applications and services for your global wireless solutions.

Steve Loebrich
Director of Product and Solutions Marketing
Aviat Networks

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TL 9000 and ISO 9001 Certification Impact on Wireless Communication

With full company recertification to the TL 9000 and ISO 9001:2008 standards, wireless communication network operators once againAviat Networks' TL 9000 and ISO 9001:2008 certifications can be sure that Aviat Networks has been fully qualified to supply them with the highest-quality products, services and solutions available to the global wireless solutions industry. In late 2011, Aviat Networks passed a full companywide audit for TL 9000 and ISO 9001:2008 industry standards for quality management. Awarded by Bureau Veritas Certification North America, the quality certifications are valid for three years. During this time, Aviat Networks will be making even more continuous improvements in quality for wireless communication network operators to use in their telecommunications solutions and wireless network services.

These certifications provide a guarantee to our customers that we can deliver on more than 90 telecommunications solutions-specific business methods and assurances—something you cannot get from a non-TL-certified vendor. In addition, we are the only specialist microwave solutions provider that is certified TL 9000 as well as one of only four TL 9000 certified microwave solutions providers overall. This is a major achievement for a company of Aviat Networks’ size.

Most importantly for mobile broadband solutions providers and other wireless communication providers TL 9000 is the telecommunications solutions industry’s “seal of good housekeeping.” It is a much more stringent standard to meet and a big step beyond ISO 9001:2008 certification, which can generically apply to a broad range of wireless technology as well as non-tech companies. An increasing number of customers worldwide now make TL 9000 and ISO 9001:2008 certification requirements for every wireless solution provider.

The TL 9000 quality management system was developed by QuEST Forum to meet the supply chain quality requirements of the global communications industry. Built on ISO 9001, it is designed for the communications industry. TL 9000’s purpose is to define unique communications quality system requirements for development, design, production, delivery and service. It specifies measurements for companies to help evaluate the effectiveness of quality improvement and implementation programs.

Scott Graham
Director Corporate Quality
Aviat Networks

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Wireless ‘Feel Good’ Factor

It has been satisfying to help a ‘go-getting’ wireless company from down-under win an award recently. Adelaide-based MIMP Connecting Solutions won the environment and energy efficiency category at the NECA (National Electrical and Communication Association) excellence awards using our Eclipse microwave radios.

MIMP won the award with a unique comms solution that links the Adelaide Zoo with the Warrawong Wildlife Sanctuary. The sanctuary is home to 100 species of birds and native mammals, most of which are nocturnal and endangered.

The Zoo’s old system ran at 256 kilobit-per-second at a very high cost until MIMP installed four environmentally friendly microwave radios. They now have a 32 Mb/s full-duplex connection between the sites with minimal ongoing costs. Now, the radios use less power than a light bulb.

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4 Realities about Rain Fading in Microwave Networks

Image of a thunderstorm line (in dBZ) seen on ...

Image via Wikipedia

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.

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FCC: New Spectrum for Wireless Backhaul, Relaxed ACM Rules

Logo of the United States Federal Communicatio...

Image via Wikipedia

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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Why Bigger is not Always Better for Mobile Backhaul!

Terrestrial microwave radio system with two an...

Terrestrial microwave radio system with two antennas employing space diversity. (Image via Wikipedia: Photo credit David Jordan)

Antenna gain is directly related to the size (diameter) of the antenna, and wireless transmission engineers looking for more system gain to improve link performance on long or tough paths in frequency bands below 10 GHz may resort to using very large antennas with diameters of 12 feet (3.7 m) or more. However, bigger is not always better. In fact, large antennas should only be used under the most unusual of circumstances.

Use of large, oversized antennas was commonplace during the 1960s and 1970s, for analog FM-FDM heterodyne microwave communication high-capacity links operating in the L6 GHz band. This was for good reason. Communications paths consisting of multiple radio links required very high receive signal levels, and fade margins of up to 50 dB, on each link to meet end-to-end noise objectives. The large antennas helped cut baseband thermal noise by more than 3 dB, which is half that of smaller antennas. Many of these paths were relatively short and many of these analog wireless links employed frequency diversity, so higher fade margins were needed to reduce outage—especially in N+1 hops. This reliance on large antennas is often still prevalent in the minds of many wireless transmission engineers.

Today’s Digital Microwave Systems

In contrast to old analog systems, digital microwave operates essentially error-free (i.e., with a bit error rate of 1 in 1,013 transmitted bits), even with much smaller fade margins. Adequate path clearance, optimal selection of diversity arrangements using smaller antennas and the precise alignment of antennas are far more effective to ensure that error performance objectives for microwave communications are met.

Big Antennas = High TCO

So because big antennas are not really needed to ensure high path availability, they do directly impact the total cost of deploying and operating a microwave link, namely:

  • Wind Loading—There is more wind loading because of the larger surface area. A 12-ft antenna has 45 percent more loading (e.g., 1,400 lbs wind load in a 70mph wind) compared to a 10-ft antenna (e.g., 980 lbs wind load). This means the microwave tower needs to be stronger to be less susceptible to the sway that results in antenna misalignment. Stronger towers mean more costly new towers, or expensive upgrades to existing towers
  • Beamwidth—Beamwidth of a 12-ft dish is 25 percent narrower compared to a 10-ft antenna, which further increases the tower’s rigidity requirements and thus cost
  • Non-Diversity vs. Diversity—Large 12-ft antennas are sometimes justified by assuming that the single large dish is more cost-effective and/or has performance characteristics as good as two smaller diversity dishes. A single 12-ft dish with its 1,400-lb single-point wind load—and narrower beamwidth—puts far more stress on a structure than dual 8-ft diversity dishes with a distributed wind load of 1,260 lbs (2x630lbs) and much wider beamwidths. Smaller diversity dish arrangements also increase the wireless link’s performance by reducing multipath outage by more than 80 percent compared to a single 12-ft dish deployed in a non-diversity hop
  • Antenna Decoupling and Alignment—The smaller beamwidth of larger antennas also increases the difficultly to align accurately, and the risk of antenna decoupling due to angle-of-arrival variations during nocturnal atmospheric (k-factor) changes. Antenna decoupling, directly proportional to path length, is increased on those longer paths in difficult geoclimatic areas that attract the use of 12-ft dishes. It can be a death spiral—the longer, more difficult paths that attract the use of larger, narrower beamwidth antennas are those that are even more sensitive to the resulting geoclimatic conditions!
  • Aesthetics—Bigger isn’t better when deploying dishes on towers, buildings and—especially—mountaintop sites, due to aesthetic concerns, building/tower owners’ concerns and local planning limitations. These can often be mitigated by using smaller antennas
  • Deployment Costs—The overall deployment cost differential between a single 10-ft and 12-ft antenna can exceed $10,000 when transport, installation and ancillary hardware are taken into consideration, and this does not include the potential cost of added tower strengthening and increased monthly tower lease charges

So before you consider using large 12-ft+ antennas, think again and consider the bigger picture. You may well end up spending a lot more money for a path that may perform more poorly than it would have if smaller antennas had been used.

For more tips, we’ve also included some wireless transmission engineering guidelines for antennas and other wireless equipment.

Stuart Little
Director of Corporate Marketing, Aviat Networks

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Wireless Services: Stepping Outside the ‘Box’

Map of Nigeria

Nigeria, in the heart of West Africa, is home to leading mobile operator MTN Nigeria and the hottest wireless carrier market on earth.

Customers are looking for partners who can do more than just provide them with “boxes.” To really partner with customers, sometimes you have to step outside of the box. Providing a comprehensive, advanced Spares Management Program solution to MTN NigeriaAviat Networks’ largest customer and a major Tier 1 mobile network operator in Africa—is a prime example of what can be accomplished when stepping outside of the box.

Challenging Environment

As many are aware, Africa represents a challenging operating environment where on a daily basis mobile operators have to contend with power outages, lack of infrastructure and a shortage of trained personnel. Due to these issues, MTN Nigeria was experiencing significant challenges with its spares management related to its overall installed base of network equipment. This included having more spares than were needed but never having the right spare in the right place at the right time.

Even though the customer had a large supply of spares as part of capital expenditures, it was actually very difficult to keep track of the physical inventory. In this situation, MTN Nigeria asked its suppliers to manage the problem. Each supplier was to take accountability for owning and managing the problem for the customer.

For more, see the complete customer success story.

Ross Gillette
Director of Services, Africa, Aviat Networks

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