Know Your Microwave Backhaul Options

If you look in the November issue of MissionCritical Communications, you will see an article by Aviat Networks director of marketing and communications, Gary Croke. In his article “Know Your Microwave Backhaul Options,” Gary covers:

  • Benefits of using indoor, outdoor and split-mount microwave radios in various scenarios
  • Rationale for choosing microwave over fiber (especially for LTE)
  • Deployability of microwave
  • Software-upgradeable capacity for “pay-as-you-grow” capex scalability
  • Cost contribution of towers over the first 10 years of LTE implementation
  • And more

You can read Gary’s article (on page-30) here—MissionCritical Communications—November 2012.

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Evolution of Trunking Microwave Radios

Aviat WTM 6000 trunking microwave radio

Aviat WTM 6000 trunking microwave radio

Back in the day, trunking microwave radios were huge power-hungry beasts that consumed vast quantities of power and space at equal rates. They were complex “animals” that took days to install and hours to configure. Then they had to be looked after like well-loved but aged members of the family—with care, all due respect and consideration. Over time, components went out of adjustment and had to be brought back into line through various tuning routines, but overall they did their job as the super-reliable backbone of the POTS (i.e., Plain Old Telephone Service).

Jump forward a few decades and the latest trunking microwave solutions are elegant and graceful—almost svelte. With their current high levels of electronic integration, a complete repeater system can stand in a single rack space—unheard of until the most recent products. Furthermore, these new systems consume dramatically less power—a typical 3+1 system (i.e., four transceivers) consumes less than 400 watts. So now, backbone operators can save significantly on operating expenditure because of decreased space and power requirements at their microwave radio shelters.

Evolving microwave systems from analog to digital microwave systems carrying digital payloads was a rocky and dangerous path. The next migration from TDM payloads to IP payloads appears to be just as treacherous. How can a traditional TDM backbone radio, typically configured with N+1 radio protection switching, be reconfigured to transport a non-TDM payload that does not suit N+1 switching? IP transport is a completely different environment altogether! Luckily, trunking radio system designers have not ignored the Internet revolution and are perfectly aware of these challenges. In fact, well-appointed trunking microwave radio systems allow a graceful evolution from TDM to IP, with capability to transport both types of traffic simultaneously—and with their own ultra-reliable protection schemes!

Today, trunking microwave radios can support both TDM and IP seamlessly, offer robust radio performance and highly reliable switching and really do make it easy for operators to design mission-critical backbone networks. They offer mean time between failure (MTBF) reliability figures into the hundreds-of-years and highly integrated yet modular designs, which make expansion very straightforward. Before deciding on a trunking microwave radio, consider if the system:

  • Allows easy migration from TDM to IP with a minimal amount of replacement materials
  • Can expand to an expected maximum channel capacity (for example, six channels) without needing additional racks, etc.
  • Enables repeater configurations within one rack
  • Has a field-proven heritage of reliability and performance

Terry Ross
Senior Product Manager
Aviat Networks

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Best Practices for Ultra Low Latency Microwave Networks

Theoretical Chicago-NY microwave networks using existing towers compared to existing optical network

For discussion purposes of ultra low latency, two theoretical ultra low latency microwave networks are compared to an existing optical Chicago-NY network.

In today’s ultra-competitive High Frequency Trading markets, speed is everything, and recently wireless technologies, and specifically microwave networking, have been recognized as a faster alternative to optical transport for ultra-low latency financial applications.

Even though microwave technology has been in use in telecommunications networks around the world for more than 50 years, new developments have optimized microwave products to drive down the latency performance to the point that microwave can significantly outperform fiber over long routes, for example between Chicago and New York. This has provided a new market opportunity for innovative service providers to venture into the microwave low latency business.

Although reducing the latency of the equipment is an important consideration, the most important metric is the end-to-end latency. Many factors that influence overall end-to-end latency require a deep understanding of the technology and how this is applied in practice.

This white paper will show that to achieve the lowest end-to-end latency with the highest possible reliability and network stability not only requires a microwave platform that supports cutting edge low latency performance but also a combination of experience and expertise necessary to design, deploy, support and operate a microwave transmission network.

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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 is Packet Microwave?

“Packet Microwave” radio systems continue to enjoy a lot of coverage and hype within our market. But it helps to understand exactly what packet Microwave is, including its benefits and limitations, and how Packet Microwave compares to Hybrid radios. We created a white paper a while ago to address these issues, and since it is still relevant today we are highlighting it again.

The paper provides a clear definition of this technology and also answers the following questions:

  • What features should you expect from Next Generation microwave radios?
  • How does Packet Microwave it differ from Hybrid microwave transport?
  • Is Packet Microwave ‘All-IP’ or ‘IP-Only’?
  • Do hybrid systems meet the requirements of packet microwave?
  • What is the best approach for operators trying to choose a microwave solution?



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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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Antennas: Why Size is Important for This Wireless Equipment

Antenna tower supporting several antennas. The...

Image via Wikipedia

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.

Ian Marshall
Regulatory Manager, Aviat Networks

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