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

Posted on by Aviat Networks

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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10 Things to Know About the Status of Asymmetrical Wireless Backhaul

Posted on by Aviat Networks
Paging through the radio 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.

Saving Spectrum?
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.

Ian Marshall
Regulatory Manager
Aviat Networks

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Diverse Wireless Network Topologies Cost Savings

Posted on by Aviat Networks

To compare how different wireless backhaul network topologies perform under the same operating scenario, let’s analyze how a traditional hub-and-spoke and a ring configuration compare in connecting the same six sites (See table below). For the hub-and-spoke configuration, each cell site is provided 50 Mbps capacity in 1+1 protection. With five links and no path diversity, full protection is the only way to achieve five nines reliability. In this configuration, 10 antennas are employed, which average a large and costly 5.2 feet in diameter. Total cost of ownership for this six-site network is close to $700,000 for five years.

TCO Comparison by Topology

For a ring design for the same six sites, throughput of 200 Mbps is established to carry the traffic for each specific hop and any traffic coming in that direction from farther up the network. Designed to take advantage of higher-level redundancy schemes, the ring configuration only requires antennas that average 2.3 feet in diameter, which are much lower in cost compared to the antennas in the hub-and-spoke configuration. And even though the ring configuration requires 12 antennas and six links, its overall TCO amounts to a little under $500,000 over five years—30 percent less than TCO for the hub-and-spoke design for the same six sites.

This comparison is based upon deployments in the USA, where most operators lease tower space from other providers.

Gary Croke
Senior Product Marketing Manager
Aviat Networks

 

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Are Auctions an Appropriate Way to Allocate Microwave Spectrum?

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frequency auction imageRecently the U.S. Congress requested information from the FCC regarding the usage of the 11, 18 and 23GHz microwave point to point bands. This move is seen by many industry watchers as the first step in preparing these bands for auctioning.

Auctioning spectrum is seen by many in the political establishment as a good way of raising large sums of money.  The 3G auctions in Europe raised $30 billion in the U.K. and $45 billion in Germany and although these figures will probably never be reached again, the attraction for governments trying to balance the books in an economic downturn is clear to see. However, these figures were for cellular access spectrum and there is evidence of microwave spectrum auctions being priced too high for operators and no bids being received, e.g. the original 28GHz auction in the U.K during 2000-2002. But even if the bidding process itself is successful, is granting large amounts of spectrum to a single operator the right way to allocate microwave spectrum?

Let’s look a little deeper into how microwave spectrum is used and allocated in most cases today in licensed common carrier frequency bands. An operator wanting to install a microwave link between points A and B would seek to obtain an individual license for that link in that specific location and frequency. This allows others to apply for other frequencies or even the same frequency in different locations.  This approach maximizes use of the available spectrum.

Now let’s look at the block licensing approach. Here a block of spectrum (either on a national or regional basis) is allocated to one user. Block allocations on a regional basis make sense for multipoint applications like fixed wireless access or mobile network applications. However, in the case of point to point (PTP) allocation a block license holder may not have requirements for that entire spectrum, but because it is now their spectrum, no one else can gain access, often resulting in under utilization. This is the situation currently with the 38GHz band in the U.S. and is leading to some in the industry to push for the availability of additional spectrum.

Another example of this is the 28GHz LMDS band, where service take up has been very low, but has effectively blocked out this band from other uses/users. Another concern for the block licensing approach and one that affects equipment vendors is that with fewer operators there are fewer equipment contracts thus leading some manufacturers to be “frozen” out of the market. This will ultimately reduce choice for all and reduce innovation and competition.

Referring back to the announcement, it makes no mention of what would happen to the holders of existing link licenses who will have engineered their networks based upon the current rules. What would happen to these links should that band now be auctioned off as a block? Spectrum auctions also break the U.S. into many smaller regions, with each regional block license being auctioned to the highest bidder. This leads to the question of demarcation and coordination between adjacent regions, particularly for links that may need cross-regional boundaries.

All in all, it would appear that based on evidence to date, auctioning FCC Common Carrier microwave spectrum will be tremendously complicated and likely not in the long term interests of the industry.

Ian Marshall
Regulatory Manager
Aviat Networks

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