April 13, 2012
As we blogged last summer, the FCC has released 650 MHz of new wireless technology spectrum for Fixed Service wireless communication technology operators. Now Comsearch, a leading provider of spectrum management and wireless engineering services in the US, has highlighted this issue in its latest online newsletter, with an article that includes some very informative coverage maps showing the zones where the new bandwidth is available.
These maps are excellent at conveying the limitations of the newly released spectrum for microwave link applications in the 7 GHz (6.875–7.125) and 13 GHz (12.7–13.1) bands. After taking into account the zones that are reserved for existing Fixed and Mobile Broadcast Auxiliary Service (BAS) and the Cable TV Relay Service (CARS) users, these new bands are only available in about 50 percent of the US land mass covering only 10 percent of the population.
What do you think? Should the FCC loosen the spectrum sharing rules even more for 7GHz and 13GHz bands? Take our poll and tell us:
March 30, 2012
Currently, 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.
Senior Network Engineer
March 15, 2012
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.
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.
Senior Product Marketing Manager
October 27, 2011
This white paper was extremely popular when we featured it in our eNews newsletter recently. Now it’s time to share it with a wider audience.
It talks about how there are several considerations when establishing realistic outage or reliability objectives for and how the effects of long-term and short-term outages differ when it comes to microwave path engineering.
September 22, 2011
Our partner, Symmetricom, recently announced the launching of a new segment of their SyncWorld ecosystem for microwave backhaul. Our hat’s off to them; this is great news for Symmetricom and the new players that are now on board. We boarded this train awhile back. After a couple years of collaborative testing between us, we first joined the ecosystem when it was initially launched in March at CTIA 2011.
So, what have we learned since then you might ask?
Well for one, packet based timing is still growing in interest, evaluation, and deployment. Customers around the world — including mobile operators, state and utility providers and others – are increasingly looking for timing solutions that operate over their Ethernet fiber and microwave network as effectively as their TDM timing solutions do. A recent Heavy Reading analyst report projects close to 2 million cell sites will have deployed the two most dominant solutions, IEEE 1588v2 and Synchronous Ethernet (SyncE), by 2015.
Secondly, we’ve learned this is by no means the technology race it started out to be. Remember when Blu-ray and HD–DVD were competing a few years ago? Or perhaps that has well faded into memory. Well, I still recall the industry buzz a couple years ago about whether Synchronous Ethernet (SyncE) was going to kill IEEE 1588v2, or vice-versa. Who was going to come out on top?
Telecom watchers and players are always primed for a tech battle it seems. Well lo and behold; this battle has become more of an alliance, as of late.
The dominant discussion today is now about how BOTH these technologies can co-exist, and where best to deploy them in a network, either side by side or in parallel, with one backing up the other. Hmmm, now that’s an interesting conclusion to a tech battle.
Case in point, a couple of our customers are planning to deploy both technologies to take advantage of their respective strengths and are in the process of doing just this. See this whitepaper for more information about synchronization over microwave backhaul or maybe this one for insight into deploying IEEE1588v2 synchronization.
So, with the reality today that packet timing is still growing and that options for packet timing (including TDM, 1588v2, SyncE, and GPS) will continue to co-exist for a long time, it becomes even more critical to seek experience when it comes to planning your sync migration.
An ecosystem is probably a good place to start, especially with those players that have been at it for some time.
Sr. Manager, Solutions Marketing
August 23, 2011
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:
More information about assessing rain-induced attenuation is available in our white paper, Rain Fading in Microwave Networks.
July 20, 2011
There is no one-size-fits-all wireless network backhaul solution. What will work for some operators’ mobile backhaul will not work for others. Many operators have large installed bases of TDM infrastructure, and it is too cost-prohibitive to uninstall them wholesale and jump directly to a full IP mobile backhaul. There is going to be a transition period.
The transition period will need a different breed of wireless solutions. Fourth Generation Hybrid or Dual Ethernet/TDM microwave radio systems provide comprehensive transmission of both native TDM and native Ethernet/IP traffic for the smooth evolution of transmission networks. They will enable the introduction of next-generation IP-based services during this transition period.
We will explore this category of digital microwave technology for wireless backhaul, which is becoming ever more important as the 4G LTE wireless revolution gets underway with all due earnestness, even while the current 3G—and even 2G—networks continue to carry traffic for the foreseeable future.
Our current white paper builds on Aviat Networks‘ previous April 2010 white paper titled “What is Packet Microwave?” and provides market data from recent industry analyst reports that demonstrate the significant and continuing role of TDM in mobile backhaul networks and some of the prevailing concerns of operators in introducing Ethernet/IP backhaul services.
If you’d like to talk to someone about the ideal wireless network backhaul solution for you, please click here.
June 10, 2011
TDD, or Time Division Duplex, where a single radio channel is used to send and receive data, has been a common technique employed in unlicensed microwave transmission bands, such as 2.4 and 5.8GHz. The advantage of TDD is a simplified and lower cost design, often based upon 802.11 standards. In contrast, FDD, or Frequency Division Duplex, where data is transmitted in one frequency channel and received in another (separated by anywhere from less than 100 to more than 1,000 MHz) has been the staple of licensed frequency bands between 2 and 38 GHz worldwide.
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.
Regulatory Manager, Aviat Networks