May 4, 2011
Successfully Implementing a 193-km Microwave Link over Water to Deliver 99.9995 Percent Availability
For most designers of microwave transmission paths, engineering a reliable link over water can be a daunting task. Reflections off the water surface can play havoc with the received signal, leading to high levels of interference resulting in fading and ultimately a high level of errors and signal interruptions. For these types of paths, performance calculations using commercially available software planning tools will be insufficient to ensure superior path performance. In these cases, experience and understanding of the key parameters that influence microwave performance are critical.
Recently, Aviat Networks and our agent, Telecomunicaciones y Sistemas S.A. (TELSSA), deployed an Eclipse microwave link for Central American Corporation for Air Navigation Services (COCESNA) in Honduras that crosses over 193 km, most of which is over water. With careful design and installation, this link is now operating successfully.
COCESNA is responsible for the air traffic control over Central America territory and oceanic areas, therefore, availability of service is a critical issue.
Key Design Considerations
A key design criteria for designers to keep in mind is the prevalent propagation conditions to meet or exceed the expected Error Performance Objective (EPO), particularly for the design of very long terrestrial microwave systems traversing large and warm bodies of water, like the ones prevailing in Central America. To meet the specified EPO for any path, designers must have a thorough understanding of the following key aspects:
a) The effect of antenna diameter
b) Precise alignment
c) Accurate diversity spacing to counter reflections
d) Deterministic prediction of the variations of atmospheric conditions
e) Multi-path propagation delay
With these parameters in mind, achieving a microwave path that delivers fiber-optic level error performance over water is possible. The mathematical models to predict the link’s EPO don’t have a meaning if the design variables above are not fully grasped, respected and implemented.
Understanding Local Propagation Conditions
Understanding the behavior of the atmospheric conditions determines the type of design needed to accommodate how the link will perform over time in any region, regardless of the temporal, seasonal and climatic variations along the path. In Central America, with its warm days and nights, standard propagation is prevalent, because the troposphere in this region is “assumed” to be thermodynamically unstable and in a state of constant turbulent mixing—ducting (a key propagation impairment over long paths) has not been observed.
Under these conditions and based on Aviat Networks’ experience, long (exceeding 80 km), over-water, semi-horizontal and windy paths exhibit excellent performance, even with relatively low ground elevations above mean sea level at both sites.
For the 6GHz, QPSK, 40 Mbps, 193-km path in Honduras, which was more than double the length of a typical long microwave path, experience, propagation considerations and exact installation were crucial in the design. Even though the path inclination was relatively small, Aviat Networks’ prediction of the propagation conditions’ “soft” fluctuations along the path determined that the link was viable. To span such an enormous path, sufficient site elevation is a must to prevent diffraction or blockage. In this design, Site A, “Las Cruces,” is at 1600 meters above mean sea level and Site B, “Dixon Hill,” at 250 meters on Roatan Island. The reflections and delay were optimally neutralized with precise antenna spacing and up-tilt.
To make the design even more unusual and challenging, the short radar tower facility at Site B Dixon Hill lacked space to mount a second space diversity antenna. As a result, Aviat Networks/TELSSA installed the second antenna on a separate structure, located 15 meters in front of the main antenna. Consequently, the design had to take into account and negate any additional signal delay arising from the horizontal antenna offset.
The resulting link was designed to provide an overall availability of 99.9995 percent availability, equating to just 32 seconds of accumulated outage every year.
Once the design was done, the challenge was not over. Ensuring superior link performance took careful and exact installation, with precise alignment of the antennas by an experienced crew to achieve a stable received signal level. A few days after the initial alignment, the crew revisited the 1600-meter site to adjust the antenna installation to counter very strong wind shears that threatened to cause instantaneous twisting of both antennas that could degrade performance.
With the design and installation complete, further adjustments were made for local weather conditions. The microwave link was then monitored over several days to measure actual performance to verify that it met or exceeded the design requirements.
The measured unfaded link RBER (Residual Bit Error Rate) performance test showed a result of 1×10-13, or just one bit error for every trillion bits received. This level of error performance is equivalent to what you would expect if you connected two Eclipse terminals on a bench, directly connected by a cable! The link is also exceeding the 6-9’s availability target.
“This IP link was a challenge for us from the moment of the project’s inception, but we have always relied on link engineering support from Aviat Networks to complement installations carried out by our engineers and technicians—who have great experience in complex hops—which has led to excellent results,” says Jose Raul Gomez, CEO TELSSA.
At 193 km, this Eclipse link is probably the longest all-packet microwave link in the world and demonstrates that making challenging paths work well takes experience, superior product performance and quality, careful design and precise installation, with results that can—and did, in this case—exceed customer expectations.
Senior Network Engineer