November 23, 2016

Case Study: Longest Microwave Link in a Mobile Phone Network

Recently, the longest microwave link (189 km) ever turned up in a public mobile phone network went live in Tonga. It’s part of operator Digicel’s Pacific network. Engineers at Aviat Networks thrive on challenges. We go farther for our customers—in products, service and distance. For example, Aviat Networks previously implemented a very long all-IP backhaul connection across 193 km of open water in Central America for a private network application.

For this network in Tonga, microwave remained the only practical answer to hooking up remote island populations. In addition to the 189km microwave link, this network includes a unique site built on an uninhabited volcanic island. End result: capacity increase to 200 Mbit/s (vs. 20 Mbps via satellite) on Aviat CTR microwave networking solutions and latency reduction to 5 milliseconds (vs. 500 milliseconds).

Customer Problem

The Kingdom of Tonga is an archipelago stretching more than 4000 km with population on three main island groups: Tongatapu, Ha’apai and Vava’u. 75,000 residents on Tongatapu have broadband service from the Southern Cross Cable (i.e., 32 pairs of 10 Gbps fiber). However, the other island groups remained at slower speeds restricted by the limitations of satellite connectivity: a few Mbps with high latency and significant operational costs.

Digicel Pacific approached Aviat Networks with this problem. They presented an opportunity to build a nationwide terrestrial microwave network that greatly enhances what Digicel offers Tongan customers.

Engineering Challenge

Microwave radio has long been, and will always be, a key technology for island nations. The cost of undersea fiber remains prohibitive. Satellite has traditionally provided long haul connectivity for locations where characteristics of microwave propagation cannot go the distance. For example, if distances remain too great or transmission sites lack elevation.

The challenge from a microwave transmission perspective remains linking these islands over 170 km and 130 km, respectively. From sites with very little elevation. For a viable solution, we needed a site with significant height for a new microwave repeater.

Fortunately for Digicel, Tonga has a perfect radio site—a 1000m high extinct volcano: Kao Island.

Engineering Culture

As a transmission system design engineer I do not often have opportunity to apply the “R” in “R&D.” My primary role: design microwave links within well-established limits governed by international recommendations of the ITU. This guidance allows path designs to be quick, accurate and with conservative, predictable outcomes. However, due to fixed factors like geography and economics, sometimes the engineering boundary needs to be tested.

Key challenge of this project remained the 189km link. Not only the distance but also lack of height to provide predictable propagation. (For transmission design nerds reading this: no line of sight when considering minimum k).

To test the design of the 189km link and ensure confidence of our team and customer, we investigated all available research. Three primary aspects existed in our approach:

1. Published material like Trevor Manning’s Microwave Radio Transmission Design Guide
2. Performance data gathered from similar paths in similar regions
3. Drawing knowledge from our experienced engineers

Written material, alongside path design tools like Pathloss, offers a well-defined approach to transmission engineering. And influencing factors which define a design.

Knowledge of the Region

However, intimate knowledge of local and regional atmospheric conditions—and the effect on microwave propagation—remained central to our design. Aviat has designed and installed numerous long links greater than 100 km in the South Pacific. And all operate beyond our design expectations both in terms of availability and capacity: 145 km in Vanuatu, 110 km in Fiji, and 140 km in Papua New Guinea. Analyzing the performance of these links using Aviat’s ProVision element management system enabled us to carefully characterize the atmospheric conditions. Both seasonally and during times of anomalous propagation. We needed to know refraction and multipath diffraction intimately.

For customers, working with a microwave specialist like Aviat Networks has the advantage of breadth of experience accumulated over more than 60 years of company history. Our history began with the first microwave links in the world with Farinon and Lenkurt and developed through Harris and Stratex Networks. Each phase of our history provided more experience with microwave propagation. A great place to develop as a transmission engineer when working alongside engineers like Dick Laine.

Building the Longest Microwave Link

Building a nationally significant telecommunications site atop a volcano on an uninhabited island also presents challenges. We needed to carefully evaluate an appropriate site to support the tower, provide helicopter access and convey solar energy to run the site autonomously.

Each piece of the complete solution had to be lifted onto site by helicopter from a nearby barge. Then assembled using hand tools and whatever we could lift or carry to site. The towers used a grillage foundation. So excavated material was used to cover the foundations. Heavier items like battery cabinets and generators added further mass to the tower foundations.

Also, cyclones remain a key factor in network availability in the Pacific. Our experience shows it imperative to engineer all elements of the build to the correct standards. And to minimize tower loading and heights. The cyclonic rated towers were designed as low as possible. And to provide critical antenna diversity to support such a long microwave link.

The ultra-reliable autonomous power system comprises dual redundant power systems of solar and batteries. Each backed up by a single generator.

Network Monitoring

The complexity of this network—both the locations and RF propagation—means you need to closely monitor it. Using Aviat ProVision EMS combined with the DC Power System manager and high resolution cameras, the entire network exists under constant evaluation for optimization and issue mitigation.

The enhanced visibility and remote video storage also enables Digicel to view anything that happened in the past. That way they know the requirements for any maintenance.

Aviat’s Solution: CTR 8300 and ODU 600

Small but powerful, CTR 8300 perfectly fit this project. Its small form factor enabled us to minimize space in the outdoor cabinets. And provide completely separate parallel networks for ultimate reliability. The high-power ODU 600 combined with CommScope cyclone-rated antennas provided ample transmit power while keeping actual antenna diameter as small as possible. This reduced wind loading.

The latest in modem technology, the CTR 8300 was put to the test combating the multipath reflection experienced over such a long path. Analysis over the last four months shows multipath and other characteristics exist as predicted. Critical antenna spacing and alignment provide a robust link providing more than 99.999 percent availability.

Get a Long Microwave Link of Your Own

If you would like to discuss a long-range microwave link of your own, please provide your contact info and a few details. Then we’ll have a microwave transmission expert contact you shortly.

You can also read more about this story in Aviat’s press release section.

-Aaron Prior
Sales Support Engineer
Aviat Networks