Terrestrial Microwave vs Geostationary Satellite

Any radio frequency above 1 GHz (1 x 10⁹ Hz), is considered a “Microwave” frequency. These frequencies have been favourably used as transmission mediums for both terrestrial telecommunication- and satellite telecommunication links. In fact, a geostationary (GEO) satellite can be thought of as a sort of “Microwave repeater” in the sky.

The “microwave frequencies” used by GEO satellites can be classed as follows for commercial applications:

• C – Band (4/6 GHz)
• K_u – Band (10/14 GHz)
• K_A  - Band (18/31 GHz)

The best frequency to use as far as reliability/availability is concerned, is the C-Band as this band is the least affected by atmospheric conditions. Both the K_u and K_a bands suffer from system outages due to thunderstorms both at the local terminals and the remote hub sites (think about the outages experienced by satellite TV subscription services like DSTV).  Due to the large distance between the GEO satellite (36 000 Km) and the hub site and receiver sites, it becomes impractical to ensure enough system gain by using bigger antennas. The only justification for satellite links is the provisioning of a service that needs to be deployed quickly, anywhere and be of a temporary nature.

Something to consider when looking a satellite communications is the round trip delay of a GEO satellite system. A signal takes 250 milliseconds to traverse from a sending station via the satellite to the hub and another 250 milliseconds to get an acknowledgement back via the same route. This causes some difficulties with applications such as voice (delay) and data. Special protocol spoofing mechanisms and acceleration schemes are used to alleviate these issues but these mechanisms just add to the cost and complexity of the system and place protocol overheads on data to be sent via the link.

A typical Very Small Aperture Terminal (VSAT) system shares the available GEO satellite’s bandwidth amongst many VSAT users, limiting the useful bandwidth per site.  The monthly costs associated with a VSAT service are inhibitingly high and upgrades to more bandwidth can be costly and sometimes not even possible.

All the technical issues mentioned above are negated by terrestrial Microwave links. Insofar as cost is concerned, a simple calculation as to the price per Megabyte of bandwidth required will show terrestrial Microwave to be superior in most instances. Bandwidth upgrades are easily done with minimal financial or technical impact and “triple play” traffic is handled much better.

Matie Strydom (Chief Technical Officer)

Wireless in Africa – Survival of the fittest

There is a lot of activity going on across Africa to build fibre infrastructure to link to the undersea cables on the east and west coasts. The rule of thumb is that wireless infrastructure is initially deployed as the primary data connectivity medium until fibre is laid in an area, which can sometimes take years.

In extreme cases, wireless will continue to be the primary connectivity medium, and in other cases once fibre is laid it becomes the secondary, failover solution. But whichever the case, the wireless infrastructure continues to play a significant role in Africa.

Wireless connections to undersea cables

Undersea cables provide such huge capacity that ISPs across Africa are being forced to upgrade their wireless infrastructure to carry this through to their customers. This means increasing their spectrum requirements, and many cannot afford to do this. As a result a lot of smaller ISPs are being acquired by the larger players that have the spectrum.

Funding is another obstacle ISPs and telcos are facing in African markets, because the local banks do not have the resources to provide them with finance. So the only way to fund projects is to partner with an international investment company or make arrangements with suppliers to defer payment for the equipment they need to purchase.

Deep understanding of the local environment

Building any form of telecommunication infrastructure in Africa requires flexibility and a deep understanding of the local environment. For example, in Angola equipment has to be flown in because bridges have not been rebuilt since they were destroyed during the civil war. It is impossible to get from one end of Angola to another except by air, which can sometimes involve crossing borders in and out of the country several times.

The ability to build relationships with local partners and in some cases local government to get insight into the areas is also crucial. In some countries spectrum regulations do not comply with the guidelines set down by the International Telecommunications Union (ITU). This means implementing wireless solutions that fit in with the spectrum allocations of specific countries.

It is also no good assuming that available spectrum is unlicensed because it says so in the ITU guidelines and finding out after the solution is implemented that it requires a license in that particular country.

Flexibility the name of the game

It sometimes happens that the spectrum allocated for a specific use is not ideal for the requirement, but there is no other choice. In addition, the policing of spectrum frequencies to reduce interference is not as efficient as it is in South Africa. Where this is the case, unlicensed spectrum is sometimes a better option because the equipment is designed to cope with a certain amount of interference whereas equipment for licensed frequencies is more sensitive.

Almost all connectivity infrastructure deployments in other parts of Africa require tailored power supply solutions. This might involve solar panels, generators, or wind turbines or a combination of these, depending on the solution, the location, the available space on the base station or high site being used, and weather conditions. Solar panels, for example, are not ideal in some tropical regions where there is a lot of rainfall. 

Physical access is generally a challenge across the rest of the continent, especially when transporting the equipment needed to implement any telecommunication solution. In Sierra Leone, for example, equipment has to be flown in to Lungi Airport and then taken across a small stretch of water to get to the capital, Freetown, which is only a few kilometers away as the crow flies.

Customs clearance is another issue, because tariffs vary from country to country, and different officials interpret the tariff codes on equipment documentation differently. This could have a significant impact on the duties that are payable and in some cases has been known to increase the cost of projects by as much as 50%.

 

Moral of the story? When doing business in Africa you need to know your environment because it truly is survival of the fittest, and if you don’t know what makes you fit, your revenue will be at risk. 

RF connectivity over water - PetroSA

PetroSA is a subsidiary of the Central Energy Fund which is a wholly state-owned entity and is responsible for exploration and production of natural oil and gas.  They needed a ship-to-shore wireless solution to link a refinery to an oil rig 80 kilometres offshore to support voice and data services, and the only way to get to the rig was by helicopter.

The uniqueness of the solution became apparent as the initial site survey unveiled the peculiar conditions in which the telecommunications link needed to be established. The link comprised of a point-to-point solution providing 20 Mbps of aggregate capacity.  Advanced RF equipment had to be procured that could operate in the niche 1.4GHz frequency range  to address the issue of reflection off the water, which would otherwise have distorted the signal and caused the connection to fail (this is known as the ducting phenomenon).

The challenges with a project like this include:

• The vast amount of water and the curvature of the earth made alignment     difficult and special equipment had to be used to overcome the ducting effect;
• Movement of the buoy on rough seas proved challenging when it came to installing, aligning and maintaining the equipment;

• Special permission had to be obtained and a unique design by structural engineers was required to mount the 3 metre antennas on to the platform;

• The execution area was considered highly hazardous because of the fact that production deals mainly with oil and gas which is highly flammable, which meant that the equipment had to be deemed safe for long term use as well.

The benefit of the new system, however, was that the process control systems used by PetroSA was based on old analogue technology which could not be replaced at the time.  These systems were supported through various telecommunications systems which have since then, become obsolete.  The integrated and modernised telecommunications put in place by Comsol however, could accommodate these legacy process control  systems.