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)