The Effects of Fixed Wireless Access Using C Band Spectrum

A shortage of traditional terrestrial infrastructure has led to Fixed Wireless Access (FWA) becoming an important communications technology in a number of countries. FWA services, including WiMax, must not harm existing communications infrastructure, and spectrum allocated to FWA should be coordinated and allocated appropriately. The 3.4 - 4.2 GHz (C band) spectrum range is allocated to Fixed Satellite Service (FSS). Any allocation of this spectrum to FWA services results in significant interference to satellite services, and will severely impact the communications infrastructure.

Current uses of C band

The C band is often used to provide fixed communications and broadcast services. The band also provides a number of important strategic services including maritime communications, aeronautical services along with robust and reliable Virtual Private Networks (VPNs) for government and private corporations. It also provides vital connectivity services such as backhaul and rural telephony to remote areas. C band is the focus of next-generation satellite services in many rural areas. Rugged IP networks for banks and other businesses are being developed using C band spectrum.

Many regions rely heavily on the use of C band satellite capacity. Other frequencies such as Ku band (11/14 GHz) are readily available in other parts of the world and some regions rely heavily on it for backhaul services. However, the lack of Ku-band satellite coverage and terrestrial fibre in some regions means that there is no other option than the C band for offering many essential services. Users of receive-only satellite antennas have no need for licences and are not registered with the regulator. This means that the total usage of C band satellite services is impossible to measure.

What If My Administration Has Licensed Wimax in C band?

While protection of satellite services demands that FWA not be licensed in any part of the C band, some countries have done so. In these cases, there is no need to support an IMT identification for those parts of the band. Instead, the FWA licence can be retained under the international mobile allocation for the band – the only regulatory instrument required.

Dangers of FWA interfering with C band satellite

Satellite services operating at C band will be damaged when forced to co-exist with FWA services. Problems will occur, including signal delays, synchronization loss, black-out periods and total loss of transmission. In order to co-habit in the same frequency bands, the two technologies require an exclusion zone of 100-150km, making co-habitation difficult in most cases. Importantly, there are also military users of satellite C band services whose locations are often not made available. In many countries, this makes co-habitation impossible.

Moving to other satellite frequencies

Moving customers to other satellite frequencies is not a viable option. Both Ku-band and Ka-band systems suffer from rain attenuation problems not suffered by the more robust C band systems. Areas which experience heavy rainfall have no option other than C band. Ku-band satellite capacity over many regions is in short supply and expensive. Ka-band capacity is also very limited. The scarcity of alternative satellite capacity at Ka band and Ku band also leads to much higher costs for both.

Portability Worsens Problems

The concept of FWA as a substitute for a mobile service ignores some fundamental technical differences. FWA can provide connectivity where traditional (wired) infrastructure does not exist, and can do so with minimal spectrum. Mobile networks offer premium connectivity with the benefits of mobility. By their nature, mobile systems rarely share spectrum with any other services due to the extreme difficulty in preventing interference between them. Fixed networks can co-exist to a limited extent with other systems, provided that careful planning is used to predict and limit harmful interference. However, widespread deployment of FWA should be permitted in bands that are not shared with other services, as this removes the constraint for pre-planning and allows a more spectrally-efficient network.

Solution: working together with FWA

Satellite and FWA networks can co-exist to offer a variety of important communications services, provided that their discreet frequencies are properly co-ordinated. Satellites provide important backhaul services for FWA systems in many cases and must be allowed to continue to do so. This can only be achieved through the proper coordination of spectrum and by not allowing FWA to inhabit bands already allocated to FSS.

Thus, while C band satellite services must use the 3.4 - 4.2 GHz band, a number of lower frequencies can be used to provide FWA services. The use of lower frequencies allows for less densely-constructed networks. FWA networks have been successfully implemented at 800/900 MHz, at 1800 MHz and at 2500 MHz in parts of the world, and these frequencies offer much better propagation characteristics.