1  Introduction

This paper looks at the feasibility of using 3400 - 4200 MHz (C-band) for terrestrial services such as WiMAX or IMT based on the need to adequately protect existing satellite operations in the same band.

Inmarsat’s network of eleven geostationary satellites operate feeder and telemetry links in C-band. Many other satellite systems use C-band for a wide range of vital communications. Before deploying terrestrial systems in C-band, it is essential to establish whether sharing is feasible with these satellite services. 

2  Required exclusion zones

Sharing between terrestrial systems and satellite earth stations has been studied by the ITU-R and a number of other organisations, such as the CEPT and the WiMAX Forum. The results of these studies are consistent and show that significant exclusion zones are required around each earth station to protect against interference from terrestrial stations.  The typical required separation distances are:

• 10s of km for long term co-channel interference (up to >100 km)

• 100s of km for short term co-channel interference (up to ~400 km)

• Several km for adjacent band operations due to unwanted emissions (long term interference)

• Several km for adjacent band overloading of FSS receivers

Due to the size of the required exclusion zones, the studies all conclude that where there is ubiquitous deployment of FSS, or FSS earth stations are in unknown locations, sharing is not feasible.

These conclusions are confirmed by several cases of actual severe interference from BWA deployments into FSS earth stations and by practical tests, such as those conducted by SUIRG (Field Test Report - February 2008).

In principle, it could be feasible to deploy terrestrial stations closer to FSS earth stations by the use of mitigation techniques.  A number of mitigation techniques have been suggested, such as down-tilting base station antennas, restricted terrestrial service coverage, use of adaptive antennas on base stations, site shielding and the use of front-end filtering. However, all of these techniques have limitations and so far, there is no evidence that any mitigation techniques could significantly improve the feasibility of sharing.

3  Earth station operations in CITEL

In many CITEL countries, C-band earth stations are already deployed with distances between neighbouring stations that are less than the typical required exclusion zones. Hence, any given location will likely be too close to at least one earth station to allow the operation of a terrestrial station.  The available areas for deployment of terrestrial systems are therefore very limited.  The density of earth station deployment tends to be greatest in cities, making the deployment of terrestrial systems particularly difficult in those areas.

In many countries, C-band is used by receive only earth stations which do not need to be licensed and thus their locations may be unknown, making coordination impossible.  Without protection through coordination, these earth stations would be particularly vulnerable to interference.

4  Continued need for satellite access to C-band

It is vital for satellite operators that spectrum in this band remains available.  It would not be feasible to transition operations to other parts of C-band, or other FSS bands for several reasons:

(1)   Satellites have fixed and limited tuning range and long life times.

(2)   There is insufficient free capacity in other bands to accommodate satellites currently operating in C-band. Hence, satellites forced out of any part of C-band may not be able to coordinate access to other bands.

(3)   Ku-band and Ka-band are much more vulnerable to propagation impairments, leading to poorer availability.  Also, it is technically challenging to operate global beams antennas, which are necessary for Inmarsat and other operators.

(4)   Inmarsat, other satellite operators and earth station operators have invested billions of US dollars in its networks, which would require replacement to operate in other bands, at a cost of similar magnitude.

(5)   As the satellite business continues to grow, so does the demand for C-band spectrum.

Extended C-band was made available for satellite use largely with the purpose of allowing expansion from congested core C-band.  The use of extended C-band has been successful in relieving some congestion in C-band, and will be even more necessary in the future as requirements for C-band satellite spectrum continue to expand.

5  Conclusions

If C-band were to be used by terrestrial systems, large and important geographic areas would have to be excluded to ensure adequate protection of earth stations. This would make the use of this band for terrestrial systems impracticable.

If terrestrial systems were nevertheless to be deployed in the small areas where protection of existing earth stations can be assured, it would severely constrain the ability to satisfy the demand for new earth stations and important satellite services.

Jonas Eneberg
Senior Manager
Spectrum Regulatory Affairs
Inmarsat