WiMAX Interfering with Satellite Communications
The Satellite Users Interference Reduction Group (SUIRG) has published the results of tests it has carried out which is warning that WiMAX communications pose a significant interference threat to satellite signals transmitted in the satellite C-band frequency.
The C band is highly associated with TVRO satellite reception systems, commonly called "big dish" systems since small receiving antennas are not optimal for C-band systems. Typical antenna sizes on C-band capable systems ranges from 7.5 to 12 feet (2.5 to 3.5 meters) on consumer satellite dishes, although larger ones also can be used. A typical C band satellite uses 3.7–4.2 GHz for downlink, and 5.925–6.425 GHz for uplink. C band is primarily used for open satellite communications, whether for full-time satellite TV networks or raw satellite feeds, although subscription programming also exists.
The primary objective of the research was to measure interference levels generated by fixed WiMAX transmissions into an FSS satellite receiving station. The method employed taking measurements of C/N (carrier/noise), I/N (interference/noise), BER (bit error rate), and spectrum plots of a satellite down link video channel.
Testing was performed in 2 phases:
- Phase 1: The FSS antenna remained in a fixed location while a WiMAX base unit was moved to several locations operating at various angles and distances from the FSS antenna to simulate subscriber waveforms. This test modeled WiMAX subscribers in a nomadic deployment affecting FSS. Tests conducted within the immediate area (up to 1 km away) showed that the digital signal was rendered unacceptable for use.
- Phase 2: The WiMAX base antenna was fixed at a height of approximately 50 meters on top of a water tower. The FSS antenna was positioned at several different locations and at various angles and significantly greater distances from the WiMAX antenna (up to 12 km) than during Phase 1 testing. This was to model WiMAX base units being deployed on cellular towers.
The trade group says that the results of the testing showed that the WiMAX transmit signal could cause significant problems to a satellite digital signal well in excess of 12 km distance. At the extreme measurement distance, the video program was fully operational with the WiMAX carrier centered on the video carrier. However, the data BER was degraded from a nominal 10-8 to a BER of 10-4. This is an unacceptable quality of service in the digital telecommunications industry.
Subsequent calculations based on the initial measured data, and scaling with ITU criteria for WiMAX output power along with additional path loss, resulted in a required separation distance of 278 km to reduce the level of interference to meet the -10 dB specification. Combining the two analyses, from a flat non-blocking terrain to a wooded hilly terrain, results show that the criteria whereby FSS antennas cannot co-exist with WiMAX systems ranges from 50 to over 200 km dependent upon the local terrain and the WiMAX output levels.
"The C-band is in many ways the lifeline of the satellite industry and protecting that spectrum from the threat of interference posed by sharing it with Broadband Wireless Access services is of paramount importance," said Robert Ames, SUIRG President. "The tremendous support of the SUIRG member companies and the industry as a whole in making this test a viable platform to aid decision makers at WRC-07 was extremely gratifying. The results of the test are a firm testament to the need for clearly defined spectrum allocation."
Posted to the site on 6th March 2008
