Tests confirm WiMAX interference threat to C-band signals, says SUIRG study

WiMax communications pose a significant interference threat to satellite signals transmitted in the C-band frequency, according to a new study from the Satellite Users Interference Reduction Group (SUIRG).

SUIRG testing conducted in the last quarter of 2007 conclusively found the incompatibility of C-band spectrum sharing between fixed frequency service (FSS) satellite transmissions and WiMAX services, the group said.

The primary objective of the test was to measure interference levels generated by fixed WiMAX transmissions into an FSS satellite receiving station. The method employed taking measurements of carrier/noise (C/N), interference/noise (I/N), bit error rate (BER) and spectrum plots of a satellite downlink video channel. Testing was performed in two 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 0.62mi away) showed that the digital signal was rendered unacceptable for use.

Phase 2: The WiMAX base antenna was fixed at a height of about 164ft 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 than during Phase 1 testing (up to 7.45mi). This was to model WiMAX base units being deployed on cellular towers.

The results of the testing showed that the WiMAX transmit signal could cause significant problems to a satellite digital signal well in excess of 7.45mi 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 172mi to reduce the level of interference to meet the -10dB specification. Combining the two analyses, from a flat nonblocking terrain to a wooded hilly terrain, results show that the criteria whereby FSS antennas cannot co-exist with WiMAX systems ranges from 31mi to more than 124.25mi depending on the local terrain and the WiMAX output levels.

For more information, visit: www.suirg.org.