Readers may remember that when the FCC opened the 3650 MHz band for wireless data services, there was concern that transmitters in this band would interfere with C-band satellite reception using the 3700 to 4200 MHz band. A recent report from the Satellite Users Interference Reduction Group showed C-band fixed wireless/WiMAX systems caused significant problems to C-band satellite reception sharing the same frequencies.
The Field Test Report: WiMAX Frequency Sharing with FSS Earth Stations describes the testing in detail. Phase 1 of the test used a fixed satellite service (FSS) antenna at a fixed location while a WiMAX base unit was moved to several locations operating at various angles and distances from the FSS antenna. This test simulated the way subscriber units may be deployed. Tests conducted up to 1 km away showed interference made the satellite signal unacceptable for use.
Phase 2 testing used a WiMAX base antenna installed approximately 50 meters above ground on top of a water tower. The FSS antenna was set up at several locations around the tower at various angles and distances from the water tower. “This was to model WiMAX base units being deployed on cellular towers,” the Field Test Report said. “The results of the testing showed that the WiMAX transmit signal could cause significant problems to a digital signal well in excess of 12 km away. At the extreme measurement distance, the video program was fully operational with the WiMAX carrier centred on the video carrier. However, the data BER was degraded from a nominal 10-8 to a BER of 10-4.”
Combining two analyses, one based on calculations assuming flat, non-blocking terrain and the other on measured data based on the actual local and surrounding terrain showed FSS antennas cannot co-exist with WiMAX systems over a range from 50 to over 200 km, depending on the local terrain and WiMAX output levels.
WiMAX services such as those offered by Sprint and Clearwire use frequencies in the 2.5 GHz band, which doesn’t create any problems for C-band satellite reception. However, wireless Internet service providers (WISPs) that want to use the 3650 MHz band in the United States will have to operate outside certain exclusion zones. These are listed on the FCC Web site, but it isn’t easy to search the site using ZIP Codes, GPS coordinates or city names to find the names of FSS operations that have to be protected.
Zing Wireless and WirelessGuys have set up a free mapping site at zing.naviciti.com to help solve this problem. For more information on the site, see “Wireless ISP Innovators Create a 3.65 GHz Virtual Map of Exclusion Zones to Aid Adoption in the U.S.”
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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