UMB could interfere with satellite TV
ByThe introduction of UWB (ultra wide band) devices, namely wireless USB hubs, have the potential to interfere with satellite TV services, concluded a new study conducted by the Department of Electronic and Information Engineering of the Hong Kong Polytechnic University for Hong Kong regulator, Office of the Telecommunications Authority.
The test included the deployment of UWB devices in different proximities of a C-band satellite TV dish in order to gauge the potential over-the-air interface between the two devices.
The study found that when a UWB device was transmitting inside the main lobe of the dish antenna at a distance of less than 5 metres, the television signal was seriously distorted and the picture and sound qualities were unacceptable. The study also found that when three UWB devices are deployed at the same time, interference was higher than a single device and the impact was noticeable.
“UWB devices did cause interference to the station when they were operated in close proximity and particularly within the main lobe of the antenna of the satellite receiving station. More UWB devices also caused more interference because of their aggregated effect,” the report said. “If UWB devices with output power spectral density of -41.3 dBm/MHz are to operate at the C-band, we should not allow such devices to go near to the C-band TV satellite receiver.”
The study added however, that when a UWB device is moved outside the main lobe of the satellite dish, and to the side, the interference was dramatically reduced but still noticeable in close distances.
“If they are as close as 2m side-by-side away from the dish antenna, the TV picture may be frozen. If they are placed 5m away, interference may not be noticeable but the fade margin will be degraded by approximately 2~4 dB,” the report said. Only when the devices are 10 metres away from the satellite dish, does the interference effect become negligiable. “So, preferable a ‘No UWB Device’ zone with radius at least 10m should be declared surrounding the dish antenna of satellite receiver.”
According to the report, current UWB devices are based on the Multi-band Orthogonal Frequency Division Multiplexing (MB-OFDM) UWM technology first proposed by the WiMedia Alliance and subsequently adopted by the ISO/IEC, the USB Implementers Forum, Bluetooth Special Interest Group and Wireless USB Promoter Group.
This standard defines the specifications for the transceivers disseminating data at up to 480Mbps within the UWB spectrum of 3.1 to 10.6 GHz. The problem is that the spectrum band between 3.4 to 4.2 GHz is mainly by fixed satellite receiving stations, including the Satellite Master Antenna Television (SMATV) and Television Receive Only (TVRO) systems, both of which are widely distributed in Hong Kong.
While the potential for interference is confirmed, the report concludes that “if appropriate interference mitigation techniques are employed and operated outside a pre-defined ‘No UWB Device’ zone or limiting the spectral density of the emissions in the C-band down to -70 dBm/MHz as suggested by the spectrum regulators of Hong Kong [10] and many other countries, the interference impacts caused by the UWB device on the C-band satellite receiving stations should not be noticeable in normal circumstances.”
“In real world deployments, such UWB devices are usually used indoor and are very unlikely operated in front of and closed to the antenna of a satellite receiving station.”
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