Divided over spectrum

05 Sep 2007

As if it hasn't made enough noise in the telecoms industry, WiMAX is now stirring new controversy. This time the debate is centered on the industry's most valuable asset: spectrum.

As regulators around the world start making plans for issuing licenses for broadband wireless access (BWA), WiMAX proponents have been lobbying market by market to reassign 56% of the frequencies below 5-GHz for BWA licenses, particularly in the 3.4- to 3.7-GHz extended C-band range used by satellite operators, to give them extra capacity.

Such a move, however, faces strong opposition from the satellite and broadcasting industries, which claim that WiMAX and satellite TV just can't get along with each other in the C-band due to substantial interference issues.

Peter Jackson, CEO of Asia Satellite, says satellite systems operating in C-band (3.4-4.2 GHz) suffer severe interference in places where BWA systems like Wi-Fi and WiMAX share the same spectrum. He adds that tests carried out by the satellite and broadcasting industries show that satellite operations in places such as Australia, Bolivia, Fiji, Hong Kong, Pakistan and Indonesia already have been negatively affected. In certain cases, he says, satellite services have been wiped out.

'What happened in some countries is that regulators there just issued the licenses without doing any tests,' he explains. 'After the WiMAX vendor put up all the equipment, they found out about that problem of loss of TV signals.'

The same issue will surface if 3G and planned 4G mobile systems are allowed to use the frequencies used in the C-band for satellite downlink services. This is under consideration by some regulators.

Sharad Sadhu, head of transmission technology and spectrum at the Asia-Pacific Broadcasting Union, says the interference occurs because the received BWA/WiMAX signal is approximately 10,000 times stronger than the satellite signal at the receiver station.

'In the presence of a strong BWA signal, you can't receive any satellite signals,' he says. He added that BWA/WiMAX equipment operating in the extended C-band can affect satellite reception in the non-overlapping part of the 3.4- to 3.7-GHz or entire C-band (3.4- to 4.2-GHz), and through saturation of the low-noise amplifiers or low-noise block converters of the satellite receiver antennas.

Mitigation solution

BWA and WiMAX vendors, on the other hand, argue that WiMAX and satellite operators can co-exist in the C-band by employing mitigation techniques such as establishing 'exclusion zones' and minimum distances between base stations and satellite earth stations.

'Studies have shown that as long as there is enough physical separation between satellite ground stations and WiMAX equipment, co-existence without any appreciable interference is possible,' says Dr Ray Owen, head of technology, home and networks mobility for Southeast Asia at Motorola.

WiMAX and satellite operators can coordinate geographical usage and coverage in the 3.4- to 3.7-GHz band to enable co-existent services. This involves understanding where a country's C-band satellite services exist; where it is possible to deploy WiMAX and not interfere with the C-band services; and then determining the areas where WiMAX would make economic sense to deploy, he says.

However, Jackson at Asia Satellite says these solutions, proposed by the ITU and some regulators, have proven impractical in real-world tests, especially in Asia, because of the sheer volume of installed C-band receiving dishes (both registered and non-registered) and the omni-directional characteristic of WiMAX transmitters.

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