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Is the 5 GHz Unlicensed Band Really Interference Free?

Mon, 09/24/2001 - 7:28am
David A. Case NCE

One of the biggest concerns for the operators using unlicensed systems is that they must operate without any real protection as second or third tier services in their assigned bands. The various licensed services have rights when it comes to band usage verse not having any rights for those operating unlicensed systems.

For example there have been recent cases where 2.4 GHz WLAN systems have interfered with a licensed service such as amateur radio. Under the FCC or Canadian rules, the unlicensed equipment must address the problem. The solution could be simple such as selecting another channel or require a more complex solution such as re aligning antennas, or purchasing special pass filters for the radio operator or worse is forced off the air by the FCC.

The unlicensed devices operating in the 2.4 GHz band operate as a third party service and some institutes or organizations have been reluctant to deploy 2.4 GHz for fear that their systems may be forced to be shutdown because of complaints by amateur radio operators or such. In fact I have read that some university ad-hoc group is looking to the 5 GHz band in belief that the interference issues will not exists for such systems as they do in the 2.4 GHz band.

Unfortunately for those hoping that the deployment in the 5 GHz band will solve the interference problems may be in for a surprise. In fact I recall a press release from one group stating that since the amateur radio operators are not using the 5 GHz frequency band there will be no interference issues to address, unfortunately they are quite wrong. In fact the current licensed users of the 5 GHz band are not looking closely as these devices unlicensed to be placed into service in their bands.

Unlicensed Bands

Let's take a look at the various bands the unlicensed systems have been allocated under the US and Canada Regulatory agencies.

For use by Unlicensed National Information Infrastructure devices.

5150 - 5250 MHz Indoor use only 250mW EIRP
5250 - 5350 MHz Indoor/Outdoor 1W EIRP
5725- 5825 MHz Indoor Outdoor 4 W EIRP*

For Spread Spectrum devices

5725 - 5850 MHz Indoor Outdoor 4 W EIRP

For low power devices

5725 - 5850 MHz 50,000 uV

The operational restrictions on these devices are as follows: The device must not cause harmful interference and it must accept harmful interference, including interference that could severely degrade performance.

These devices operating in the 5 GHz band have no more protection then the 2.4 GHz or 900 MHz devices. Though for now with less unlicensed devices operating in the 5 GHz bands and there will be less background noise from competing devices (for now), as the deployments increase so will the chance of interference from other Part 15 or RSS-210 devices.

Despite some of the rumors and comments I seen on various wireless chat groups, neither the FCC nor Industry Canada is going to set up an arbitration board to deal with interference to Part 15 devices from other Part 15 devices in the 5 GHz band.

Licensed User.

To understand where the possible interference complaints could come from, we need to look at the primary users of the band. The bands are divided into two categories, one regulated by National Telecommunications and Information Agency (NTIA), which address the US Government frequency use, and the other for the non-government allocated spectrum, which is regulated by the Federal Communications Commission (FCC).

The 5000 to 5250 MHz band is used on a primary basis by Aeronautical Radio Navigation and other government services (NTIA). Other licensed services in that band include operation of FCC Part 87 Aeronautical Radio Location Devices and FCC Part 25 Satellite Communications Devices including Digital Broadcast Satellite services.

A look at the 5250 to 5350 band will find that the primary service is radiolocation (NTIA) and (FCC). Also the 5250 - 5470 MHz frequency band from an international perspective is used by the various Earth Exploration Satellite Services.

On the bottom band edge of the 5725 - 5850 ISM band NTIA has Radio Location Services and the FCC has amateur radio services operating. On the upper band edge of 5850 bands, NTIA has Radio Location Services and the FCC has Intelligent Transportation Services and amateur radio services.

With both these services right next to the band edge the FCC has limited the out of band emissions for the unlicensed services to at both the lower and upper band edges to maximum field strength of 500 uV/m.

In the actual 5725 - 5850 MHz ISM bands the following license services currently operate.

On the government side Radio Location (G2) and other government services. On the non-government side the following services operate in this band as well, the include Amateur Radio, Amateur Satellite (Space to Earth), Land Mobile and ISM Devices (non licensed).

The Problem.

The problem basically boils down to the licensed services sharing bandwidth with unlicensed services. There are several causes for concern on both the licensed services and for the unlicensed services.

A good example is how the various radiolocation and government radar system operators look at the problem. One concern is how the 5 GHz WLAN systems will listen before they talk to see if the band is in use. On the WLAN side the primary concern was to address other WLAN and not systems such as warning or target tracking radar.

The catch is since the military specifications of the radar are classified, how is the information needed going to be transferred to the commercial side to avoid interfering with these systems. Yet the unlicensed system cannot interfere with these systems and since some of the radar systems are mobile it makes the situation more difficult

Unlike the problems we have seen (however slight in the 2.4 Ghz band), the fact that we are sharing the band with licensed government and private services could make spectrum management become a key part of the installation and operation of these systems.

The problems are not only WE based only. The European Space Agency is far from a fan of Hyperlan and is lobbying for restrictions on it's operation and power not only in Europe, but on a world wide basis in the International Telecommunication Union - Radio and I suspect it will lobby at the World Radio Conference 2003 as well.

With the amateur operators using part of the bands, we still risk the possibility of our high power outdoor systems interfering with their services, specially the downlinks from amateur satellites which use transmit at extremely low power.

We also have to contend with the makers of ISM devices in the upper band who have no restriction on the power their devices can operate it, which could cause severe interference to the unlicensed systems.

Solving the Problem

The problem is not without some solutions. The first is to understand who is operating on these bands and how the unlicensed system be deployed to reduce or eliminate interference.

Remember according to the FCC, the definition of harmful interference is the amount of signal degradation a license service will accept before it determines the signal is non usable by them. That means if an amateur operator has a little RF snow on his video monitor and does not like it, he can complain to the FCC.

The second way to address the issue will be through the various working groups such as 802.11 or the IEEE EMC Society who can address some of the interference issues.

On the international arena, the US Delegation to ITU-R Joint Task force Group 4-7-8-9 is working to tone down the or remove the restrictions that ESA wants to place on these type of unlicensed devices.

Several of the workgroups under this task force have commissioned studies on the potential interference of WLAN and U-NII devices to radar and radiolocation equipment. Currently some of the studies are encouraging that the levels of potential interference to some systems are not a great as first thought, but more research is still needed.

The best way however to deal with the possible problems in the 5 Ghz band is to develop a plan of action to address the various interference scenarios before they escalate to the FCC or similar agencies.

David A. Case is a member of Wireless Design & Development's Editorial Advisory Board.


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