Motorola recently presented a new proposal for unlicensed TV white space devices that offers far more protection to DTV reception than the Microsoft and Philips devices recently tested by the FCC.
Broadcast organizations are united in their opposition to unlicensed devices on TV channels. The FCC tests of devices relying on sensing to determine whether a channel was in use showed they did not protect DTV receivers from interference. However, broadcasters have not opposed the use of TV “white space” for licensed devices that could be identified and shut down if they caused interference. Does the Motorola proposal offer sufficient protection for over-the-air reception?
The use of geolocation at the base station to determine what channels are clear avoids the problem with sensing receivers being hidden from signals available at other nearby locations. Client units look for a beacon and control signal from the Motorola TV white space (TVWS) access point and periodically download an updated channel list from it.
Motorola recommends that geolocation be required for all TVWS devices operating with output power above 10 milliwatts (mW). No devices, at any power, would be allowed on TV channel 14-20 without geolocation.
As previously reported, wireless microphone users have joined broadcasters in fighting against unlicensed TVWS devices. Motorola’s recommendation would provide for third party administration of a database of Part 74 ad-hoc operations and wireless mikes at major events and protect those frequencies for the duration of the event. The database would also allow for protection of cable headends receiving distant TV signals. All devices would be required to detect a beacon signal that could be used to protect users of wireless microphones and other incumbents from interference even if the channel was not listed as in use in the database, as would be the case for ENG wireless microphones.
Under Motorola’s proposal, all devices would have to include sensing, but it would not be the sole method for protecting spectrum users. The sensing would also help prevent TVWS devices from interfering with each other.
Will broadcasters accept unlicensed TVWS devices if they comply with Motorola’s recommendations? The risk of interference is certainly less than with sensing only devices. The Motorola TVWS presentation does not address device security. How difficult would it be for someone to disable the geolocation/protection features? Broadcasters and other users of the TV spectrum would want to know who would administer the database, a critical part of the system. How would it be updated—over the Internet, over-the-air, or both? How often would it be updated? Would it be handled locally or nationally? Would TVWS devices shutdown automatically if an updated database was not received after a certain amount of time? If devices under 10 mW are excluded from geolocation requirements, will proponents come back in a year and try to get the power limit raised, until interference does result? Will manufacturers comply with the power limits? We’ve seen that hasn’t been the case with the FM radio transmitters sold for use with portable media players.
Properly implemented, the database solution could offer real-time protection that wouldn’t be possible even under a licensed TVWS scenario. For example, if protection criteria are changed (see the next story on OMVC’s concerns about interference to mobile and handheld DTV reception), the database could be updated to reflect this. It could also be updated if it turns out the proposed protection levels are not sufficient. These protections will only work if devices shut down if they fail to receive updated databases or if the database received is corrupt. The system could even be adopted to allow a hybrid licensed/unlicensed mix of TVWS systems in rural areas. Backhaul links could be given higher priority and protection in the database.
The other question, of course, is whether TVWS proponents will accept Motorola’s solution. Sensing will still be required and as we’ve seen from the FCC testing, it isn’t easy to accomplish. A nationwide system will have to be set up to administer the database. Fortunately the Internet provides an easy way to distribute it, but the devices will have to be connected to the Internet (directly or indirectly through a host PC) to receive it. Over-the-air database distribution would require additional spectrum or bandwidth, perhaps on broadcast DTV signals. Devices will cost more if they have to include both geolocation and database control, although Motorola says the extra device memory required is “nominal and inexpensive.”
A final comment—with technology like this available that offers far better protection than sensing alone, why even consider risking the TV spectrum with failed TVWS systems that use sensing alone?
