The uphill battle for inexpensive cognitive devices

Ofcom recently published conclusions from the Digital Dividend Review, based on responses from the public consultation phase. In this, they state that “beacons were the least appropriate and did not merit further investigation”, “detection alone was very difficult to implement and unlikely to be used for the foreseeable future”, and that “the most important mechanism in the short to medium term will be geolocation”. However, they have also concluded that the transmit power and receive threshold levels should be lower than originally envisaged.

 

Comparing the U.S. and U.K. decisions

Back in November 2008, the FCC adopted rules to allow unlicenced transmitters to operate in UHF-band whitepaces. The key points here included the provision of a 4W limit for fixed-location transmitters, 100mW for personal portable devices (40mW on adjacent channels), and a -114dBm threshold for sensing DTT and wireless microphones. However, Ofcom has concluded that their sensing threshold should be -120dBm for a 8MHz DTT channel (in the US, the DTT channel width is 6MHz) , -126dBm for a 200kHz wireless microphone channel, and that the transmit power be limited to a maximum of 50mW.

 

What this means:

Lets put this in perspective. A really good wireless microphone receiver specifies a sensitivity of -105dBm for 12dB signal-to-noise-and-distortion (SINAD) and in a venue, you never want to be operating at that extreme to avoid the likes of Brian Johnson sounding even raspier than expected. What Ofcom’s conversative sensing thresholds and low transmit power rulings will do is significantly ramp up the difficulty and expense associated with developing and deploying new cognitive device wireless networks. That extra 12dB over the FCC’s -114dBm threshold costs a lot; high performance receiver parts, possible restrictions on frequency-agility due to the need for high-spec bandpass filters, and clever signal-chain software. The 50mW transmitter power restriction won’t support significantly long range services even in the UHF band either. The combination of these restrictions will really reduce potential for launching new services. Guess we’ll just have to be even cleverer about how we do it.