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Cognitive radio: An opportunity lost?

Posted: 16 Nov 2004 ?? ?Print Version ?Bookmark and Share

Keywords:cr? cellphone? wireless? network? device?

The fundamental uniqueness of cognitive radio technology is its ability to adapt to a wireless spectrum or network environment automatically to accomplish some task. The task may vary from something simple, like making a call, to something more complex, like unobtrusively accessing a remote database.

When a multimode mobile phone is turned on, it automatically tries to find a cellular network to associate with by searching for a control channel. The phone moves through the possible spectrum segments until it finds a compatible network (AMPS, cdma2000 and GSM). Then it uses established protocols to associate with the network and complete authentication to acquire service.

The promise of CR goes beyond making good use of well-established and well-behaved spectrum environments. A CR could be given the ability to sense spectrum usage patterns and establish and maintain new network connections with other cognitive devices in spectrum slots where no existing or authorized network exists.

Within the current regulatory regime for commercial radio in the United States, wireless systems are assigned spectrum according to functional (point-to-point, point-to-multipoint, broadcast, satellite etc.) and technological categories (analog or digital tv channels, mobile-phone channels, unlicensed devices using digital modulation or frequency hopping etc.). The resulting patchwork of frequencies, technologies and applications leaves large segments of spectrum virtually unoccupied and others densely occupied only in major metropolitan areas.

CR technology offers the opportunity to recover that spectrum and put it to good use. For example, TV channels offer particularly good opportunities for cognitive technology to do its adaptive thing. Analog and digital TV signals have relatively unique spectrum signatures that are easy for a CR to identify. TV signals are continuously present when the TV station is in operation, so a CR does not have to wait for the signal to show up to identify it. Finally, TV signals are rigidly assigned to 6MHz-wide channels with fixed center frequencies, so a CR does not have to look everywhere in the spectrum below 1GHz to discover white space, but just in this predefined array of channels.

Despite this, the FCC, in the current NPRM, "Unlicensed Operation in the TV Broadcast Bands," ET Docket No. 04-186, has chosen to leave CR technology completely out of the picture in terms of the rules the commission has proposed. To be fair, the FCC does invite comment on the possibility of using spectrum sensing and other cognitive techniques to adaptively access and use unoccupied TV channels.

But when push comes to shove, the FCC prefers to propose rules based on geolocation (GPS plus a database identifying empty TV channels); on a control channel that would define for a geographic region what channels are unoccupied; or on a professional installer who would do the necessary work to determine if a fixed wireless ISP-type network could safely occupy an apparently empty channel.

Perhaps the FCC believes spectrum sensing, the CR feature that identifies an empty spectrum slot, is not ready for prime time. The question may be: Can a spectrum-sensing device detect a TV station with enough distance margin to avoid interfering with its protected service when the device begins transmitting?

My own analysis indicates that a well-designed cognitive device using spectrum-sensing technology can identify a broadcast DTV signal well beyond the range at which the devices proposed in the NPRM can cause interference.

The analysis establishes that DTV signals can be reliably detected on the order of 19dB to 24dB below proposed protection limits for full-service DTV field strength. Even with the unlicensed device operating at full transmit power, the range of the unlicensed signal is insufficient to violate the NPRM's 23dB desired-to-undesired co-channel interference protection rule.

The sensing performance is based on traditional spectrum analysis focused on DTV pilot tone detection in a 10kHz bandwidth, using a receiver with a 5dB noise figure and sufficient averaging to minimize the measured pilot tone signal-level variation due to noise.

Protection margins when unlicensed devices operate at less than the full power permitted by the proposed rules will be even greater than those shown. For instance, unlicensed devices might use another cognitive technique, link power control, to reduce the power transmitted from one device to another when the signal at the receiving device reaches a specific threshold. So applying spectrum sensing to identify occupied TV channels is a technically defensible approach to protecting TV broadcast interests.

- John Notor

System Architect

Cadence Design Systems Inc.

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