1. IEEE P802.22 Wireless RANs Date: 2008-12-21
March 2008
doc.: IEEE /0xxxr0
December 2008
IEEE P Wireless RANs Date:
Pros and Cons of RF sensing in TV bands (Comments on the FCC R&O )
Authors:
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Gerald Chouinard, CRC
Gerald Chouinard

2. March 2008
doc.: IEEE /0xxxr0
December 2008
DTV Sensing
DTV sensing was studied by as a means of detecting the presence of DTV incumbents in the TV bands
In order to make sensing sufficiently reliable in detecting incumbent DTV operation, the sensing threshold has been set to a very low value (-114 dBm) rendering sensing unreliable for the unlicensed systems because DTV signals will be detected when it is not needed:
DTV signals will be detected beyond the protected contours where DTV receivers are not protected
DTV signals will be detected from very distant DTV transmitters because of the statistical nature of the propagation channel
Sensing may no longer be necessary since access to an incumbent database is compulsory for protecting DTV incumbents
Broadcasters agree that the incumbent database should be the prime means of protection over sensing
Gerald Chouinard, CRC
Gerald Chouinard

3. Co-channel sensing of DTV incumbent
March 2008
doc.: IEEE /0xxxr0
December 2008
Co-channel sensing of DTV incumbent
F(1,1) 589 km
F(10,1) 520 km
F(10,10) 435 km
F(50,1) 437 km
Sensing threshold:
S/N = -19 dB at sensing detector
118 km
F(50,10) 361 km
Sensing CPE
Required DTV sensing threshold= -114 dBm to compensate for blockage
DTV TX
Probability of signal exceeding 19 dB(uV/m) = %
Sensing threshold= 22 dB below protected field strength level
DTV protected
noise-limited contour
41 dB(uV/m) F(50, 90)
Gerald Chouinard, CRC
Gerald Chouinard

4. Potential denial of service from sensing
March 2008
doc.: IEEE /0xxxr0
December 2008
Potential denial of service from sensing
Sensing of DTV
An unlicensed device using an ATSC DTV-like signal would have a de-facto precedence over other unlicensed devices because they would be seen as incumbents.
The database would allow avoiding such potential denial of service for devices that can access it. Sensing results will have to be discarded.
Sensing-only devices would not have such capability
Sensing of wireless microphones
Unlicensed devices using a signal format similar to wireless microphones would have a de-facto precedence over other unlicensed devices because they would be seen as incumbents
Analog FM microphone
Digitally modulated microphone
Constant Wave simulating an idle microphone
Gerald Chouinard, CRC
Gerald Chouinard

5. Potential denial of services from sensing (cont’d)
March 2008
doc.: IEEE /0xxxr0
December 2008
Potential denial of services from sensing (cont’d)
Sensing of wireless microphones (cont’d)
Any device using a signal format similar to a wireless microphone would have a de-facto precedence on other unlicensed devices because they would be seen as incumbents
The FCC seems to have decided to recognize any wireless microphone operations, be they Part 74 (i.e., operated by broadcasters and for special events), as well as any other wireless microphones on the market.
The latter ones will be de-facto unlicensed devices but without the requirement for sensing and access to the database
It will be hard for the sensing to make a difference between one of these microphones in idle mode and a simple oscillator. This will likely open the door to very low cost devices producing denial of service for any other unlicensed devices (e.g., low-cost oscillators available on eBay)
It is unclear how the regulators will be able to control access to these devices which would work under the same conditions as the de-facto unlicensed wireless microphones
These devices will only need to emit very low signal levels to upset the operation of unlicensed devices: -25 dBm EIRP at 1 km line-of-sight
Gerald Chouinard, CRC
Gerald Chouinard

6. Potential denial of services from sensing (cont’d)
March 2008
doc.: IEEE /0xxxr0
December 2008
Potential denial of services from sensing (cont’d)
Sensing in presence of other unlicensed devices
The level of out-of-band emission allowed by Part a is 200 uV/m in 120 kHz bandwidth at 3 m
This corresponds to 46 dB(uV/m) in 120 kHz and 63 dB(uV/m) in 6 MHz
The sensing threshold of -114 dBm corresponds to a field strength of 19 dB(uV/m) for the reference 0 dBi antenna gain and 0 dB coupling loss
Sensing will be impacted by the allowed out-of-band emission of another unlicensed device if the device is at less than 473 m from the sensor in line-of-sight condition if the sensing bandwidth is 6 MHz and 86 m if the sensing bandwidth is 200 kHz
All unlicensed devices on all frequencies will need to use synchronized quiet periods in their transmission to allow sensing of incumbents
To avoid that other unlicensed devices hide the presence of incumbents in the channel being sensed (see next slide)
To avoid other unlicensed devices operating on other TV channels generating sensing false detections from their out-of-band emission if they are located relatively close to the sensor.
Gerald Chouinard, CRC
Gerald Chouinard

7. Potential denial of services from sensing (cont’d)
March 2008
doc.: IEEE /0xxxr0
December 2008
Potential denial of services from sensing (cont’d)
Sensing in presence of other unlicensed devices (cont’d)
In the work of , it has been assumed that all WRAN devices in a channel and its adjacent channels will need to have their quiet periods synchronized in order to avoid that a WRAN operation hides the presence of a broadcast incumbent operation to another WRAN cell
In the work of , it has also been assumed that the WRAN devices would have to move out of the channel if RF power was detected by the energy detector whether this RF signal could be classified or not (i.e., recognized as a known signal by the detection algorithm or not)
This was to cover for the case where an unknown signal would hide the presence of an broadcast incumbent signal
In particular, this will cover for the case where an unlicensed device, having a short sensing and interfering range (i.e., close to the ground), is located close to another unlicensed device that has a larger sensing and interference range (higher power or antenna height). The presence of the first device would occult the eventual existence of incumbents to the second device because of the desensitization of its sensing RF path.
Gerald Chouinard, CRC
Gerald Chouinard

8. Impact of not doing DTV Sensing
March 2008
doc.: IEEE /0xxxr0
December 2008
Impact of not doing DTV Sensing
The sensing RF chain will be simpler
Antenna:
The sensing antenna for wireless microphones will be much simpler, e.g., a vertical monopole giving an omnidirectional pattern for vertical polarization with a reasonable gain (0-2 dBi)
No need for a complex horizontally polarized sensing antenna: good omnidirectional pattern is more difficult to achieve with horizontal polarization (needs something like crossed drooping dipoles)
Current Functional Requirement Document asks for an omnidirectional antenna with 0 dBi gain in all polarizations: never been verified for feasibility
Low-noise amplifier, frequency converter, filter
Bandwidth of the RF chain does not need to be 6 MHz (or wider to capture N and N+/-1 in a single sensing). It could be narrowband if the practice of starting to populate a TV channel with a specific microphone sub-channel was established or even better if a wireless microphone beacon was to be used.
Using a narrowband filter would reduce the noise level and improve the SNR
Simpler narrowband amplifier, converter and filter could be used.
Gerald Chouinard, CRC
Gerald Chouinard

9. Impact of not doing DTV Sensing (cont’d)
March 2008
doc.: IEEE /0xxxr0
December 2008
Impact of not doing DTV Sensing (cont’d)
The sensing RF chain will be simpler (cont’d)
Sensing detector:
Simpler narrowband microphone (or TG1 beacon) detector could be used
Complex sensing schemes for DTV would not be needed
Detector would be optimized to only detect wireless microphones (or TG1 beacon)
The MAC messages to request sensing and report on the detected signals would be simpler
MAC messages in both directions would be limited to wireless microphone (orTG1 beacon) sensing
The CPE spectrum automaton would be simpler
The BS spectrum manager would be simpler
Requesting information from the database on existence of DTV stations and registered wireless microphone operation
Managing the sensing reports for wireless microphones (or TG1 beacon)
Gerald Chouinard, CRC
Gerald Chouinard

10. March 2008
doc.: IEEE /0xxxr0
December 2008
Conclusions
DTV sensing at the proposed low signal threshold (-114 dBm) is unreliable, prone to denial of service and unnecessary since DTV protection will be first ensured through database access
Wireless microphone sensing at the low signal threshold (-114 dBm) is unreliable and prone to very easy denial of service.
IEEE has developed a standard for a signal beacon to indicate the presence of wireless microphone operation
This beacon is to emit a digitally encoded signal at 250 mW that will carry all the necessary information for identifying wireless microphone operation including its location and encrypted signature for certification
This beacon can be sensed in a known 200 kHz BW to improve sensitivity
The beacon can be placed at a more prominent place than the wireless microphone receivers to maximize the ‘bubble of protection”
Technical parameters of the beacon were developed to provide protection from unlicensed devices up to 4 W EIRP at any antenna height (sensing range of the unlicensed device is larger than its interference range)
Gerald Chouinard, CRC
Gerald Chouinard

11. December 2008
References
IEEE P , Draft IEEE Standard for Local and metropolitan area networks, Part 22.1: Methods to Enhance Protection of Low-Power, Licensed Device Operation in the TV Broadcast Bands from Harmful Interference from License-Exempt Devices Operating in those Bands, 05/05/2008
Gerald Chouinard, CRC