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March 2007
Monisha Ghosh, Philips
Slide 1
doc.: IEEE 802.22-07/0125-00-0000
Submission
DTV Signal Sensing Using Pilot Detection
IEEE P802.22 Wireless RANs Date: 2007-03-11
Authors:
Notice: This document has been prepared to assist IEEE 802.22. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein.
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Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair Carl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at [email protected].>
Name Company Address Phone email Monisha Ghosh Philips USA 914-945-6415 [email protected]
Vasanth Gaddam Philips USA 914-945-6424 [email protected]
Gene Turkenich Philips USA 914-945-6370 [email protected]
March 2007
Monisha Ghosh, Philips
Slide 2
doc.: IEEE 802.22-07/0125-00-0000
Submission
Sensing the DTV pilot
• The DTV pilot is an unique feature that can be used to sense the presence of the DTV signal.
• Standard approach:– Narrow-band filter (~ 10 kHz) centered around nominal DTV pilot
location. – Calculate energy of filtered signal, compare to a threshold.
• Problems:– Pilot could be in a deep fade: quite common.– Threshold is susceptible to noise uncertainty.– Uncertainty in pilot location: could require a 100kHz bandwidth filter.
The larger the filter bandwidth, the worse the performance.
• Challenge: Can DTV pilot detection be made to provide robust sensing at SNR = -20 dB?
March 2007
Monisha Ghosh, Philips
Slide 3
doc.: IEEE 802.22-07/0125-00-0000
Submission
Alternate approaches
• Pilot-energy detection: can be made more robust by narrowing the filter-bandwidth.– Can this be done without compromising the detectability of signals
with large frequency offsets?
• Pilot location detection, rather than pilot energy.– More robust to thresholds: insensitive to noise uncertainty.– Performance is determined by pilot energy, however increasing
sensing time improves performance.
• Two methods proposed in this presentation:– FFT-based approach: pilot-energy and location detection.– Loop-based: pilot-location detection.
March 2007
Monisha Ghosh, Philips
Slide 4
doc.: IEEE 802.22-07/0125-00-0000
Submission
FFT-based pilot detection
• Demodulate signal to baseband, using nominal pilot position.
• Filter, using a low-pass filter. We used a BW of +/- 20 kHz.
• Subsample the signal. We reduced the sample rate from 21.52 MHz to 53.8 kHz (1/400).
• Take FFT of the subsampled signal. The FFT size will depend on the dwell-time. With parameters above:– 1 ms dwell will allow 32-point FFT.
– 5 ms dwell will allow 256-point-FFT.
• Pilot-energy detection: find maximum of FFT output-squared, and compare to a threshold. (NOT average over all FFT bins, but maximum).
• Pilot-location detection: compare location of maximum of FFT-output between multiple dwells.
March 2007
Monisha Ghosh, Philips
Slide 5
doc.: IEEE 802.22-07/0125-00-0000
Submission
Frequency Domain
0
x(t)
0
y(t)
53.8 kHz -53.8 kHz
……
21.52 MHz -21.52 MHz
……
March 2007
Monisha Ghosh, Philips
Slide 6
doc.: IEEE 802.22-07/0125-00-0000
Submission
Pilot Location Detection
• Let no. of dwells = N.• Let be the location of the maximum of the FFT-output
averaged over the first N/2 dwells.• Let be the location of the maximum of the FFT-output
averaged over the second N/2 dwells.• Detection statistic:
• If D < NT , signal present.
• Other variations:– Any PSD algorithm can be used instead of FFT.– Other averaging intervals could be used.
• PFA using this method is extremely low and robust. Threshold value NT will depend on the FFT-size.
)1(maxf
)2(maxf
)2(max
)1(max ffD
March 2007
Monisha Ghosh, Philips
Slide 7
doc.: IEEE 802.22-07/0125-00-0000
Submission
Signals From MSTV
Signal 1 WAS_3_27_06022000_REF.ASC
Signal 2 WAS_51_35_05242000_REF.ASC
Signal 3 WAS_311_36_06052000_ref.ASC
Signal 4 WAS_311_48_06052000_ref.ASC
Signal 5 WAS_68_36_05232000_ref.ASC
Signal 6 WAS_49_34_06142000_opt.ASC
Signal 7 WAS_311_35_06052000_ref.ASC
Signal 8 WAS_06_34_06092000_ref.ASC
Signal 9 WAS_49_39_06142000_opt.ASC
Signal 10 WAS_47_48_06132000_opt.ASC
Signal 11 WAS_32_48_06012000_OPT.ASC
Signal 12 WAS_86_48_07122000_ref.ASC
March 2007
Monisha Ghosh, Philips
Slide 8
doc.: IEEE 802.22-07/0125-00-0000
Submission
Performance with 1 ms dwell time, 10 dwells
32-point FFT
March 2007
Monisha Ghosh, Philips
Slide 9
doc.: IEEE 802.22-07/0125-00-0000
Submission
Performance with 5 ms dwell time, 6 dwells
256-point FFT
March 2007
Monisha Ghosh, Philips
Slide 10
doc.: IEEE 802.22-07/0125-00-0000
Submission
Performance with 5 ms dwell time, 10 dwells
256-point FFT
March 2007
Monisha Ghosh, Philips
Slide 11
doc.: IEEE 802.22-07/0125-00-0000
Submission
32-point FFT for Signal 7 (WAS_311_35_06052000)
Strong pilot, easily detected.
March 2007
Monisha Ghosh, Philips
Slide 12
doc.: IEEE 802.22-07/0125-00-0000
Submission
32-point FFT for Signal 9 (WAS_49_39_06142000)
Faded pilot, hard to detect
March 2007
Monisha Ghosh, Philips
Slide 13
doc.: IEEE 802.22-07/0125-00-0000
Submission
256-point FFT for Signal 9 (WAS_49_39_06142000)
Faded pilot,easily detected
March 2007
Monisha Ghosh, Philips
Slide 14
doc.: IEEE 802.22-07/0125-00-0000
Submission
Threshold sensitivity for signal energy detection
March 2007
Monisha Ghosh, Philips
Slide 15
doc.: IEEE 802.22-07/0125-00-0000
Submission
Threshold sensitivity for pilot energy detection
March 2007
Monisha Ghosh, Philips
Slide 16
doc.: IEEE 802.22-07/0125-00-0000
Submission
Threshold sensitivity for pilot location detection
March 2007
Monisha Ghosh, Philips
Slide 17
doc.: IEEE 802.22-07/0125-00-0000
Submission
Summary Of FFT-based detection
• SNR = -20 dB, pilot-energy detection: PD = 100% for all 12 channels, PFA = 2%, with ten 5 ms dwells.
• SNR = -20 dB, pilot-location detection: PD = 100% for 11 channels, and 90% for 1 channel, PFA = 3%, with ten 5 ms dwells.
• Thresholds for pilot-energy and pilot-location detection are less sensitive than threshold for signal-energy detection.
March 2007
Monisha Ghosh, Philips
Slide 18
doc.: IEEE 802.22-07/0125-00-0000
Submission
Loop-based pilot detection
• Two carrier recovery modules, each preset to a different frequency.
• The difference of the loop outputs of the two carrier recovery modules is compared against a threshold:– Signal is assumed to be present if the difference is less than the threshold.
• Not very sensitive to the threshold:– We used very conservative thresholds.
• Probability of false alarm is very low (close to 0) for a large range of threshold values.
• Sensing time can be reduced by adjusting the loop parameters.
March 2007
Monisha Ghosh, Philips
Slide 19
doc.: IEEE 802.22-07/0125-00-0000
Submission
Loop output behavior in the absence of DTV
• Not much variation from the initial frequency setting for the two loops.– Loops do not converge.
March 2007
Monisha Ghosh, Philips
Slide 20
doc.: IEEE 802.22-07/0125-00-0000
Submission
Loop output behavior in the presence of DTV
• Monotonic behavior in loop output in the presence of DTV signal for wide range of gain settings and SNRs.– Large threshold margin for reliable detection.
March 2007
Monisha Ghosh, Philips
Slide 21
doc.: IEEE 802.22-07/0125-00-0000
Submission
Performance after 75 ms, PFA << .01
March 2007
Monisha Ghosh, Philips
Slide 22
doc.: IEEE 802.22-07/0125-00-0000
Submission
Conclusions• Pilot detection is a robust means of sensing, down to -20 dB.
– Two methods presented: FFT-based and loop- based.
• Pilot-location detection is extremely robust against noise uncertainty.
• Pilot-energy detection, using the FFT based method, essentially reduces the bandwidth of the filter and hence performance is improved, even for severely faded pilots.
• Both pilot energy and pilot location detection can be used with multiple short dwell times to deliver robust sensing, thus avoiding the need for a single long quiet period.– QoS can be maintained for voice traffic.