System Design Issues in building a Cognitive Radio Network: IEEE 802.22

Suhas Mathur, WINLAB, Rutgers University Summer internship @ Qualcomm 10th October 2006

System Design Issues in building a

Cognitive Radio Network: IEEE 802.22

Detection of DTV signals at very low SNR using PN sequences

Joint work with Steve Shellhammer (Qualcomm) & Rahul Tandra (U.C. Berkeley)

“What I did in my summer internship”


•Apparent scarcity of spectrum•Spectrum use model flawed – allocated but not utilized•J. Mitola coins ‘cognitive radio’ -Ph.D. thesis (May 2000)

•FFC issues NPRM for TV bands (May 2004)

•802.22 is born (Sept 2004)

•How to protect the ‘incumbent’ ?•May 2006: v0.1 of the 802.22 Draft is shipped•Sept. 2006: FCC Docket: Retail by Feb 2009.•Sensing specs. still not met, broadcasters dissatisfied•Unlicensed economic model still attractive

–3G auction: $17 Bn , $34 Bn , $46 Bn

A brief history of Cognitive Radio and IEEE 802.22

3Suhas Mathur, WINLAB, Rutgers University Summer internship @ Qualcomm 10th October 2006

802.22 deployment scenario

802.22: Fixed wireless broadband access network operating in TV bands (Channel 2 -51)

Cellular system, central BS, CPEs (Consumer Premise Equipment)

Targeted at but not limited to rural deployment

Strict requirements on protections of existing licensed services


Incumbent protection numerology

Parameter TV broadcasting

Part 74 devices

Channel detection time < 2 sec < 2sec

Incumbent detection threshold

-116 dBm -107dBm

Required prob(detection) 90% 90%

Max. False alarm allowed 10% 10%


Why do we need to detect at such low SNR?



Shadow faded CPE



Shadow faded CPE

INTERFERENCE


‘Hidden incumbent’ problem• An in-band Incumbent

appears at a location where BS cannot sense it but CPE can.

• CPE wishes to inform BS about incumbent but may loose sync to BS due to interference from incumbent

• BS continues to transmit on the channel occupied by incumbent, causing interference to it.


‘Hidden cell’ problem


Sharing licensed spectrum

Vertical sharing

Horizontal sharing


Horizontal spectrum sharing• An unlicensed user must

adhere to FCC guidelines for protection of incumbents.

• But other unlicensed systems on the TV bands do not enjoy the same protection.

• This means a Cognitive Radio must not only detect, but also classify signals.

• Simple narrowband power detection [Tandra & Sahai] does not achieve this objective


Part II

Detection of DTV signals at very low SNR using PN sequences


Outline of Part II

• The ATSC standard and PN sequences• Optimal & approximate detectors• Problem with detector a proposed detector• Our detection algorithms• Effects of multipath fading on PN

correlation• Fundamental limits in PN seq. detection• Sensitivity to pilot estimate• A comparison of PN and pilot detection• Pilot spectral line detection


ATSC DTV numerology

• Symbol rate: 10.76 Msymbols/sec

• Data Segment SYNC: A ‘1001’ pattern at the beginning of each segment

• Data Field SYNC: An entire segment containing PN sequences: PN511 + PN63 + PN63 + PN63


ATSC framing structure

A single VSB Data Segment

The Data Field SYNC


Typical ATSC DTV spectrumPilot tone

6 Mhz = 1 DTV channel

f_IF = 5.38 Mhz

VSB spectrum at IF

Nyquist Rolloff = 0.1152


Simulation Methodology

DTV signal captures

(50 DTV signals captured at high SNR,

provided to us by MSTV Corp.)

+X

Scale

Gaussian Noise

Pull out reqd. # of samples Detection

algorithm

Threshold calibration

Signal at desired SNR

Multipath fading, noise

Test locations in NY, Washington


The optimal PN seq. detector

• Assumption: exactly 1 PN sequence present

• Generalized Likelihood Ratio Test (GLRT) in favor of H1 iff:

• is the MLE given by:

• GLRT is:


Approximate detector• Neglect the signal in alternate hypothesis (low

SNR)

• is the MLE given by

• GLRT is:

• Threshold can be found easily by using:

simply correlate and compare max

value with


A detector proposed by France Telecom

• Correlate the received signal, r(n), with the known PN sequence

1

0

)()(N

nkn nxrkp

• Use a simple low pass filter to estimate the mean and the variance of the correlator output

• An ATSC DTV is declared detected when

2))()()(1()1()(

)()1()1()(

kpkkk

kpkk

)'()'( 2 kck

( and c are constants set by the BS)


The Problem with this Detector

• The estimate of the mean approach zero even for noise only input

• This implies that the test statistic can approach zero even for a noise only input signal

• This results in false alarms. The detector threshold ‘c’ cannot be calibrated for a given false alarm probability pFA.

2( ) ( ) / ( )T k k k

( ) [ ( )] ( ) ( ) 0k E p k E r n k x n


Test statistic with noise-only input

Output of the correlator when only filtered receiver

noise is fed as input.

Ratio shows false peaks because mean goes very close to

zero.

)(/)( 2 kk

Correlator output Detector output


Detector operation with DTV signal + Noise (SNR = 0 dB)

Output of the correlator over duration of 1 ATSC field when DTV signal + noise is fed as input. The

peak indicated the position of the PN sequence.

Ratio again shows false peaks because mean goes very close to zero

)(/)( 2 kk



A simple ‘fix’

• Take absolute value of the correlator output before feeding as input to the detector.

• Mean cannot go arbitrarily close to zero

• The ratio is random for noise input.

)()1()1()( kpkk

1

0

)()(N

nkn nxrkp

)(/)( 2 kk


Modified Detector with noise input

Absolute value of the correlator output when only filtered receiver

noise is fed as input.

Ratio is random)(/)( 2 kk



Modified detector with DTV Signal + Noise (SNR = 0 dB)

Absolute value of correlator output over duration of 1 ATSC field when DTV signal

+ noise is fed as input. The peak indicated the position of the PN

sequence.

Ratio shows peak at location of PN sequence

)(/)( 2 kk



A simple test statistic• Correlate received signal with the following

sequence:

[PN511 + PN63 + 63 zeros + PN63]

• Find the tallest peak in the correlator output• Compare its magnitude with threshold


Performance of a simple correlator


The effect of multipath

Phase reversal

48.4 ms



Multiple peaks

6 s

~ 2km dominant echo



Multiple peaks

6 s

Can we utilize multipath in a positive way ?

(Lessons from CDMA ?)

~ 2km dominant echo


Fundamental limits in PN detection

• Using a larger received signal for detection does not always improve performance.

multipath fading

noise enhancement

Multipath, jitter and small variations in clock frequency cause timing offsets resulting in misaligned peaks by +/-1,2 samples


Using the Data Segment SYNC• The DATA Segment SNYC is a 1001 pattern• Very non unique but much more frequent (every 77.3 s=1 seg.)

• Present in noise and data quite often, but can we use its periodicity?

• A long sequence of the following form can be used for correlation:

[1001 (828 zeros) 1001 (828 zeros) … N times ]


Performance of ‘1001 correlator’


Aligning peak polarities helps! (sometimes)


The issue of Multiple antennas

• The height and polarity of a PN correlation peak depends on the instantaneous fade.

• Two or more independent fades would provide different correlation peaks.

• If multiple antennas can provide statistically independent fading, they can help.

• A simple system would be to run 2 parallel PN detectors and OR their decisions. i.e.

Miss detection = (Miss detection 1) AND (Miss detection 2)

PN sequence detection

• Power detector is affected by shadowing

• Therefore multiple sensors located in INDEPENDENT shadow fades would help

• Existence of such independent CPEs not guaranteed (incumbents not happy with the idea)

• Multiple antenna on a single CPE do not help (same shadow fade)

Power detection


Multiple antennas

• Running 2 parallel detectors

• Assumption: Fading processes at two antennas are statistically independent

• We use fields from widely time-separated part of a DTV signal capture to satisfy independent fade assumption

• At 600 Mhz, /2 = 25 cm. A practical antenna array can be built.


The effect of pilot estimation error

• The pilot frequency is used in downconverting a passband signal to recover the baseband transmit signal.

• How sensitive is PN detection to the estimate of the pilot frequency?

• We attempt to measure the effect of pilot estimation error on a PN correlation peak





• At low enough SNR even 0.2% error in pilot frequency estimate can be disastrous for PN correlation

• Pilot estimation needs to be accurate

• Why not use the pilot line as a means for DTV signal detection?


Part III

Spectral line detection of a DTV pilot[Extension of ‘Narrowband pilot energy detection’ done by

Rahul Tandra]


Power spectrum of a DTV signal at -22 dB


Performance of pilot spectral line detection

• Signal shows a strong pilot• Detector performance is

very good even at -25 dB !

strong pilot

Pilot energy detection

[Tandra]



• Signal shows a moderately strong pilot

• Detector begins to fail at -21dB

Moderate pilot



• Pilot is almost completely absent from signal

• Detector begins to fail at -16 dB

Very weak pilot


Good, moderate and bad pilots


Doubling the listening time

Very weak Pilot


Doubling the listening time

Moderate Pilot


Some gains from listening longer


Noise Uncertainty

• For signals with weak pilots, uncertainty in the noise power estimate can drastically change performance.

• We assume = +/- 1 dB.

• How does performance change?


Noise Uncertainty

• Signal shows a strong pilot• Detector performance worse

by approx 2.5 dB. • Detector begins to fail at -24

dB

strong pilot


Noise Uncertainty

• Signal shows a moderately strong pilot

• Worse by ~2 dB• Detector begins to fail at -17

dB

Moderate pilot


Noise Uncertainty


• Worse by ~2 dB• Detector begins to fail at -12

dB

Very weak pilot


Noise Uncertainty


• Worse by ~2 dB• Detector begins to fail at -12 dB• If we listen for double the time

detector begins to fail at -14 dB

Very weak pilot


A comparison of PN detection and pilot energy detection

PN Sequence Pilot Energy Detection

Occurs once in 24.2 ms Always present in time

Frequency content over entire DTV spectrum

Present at a specific frequency (14 known pilot freq. location)

Power in PN sequence is 1/313rd of the total energy in signal (25 dB below)

Power in pilot is 11 dB below avg signal power.

Need to sense for longer to average out noise

Need to sense for shorter duration.

Cannot improve performance by sensing for infinitely long duration.

Noise uncertainty results in an SNR wall effect below which signal cannot be sensed.

Requires knowledge of pilot frequency Does not require knowledge of PN seq.

Achieves signal classification. Does not achieve signal classification


Proposed dual mode DTV detector

• Pilot detection detects pilot and triggers PN detection for signal classification (confirms DTV)

• Multiple antennas (help both pilot and PN)• Detection quiet periods are kept small.


Conclusions and future directions

• PN Correlation detector performance limited by multipath fading, noise and jitter.

• Difficult to accurately estimate multipath at low SNR.

• Multiple antennas on a single detector can help (statistically independent fading processes)

• Pilot spectral line detection is promising but does not classify signal as DTV.

• A ‘dual mode’ detector utilizing – Pilot spectral line detection for detection and – PN correlation (with multiple antennas) for classification.

can achieve quick and reliable DTV incumbent protection


The internship experience

• Worked in the standards division of Corporate R&D

• Some exposure to systems, IEEE standards procedures (tedious).

• Focus on practical feasibility. • Economic & political side of technology.• Overall – a satisfying, busy 3 months (no time to work on WINLAB research

simultaneously )


Thank you


Backup slides


Incumbent protection numerology

Required prob(detection) = 90 % False alarm allowed = 10%


Hidden wireless microphones

• An example of hidden incumbents

• Microphones are low power devices and can appear and disappear on a finer time scale.

• Wireless microphones are likely candidates for hidden incumbents.

• Propagation curves show that it Is virtually impossible for a BS to detect a microphone at the edge of a WRAN cell.

Distance from wireless mic (km)

Rec

eive

d po

wer

(dB

m)






Do multiple antennas help?

• The height and polarity of a PN correlation peak depends on the instantaneous fade.

• Two or more independent fade would provide different correlation peaks.

• If multiple antennas can provide statistically independent fading, they can help.

• A simple (suboptimal) system would be to run 2 parallel PN detectors and OR their decisions. Therefore:

Miss detection = (Miss detection 1) AND (Miss detection 2)


Multipath Fading

Source: Gorka Guerra, Pablo Angueira, Manuel M. Vélez, David Guerra, Gorka Prieto, Juan Luis Ordiales, and Amaia Arrinda ‘Field Measurement Based Characterization of the Wideband Urban Multipath Channel for Portable DTV Reception in Single Frequency Networks’ IEEE TRANSACTIONS ON BROADCASTING, VOL. 51, NO. 2, JUNE 2005 171

Documents

System Design Issues in building a Cognitive Radio Network: IEEE 802.22