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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 1 Spectrum Sensing for DVB-T OFDM Systems IEEE P802.22 Wireless RANs Date: 2008-04-30 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. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEEs name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEEs sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.22. 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 Chairhttp://standards.ieee.org/guides/bylaws/sb-bylaws.pdf Carl R. StevensonCarl 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 patcom@iee.org.patcom@iee.org >

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 2 Outline Basic approach TDSC Method Timing Issue CP Method Simulation parameters Simulation results Conclusions

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 3 The n th sample of the l th OFDM symbol can be expressed as N: Number of subcarriers H[k]: Complex channel gain of the k th subcarrier X l [k]: Data symbols on the k th subcarrier of the l th OFDM symbol f Δ : Frequency offset normalized to the subcarrier spacing w l [n]: Complex AWGN process Basic Approach (1)

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 4 Basic Approach (2) If these two OFDM symbols have the same pilot tone positions, after some calculations and reasonable approximations Define the Time Domain Symbol Cross-Correlation (TDSC) function of two OFDM symbols as P: the set of pilot tone positions,

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 5 TDSC Method In DVB-T, every fourth OFDM symbols have the same pilot tone position. Define C(v) as the accumulated TDSC function, where v = l-m is the symbol index difference of two OFDM symbol A: the number of pilot position sets, here A = 4. S v : the number of TDSC accumulated for C(v).

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 6 TDSC Method In DVB-T, we will compute C(4), C(8), C(12), …. These accumulated TDSC functions have different phase and hence we cannot simply add them together. In order to solve this problem, lets define and let decision statistic be where a v is combining ratios.

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 7 Combining Ratio We use an intuitive criterion to find the combining ratios a v. We choose a v such that the Kullback-Leibler divergence is maximized for two hypotheses H 0 and H 1. According to the Central Limit Theorem, the probability distributions of the test statistic for both hypotheses approach complex Gaussian distribution For two complex Gaussian random variables, the Kullback-Leibler divergence is given by

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 8 Combining Ratio By computing for all v, the optimal combining ratios are obtained. It is too complex to solve. As a result, we assume that which is reasonable when the SNR is very low. Then, a suboptimal solution is given by a v = S v S v+d Where S v is the number of TDSC accumulated for C(v).

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 9 Timing Issue We dont have correct timing when performing spectrum sensing. Due to the CP nature of OFDM, as long as the sampling instance falls in the CP, the TDSC equation is correct. If the length of CP is L, and we try a sampling instance every L samples, at least one sampling instance will fall in CP for [N/L]+1 consecutive trials. We use the maximum amplitude over these timing trial as decision statistic. N-LL L L-1

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 10 CP Method Define where A is the number of OFDM symbols used. Decision statistic

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 11 Simulation Parameters The number of subcarriers is N = 2048. The wireless channel is a Rayleigh multipath fading channel with exponential power delay profile recommended in DVB-T standard. Channel bandwidth is 8 MHz. The timing offset is 300 samples and the frequency offset is set to be 7.324 KHz which corresponds to 2.5 subcarrier spacing for the 2K modes. The sensing time is 50 ms which corresponds to 216, 210, 198, and 178 symbols for CP equaling to 1/4, 1/8, 1/16, and 1/32 of the subcarrier number. P FA = 0.01.

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 12 Simulation Results – CP = 1/4

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 13 Simulation Results – CP = 1/8

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 14 Simulation Results – CP = 1/16

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 15 Simulation Results – CP = 1/32

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doc.: IEEE 802.22-08/122r0 Submission April 2008 Hou-Shin Chen and Wen Gao, Thomson Inc.Slide 16 Conclusions The performances of the CP method have a dramatic degradation when the length of CP is decreased from 1/4 to 1/32 of the carrier number. The performances of the TDSC method are almost the same for all kinds of CP. Indeed, the performance of the shorter CP is a little better than that of the longer CP because there are more OFDM symbols for shorter CP in the same sensing time. The TDSC method has higher complexity. However, it outperforms the CP method for all kinds of CP and for CP is 1/32, it outperforms the CP method for 7 dB.

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