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**Spectrum Sensing for DVB-T OFDM Systems Using Pilot Tones**

July 2007 Spectrum Sensing for DVB-T OFDM Systems Using Pilot Tones IEEE P Wireless RANs Date: Authors: Notice: This document has been prepared to assist IEEE 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 IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s 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 Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures 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 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at > Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Outline Basic approach Practical situation Decision statistic**

July 2007 Outline Basic approach Practical situation Decision statistic Pilot pattern of the DVB-T OFDM systems Simulation parameters Simulation results Conclusions Hou-Shin Chen and Wen Gao, Thomson Inc.

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July 2007 Basic Approach (1) The nth sample of the lth OFDM symbol can be expressed as where N: Number of subcarriers H[k]: Complex channel gain of the kth subcarrier Xl[k]: Data symbols on the kth subcarrier of the lth OFDM symbol Let P denote the set of positions of the pilot subcarriers, then Hou-Shin Chen and Wen Gao, Thomson Inc.

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July 2007 Basic Approach (2) Let Rxl,m [n] = E[xl[n]xm*[n]] where E[·] is the expectation operator, then which is the correlation of the lth and mth OFDM symbols. Rx[l,m] is used as the test statistics to implement spectrum sensing for OFDM systems employing pilot tones. Hou-Shin Chen and Wen Gao, Thomson Inc.

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July 2007 Practical Situation In practice, we are lack of the frame timing information and the effect of frequency offset should be considered. Let D = N/L which is the ratio of the subcarrier number and the CP length. In DVB-T, D can be 4, 8, 16, and 32. In the receiver end, if we try D points which are equally spaced by L samples as initial sampling instances, there will be one point which is a correct sampling instance. Denote r[n] the samples of the received signal. Let where M=N+L and d = 0, 1, 2, …, D-1 In the correct frame timing where fΔ is carrier frequency offset normalized to the subcarrier spacing Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Decision Statistic Define**

July 2007 Decision Statistic Define which is the sum of correlations of two OFDM symbols which have the same index (time) difference. Pilot Correlation Method (PC Method) for a fixed v or Second Order Pilot Correlation Method (SOPC Method) as decision statistic where V: Maximum symbol index difference of the OFDM symbol correlation α: A fixed integer selected according to pilot frame structure Sv: Combining ratio Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Pilot Tones of DVB-T OFDM Systems**

July 2007 Continued pilots are inserted in the same positions for every OFDM symbol and the subcarrier spacing between any two nearby pilots are not fixed. Scattered pilots are inserted every 12 subcarriers and thus there is an 11 subcarrier spacings between two consecutive pilots. The positions of scattered pilots are shifted for 3 subcarriers for every other OFDM symbol so that the positions of scattered pilots are repeated every 4 OFDM symbols. Hou-Shin Chen and Wen Gao, Thomson Inc.

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**CP Method Define where A is the number of OFDM symbols used.**

July 2007 CP Method Define where A is the number of OFDM symbols used. Decision statistic Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Simulation Parameters**

July 2007 Simulation Parameters The wireless channel is a is a Rayleigh multi-path fading channel with exponential power delay profile and root mean-square (RMS) delay-spread of 5.2 ms. The timing offset is 300 samples and the frequency offset is set to be KHz which corresponds to 2.5 subcarrier spacing for the 2K modes. The sensing time is 50 ms which corresponds to 117, 130, 137, and 142 symbols for CP equaling to 1/4, 1/8, 1/16, and 1/32 of the subcarrier number. Sv is chosen to be the square root of the samples that are used to average. Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Simulation Results – CP Method**

July 2007 Simulation Results – CP Method Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Simulation Results – PC Method**

July 2007 Simulation Results – PC Method Hou-Shin Chen and Wen Gao, Thomson Inc.

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**Simulation Results – SOPC Method**

July 2007 Simulation Results – SOPC Method Hou-Shin Chen and Wen Gao, Thomson Inc.

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July 2007 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 PC method and SOPC 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 CP method and the PC method has approximately the same complexity and the performance of the CP method is better than the PC method only for CP is 1/4. The SOPC 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 8 dB. Hou-Shin Chen and Wen Gao, Thomson Inc.

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