1. 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:
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Hou-Shin Chen and Wen Gao, Thomson Inc.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

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

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

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

13. 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.