A Soft Decision Decoding Scheme for Wireless COFDM with Application to DVB-T

1. A Soft Decision Decoding Scheme for Wireless COFDM with Application to DVB-T Advisor : Yung-An Kao Student : Chi-Ting Wu 2005.03.01

2. Yong Wang, JianHua Ge, Bo Ai, Pei Liu, ShiYong Yang, “ A Soft Decision Decoding Scheme for Wireless COFDM with Application to DVB-T,” Consumer Electronics, IEEE Transactions on ,Volume: 50 ,Issue: 1 ,Feb 2004 Pages:84 - 88

3. Outline • Introduction • Soft decision decoding in DVB-T system • Proposed CSI estimation method • CCI detecting and erasing technique • Simulation results • Conclusion

4. Introduction • Basic idea of OFDM is to split the data onto subcarriers to combat multipath channel • Fading magnitude differs from subcarrier • In DVB-T system, co-channel interference (CCI) causes signal distortion • Analog signals

5. RS code Convolution code VHF:30~300 MHz UHF:300~3000 MHz Soft decision decoding in DVB-T system (Tx) QPSK 16QAM 64QAM 1/32, 1/16, 1/8, 1/4 2k or 8k mode DVB-T signals will be transmitted in VHF/UHF channels with frequency spacing of 6~8 MHz

6. Soft decision decoding in DVB-T system (Rx) • CSI is often defined as Carrier-to-Noise Ratio (CNR) • CCI can also be referred to as frequency selective interference • The reliability could be treated as a priori information

7. Proposed CSI estimation method (1/4) • Several methods of CSI generation • Square of the channel frequency response it doesn’t take account of the effect of noise power • Interpolating the MSE of the pilot subcarriers it assumes that the channel is varying slowly enough, not suitable for mobile channel Properly defined metric distance between the received data and hard decision constellation

8. Proposed CSI estimation method (2/4) l : symbol index k : subcarrier index N : Gaussian noise with power Y : FFT output signal X : transmitted signal H : channel frequency response if

9. Proposed CSI estimation method (3/4) • In general, CNR for k-th subcarrier can be defined as • compare to both equations • where <·> denotes time-averaging function over the OFDM symbol and

10. Bit decision metric mk,l(i is equal to 2,4,6 for QPSK, 16QAM, 64QAM) Proposed CSI estimation method (4/4) • In fast time-varying channel, we should take account of the factor of Doppler shift. • Channel correlation estimator is to detect the correlation of the Hk,l in time direction and corresponding iterative factor • The shorter the minimum coherence time, the greater the iterative factor should be. • Therefore, we could obtain the data reliability metric

11. CCI detecting and erasing technique • There’s no extra spectrum for DVB-T, so analog and digital signals have to co-exist • Utilize the property of CCI to detect the distorted subcarrier positions and erased them • We can detect according to the amplitude deviation of the estimated Hk,l in time direction

12. Simulation results 2k mode DVB-T system ½ inner code rate 1/16 guard interval symbol interleaver only 4-bit quantization Trace back length = 80 Rayleigh fading channel 64 QAM BER comparison of CSI generation method under Rayleigh fading channel

13. Simulation results 2k mode DVB-T system ½ inner code rate 1/16 guard interval symbol interleaver only 4-bit quantization Trace back length = 80 Rayleigh fading channel QPSK Doppler shift = 100Hz BER comparison of different iterative factor under Rayleigh fading channel with fDmax=100 Hz

14. Simulation results 2k mode DVB-T system ½ inner code rate 1/16 guard interval symbol interleaver only 4-bit quantization Trace back length = 80 Rayleigh fading channel 64 QAM Interference = 60/1705 = 3.51% CNR of signal = 40dB BER comparison of soft-decision decoder with and without CCI erasing under Rayleigh fading channel with PAL interference

15. Conclusion • The size of soft-decision metric caries adaptively in accordance with CSI and CCI • By selecting suitable iterative factor, the CSI could be used for mobile reception • This method has been used in BDB-T(Terrestrial Broad-band Digital Broadcasting) and HDTV in China