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Synchronization of Turbo Codes Based on Online Statistics

This research paper explores the necessity of timing synchronization in turbo coded systems, and proposes an algorithm based on online statistics to compensate for coding gain loss. The paper also discusses the joint estimation of SNR and timing offset, over-sampling and interpolation techniques, and presents simulation results and conclusions.

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Synchronization of Turbo Codes Based on Online Statistics

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  1. Synchronization of Turbo Codes Based on Online Statistics Jian Sun and Matthew C. Valenti jian@csee.wvu.edu Wireless Communication Research Laboratory Lane Dept. of Comp. Sci. and Elec. Engr. West Virginia University May 14 2003

  2. Outline • Necessity of timing synchronization • Concepts • Effective signal-to-noise ratio (SNR) • Online statistic • Algorithm to compensate coding gain loss • Over-sampling and interpolation • Joint estimation of SNR and timing offset • Simulation results and conclusions • Future work

  3. Necessity of Timing Synchronization in Turbo Coded Systems • Turbo codes are attractive because of its near-Shannon-limit BER performance in very low SNR environments. • Traditional synchronization methods will fail because of low SNR. • Improper synchronization will render the turbo coded system great loss of coding gain. • Prior work done by Mielczarek and Svensson [1] • Viewing turbo decoders as SNR improvers • Post decoding combination • Two turbo decoders in parallel [1] B. Mielczarek and A. Svensson, “Timing error recovery in turbo coded systems on AWGN channels," IEEE Trans. Comm., vol. 50, pp. 1584-1592, October 2002.

  4. BER Performance of Turbo Coded System with Fixed Timing Offset • Timing offset is the difference of the actual sampling time and the symbol’s ending point. • Simulations show the effect of fixed timing offset on BER performance. • Simulation settings • Following cdma2000 encoder structure • Coding rate = 1/3 • Interleaver size = 1530 • Constraint length = 4 • 10 iterations • Raised-cosine pulse shaping with roll-off factor 0.5 0 10 -1 10 τ = 0.2T -2 10 τ = 0.1T -3 10 Bit Error Rate -4 10 1 dB -5 10 τ = 0 0.25 dB -6 10 -7 10 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Eb/N0 (dB)

  5. Effective SNR • Received signal • Effective SNR • Mean squared error of ISI Energy per symbol Additive noise Intersymbol interference Transmitted data Pulse shaping function

  6. Effective SNR and Timing Offset • Simulation settings: • Uncoded BPSK signal in AWGN channel • Raised-cosine pulse shaping with roll-off factor 0.5 • The effective SNR is a function of both channel SNR and timing offset. • The shape and position of the curves changes with the channel SNR. 3 2 SNR = 2 dB 1 0 Effective SNR (dB) -1 SNR = 1 dB -2 SNR = 0 dB -3 -4 -5 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 Timing Offset (T)

  7. Joint Estimation of SNR and Timing Offset • Effective SNR is a function of both timing offset and SNR on channel. • With multiple values of effective SNR, we can solve the equation for timing offset and SNR simultaneously. • Assuming semi-static channel, we need to estimate the effective SNR on basis of each frame without knowledge of transmitted data. • To generate multiple effective SNR estimates, multiple samples of each symbol in a frame are taken.

  8. Over-Sampling and Interpolation • Decoder infrastructure { r } s Online 1 , n 1 Channel Received Estimation Joint r ( t ) Signal Matched Estimation Filter s Algorithm { r } Online 2 2 , n Channel Estimation SNR Timing Estimation Estimation Interpolated Decoded Samples Data Inter- Turbo polator Decoder

  9. Interpolation Method • Linear interpolation • Information loss due to improper timing cannot be recovered • Generate satisfactory results with sufficient large number of samples per symbol • More subtle interpolation methods • Boost up noise which is dominant in the environment where turbo codes are implemented • Do not improve SNR r2 r0 r1 0 t1 t2 Linear interpolation

  10. Online Statistics • Summers and Wilson’s approach [2] • The calculation does not require knowledge of the data. • The statistic itself is a random variable. • The statistic is a one-to-one map to SNR. • The statistics can be used to calculate the effective SNR associated with sample sequences • The joint estimation algorithm can be implemented with statistics directly. [2] T. A. Summers and S. G. Wilson, “SNR mismatch and online estimation in turbo decoding,” IEEE Trans. Comm., vol. 46, pp. 421-423, April 1998.

  11. Joint Estimation Algorithm • Curve fitting algorithm • Minimum mean-squared error approach • Needs to store all possible curve shapes • Vulnerable to fluctuation of statistic values • Linear approximation • Solution forβ0andτ

  12. Simulation Settings • Turbo encoder • Interleaver size = 1530 • Constraint length = 4 • Code rate = 1/3 • Encoder structure follows cdma2000 specifications • Semi-static AWGN channel • BPSK modulation • Raised-cosine roll-off pulse shaping with roll-off factor 0.5 • Timing offset is a random variable with uniform distribution between -0.5T and 0.5T, but it is fixed in each frame • Timing offset differs from frame to frame • Turbo decoder • 10 iterations are conducted before hard-decision is made • Linear approximation is used for joint estimation algorithm • Linear interpolation is used to regenerate signals

  13. BER Performance of Turbo Coded System with 2 Samples/Symbol • About 0.2 dB loss due to linear interpolation • About 0.7 dB loss due to error in estimation of timing offset • About 0.8 dB loss due to error in joint estimation of channel SNR and timing offset. 0 10 Estimated Timing and SNR -1 Known SNR, Estimated Timing 10 Known Timing and SNR Perfect Timing -2 10 -3 10 Bit Error Rate -4 10 -5 10 -6 10 -7 10 -8 10 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Eb/N0 (dB)

  14. BER Performance of Turbo Coded System with 4 Samples/Symbol 0 10 • Almost no loss due to linear interpolation. • Less than 0.2 dB coding gain loss from error in joint estimation of channel SNR and timing offset. • Implementation with one turbo decoder and low additional complexity and latency. Estimated Timing and SNR Known Timing and SNR -1 10 Perfect Timing -2 10 -3 Bit Error Rate 10 -4 10 -5 10 -6 10 -7 10 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Eb/N0 (dB)

  15. Possible Improvements • Simulation results indicate that BER performance can be improved without changing the decoder structure by • Using better estimators that make estimation with smaller variance; • Using better interpolators that recover more information by combining samples without loss of SNR. • Refine estimation using turbo principles. • The iterative nature of turbo decoder enables us to rebuild reliable decision using intermediate decoding results. • The intermediate decoding result can be directed back to form a loop. • The joint estimation of channel SNR and timing offset can be refined with decision-feedback.

  16. Conclusions • Effective SNR can be used to evaluate loss in coding gains for turbo coded systems caused by improper timing. • With slight additional complexity, the proposed system can recover coding gains to 0.2 dB within the performance of perfect timing situation with 4 samples per symbol.

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