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PAPR Reduction Methods for Noncoherent OFDM-MFSK

PAPR Reduction Methods for Noncoherent OFDM-MFSK. 3rd COST 289 Workshop Aveiro, Portugal, July 12-13, 2006. Matthias Wetz, Werner G. Teich, Jürgen Lindner. matthias.wetz@uni-ulm.de http://it.e-technik.uni-ulm.de. Motivation. Fast time variant channels for data transmission to and from

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PAPR Reduction Methods for Noncoherent OFDM-MFSK

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  1. PAPR Reduction Methods for Noncoherent OFDM-MFSK 3rd COST 289 Workshop Aveiro, Portugal, July 12-13, 2006 Matthias Wetz, Werner G. Teich, Jürgen Lindner matthias.wetz@uni-ulm.de http://it.e-technik.uni-ulm.de

  2. Motivation • Fast time variant channels for data transmission to and from • high speed trains • Security relevant data requires robust transmission scheme • Combination of OFDM and noncoherently detected MFSK offers • high data rate and robustnes • A problem of multicarrier transmission is a high PAPR Subcarrier phases for noncoherent OFDM-MFSK are arbitrary Use the phases to reduce the PAPR

  3. Outline • Motivation • Basic OFDM Transmission Model • A Robust Transmission Scheme - OFDM-MFSK • PAPR Reduction Algorithms • Influence on the Spectrum of the Transmit Signal • Conclusions

  4. OFDM Transmission Model par ser iΔt cyclic ext. IDFT CODmod TF s(t) s(i) Coding ser par x(k) h(t) Channel AWGN par ser n(t) rem. cyclic ext. iΔt DFT DET RF g(t) Detection (Decoding) ser par

  5. OFDM-MFSK Δf … 00 01 11 10 00 01 11 10 OFDM-Subcarriers (Frequency) • OFDM-4FSK: Subcarriers are grouped into groups of four • 4FSK modulation over each group • One out of four carriers is occupied • Gray coding • Coherent and noncoherent detection possible + For noncoherent detection no CSI is necessary + Very robust against time variant channels + Subcarrier phases are arbitrary and can be used for PAPR reduction

  6. Peak-to-Average Power Ratio Definition PAPR: Unfavourable superposition of subcarriers in OFDM Very high PAPR of time domain signal Problem: Transmit amplifier has saturation limit Nonlinear distortion (Out of Band Radiation) High backoff necessary (amplifier inefficient) Noncoherently detected OFDM-MFSK Subcarrier phases can be chosen arbitrarily so that PAPR is reduced No side information necessary

  7. PAPR Reduction Goal: Find optimum subcarrier phases for each possible OFDM symbol, so that PAPR is minimum Problem: N=256 and OFDM-4FSK possible OFDM symbols, possibilities to assign phase, if two phases for each subcarrier are considered Exhaustive search impossible Worst case: All subcarrier phases are the same Subcarriers add coherently PAPR = N/M = 256/4 = 18 dB

  8. 1 0.9 0.8 0.7 0.6 random continuous phases [0, 2π) 0.5 CDF(z)= P(PAPR<z) 0.4 0.3 0.2 random discrete phases 0 or π 0.1 5 6 7 8 9 10 11 z=PAPR[dB] PAPR Reduction Methods Cumulative Distribution Function (CDF) • First approach: • Random phases • Allow only 0 or π

  9. Selected Mapping • Introduced by Bäuml, Fischer and Huber (´96) • Assign random subcarrier phases to each symbol several times • Transmit OFDM symbol • with lowest PAPR • When applied to noncoherently • detected OFDM-MFSK, no • side information is needed 1 Selected Mapping 0.9 best of 2 symbols Selected Mapping 0.8 best of 4 symbols 0.7 Selected Mapping best of 10 symbols 0.6 random continuous with discrete random phases [0, 2π) phases (0 or π) is chosen 0.5 CDF(z) 0.4 0.3 0.2 random discrete phases 0 or π 0.1 5 6 7 8 9 10 11 z=PAPR[dB]

  10. Time-Frequency Domain Swapping • Introduced by Ouderaa et al. (´88) • Swapping between time and frequency domain • Iterative reduction of • PAPR • Stop when PAPR is not • decreasing any more • Parameter: time domain • clipping level CL random starting phases build spectrum with fixed amplitudes and variable determine phases FFT IFFT amplitude clipping in time domain

  11. Time-Frequency Domain Swapping (cont´d) • Good performance • Very high complexity: • up to several hundred • iterations per symbol 1 CL=0.95 0.9 random phases 0.8 CL=0.8 0 or π CL=0.9 selected mapping 0.7 best of 10 symbols 0.6 CDF(z) 0.5 0.4 0.3 0.2 time-frequency domain swapping algorithm 0.1 3 4 5 6 7 8 9 10 z=PAPR[dB]

  12. Sequential Algorithm random starting phases IFFT • Subcarrier phases are • systematically changed to • reduce PAPR • Subcarrier phases are flipped • sequentially • One extra IFFT per occupied • subcarrier PAPR evaluation flip φn IFFT PAPRnew < PAPR ? yes no PAPR=PAPRnew accept φn discard changes next subcarrier n

  13. Sequential Algorithm (cont´d) • Better performance than selected mapping • Lower complexity than • swapping algorithm 1 selected mapping best of 10 0.9 0.8 good trade off complexity / performance 0.7 selected mapping best of 65 0.6 swap algorithm CL=0.9 CDF(z) 0.5 0.4 random phases 0/π 0.3 sequential algorithm 0.2 0.1 3 4 5 6 7 8 9 10 z=PAPR[dB]

  14. Complexity Comparison • Random Phases: • PAPR 6-10.5dB • Selected Mapping (best of 10 symbols): • PAPR 5.8-7.8dB • 10 FFTs in total • Sequential Algorithm: • PAPR 5.1-6.9dB • 1 extra FFT per occupied subcarrier • 65 FFTs in total • Time-Frequency Domain Swapping (CL=0.8): • PAPR 3.8-6.5dB • About 200 FFTs in total In general, better performance means higher complexity Remaining problem: PAPR reduction has to be done for each symbol

  15. NL Zonal Filter s(t) s‘(t) s‘BP(t) Model of a Nonlinear Transmit Amplifier • Nonlinearity causes distortion at harmonic bands of carrier frequency • Zonal filter limits signal to be a bandpass signal • Nonlinearity can be modeled in the lowpass domain

  16. 1.5 1.5 1 1 0.5 0.5 0 0 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 Model of a Nonlinear Transmit Amplifier Aout Aout Ain Ain Transformation of the characteristics into lowpass domain Soft limiter in bandpass domain: amplitude saturates

  17. Transmit Spectrum with Nonlinear Distortion random subcarrier phases 0/π 10 0 -10 3dB IBO -20 -30 PSD [dBr] 6dB -40 -50 9dB -60 -70 -80 -90 -100 -500 -400 -300 -200 -100 0 100 200 300 400 500 Δf f/ • Simulation parameters: • Raised cosine transmit filter α=0.2 • 160 used subcarriers • Reference point: Interference in next channel after neighbour channel < -70dBr

  18. Selected Mapping (best of ten) Sequential Algorithm 10 10 0 0 -10 -10 3dB IBO 3dB IBO -20 -20 -30 -30 PSD [dBr] 6dB -40 -40 PSD [dBr] 5dB -50 -50 -60 -60 6dB 7dB -70 -70 -80 -80 -90 -90 -100 -100 -500 -400 -300 -200 -100 0 100 200 300 400 500 -500 -400 -300 -200 -100 0 100 200 300 400 500 Δf Δf f/ f/ Transmit Spectrum with Nonlinear Distortion • Further reduction possible with swapping algorithm but improvement is small

  19. Summary and Conclusions OFDM-MFSK was presented • Noncoherent detection possible • Robust transmission scheme • Subcarrier phases can be used for PAPR reduction PAPR reduction algorithms were analysed • Selected Mapping • Time-frequency domain swapping • Sequential algorithm Influence on the spectrum of the transmit signal • Effects of different PAPR reduction methods were compared

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