PAPR Reduction Methods for Noncoherent OFDM-MFSK

Transcription

1 3rd COST 289 Workshop Aveiro, Portugal, July 12-13, 2006 PAPR Reduction Methods for Noncoherent OFDM-MFSK Matthias Wetz, Werner G. Teich, Jürgen Lindner

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 Information Technology University of Ulm 2

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 Information Technology University of Ulm 3

4 OFDM Transmission Model COD mod Coding ser par IDFT cyclic ext. par ser s(i) iδt TF s(t) x(k) Channel h(t) DET Detection (Decoding) ser par DFT rem. cyclic ext. par ser iδt RF g(t) AWGN n(t) Information Technology University of Ulm 4

5 OFDM-MFSK Δf 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 Information Technology University of Ulm 5

6 Peak-to-Average Power Ratio Definition PAPR: ( t) ( max s ) 1 T T 0 t s () t 2 2 dt 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 Information Technology University of Ulm 6

7 PAPR Reduction Goal: Problem: Find optimum subcarrier phases for each possible OFDM symbol, so that PAPR is minimum N=256 and OFDM-4FSK / 4 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 Information Technology University of Ulm 7

8 PAPR Reduction Methods 1 Cumulative Distribution Function (CDF) First approach: Random phases Allow only 0 or π CDF(z)= P(PAPR<z) random continuous phases [0, 2π) random discrete phases 0 or π z=papr[db] Information Technology University of Ulm 8

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 Selected Mapping best of 2 symbols When applied to noncoherently detected OFDM-MFSK, no side information is needed CDF(z) Selected Mapping best of 4 symbols Selected Mapping best of 10 symbols with discrete random phases (0 or π) is chosen random continuous phases [0, 2π) random discrete phases 0 or π z=papr[db] Information Technology University of Ulm 9

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 ϕ n IFFT determine phases ϕ n FFT amplitude clipping in time domain Information Technology University of Ulm 10

11 Time-Frequency Domain Swapping (cont d) Good performance Very high complexity: up to several hundred iterations per symbol CL= CL=0.9 CL=0.8 selected mapping best of 10 symbols random phases 0 or π CDF(z) time-frequency domain swapping algorithm z=papr[db] Information Technology University of Ulm 11

12 Sequential Algorithm random starting phases IFFT PAPR evaluation flip φ n IFFT PAPR new < PAPR? yes no Subcarrier phases are systematically changed to reduce PAPR Subcarrier phases are flipped sequentially One extra IFFT per occupied subcarrier PAPR=PAPR new accept φ n discard changes next subcarrier n Information Technology University of Ulm 12

13 Sequential Algorithm (cont d) Better performance than selected mapping Lower complexity than swapping algorithm selected mapping best of 10 good trade off complexity / performance CDF(z) swap algorithm CL=0.9 selected mapping best of sequential algorithm random phases 0/π z=papr[db] Information Technology University of Ulm 13

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

15 Model of a Nonlinear Transmit Amplifier NL Zonal Filter s(t) s (t) s BP (t) 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 Information Technology University of Ulm 15

16 Model of a Nonlinear Transmit Amplifier A out A out A in Soft limiter in bandpass domain: amplitude saturates A in Transformation of the characteristics into lowpass domain Information Technology University of Ulm 16

17 Transmit Spectrum with Nonlinear Distortion 10 random subcarrier phases 0/π PSD [dbr] dB IBO 6dB 9dB f/δf Simulation parameters: Raised cosine transmit filter α= used subcarriers Reference point: Interference in next channel after neighbour channel < -70dBr Information Technology University of Ulm 17

18 Transmit Spectrum with Nonlinear Distortion 10 Selected Mapping (best of ten) 10 Sequential Algorithm 0 0 PSD [dbr] dB IBO 6dB PSD [dbr] dB IBO 5dB dB dB f/δf f/δf Further reduction possible with swapping algorithm but improvement is small Information Technology University of Ulm 18

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 Information Technology University of Ulm 19