Peak-to-Average Power Ratio (PAPR)

1. Peak-to-Average Power Ratio (PAPR) • One of the main problems in OFDM system is large PAPR /PAR(increased complexity of the ADC and DAC, and reduced efficiency of RF power amplifier, and etc.) • An OFDM signal consists of a number of independently modulated subcarriers, which can give a large PAPR /PAR when added up coherently.

3. PAPR (Cont.) The crest factor

4. Reducing PAR techniques • Signal distortion techniques [Clipping (rectangular) and Peak windowing (Cosine, Kaiser, Hamming)] * window length increase -> reduce out of band radiation but increase BER • Probabilistic techniques (Partial transform Sequence (PTS), Selective Mapping (SLM)) • Coding techniques (Block coding) * no good codes for practical value of N>64 and larger constellation size ( >4 )are known.

6. Clipping

7. Smart Clipping

9. Selective Mapping (SLM) In SLM , transmitter selects one of the smallest PAR OFDM signal by using phase rotation.

11. Partial Transmit Sequence (PTS) In PTS, the data symbols are broken into several Sub-blocks. These sub-blocks are added and transmitted with optimized phase rotation factors.

12. PTS (cont.)

13. Drawbacks of techniques for reducing PAPR • Reducing data rate. • (the side information, coding rate) • Increasing the out of band radiation and BER. • (clip the peak power signals) • Increasing systems complexity. • (PTS, SLM)

15. OFDM Systems

16. OFDM Transceiver RF Tx DAC Binary Input Data Add Cyclic extension & Windowing QAM mapping Pilot Insertion S - P P - S Coding Interleaving IFFT FFT Remove Cyclic extension QAM demapping Channel Correction P - S S - P Decoding De-Interleaving Binary Output Data Timing & Freq. Sync. RF Rx ADC

17. OFDM based Applications • Wireless LAN standards using OFDM are • HiperLAN-2 in Europe • IEEE 802.11a, .11g • OFDM based Broadband Access Standards are getting defined for MAN and WAN applications • 802.16 Working Group of IEEE • 802.16 -- single carrier, 10-66GHz band • 802.16a, b -- 2-11GHz, MAN standard

18. IEEE 802.11a Overview • Carrier frequency= 5 GHz • Total allotted bandwidth= 20 MHz x 10 = 200MHz • Size of the FFT= 64 • Number of data subcarriers= 48 • Number of Pilot subcarriers= 4 • FFT period= 3.2 µs • Channel bandwidth used= 64/3.2 µs => 20 MHz

19. Typical Configuration • 52 subcarriers, 64 point FT/IFFT • Symbol time 4 µs • Guard time 800 ns • BPSK, QPSK, 16-QAM, 64-QAM • Coding rates 1/2,3/4,2/3 • Bit rates 6,12,18,24,36,48,54 Mbps • Channel spacing 20 MHz • Tolerable delay spread about 250 ns at 24 Mbps

20. DFT (FFT) as Signal Generatorfor Complex Sinusoids

21. DFT (FFT) As Signal Analyzer for Complex Sinusoids

22. Radix-2 FFT Flow Diagrams

23. OFDM Modulation With IFFTand Interpolator

24. OFDM Demodulation With FFT

25. OFDM Transceiver

26. Linear Versus Circular Convolution

27. Fast Circular Convolution with the FFT

28. Reserve Frequency Bins For Clipping Pulses

29. Selecting Reserve Frequency Bins

30. Reserve Bin Canceller Clipping at 2.5 s (8 dB)

31. Statistics for Clip at 2.5 (8 dB)

32. Reserve Bin Canceller Clipping at 2.2 s (6.9 dB)

33. Statistics for Clip at 2.2 (6.9 dB)

34. Reserve Bin Canceller Clipping at 2.0 s (6 dB)

35. Statistics for Clip at 2.0 (6 dB)