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Peak-to-Average Power Ratio (PAPR)

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.)

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Peak-to-Average Power Ratio (PAPR)

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  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.

  2. PAPR (Cont.) The crest factor

  3. 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.

  4. Clipping

  5. Smart Clipping

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

  7. 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.

  8. PTS (cont.)

  9. 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)

  10. 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

  11. Selection of OFDM parameters • Bandwidth, bit rate, delay spread • Guard time Tg • 2 to 4 times delay spread  2 to 4 • depends on the order of modulation employed • Symbol duration > Guard time to maximize SNR • More subcarriers, smaller spacing, implementation complexity, more sensitivity to phase noise & frequency offset, high PAPR • Symbol duration  5 x Guard time ( 1-dB SNR loss ) • Ts = 5 x Tg  Tofdm = Ts + Tg • Subcarrier spacing f = 1 / Ts • Number of subcarriers = 3-dB BW / f

  12. Example : • Bit rate = 20 Mbps • Tolerable delay spread = 200 ns • Bandwidth < 15 MHz • Tg = 800 ns • Tofdm = 5 x Tg + Tg = 4.8 sec • f = 1 / 4 sec = 250 KHz • Number of bits in one OFDM symbol = 20 Mbps x 4.8 sec = 96 • 16-QAM with rate ½ Conv. Coding  2 bits / symbol / subcarrier  48 subcarriers  48 x 250 KHz = 12 MHz < 15 MHz • QPSK with rate ¾ coding  1.5 bits / symbol / subcarrier  64 subcarriers  64 x 250 KHz  16 MHz > 15 MHz • 64 point IFFT / FFT  16 zero subcarriers  oversampling Given requirements

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