Presentation on theme: "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."— Presentation transcript:
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.
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.
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)
16 OFDM Transceiver Coding Binary Input Data Interleaving QAM mapping Pilot Insertion S - P IFFT FFT Decoding De-Interleaving QAM demapping Channel Correction P - S Binary Output Data S - P P - S Add Cyclic extension & Windowing DACRF Tx Remove Cyclic extension Timing & Freq. Sync. ADCRF Rx
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 Generator for Complex Sinusoids
21 DFT (FFT) As Signal Analyzer for Complex Sinusoids