PAPR Reduction Method for OFDM Systems without Side Information Pornpawit Boonsrimuang, Pisit Boonsrimuang Kazuo Mori, Tawil Paungma and Hideo Kobayashi

Contents Research Topic Objective of Research Conventional Methods Proposal of PAPR Reduction Method Performance Evaluation Conclusions

Proposal of PAPR Reduction Method Research Topic Proposal of PAPR Reduction Method for OFDM Signal without Side Information OFDM: Orthogonal Frequency Division Multiplexing

Target of Research Field Next Generation of Wireless LAN Outdoor 100Mbit Indoor Gigabit Next Generation of Wireless LAN

WiMAX(Worldwide Interoperability for Microwave Access) Advantages of OFDM Enable the higher data rate transmission Robustness to multi-path fading Efficient usage of frequency bandwidth The OFDM has already been adopted as the standard transmission techniques in ADSL (Asynchronous Digital Subscriber Line) Terrestrial Digital TV WiMAX(Worldwide Interoperability for Microwave Access) Wireless LAN(Wireless Local Area Network) Candidate transmission technique for the next generation of wireless LAN systems

Structure of OFDM System System Overview (a) OFDM spectrum s(t) Serial to Parallel (S/P) ... ... Parallel to Serial (P/S) Mod IFFT Add GI AMP Transmitter vs. (b) conventional FDM spectrum Fading Channel H(w) Channel Noise n(t) Receiver r(t) Parallel to Serial (P/S) ... ... Serial to Parallel (S/P) Equalization Remove GI Demod FFT Channel estimation

Disadvantage of OFDM Larger peak to averaged power ratio (PAPR) Larger PAPR will cause the degradation of BER performance and expansion of spectrum re-growth in the non-linear channel PAPR: Peak to Averaged Power Ratio

Comparison of OFDM Signal with Single Carrier Signal 2 2 1.5 1.5 Amplitude of OFDM Time Domain Signal 1 Amplitude of OFDM Time Domain Signal 1 0.5 0.5 100 200 300 400 500 100 200 300 400 500 Sample Number Sample Number (a) OFDM signals in time domain (b) Single carrier signal in time domain Amplitude of OFDM time domain signal is fluctuated much larger than that for single carrier signal

Operation of OFDM Signal at Lower IBO in Non-Linear Channel -10 -8 -6 -4 -2 2 4 Input Power (dB) Output Power (dB) r=2 r=4 r=30 r=6 Linear where F[ ] represent the AM/AM conversion characteristics of non-linear amplifier

Operation of OFDM Signal at Higher IBO in Non-Linear Channel Inter-modulation Noise -10 -8 -6 -4 -2 2 4 Input Power (dB) Output Power (dB) r=2 r=4 r=30 r=6 Linear where F[ ] represent the AM/AM conversion characteristics of non-linear amplifier

Inter-Modulation Noise Occurred at Non-Linear Amplifier Transmitter structure of conventional OFDM ... Parallel to Serial (P/S) Mod Serial to Parallel (S/P) s(t) Add GI IFFT AMP AMP Non-Linear (a) Input of power amplifier (b) Output of power amplifier

Disadvantage of OFDM Technique OFDM with N sub carriers has PAPR ratio of 10log N [dB] N sub Carriers Larger PAPR Required Low Input Back-Off Inefficient Usage of Power Amplifier

Several methods have been proposed to reduce the PAPR of OFDM signal Improvement of PAPR Several methods have been proposed to reduce the PAPR of OFDM signal Data coding technique The main disadvantage of this method is to degrade transmission efficiency Clipping technique BER performance Phase alignment technique The main disadvantage of this method is required to transmit side information which leads the degradation of transmission efficiency

Conventional PAPR Reduction Methods Based on Phase Alignment Technique The Selected Mapping (SLM) and Partial Transmission Sequence (PTS) methods which control the phase of data sub-carrier SLM and PTS are required to inform the phase information to the receiver as the side information (SI) for the correct demodulation Although the conventional method could improve the PAPR performance relatively, the transmission efficiency and system complexity would be degraded because of the necessity of transmission of side information

Objective of Research Improve PAPR and BER performances without degradation of transmission efficiency Proposal of PAPR Reduction Method for OFDM Signal Based on Phase Alignment Technique which Requires No Side Information

Proposal of PAPR Reduction Method for OFDM Signal without Side Information Use the common weighting factor over one frame including the preamble symbols The time-frequency domain swapping algorithm is employed in the determination of common weighting factor The common weighting factor can be estimated together with the channel frequency response by using preamble symbols The common weighting factor can be removed from the received data symbols by using the frequency domain equalization Proposed method could improve the PAPR performance without Side Information

Structure of Proposed PAPR Reduction Method Weighting Factors for Reducing PAPR 1st Preamble Symbol 2nd Preamble Symbols Common weighting factor over one frame 1st Data Symbol One Frame Lth Data Symbol 1 2 N-1 Number of sub-carriers Common weighting factor will be assigned for each sub-carrier over one frame

Amplitude (Time Domain) Amplitude (Time Domain) Determination of Weighting Factor by Using Time-Frequency Domains Swapping Algorithm Predetermined threshold level (S) Amplitude (Time Domain) FFT Amplitude (Time Domain) Time domain Frequency domain Basic concept of Time-Frequency Swapping Algorithm

Flowchart of Time-Frequency Domains Swapping Algorithm 3 Calculation of Time-domain Signal 2 S mi (t) 1 Threshold level(S) N-1 N-FFT Calculation of Error Signal 2 FFT ei (t) 1 N-1 N-FFT Determined Phase (weighting factor)

Basic Concept of Proposed Algorithm N-1 1 2 3 4 5 6 N-2 N-1 Frequency Domain IFFT 10 20 30 40 50 60 -1 1 2 Sampling time Amplitude S Threshold Level Time Domain

Basic Concept of Proposed Algorithm N-1 1 2 3 4 5 6 N-2 N-1 Frequency Domain FFT 10 20 30 40 50 60 -1 1 2 Sampling time Amplitude Time Domain

Basic Concept of Proposed Algorithm N-1 1 2 3 4 5 6 N-2 N-1 Frequency Domain FFT 10 20 30 40 50 60 -1 1 2 Sampling time Amplitude Time Domain

Basic Concept of Proposed Algorithm N-1 1 2 3 4 5 6 N-2 N-1 Frequency Domain FFT 10 20 30 40 50 60 -1 1 2 Sampling time Amplitude Time Domain

Basic Concept of Proposed Algorithm N-1 1 2 3 4 5 6 N-2 N-1 Frequency Domain IFFT 10 20 30 40 50 60 -1 1 2 Sampling time Amplitude Time Domain

Basic Concept of Proposed Algorithm N-1 1 2 3 4 5 6 N-2 N-1 Frequency Domain FFT 10 20 30 40 50 60 -1 1 2 Sampling time Amplitude S Time Domain

Amplitude of OFDM signal in the time domain 1 3 2 120 80 60 40 20 100 Time Sample Number (a) Before T-F Swapping Algorithm Amplitude 1 0.5 1.5 120 80 60 40 20 100 Time Sample Number (b) After T-F Swapping Algorithm Amplitude

Determination of Weighting Factor Error signal in frequency domain OFDM signal in frequency domain Determined weighting factor Average for the weighting factors obtained for all symbols Common weighting factor over one frame

(a) Original OFDM Signal (b) Phase Rotation due to weighting factor Scatter Diagram of Preamble and Data Symbols -1 1 (a) 1 -1 (b) (a) Original OFDM Signal (b) Phase Rotation due to weighting factor

Structure of Transmitter for Proposal Method / P MOD Xn Data & Preamble N - 1 I F T + GI SSPA D A U C yk X i Number of predetermined iteration or PAPR < Target No Yes ³ Calculate error signal FFT Determination of common weighting factor

Structure of Receiver for Proposed Method / C & A Estimation Preamble X DEMOD DATA - GI S P FFT 1 ˆ n H Received Data Frequency Domain Equlization Estimated Channel Response Demodulated Data : Weighting Factor at n-th Sub-Carrier

in Performance Evaluation Multi-path fading model List of Parameters in Performance Evaluation Modulation 64QAM Demodulation Coherent Allocated bandwidth 5MHz Number of FFT points 256 Number of sub-carriers 64 Symbol duration 12.8 s Guard interval 1.28 s Non-linear amplifier SSPA Non-linear parameter of SSPA r =2 Number of data symbols in one frame 12 Number of preamble symbols in one frame 2 Multi-path fading model Power delay profile Exponential Number of delay paths 16 Decay constant -1 dB

PAPR Performance Modulation Method =64QAM Conventional Proposed

PAPR Performance vs. Number of Iterations Modulation Method =64QAM Conventional Proposed

BER Performance Modulation Method =16QAM Conventional Ideal Proposed

BER Performance Modulation Method =64QAM Conventional Ideal Proposed

Conclusions Proposed the PAPR reduction method for OFDM signal without side information Use the common weighting factor over one frame including the preamble symbols The time-frequency domain swapping algorithm is employed in the determination of common weighting factor The common weighting factor can be removed from the received data symbols by using the frequency domain equalization The common weighting factor can be estimated together with the channel frequency response by using preamble symbols

Conclusions The proposed technique can achieve the better PAPR performance and better BER performance than the conventional OFDM in the channel of non-linear amplifier and multi-path fading