The University of Texas at Austin EE 381V Wireless Communications Lab Graduate Course Project PAPR Reduction Techniques in OFDM Systems Nachiappan Valliappan & Rajaganesh Ganesh The University of Texas at Austin

Objectives Understand the effects of high PAPR in multicarrier systems Investigate performance of available PAPR reduction techniques Identify criterion for PAPR reduction technique selection

Instrument Specs NI 5660 – RF Signal Analyzer Input power +30 to -130 dBm (provides up to 50 dB of input attenuation) Digitizer 64MS/s NI 5670 – RF Vector Signal Generator Output average power -145dBm to +13dBm Maximum allowable peak envelope power +17dBm 1dB Gain Compression point dependent on temperature, frequency etc.

Instrument Specs NI 5670 – RF Vector Signal Generator Table 1 [1]

System Design Symbol rates supported 1Msps, 2Msps,5Msps,10Msps,12.5Msps Channel coding Rate 2/3 convolutional code Modulation schemes supported BPSK, 4-QAM, 16-QAM Pulse Shaping Raised cosine pulse shape with roll-off 0.5

System Design Passband Bandwidth 1MHz, 2MHz, 5MHz, 10MHz, 12.5MHz Number of subcarriers N (= FFT Size) 64 Length of Cyclic Prefix Lc 16 PAPR Oversample Factor 4

System Design Symbol Timing Extraction Max Energy, Early-Late Gate Method Frame Timing & Frequency Offset Estimation Schmidl-Cox Algorithm Channel Estimation & Equalization IEEE 802.11a training sequence

PAPR Reduction Techniques Interleaving Amplitude Clipping & Filtering (RCF) Selection Level Mapping (SLM) Partial Transmit Sequence (PTS) Active Constellation Exchange (ACE) Tone Injection

Experiment I PAPR Measurement for unusually high PAPR Signals

Loop back Tx-Rx by an RF cable Procedure Loop back Tx-Rx by an RF cable Send a sequence of all ones (1’s) so that the max. theoretical PAPR is reached (N – Number of subcarriers) Oversample the Rx signal & calculate PAPR Compare observed PAPR with theoretical results for the different schemes Max. PAPR = 10*log10(N)

System Setup for Expt. I Data: All 1’s sequence Symbol Rate: 1 Msps Modulation scheme: 4-QAM N=64, Lc=16 No channel coding Tx average power level = - 2.2dBm PEP is just below 17dBm! Rx reference level = 20dBm

Experiment I Results

Effect of PA saturation In-band distortion 1dB compression point 13dBm @ 2.7G, 16dBm @ 2G @2GHz @2.7GHz

No PAPR scheme

RCF

Interleaving

SLM

PTS

ACE

Experiment II PAPR Measurement of a typical OFDM signal Complementary CDF (CCDF) comparison

Procedure Loop back Tx-Rx. by an RF cable Send a sequence of random bits Oversample the Rx signal & calculate PAPR for the different schemes Plot the CCDF at Tx & Rx Observe reduction in PAPR Observe changes to Tx constellation

System Setup for Expt. II Data: Random bits Symbol Rate: 1 Msps Modulation scheme: 4-QAM N=64, Lc=16 No channel coding Tx average power level = -40dBm Rx reference level = -20dBm

Experiment II Results

RCF

Effect of Tx Power Spectrum Before RCF After RCF

Effect on Tx Constellation

Interleaving

SLM

PTS

ACE

Effect on Tx Constellation

Tone Injection

Effect on Tx Constellation

Experiment III A typical OFDM system with PAPR reduction

Procedure Transmit random bits over the wireless channel Perform synchronization, offset, channel estimation & equalization Find the BER for uncoded transmissions Observe the impact of in-band distortion (esp. in RCF!) on BER

Experiment III Results

5MHz Bandwidth

10MHz Bandwidth

12MHz Bandwidth

BER vs SNR - Uncoded 4-QAM

PAPR Techniques - A Comparative Study

Tradeoff Table 2 [7] Technique Distortionless Power Increase Data rate loss RCF No Interleaving Yes SLM PTS Tone Injection ACE Table 2 [7]

Table 2 [6] Technique Processing at Tx & Rx RCF Tx: Amplitude clipping, filtering Rx: None Interleaving Tx: K IDFTs, (K – 1) interleavings Rx: Side information extraction, inverse interleaving SLM Tx: U IDFTs Rx: Side information extraction, inverse SLM PTS Tx: M IDFTs, WM–1 complex vector sums Rx: Side information extraction, inverse PTS Tone Injection Tx: IDFTs, search for maximum point in time, tones to be modified, value of p and q Rx: Modulo-D operation ACE Tx: IDFTs, projection onto “shaded area” Table 2 [6] Table 3 [7]

References [1] National Instruments, NI RF Signal Generator: NI PXI-5670/5671 Specifications, Retrieved December 3, 2010 from http://www.ni.com/pdf/manuals/371355c.pdf [2] National Instruments, 2.7 GHz RF Vector Signal Analyzer, Retrieved December 2, 2005 from http://www.ni.com/pdf/products/us/4mi469- 471.pdf [3] National Instruments, NI RF Signal Generator: Getting Started Guide, Retrieved December 1, 2005 from http://www.ni.com/pdf/manuals/371356b.pdf [4] National Instruments, NI 5670 RF Vector Signal Generator User Manual, Retrieved December 1, 2005 from http://www.ni.com/pdf/manuals/rfsg _um.pdf [5] National Instruments, 2.7 GHz RF Vector Signal Analyzer, Retrieved December 2, 2005 from http://www.ni.com/pdf/products/us/4mi469-471.pdf

References [6] National Instruments, NI RF Signal Analyzer: Getting Started Guide, Retrieved December 2, 2005 from http://www.ni.com/pdf/manuals/371237a.pdf [7] Jae Hong Lee and Seung Hee Han. An overview of peak-to-average power ratio reduction techniques for multicarrier transmission Wireless Communications. IEEE Wireless Communications Magazine, Vol. 12:pp 56-65, April 2005.