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 SpecsNI 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 SpecsNI 5670 – RF Vector Signal

Generator

Table 1 [1]

System DesignSymbol rates supported

1Msps, 2Msps,5Msps,10Msps,12.5Msps

Channel codingRate 2/3 convolutional code

Modulation schemes supportedBPSK, 4-QAM, 16-QAM

Pulse ShapingRaised cosine pulse shape with roll-off 0.5

System DesignPassband Bandwidth

1MHz, 2MHz, 5MHz, 10MHz, 12.5MHz

Number of subcarriers N (= FFT Size)64

Length of Cyclic Prefix Lc

16

PAPR Oversample Factor4

System DesignSymbol 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 TechniquesInterleaving

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

Procedure

Loop back Tx-Rx by an RF cableSend a sequence of all ones (1’s) so that

the max. theoretical PAPR is reached

(N – Number of subcarriers)Oversample the Rx signal & calculate

PAPRCompare observed PAPR with theoretical

results for the different schemes

Max. PAPR = 10*log10(N)

System Setup for Expt. IData: All 1’s sequenceSymbol Rate: 1 MspsModulation scheme: 4-QAM N=64, Lc=16No channel coding

Tx average power level = - 2.2dBmPEP is just below 17dBm!

Rx reference level = 20dBm

Experiment I

Results

Effect of PA saturation In-band distortion1dB 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 cableSend a sequence of random bitsOversample the Rx signal & calculate

PAPR for the different schemesPlot the CCDF at Tx & RxObserve reduction in PAPRObserve changes to Tx constellation

System Setup for Expt. IIData: Random bitsSymbol Rate: 1 MspsModulation scheme: 4-QAM N=64, Lc=16No 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

Table 2 [7]

Technique Distortionless

Power Increase

Data rate loss

RCF No No No

Interleaving Yes No Yes

SLM Yes No Yes

PTS Yes No Yes

Tone Injection Yes Yes No

ACE Yes Yes No

Tradeoff

Table 2 [6]

Technique Processing at Tx & Rx

RCF Tx: Amplitude clipping, filteringRx: None

Interleaving Tx: K IDFTs, (K – 1) interleavingsRx: Side information extraction, inverse interleaving

SLM Tx: U IDFTsRx: Side information extraction, inverse SLM

PTS Tx: M IDFTs, WM–1 complex vector sumsRx: Side information extraction, inverse PTS

Tone Injection Tx: IDFTs, search for maximum point in time, tones to be modified, value of p and qRx: Modulo-D operation

ACE Tx: IDFTs, projection onto “shaded area”Rx: None

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.