Kashif Sultan, Hazrat Ali, Zhongshan Zhang

Presentation by

Dr. Hazrat Ali

COMSATS Institute of Information Technology

Joint SLM and Modified

Clipping Technique

for PAPR Reduction

14 Jan, 2016 IBCAST 2016 1

Orthogonal Frequency Division Multiplexing

Peak-to-Average Power Ratio

Effect on Power Amplifier

PAPR Reduction Techniques

Selective Mapping

Conventional Clipping

Proposed Scheme

Modified Clipping

Joint SLM and Modified Clipping

Contents

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OFDM forms the basis for 4G (fourth generation wireless communication)

It is revolutionary technology in wireless communication

Long Term Evoultion (LTE) – the 4G cellular standard

LTE is based on two key technologies OFDM

WiMax

OFDM is a key broadband wireless technology which supports data rates in excess of 100 Mbps

WLAN standard 802.11a/g/n are also based on OFDM.

Orthogonal Frequency Division

Multiplexing (OFDM)

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OFDM

Block Diagram of an OFDM system

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The peaks in an OFDM signal cause challenges as they create non-uniform envelope

Due to high peaks in a signal, the power amplifier goes to its non-linear region resulting in non-linear distortion and spectral spreading

𝑃𝐴𝑃𝑅 𝑥 𝑛 = max0≤𝑛≤𝑁−1

𝑥 𝑛 2

𝐸 𝑥 𝑛 2

Peak-to-Average Power Ratio (PAPR)

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Ideally, one would prefer a high number of sub-carriers for an OFDM system

However, PAPR increases if the number of sub-carriers is increased

PAPR in OFDM System

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Effect of PAPR on Power Amplifier

High PAPR in OFDM signal results in amplifier saturation.

Loss of orthogonality

Leads to Inter Carrier Interference

Distortion

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PAPR Reduction Techniques

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PAPR Reduction Techniques

Signal Distortion Techniques

Multiple Signaling and Probabilistic Techniques

Coding Technique

Hybrid Techniques

Y. Rahmatallah and S. Mohan, “Peak-To-Average Power Ratio Reduction in OFDM Systems : A

Survey And Taxonomy,” IEEE Comun. Surv. Tutorials, vol. 15, no. 4, pp. 1567–1592, 2013.14 Jan, 2016 IBCAST 2016 9

PAPR Reduction Techniques

Signal Distortion Techniques

Clipping and Filtering

Peak Windowing

CompandingTechniques

Peak Cancellation

Multiple Signaling and Probabilistic Techniques

Selective Mapping

Partial Transmit Sequence

Tone Injection

Active Constellation Extension

Coding Technique

Linear Block Coding

Golay Sequence

Turbo Coding

Hybrid Techniques

Joint PTS-QC Scheme

Joint Clipping and CompandingJoint Clipping and ACE

Y. Rahmatallah and S. Mohan, “Peak-To-Average Power Ratio Reduction in OFDM Systems : A

Survey And Taxonomy,” IEEE Comun. Surv. Tutorials, vol. 15, no. 4, pp. 1567–1592, 2013.14 Jan, 2016 IBCAST 2016 10

PAPR Reduction Techniques

Signal Distortion Techniques

Clipping and Filtering

Peak Windowing

CompandingTechniques

Peak Cancellation

Multiple Signaling and Probabilistic Techniques

Selective Mapping

Partial Transmit Sequence

Tone Injection

Active Constellation Extension

Coding Technique

Linear Block Coding

Golay Sequence

Turbo Coding

Hybrid Techniques

Joint PTS-QC Scheme

Joint Clipping and CompandingJoint Clipping and ACE

Y. Rahmatallah and S. Mohan, “Peak-To-Average Power Ratio Reduction in OFDM Systems : A

Survey And Taxonomy,” IEEE Comun. Surv. Tutorials, vol. 15, no. 4, pp. 1567–1592, 2013.14 Jan, 2016 IBCAST 2016 11

Selective Mapping

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Selective Mapping

Multiple copies of OFDM symbol

Transmit the one with minimum PAPR

Multiplying original data block with different phase factors – prior to IFFT operation

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Conventional Clipping

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Conventional Clipping

The expression for conventional clipping can be written as,

𝑥 𝑡 = 𝑥 𝑡 𝑥 𝑡 < 𝐶𝐿

𝐶𝐿𝑒𝑗∅ 𝑡 𝑥 𝑡 > 𝐶𝐿

Where 𝑥 𝑡 is transmitted signal and ‘CL’ is the clipping level

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Conventional Clipping

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PAPR for Conventional Clipping

• PAPR performance for conventional clipping approach

• Simplest to implement

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Joint SLM and modified

Clipping

(Proposed Approach)

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Joint SLM and modified Clipping

Firstly, the conventional SLM technique is applied.

SLM improves the PAPR and control BER

The application of modified clipping further improves the PAPR

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Input DataSLM

techniqueModified Clipping

Transmit Data

Modified Clipping

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Modified Clipping

Mathematical representation for proposed clipping scheme

𝑥 𝑡 =

𝐶𝐿𝐻𝑒𝑗∅(𝑡) 𝑥 𝑡 > 𝐶𝐿𝐻 𝑥 𝑡 𝐶𝐿𝐿 < 𝑥 𝑡 < 𝐶𝐿𝐻

𝐶𝐿𝐿𝑒𝑗∅ 𝑡 𝑥 𝑡 < 𝐶𝐿𝐿

Where ′ 𝑥 𝑡 ′ is input signal, ‘CLH’ is higher threshold level and ‘CLL’ is lower threshold level

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Joint SLM and modified Clipping

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PAPR for Modified Clipping

• PAPR performance improves for modified clipping (with 2 levels of threshold)

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PAPR for Joint SLM and Modified Clipping

• Joint SLM and Modified Clipping scheme further improves the PAPR performance

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Conclusion

PAPR is a key challenge in OFDM Systems

A simple modified clipping approach has been presented which brings improvement in PAPR performance

Combining it with Selective Mapping further improves the performance

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Thank you

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