The document presents Dr. Hazrat Ali's research on reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It reviews various PAPR reduction techniques, including joint selective mapping (SLM) and modified clipping, illustrating their effectiveness in addressing amplification challenges. The proposed methods enhance performance, indicating a significant advancement in OFDM technology utilized in 4G and other broadband wireless communications.

1. 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

2.  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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3.  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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4. OFDM
Block Diagram of an OFDM system
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5.  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 = max
0≤𝑛≤𝑁−1
𝑥 𝑛 2
𝐸 𝑥 𝑛 2
Peak-to-Average Power Ratio (PAPR)
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6.  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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7. 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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8. PAPR Reduction Techniques
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9. 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.
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10. PAPR Reduction Techniques
Signal Distortion
Techniques
Clipping and Filtering
Peak Windowing
Companding
Techniques
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
Companding
Joint 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

11. PAPR Reduction Techniques
Signal Distortion
Techniques
Clipping and Filtering
Peak Windowing
Companding
Techniques
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
Companding
Joint 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

12. Selective Mapping
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13. 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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14. Conventional Clipping
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15. Conventional Clipping
The expression for conventional clipping can be written as,
𝑥 𝑡 = 𝑥 𝑡 𝑥 𝑡 < 𝐶𝐿
𝐶𝐿𝑒 𝑗∅ 𝑡
𝑥 𝑡 > 𝐶𝐿
Where 𝑥 𝑡 is transmitted signal and ‘CL’ is the clipping level
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16. Conventional Clipping
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17. PAPR for Conventional Clipping
• PAPR performance for
conventional clipping
approach
• Simplest to implement
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18. Joint SLM and modified
Clipping
(Proposed Approach)
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19. 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 Data
SLM
technique
Modified
Clipping
Transmit
Data

20. Modified Clipping
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21. Modified Clipping
 Mathematical representation for proposed clipping scheme
𝑥 𝑡 =
𝐶𝐿𝐻𝑒 𝑗∅(𝑡) 𝑥 𝑡 > 𝐶𝐿𝐻
𝑥 𝑡 𝐶𝐿𝐿 < 𝑥 𝑡 < 𝐶𝐿𝐻
𝐶𝐿𝐿𝑒 𝑗∅ 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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22. Joint SLM and modified Clipping
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23. PAPR for Modified Clipping
• PAPR performance
improves for modified
clipping (with 2 levels of
threshold)
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24. PAPR for Joint SLM and Modified Clipping
• Joint SLM and Modified
Clipping scheme further
improves the PAPR
performance
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25. 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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26. Thank you
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