Tianqing su, papr

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Technology for reduction of the high
Peak-to-Average-Power-Ratio
Tianqing Su, 21.01.2015

21.01.2015 | Paula Becker | Sensitivity of OFDM performance to Doppler shift | 2/24
Gliederung
1. Introduction
2. OFDM
3. The Peak-to-Average-Power-Radio Problem
4. PAPR Reduction Techniques
5. Comparison of PAPR Reduction Performances
6. Conclusion
1. Introduction
2. OFDM
6. Conclusion

The Evolution of Mobile Technologies
source: http:// androidauthority.com

Gliederung
1. Introduction
2. OFDM
6. Conclusion

Orthogonal Frequency Division Multiplexing(OFDM)
 OFDM is a Multi-Carrier
Modulation technique, which has
been known since 1966
 Basic idea: a single high rate data-
stream is divided into multiple low
rate data streams and is modulated
using sub-carriers which are
orthogonal to each other
Multicarrier Transmission Technique with orthogonal subcarriers, source:
http://lightwaveonline.com

OFDM Transceiver Structure
OFDM Transceiver Block Diagram, source: http://cp.literature.agilent.com

Advantages and Drawbacks of OFDM
 Advantages:
 Robustness against frequency selective fading and time dispersion
 Transmission rates close to capacity can be achieved
 Low computational complexity implementation (FFT)
 Drawbacks:
 Sensitivity to frequency offset
 Sensitivity to nonlinear amplification
High power amplifiers source: http://alibaba.com
Nonlinearity of HPA source: MilastMagazine, February, 2013

Gliederung
1. Introduction
2. OFDM
6. Conclusion

The Peak-to-Average-Power-Radio Problem
Peak-to-Average Power Ratio (PAPR) is defined as the maximum power to the average
power of the OFDM transmission
The OFDM symbol s(t) is 𝑠 𝑡 =
1
𝑁
𝑋 𝑘 ∙ 𝑒 𝑗𝑘2𝜋∆𝑓𝑡
, 0 ≤ 𝑡 ≤ 𝑇
∆f =1/T: the subcarrier spacing
Xk : complex baseband data modulating the k-th subcarrier for s(t)
The peak instantaneous power of the OFDM symbol sequence is:
𝑃𝑚𝑎𝑥 𝑠 𝑡 = max
0≤𝑡≤𝑇
𝑠(𝑡) 2
The average power of the OFDM symbol sequence is:
𝑃𝑎𝑣 𝑋0, 𝑋1, … , 𝑋 𝑁−1 =
1
𝑁 𝑘=0
𝑁−1
𝐸 𝑋 𝑘
2
Therefore, the PAPR of the OFDM symbol s(t) can be expressed as:
𝑃𝐴𝑃𝑅 𝑠 𝑡 =
𝑃 𝑚𝑎𝑥(𝑠(𝑡))
𝑃 𝑎𝑣(𝑋0,𝑋1,…,𝑋 𝑁−1)

Effect of PAPR
The high PAPR :
 introduces inter-modulation distortion and out-of-band radiation because of the nonlinearity
of the HPA
 increases BER and adjacent channel interference
 requires a large range of dynamic linearity from the analog circuits which usually results in
expensive devices and high power consumption
It is desirable to reduce the PAPR of an OFDM signal.

Gliederung
1. Introduction
2. OFDM
6. Conclusion

PAPR Reduction Techniques
PAPR Reduction techniques are dependent on PAPR Spectral efficiency, reduction capacity,
complexity of computation, etc., which are divided into three categories:
 Signal Distortion Techniques
 Signal Clipping
 Peak Windowing
 Companding
 …
 Signal Scrambling Techniques
 Selected Mapping Method (SLM)
 Partial Transmit Sequence (PTS)
 Interleaving
 …
 Coding Techniques
 Block Coding Schemes
 Sub Block Coding Schemes
 Block Coding with Error Correction

Clipping
X 𝑛 =
𝐾𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑, 𝑖𝑓 𝑋(𝑛) > 𝐾𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑
𝑋 𝑛 , 𝑖𝑓 𝑋(𝑛) < 𝐾𝑡ℎ𝑟𝑒𝑠ℎ𝑜𝑙𝑑
source: http:// rfmw.em.keysight.com
 The simplest way to reduce the
PAPR
 The peak amplitude becomes
limited to some desired level
 the clipped signal will then go
through a low-pass filter to reduce
the high frequency signals which
correspond to the sharp corners in
the clipped signal

Performance of Clipping and Filtering
source: nternational Journal of Electrical & Computer Sciences
IJECS-IJENS Vol:10 No:05
 By distorting the OFDM signal
amplitude, a kind of self-interference
is introduced
 Nonlinear distortion increases out-
of-band radiation

Peak Windowing
 To remedy the out-of-band problem of clipping
 Large signal peak is multiplied with a certain window, such as Gaussian shaped window to
minimize the out-of-band interference
 The window should be narrowband and not be too long in the time domain
source: http://rfmw.em.keysight.com

Selected Mapping Method (SLM)
 In SLM, several candidate data blocks are generated from the original data block and
the one with lowest PAPR is transmitted
 No distortion is introduced
 ⌈log_2𝑀 ⌉ bits of side information per OFDM symbol is required
 Computational complexity is increased to ensure a better performance
source: Study of Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals
by Chhavi Choudhary S.S. Rattan

Partial Transmit Sequence (PTS)
 The original data block is partitioned into V disjoint subblocks in PTS system. The
subcarriers in each subblock are rotated by the same phase factor such the PAPR of
the combination is minimized

Performance of PTS
 PAPR performance improves as the number sub-blocks increases

Comparison of SLM and PTS
 SLM produce multiple independent signals, however PTS signals are
interdependent. The complexity in PTS is reduced compared to SLM method
 PTS method also provides a better PAPR reduction performance
source: Comparative study between Selection Mapping Technique (SLM) and Partial Transmission
sequence (PTS) for PAPR reduction in OFDM signals by Md. Kislu Noman

Coding Techniques
 Basic idea: Coding schemes utilize the complementary bits that are added to the original
information bits to reduce the probability of the peak signals occurrence
 Block codes with a generator matrix G and parity check matrix H provide also error
correction capability
 Advantages: the inherent error control capability and simplicity of implementation
 Dawbacks: reduction of the transmission rate, the number of subcarriers is limited
source: PAPR Reduction in OFDM systems using Quasi Cyclic
LDPC Codes by S.P Vimal

Gliederung
1. Introduction
2. OFDM
6. Conclusion

Comparison of PAPR Reduction Performances
Type Power
increase
Implementation
Complexity
Bandwidth
expansion
Data rate loss
Clipping &
Filtering
No Low No No
PTS No High Yes Yes
SLM No High Yes Yes
Coding No Low Yes Yes
 Basic requirement of practical PAPR reduction techniques include
 the compatibility with the existing modulation schemes
 high spectral efficiency
 low complexity
 No specific PAPR reduction technique is the best solution for all multi-carrier
transmission

Gliederung
1. Introduction
2. OFDM
6. Conclusion

Summery
 OFDM is a digital multi-carrier modulation method where a large number of closely spaced
orthogonal sub-carriers to support high bit rate transmission
 The high PAPR is the main obstacle which causes non-linearity at the transmitter end
 PAPR reduction techniques are roughly divided into three categories:
 Signal Scrambling Techniques
 Signal Distortion Techniques
 Coding Techniques
 PAPR reduction capacity, Power increase, data rate, computational complexity, BER
increase at the receiver, etc. should be considered before a specific PAPR reduction
technique is chosen
source: http:// itpro.co.uk

Thank you for your
attention.
Tianqing Su
t.su@tu-braunschweig.de

Tianqing su, papr

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