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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.comNonlinearity 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 ,

∆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:

The average power of the OFDM symbol sequence is:

Therefore, the PAPR of the OFDM symbol s(t) can be expressed as:


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 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_2 bits of side information per OFDM symbol is required𝑀 ⌉ 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 Powerincrease

ImplementationComplexity

Bandwidthexpansion

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 [email protected]