Peak to Average Power Ratio.pptx

An Overview: Peak-to-Average Power Ratio Reduction Techniques for OFDM Signals

An Overview: Peak-to-Average Power RatioReduction Techniques for OFDM SignalsBy HishamOFDMBasic idea Using a large number of parallel narrow-band sub-carriers instead of a single wide-band carrier to transport information conveniently implemented using IFFT and FFT operations Advantages Very easy and efficient in dealing with multi-path It handles frequency selective channels well when combined with error correction coding Disadvantages Sensitive to frequency offset and phase noise Peak-to-average problem reduces the power efficiency of RF amplifier at the transmitterSingle carrier Vs Multi carrier Transmition

OFDM Signal

IDFT ?OFDM

Peak-to-Average Power Ratio (PAPR)One of the challenging issues for Orthogonal Frequency Division Multiplexing (OFDM) system is its high Peak-to-Average Power Ratio (PAPR)Happens Due to Using IFFT.

Why it is a problem ?In general, even linear amplifiers impose a nonlinear distortion on their outputs due to their saturation characteristics caused by an input much larger than its nominal value

Note that the nonlinear characteristic of HPA (High Power Amplifier), excited by a large input, causes the out-of-band radiation that affects signals in adjacent bands, and in-band distortions that result in rotation, attenuation, and offset on the received signal

How to solve it ?For me i think there is two ways of solving this problem:1- Solve before the IFFT ( Prevent it from happening).2- Remove it after happening.

For me it is more efficient to choose the first choose.Solutions Clipping and FilteringCompandingInterleavingCoding SchemesSelective Mapping (SLM)Partial Transmit Sequence (PTS)Active Constellation Extension (ACE)Tone Reservation and Tone Injection

Clipping and Filtering

Clipping is by far simplest technique for PAPR Reduction, but Clipping also introduces peak regrowth in OFDM signal which can be reduced by Repeated Clipping And Filtering(CAF) method.

Ex: Ref 1

CompandingThe idea of companding came from the companding of speech signal and that the OFDM signal is similar.Companding are of two types- linear and non-linear. Linear companding focuses on expanding small signals only while non-linear companding enlarges small signals as well as compresses the large ones thereby obtaining uniform distribution of signals.

Ex: Ref 2

InterleavingTo recover the signals the receiver need to know the information about which interleaver is used.

Ex: Ref 3

Coding Schemes

When FEC codes are used to mitigate the effect of the distortion techniques, the OFDM is termed as COFDM.The basic concept is that when N signals are added in phase they add up to the signal power, such arrangements can be made with different coding schemes.While these block codes reduce PAPR, they also reduce the transmission rate

Used CodingSimple Odd Parity Code (SOBC) : Ref 4 Reed-Solomon, Simplex code : Ref 5Cyclic Coding (CC) : Ref (J. G. Proakis, Digital Communications)Complement Block Coding (CBC) & Modified Complement Block Coding (MCBC):Ref 6,8Simple Block Code (SBC) : Ref 7

Selective Mapping(SLM technique).Data blocks are converted into several independent blocks and the one with lower PAPR is sent in which converting process involves multiplying data sequences to random phase sequences generated.The selected index is called side-information index (SI Index), must also be transmitted to allow recovery of the data block at the receiver sideProbability of erroneous SI detection has a significant influence on error performance of the system

Ex: Ref 9

Partial Transmit Sequence (PTS)In the PTS technique, an input data block of N symbols is partitioned into disjoint subblocks and then the signal is transmitted.There are three kinds of subblock partitioning schemes :adjacent, Interleaved and pseudo-random partitioning.Advantage is that works with an arbitrary number of subcarriers any modulation scheme. But, this scheme includes complexity and side information like SLM

Ex: Ref 10

Active Constellation Extension (ACE)

This technique deals with extending the constellation points outside the signal constellation which is then used to cancel the time domain peaksThis technique has several advantages like no loss of data, no degradation in system performance , lower BER as compared to other techniques and bears no side information like SLM.Ex: Ref 11

Tone Reservation and Tone Injection

In TR, the objective is to find the time domain signal c to be added to x the original time domain signal to reduce the PAPR.

The basic idea of TI is to extend the constellation and thus the same data point corresponds to multiple possible constellation points. The TI technique is more problematic than the TR technique since the injected signal occupies the frequency band as the information bearing signals. Moreover, the alternative constellation points in TI technique have an increased energy and the implementation complexity increases for the computation the optimal translation vector.Ex: Ref 12

CRITERIA OF THE PAPR REDUCTION IN OFDM SYSTEMSHigh capability of PAPR reductionLow average powerLow implementation complexityNo bandwidth expansionNo BER performance degradationWithout additional power neededNo spectral spillage