Linear Companding Transform for the Reduction

of Peak-to-Average Power Ratio(PAPR) of OFDM Signals

Presented By BABLE VITHAL

 

• A major drawback of OFDM signal is their high PAPR which causes serious degradation in performance.•Companding transform is well known method to reduce the PAPR without restrictions on system parameters such as no. of subcarriers, frame format and constellation type.• a new linear companding transform (LCT) with more design flexibility than LNST is proposed.•Computer simulation shows that proposed transform has a better PAPR reduction and BER performance than LNST with better PSD

Abstract

• Our main aim to increase the distance in wireless communication.

So, we are using : 1.PAN

2. LAN 3. MAN 4.WAN

• But, as we are going to increase the distance in this methods we are losing

the data rate performance.

Previous Technologies Used

• By using 3 satellites 120 degree apart we can cover total distance on earth.

• In this we are using following techniques:

Satellite Communication

1.TDMA: Multiple users will access the data in given time

slot. E.g. TV , in television if we consider single user

accessing particular channel then on that channel we can access particular program at a time.

2.FDMA: multiple users will access the data in given frequency rangeE.g. FM, in FM we can access any transmitting antenna at the same

time having different frequencies.

3.CDMA: Multiple users will access the data in given time

slot and given frequency range.Here time as well as frequency is divided.E.g. GSM sim card

It is orthogonal frequency division multiplexing OFDM is a multicarrier modulation scheme that divides

the incoming high data into parallel low data.OFDM is the technology used in 4G.The aim of OFDM to provide high data rate. OFDM splits the data stream to be transmitted into N

parallel streams of the reduced data rate and to transmit each of them on a separate sub carriers.

4. OFDM

FDM vs OFDM

The best advantage of OFDM is spectral overlapping among subcarriers is allowed.

As overlapping of subcarriers is allowed the bandwidth used becomes less that is more bandwidth is saved.

As bandwidth used is less, the time required for transmission of the signals becomes less, and data rate should be more.

Compare to FDM scheme OFDM has very high data rate.

Very high spectral efficiency.Resistance against multipath fading.Ease of filter the noise.Combining OFDM technique with other technique is very

easy. ( e.g. CT with OFDM, channel estimation technique with

OFDM)It eliminates ISI and ICI by using CP.OFDM is computationally efficient by using FFT

Advantages of OFDM

Disadvantages in OFDMHigh PAPR

Cyclic

Prefix Doppler

shift

The main drawback in OFDM is its high PAPR which causes serious degradation in performance of the OFDM system.

Cyclic prefix and Doppler shift also the minor drawbacks .

PAPR is the peak to average power ratio.The typical PAPR value for the OFDM is 12

db, which is considerable.The transmit signals in an OFDM system can

have high peak values in the time domain since many subcarrier components are added via an IFFT operation.

As the phase difference between them is very less when no of carriers increased.

PAPR

Mathematically PAPR is given by following formula. PAPR is nothing but peak power to average power comparision . If we considered only one signal among the heavy traffic of signals , PEAK will be as shown follow:

A large PAPR would drive PAs at the transmitter into saturation, producing interference among the subcarriers that degrades the BER performance and corrupts the spectrum of the signal.

To avoid driving the PA into saturation and reduce the noise produced due to high peak the following techniques are used.

1. Signal distortion techniques: a. clipping and filtering. b. Peak windowing. c. Companding Transform.

PAPR Reduction Techniques

2. Multiple Signaling and Probabilistic Techniques:

a. selective mapping (SLM) b. Partial transmission sequence(PTS) c. Tone injection 3. Coding Techniques: a. linear coding b. Turbo coding

Compared with all these techniques Companding transform (CT) is a well-known method to reduce PAPR without restrictions on system parameters such as number of subcarriers, frame format and constellation type.

This method basically applied for audio signals. Companding consist compression and expansion.We use compressing at Transmitter and expanding at

Receiver. The electronic circuit does this is called compander

and works by compressing or expanding the original signal.

Companding Transform Technique

vcvsac

Block Diagram

Input data may be anything audio, video or image .

We convert that in parallel format by using S/P

We are using constellation mapper to map the signals depending on the no of signals and the availability.

We are using IFFT to convert signal from frequency to time domain .

by using CP we are reducing the BER .But CP adds extra bandwidth to ofdm

signal.Then by using compander we are

compressing the signal which is having peak amplitude.

We consider one of the noise channel as AWGN and considering behavior of transmitted signal.

By using expander, we expand the transmitted signal depending on the receiver requirement.

and by using reverse operations we are getting the data with low PAPR and low BER.

Also computational complexity is less.

Results in matlab

In this project, a new linear CT is proposed with two inflexion points in order to increase the flexibility of companding.

Result show that the proposed transform has a higher PAPR reduction capability and better BER performance than original signal and LNST with less spectral broadening.

The proposed transform can be designed to meet excellent trade off between PAPR reduction and BER performance.

And the LCT is easy to implement.

Conclusion

THANK YOU