Abstract—Orthogonal frequency division multiple-xing

(OFDM) is an effective transmission method for cognitive radio

system. This method provides high bandwidth and robustness

against time dispersive channel. But OFDM inherits the high side

lobe power and peak to average power ratio. High side lobe power

of primary user (PU) distorts the signal of the secondary user (SU).

In this paper we reduce the side lobe power and peak to average

power ratio. To reduce the side lobe power and high PAPR we use

the multi-choice sequence method (MCS) and hyperbolic tangent

sigmoid transfer function for companding transform. By changing

the value of parameter k we can get better Peak to average power

ratio (PAPR). the BER performance of proposed companding

transform of NC OFDM signal without side lobe power reduction is

3.5x10-3 for SNR 14 that alittle batter than mu law companding

transform. the proposed method reduce PAPR more as the parameter

k increase by varing value of k from 0.3 to 0.45 can improve papr by

2.5 to 6 db with respect to original OFDM system.

Keywords—Average Power Ratio, Band Radiation Reduction,

OFDM Based Cognitive Radio.

I. INTRODUCTION

OR cognitive radio system non-contiguous orthogonal

frequency division multiplexing [10] (OFDM) become the

most attractive method for multicarrier modulation [1]-[2]

. OFDM have lots of advantages like high spectral efficiency,

multiple access capability, robustness in case of frequency

selective channel high flexibility, narrow-band interference

rejection, and simple one tap equalization. However, some

drawback of OFDM based transmission system are high side

lobe power in primary user (PU) and high peak to average

power ratio (PAPR). OFDM signal have N number of sub

carrier out of which some are occupied by primary licensed

user (PU) and remaining is enabled for secondary user (SU).

The NC-OFDM transceiver activates those sub carriers that

are not located in the band occupied by the primary users and

this can be determine by dynamic spectrum sensing and

channel estimation techniques [2].

Large spectral side lobe cause out of band (OBB) radiation

in the high power amplifier (HPA) . High PAPR can lead to

saturation in high power amplifier in SU and consequently

increase OBB radiation, distort signal and reduce the power

amplifier efficiency. Since the OBB radiation introduce the

Ajay Somkuwar, Professor & HOD Electronics and comm. Engg. MANIT

Bhopal, India

interference in the primary frequency band hence it is highly

desirable to reduce the OBB radiation as much as possible in

NC-OFDM system [3]-[8]. There are two source of OBB

radiation 1) high side lobe power 2) high PAPR in OFDM. A

number of methods have been proposed to reduce the high

side lobe including multiple choice sequences [9] (MCS).

MCS can effectively reduce the high side lobe power level

while requiring only a small amount of redundancy. In this

method several sequences are generated from the original

sequence by multiplying a phase shift. Out of these, a

sequence with lowest side lobe power is chosen for

transmission. To reduce the PAPR of the NC-OFDM signal

various method have been developed such as clipping, mu law

companding [12] ,selected mapping ,coding techniques. The

clipping is the simplest and efficient method but the BER

performance is burst [11]. A new method, companding of

amplitude of the sequence using hyperbolic tangent sigmoid

transfer function is efficient solution to reduce the PAPR and

BER is acceptable).

II. MCS METHOD

Consider the frequency domain OFDM sequence

X=[X0,…………,XN-1]T here [.]T denotes the transpose of

vector. N denotes the number of orthogonal subcarrier

modulated by phase shift keying (PSK) or quadrature

amplitude modulation (QAM). The NC-OFDM transceiver

activate those subcarrier that are nod located in PU band

using dynamic spectrum sensing and channel estimation

techniques [2].

The time domain OFDM symbol can be computed using IFFT

(1)

Where n=0,1,…..N-1 is time index and J is the over

sampling factor.

Consider the multiple sequence Xb= [X0b,……….,XN-1b]T

b=1,………..B which is generated by the original data

sequence X [9]. the average side lobe powers(b) of multiple

sequence where b=1,………B, in the adjacent PU frequency

band is

(2)

Where Fk, k=0,…….N-1, is the normalized subcarrier

frequency and Gm , m=1………..M denotes the normalized

frequency samples within the adjacent PU frequency bands.

Let the sequence Xb‟ have the lowest side lobe power hence

Peak to Average Power Ratio and Out of Band

Radiation Reduction in OFDM Based Cognitive

Radio

Ajay Somkuwar

F

4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2015) Dec. 15-16, 2015 Pattaya (Thailand)

11

this sequence is chosen for transmission among all the

multiple sequence b‟=arg min[S(a)].

To generate the multiple sequence of X we consider the

phase approach. The multiple sequence is generated by phase

rotation of original sequence. i.e. Xkb=Xkejɸ

kb where ɸkb a

uniformly distributed random variable over [0,π]. To

determine the correct sequence at the receiver we have to sent

log2B bits of explicit side information per OFDM symbol.

III. PAPR REDUCTION

Suppose the OFDM discrete time sample signal is xn given

above if N is large enough the real part and the imaginary

part will become the Gaussian distributed. The PAPR of the

OFDM symbol in term of power is given by:

(3)

Where max (|xn|2) is the maximum power of the OFDM signal

and the E(|xn|2) denotes the average power of the OFDM

signal. To reduce the PAPR of the signal xn we use the

nonlinear companding technique, wich compress the large

signal and expand the small signal simultaneously using the

tangent sigmoid transfer function [17].

The hyperbolic tangent sigmoid function is :

(4)

and the new compended signal is

(5)

V is the average value of amplitude of the OFDM signal and

is almost constant α and k are the companding parameter.

The value of the k is chosen firstly and corresponding value

of α is calculated with the constrain that the average power of

the original signal and the companded signal should be same

and to avoid the AWGN enhancement [15] i.e.

E(xn2)=E(x‟2

n) (6)

IV. PROPOSED METHOD

In this paper we consider the sequence

X(k)=[X0,………Xn-1] now this sequence is our original

sequence and we generate the multiple sequence

Xb(k)=[Xb(1),………..Xb(N-1)] where b=1,……B by using

the MCS using phase rotation

Xb(k)=X(k)*ejπȹ(b);

Now select the sequence having the lowest side lobe power

it means Xb‟(k) and the lowest side lobe power is :

This is the lowest side lobe power that belongs to the

sequence Xb‟(k). The IFFT of this sequence is xn this xn have

the lowest side lobe power.

As the real and imaginary part of of xn is Gaussian

distributed is almost constant, the true power of the

companded signal [16] is given by

(7)

Where is the probability density function using (5)we

determine the α for a given value of k. suppose kis chosen 0.4

then α is equal to 2.733.

V. THE SYSTEM MODEL

The base band model of the NC OFDM transmitter have

the QAM, serial to parallel converter, MCS block ,IFFT

block, tan sigmoid companding block, digital to anolog and

up converter block and then amplifier. The serial incoming

bits are mapped to a QAM constellation giving N parallel

constellation points representing the data. The subcarriers

located in PU band are deactivated using the method like

dynamic spectrum sensing. The MCS is used to obtain the

sequence Xb . The NC OFDM discrete time symbol are obtain

by IFFT operation. From the time domain sequence of Xb the

lowest side lobe power sequence is companded by tan sigmoid

companded operation. The parallel time domain sequence are

converted into serial stream, and this signal is converted to

analog signal x(t). the resulting are up converted to carrier

frequency and amplified by non linear power amplifier, and

then finally transmitted. Here we consider a solid state power

amplifier (SSPA)with amplitude modulation (AM)

Characteristics

Where x^(t) is the modulated OFDM signal and x0 is the

output saturation level the parameter controls the

characterstics and k is the gain of the amplifier. To reduce the

nonlinear distortion due to signal peak, the amplifier is driven

with an input back off(IBO)

P0 is the amplifier saturation power, p0=x20.

Fig 1 shows the block diagram of the NC OFDM base band

model

VI. SIMULATION AND RESULT

To show the overall performance of the proposed method

on PAPR reduction and BER we assumed that randomly

generated data are modulated by 16 QAM the number of sub

carrier are N=1024. It is assumed that 24 subcarrier is

detected in PU band and the Su band uses the remaining

subcarrier (2 band of 1000) for data transmission the sspa

parameter pis set to 10 and the saturation point of x(t)is

varied to obtain different IBO values. Fig 2 shows that the

proposed method reduce papr more as the parameter k

4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2015) Dec. 15-16, 2015 Pattaya (Thailand)

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increase by varying value of k from 0.3 to 0.45 can improve

PAPR by 2.5 to 6 db with respect to original OFDM system

given that CCDF =10-4 where CCDF is the complementary

cumulative distribution function of PAPR of OFDM signal

that can be expressed as Probability

{PAPR>PAPR0}=1-(1-exp(PAPR0))N (8)

In fig 3 the BER performance of proposed companding

transform of NC OFDM signal without side lobe power

reduction is 3.5x10-3 for SNR 14 that alittle batter than mu

law companding transform.

Fig 2 complementary cumulative distribution function OF PAPR

Fig 3 Bit error rate performance in AWGN channel.

VII. CONCLUSION

The MCS the effective technique for the reduction of side

lobe power and the tan sigmoid companding transform is the

effective method in reducing the PAPR of NC OFDM signal

when the subcarrier is large enough(N>64). This method

based on companding technique has low implementation

complexity and no constrain on modulation format and sub

carrier size. In this paper we reduce side lobe power and

PAPR more efficiently and exhibits better BER performance

than the other companding techniques. By changing the value

of k we can get better PAPR.

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