Peak to Average Power Ratio and Out of Band Radiation...
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Transcript of Peak to Average Power Ratio and Out of Band Radiation...
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)
12
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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