International Journal of Engineering Trends and Technology (IJETT) - Volume4
Issue7- July 2013
Design and implementation of an enhanced channel accessing
system for low BER
Nasreen1, K. Gouthami 2
1
Asst.Prof, Tirumala Engineering College, jonnalagadda,guntur Dt.
2
HOD, Vignan's Nirula Institute of Tech. & Sci for women,Guntur
ABSTRACT: In this project, we have
it is used for a variant of digital
introduced an advanced and novel
subscriber line (DSL). For wireless,
high-rate,
OFDM
low
complexity
MIMO
is the
and
basis
for
radio
several
transmission scheme, called STBC-
television
broadcast
SM, as an alternative to previous
applications, including the European
techniques like SM and VBLAST. The
digital broadcast television standard, as
amplitudes of the signal are scaled in a
well as digital radio in North America.
way to reduce the PAPR. Proposed
new transmission scheme employs
both APM techniques and antenna
1.1 Principle of OFDM
The principle of OFDM is to
indices to convey information and
divide
exploits the transmit diversity potential
communication channel into a number
of MIMO channels. The amplitude of
of equally spaced tones or sub-carriers.
complex OFDM signal is clipped and
Each equally spaced subcarrier carries
then scaled in such a way that the
a portion of user’s information.
the available
spectrum or
PAPR is reduced without causing
much degradation in bit error rate
(BER). This project presents QPSK
constellation
with PAPR and BER
reductions.
KEY WORDS: OFDM, BER,
VBLAST,
APM,
CLIPPING,
CLAMPING, MIMO
INTRODUCTION:
OFDM is a multimode modulation and
multiple access technique used in a
number of commercial wired and
Figure 1 Three sub-carriers within
OFDM symbol
wireless applications. In the wired side,
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International Journal of Engineering Trends and Technology (IJETT) - Volume4
Issue7- July 2013
The special property of OFDM is
OFDM enables the creation of a
that each sub-carrier is orthogonal with
very
every other sub-carrier. Moreover, the
which can be used efficiently for a
spectrum of each sub-carrier can be
wide range of services, including both
allowed
voice and data.
to
overlap.
Since
the
orthogonality is maintained, the subcarriers do not interfere with each
flexible
system
architecture,
 OFDM segments the available
spectrum according to frequency
rather than TDMA which segments
other. The orthogonal property of sub –
carrier is given by,
the available spectrum according to
time and CDMA according to
spreading codes.
∫Cos (2*pi*n*f0*t) + Cos
 OFDM can also be considered as
(2*pi*m*f0*t) dt = 0
multiple access technique such that
Where (n! = m), t – Time, f0 –
individual carriers can be assigned
frequency of transmission n, m –
to different users.
 OFDM can be combined with
constants
frequency hopping to create a
Figure 1 shows the OFDM spectrum
spread spectrum system, realizing
with
the benefits of frequency diversity
three
orthogonality
sub-carriers.
can
be
The
completely
and
interference
averaging
of
maintained, even though the signal
CDMA technique.The cyclic prefix
passes through a time dispersive
called as a guard time makes the
channel, by introducing a cyclic path.
channel
A cyclic prefix is a copy of the last part
transmitted waveforms were from
of the OFDM symbol which is
time minus infinite which prevents
pretended to the transmitted symbol.
one sub-carrier from interfering
This makes the transmitted signal
with another called Inter Carrier
periodic so that it avoids Inter Symbol
Interference (ICI).
to
behave
as
the
Interference (ISI) and Inter Carrier
Interference (ICI).
PAPR:
Benefits of OFDM
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International Journal of Engineering Trends and Technology (IJETT) - Volume4
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The crest factor or peak-to-average ratio
expensive components in the radio. In
(PAR) or peak-to-average power ratio
this section, we quantify the PAR
(PAPR)
problem; explain its severity in OFDM
is
a
measurement
of
a
waveform, calculated from the peak
systems.
amplitude of the waveform divided by
the RMS value of the waveform.It is
therefore
a
dimensionless
The PAR Problem
quantity.
When transmitted through a nonlinear
While this quotient is most simply
device, such as a high-power amplifier
expressed by a positive rational number,
(HPA) or a digital to analog converter
in commercial products it is also
(DAC) a high peak signal, generates
commonly stated as the ratio of two
out-of-band energy (spectral regrowth)
whole numbers, e.g., 2:1. In signal
and in-band distortion (constellation
processing
applications
tilting
expressed
in
it
decibels
is
often
(dB).
and
scattering).
These
The
degradations may affect the system
minimum possible crest factor is 1, 1:1
performance severely. The nonlinear
or 0 dB.
behavior
of
an
characterized
OFDM signals have a higher peak-toaverage ratio (PAR) often called a
peak-to-average power ratio (PAPR)
than single-carrier signals do. The
reason is that in the time domain, a
multicarrier signal is the sum of many
narrowband signals. At some time
instances, this sum is large and at other
times is small, which means that the
peak
value
of
the
signal
is
substantially larger than the average
value. This high PAR is one of the
most
important
implementation
challenges that face OFDM, because it
reduces the efficiency and hence
increases the cost of the RF power
HPA
can
be
by
amplitude
modulation/amplitude
modulation
(AM/AM)
and
modulation/phase
amplitude
modulation
(AM/PM) responses. Figure (1) shows
a typical AM/AM response for an
HPA, with the associated input and
output back-off regions (IBO and
OBO, respectively). To avoid such
undesirable
nonlinear
effects,
waveform with high peak power must
be transmitted in the linear region of
the HPA by decreasing the average
power of the input signal. This is
called (input) backoff (IBO) and
results in a proportional output backoff
(OBO). High backoff reduces the
amplifier, which is one of the most
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a
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International Journal of Engineering Trends and Technology (IJETT) - Volume4
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power efficiency of the HPA and may
plane for the corresponding M-PSK or
limit the battery life for mobile
M-QAM scheme and complex symbols
applications. In addition to inefficiency
are generated. These symbols are to be
in terms of power, the coverage range
transmitted independently on to the
is reduced, and the cost of the HPA is
subcarriers. To achieve this, they are
higher than would be mandated by the
fed parallel to the input of the N-point
average power
IFFT. They represent the frequency
requirements. The
input backoff is defined as Where is
domain data set. Inverse
the saturation power, above which is
transform
the nonlinear region, and sat in P in P
domain data set into its corresponding
is the average input power. The
time
amount of backoff is usually greater
Specifically, IFFT is useful for OFDM
than or equal to the PAR of the signal.
because
The power efficiency of an HPA can
waveforms with orthogonal frequency
be increased by reducing the PAR of
components.
the transmitted signal. Clearly, it
applied at the output of the IFFT block
would be desirable to have the average
and the OFDM symbols are then
and peak values are as close together
transmitted over the channel with
as possible in order to maximize the
energy per bit as Eb. The channel
efficiency of the power amplifier. In
considered here is an Additive White
addition to the large burden placed on
Gaussian Noise (AWGN) channel with
the HPA, a high PAR requires high
mean zero and variance No. At the
resolution for both the transmitter’s
receiver the inverse PAPR reduction
DAC and the receiver’s ADC, since
technique is applied and FFT block is
the dynamic range of the signal is
used to get the frequency domain data
proportional
High-
set from the time domain values. The
resolution D/A and A/D conversion
signal in frequency domain represents
places an additional complexity, cost,
the data symbols which were mapped
and power burden on the system.
to M-PSK or M-QAM. After parallel
to
the
PAR.
converts
this
domain
it
generates
PAPR
Fourier
frequency
representation.
samples
of
reduction
is
to serial conversion, these symbols are
SYSTEM MODEL
As shown in Fig. 1, the incoming data
bits are mapped onto the constellation
ISSN: 2231-5381
used to estimate the original data
values.
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We select the clipping threshold at
which the BER is degraded from 1.5
?10−3 to 3.5 ?10−3 at SNR of 10dB
and the amplitudes above this clipping
threshold are clipped. Instead of
clipping the signal further to reduce the
PAPR,
we
consider
a
reversible
process - Differential Scaling which
would reduce the PAPR but not
deteriorate the BER. Since different
ranges of amplitudes of the signal are
scaled in a different manner, it is called
Differential
Scaling.
We
have
considered three types of scaling as
described below.
the lower amplitudes of the signal by a
factor of β. This leads to increase the
average value without affecting the
peak values. Therefore, the resulting
PAPR reduces. The PAPR reduction
function can be defined as
ISSN: 2231-5381
where xp is the amplitude peak value
occurring in an OFDM symbol block,
α is the factor deciding the clipping
threshold in terms of percentage of the
peak value and β is the scaling factor
for the range [0,A) whose value is
greater than one. The values of the
parameters used are mentioned at the
end of this section.
Scale Down: In this method, we scale
down the higher amplitudes of the
signal by a factor of γ. This leads to
decrease the peak value. Although the
average value would also fall down,
Scale Up: In this method, we scale up
h(x) = αxp, if x > αxp
= βx, if x < A
= x, if A ≤ x ≤ αxp
the resulting PAPR reduces. Because
the reduction in peak power is greater
than the reduction in the average
power. The PAPR reduction function
can be defined as
h(x) = αxp, if x > αxp
= γx, if B ≤ x ≤ αxp
= x, if x < B
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where xp is the amplitude peak value
occurring in an OFDM symbol block,
α is the factor deciding the clipping
threshold in terms of percentage of the
peak value and γ is the scaling factor
for the range [β,αxp] whose value is
less than one.
CLIPPING AND DIFFERENTIAL
SCALING:
We propose in this section a new
technique
called
Clipping
and
Differential Scaling. The probability
distribution of amplitudes of the
OFDM
signal
distribution
follows
[12]
and
Rayleigh
thus
the
probability of high peaks is very less.
An upper threshold above which the
signal amplitudes do not contribute
much to the signal is determined as
follows. Using simulations, we have
determined BER for the modified
signals alongwith PAPR.
SIMULATION RESULTS:
CONCLUSION:
Finally, I have
established an efficient and most
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International Journal of Engineering Trends and Technology (IJETT) - Volume4
Issue7- July 2013
powerful communication in OFDM
from orthogonal designs," IEEE Trans.
with less PAPR and reduced Bit Error
Inf. Theory, vol. 45, no.5, pp. 1456-
Rate Probability. Using simulations,
1467, July 1999.
we obtained the values of threshold for
[5] X.-B. Liang, “Orthogonal designs
clipping and parameters for scaling
with maximal rates," IEEE Trans. Inf.
with a view to reduce PAPR without
Theory, vol. 49, no. 10, pp. 2468-2503,
degradation
BER.
We
have
Oct. 2003.
PAPR
and
BER
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performance for all the techniques
modified selected mapping technique
considered. The proposed up-down
to reduce the peak-to-average power
scaling technique is able to achieve
ratio ossf OFDM signal,” IEEE Trans.
PAPR reduction of the order of 8.5 dB
Consum. Electron., vol. 53, no. 3, pp.
from 12 dB PAPR initially
846–851, Aug. 2007.
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