International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
Diversity Combining Techniques using LDPC
Coding of OFDM
Karamjeet Singh
KomalArora
Lovely Professional University
Jalandhar, Punjab
ABSTRACT
The OFDM is the latest technology in which the
multiple signals are move from source to destination
orthogonally. All most five type technologies are
used in OFDM. Above all the technologies of
OFDM, MIMO is the most efficient and commonly
used technology. In MIMO the signal at the receiver
side is split into multiple signals. The split signals are
transmitted by multiple antennas on the receiver side
many antennas are responsible for receiving the
signals. In MIMO technology bit rate error very high
due to its unique features. In this paper new
technique is proposed to reduce bit rate in MIMO.
The proposed technique is based on LDPC filter.
LDPC filter is used in OFDM to reduce bit rate error.
The proposed technique is implemented in MATLAB
and result shows that proposed technique will reduce
bit rate error in OFDM.
KEYWORDS
LDPC, OFDM, MIMO, Simulation, Signals
1.
INTRODUCTION
In the concept of OFDM is subset of frequency
division multiplexing which is single channel
utilizing multiple sub-carriers on adjacent
frequencies.Sub carriers in OFDM system are
overlapping to maximize spectral efficiency.
Overlapping adjacent channels can interfere with one
other. OFDM system is precisely orthogonal to one
another. OFDM systems are able to maximize
spectral efficiency without causing adjacent channel
interference [7]. In the OFDM communication
systems are able to more effectively utilize the
frequency spectrum through overlapping sub-
ISSN: 2231-5381
carriers. These sub-carriers are able to partially
overlap without interfering with adjacent sub-carriers
and maximum power of each sub carrier corresponds
directly with the minimum power of each adjacent
channel. Sub-carrier is represented by different peak.
Peak of each subcarrier corresponds directly with the
zero crossing of all channels. OFDM channels are
different from band limited FDM channels how they
apply shaping filter [8]. MIMO communication uses
multiple antennas at both the transmitter and receiver
to exploit the different domains for spatial
multiplexing and spatial diversity. In the spatial
multiplexing used to increase the capacity of MIMO
link by transmitting independent data streams in the
same time slot and frequency band simultaneously
for each transmit antenna and differentiating multiple
data streams at the receiver using channel
information about each path propagation path.:
MIMO (Multiple Input Multiple Output) in contrast
to traditional systems takes advantage of multipath
propagation signals. Rather to apply different
techniques to handle with multipath signals MIMO
puts multipath signals to work. This is done by
sending and receiving more than one data signal in
the same frequency band at the same time by using
multiple transmit and receive antennas[1]. Wireless
channels are multipath fading channels, causing ISI
(Inter symbol interference), ISI occurs when a
transmission interferes with itself and receiver cannot
decode the signal correctly.
For multipath signals travelling through different
paths have their independent effect imposed by the
channel. Signal parameters on which multipath
channel have effect are independent path gain (or
loss), independent path frequency offset, independent
path phase shift (change in angle), independent path
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
time delay. To remove ISI from the signal, many
kinds of equalizers can be used. Different techniques
are used to handle the changes made by the channel;
receiver requires knowledge over CIR (Channel
Impulse Response) to combat with the received
signal for recovering the transmitted signal. CIR is
provided by the separate channel estimator. Usually
channel estimation is based on the known sequence
of bits, which is unique for a certain transmitter and
is repeated in every transmission burst. Which
enables the channel estimator to estimate CIR for
each burst separately by using the known transmitted
signal and the corresponding received signal. LDPC
are the codes which are used in channel coding for
the different enhancement in the system performance.
We are trying LDPC channel coding with BPSK
modulation scheme with MRC and EGC at the
receiver over Rayleigh fading channel. BER
Performance of System is analyzed with and without
LDPC channels coding.
2.
LITERATURE REVIEW
R. Malik,presented a paper on spread spectrum
secret military technology to 3G which describe the
spread spectrum transmission without even realizing
it. In the system used a primitive spark-gap
transmitter used an RF bandwidth that was much
wider than the information bandwidth. In today
scenario wireless communication plays an integral
part of our daily lives. Cellular phones are quite
common place and hardly imagine life without them.
It also describes the future it was not only use them
for conversation it also access the internet and also
watch the latest news updates and mange our daily
life. Using a Bluetooth link to his cellular phone his
car computer informs him of rise in engine
temperature and an imminent breakdown. With the
help of GPS locator the phone determines the nearest
service station and downloads a map to get there [2].
Moe Z.Win,presented a paper based on the ultrawide bandwidth time hopping spread spectrum
impulse radio for wireless multiple access
communication. In this paper describe the feature of
time hopping spread spectrum multiple access
systems employing impulse signal technology are
outlined and emerging design issues are described. It
also show the performance of such communication
systems in terms of achievable transmission rate and
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multiple access capability are estimated for both
analog and digital modulation formats under ideal
multiple access channel conditions [3].
Christian R. Berger, presented a paper based on the
precise timing for multiband OFDM in UWB system
large bandwidth has been generated high expectation
on the spatial resolution that it could achieve. In the
concept of synchronization in the presence of dense
multipath is challenging due to the channel fading. In
this concept shows that spatial resolution can be
realized in practical implementation how much is
reliable. In this paper defines the address practical
synchronization algorithm for multiband OFDM
UWB transmission and analyze the performance of
joint timing and also estimate the algorithm. In this
paper show the probability of mis-timing in the single
band OFDM has only diversity order one. Most
timing error is equal to or greater than diversity order
where the number of channels taps. In the benefits of
frequency hopping across multiple sub bands that
enables a diversity increase in timing performance
where NB is the number of sub bands used [4].
MalharChauhan, presented a paper based on
thedifferent techniques to reduce the PAPR in OFDM
(orthogonal frequency division multiplexing) system
is known as techniques for transmitting large data
over the radio waves. In the case of PAPR (peak to
average power ratio) for several sinusoidal leads. In
this paper reduce using the proposed techniques. In
the proposed techniques will show the better
performance for reduction in PAPR techniques it can
be used to performance. In the OFDM is multicarrier
multiplexing access techniques for transmitting large
data over the radio waves. In the mobile generation
system is expected to provide high data rate to meet
the requirements for future multimedia application.
The minimum data rate required for the 4G system is
10-20Mbps at least 2 Mbps in moving vehicles. In
this paper used the modulation techniques adopted by
the 4G mobile system is OFDM. Major problem
come in the OFDM system is PAPR peak to average
power ratio. PAPR always reduce the efficient
transmission. Different techniques used to PAPR in
OFDM system [5].
Y.Awad, presented a paper based on the OFDM
transceiver for IEEE802.20 standards which describe
the orthogonal frequency division multiplexing which
is now widely used in various high speed mobile and
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
wireless communication systems. In this paper
describe the design and implementation of
transceiver for IEEE 802.20 standard and their
variable length and targeting a low cost Xilinx
Spartan. The design includes dynamic size variable
length cyclic prefix and different window
combination options to meet the requirements of the
wide and frequency division multiplexing mode of
IEEE 802.20. The resources occupied by each part of
the design are analyzed. The paper also features test
and implementation results [6].
3.
LPDC FILTER
Low-density parity-check (LDPC) code is a
linearerror correcting code, a method of transmitting
a message over a noisy transmission channel and is
constructed using a sparse bipartite graph. LDPC
codes are capacity-approaching codes, which means
that practical constructions exist that allow the noise
threshold to be set very close to the theoretical
maximum (the Shannon limit) for a symmetric
memory-less channel. The noise threshold defines an
upper bound for the channel noise, up to which the
probability of lost information can be made as small
as desired. Using iterative belief propagation
techniques, LDPC codes can be decoded in time
linear to their block length[8].LDPC codes are
defined by a sparse parity-check matrix. This sparse
matrix is often randomly generated, subject to the
sparsity constraints. These codes were first designed
by Gallager in 1962.Below is a graph fragment of an
example LDPC code using Forney's factor graph
notation. In this graph, n variable nodes in the top of
the graph are connected to (n−k) constraint nodes in
the bottom of the graph. This is a popular way of
graphically representing an (n, k) LDPC code. The
bits of a valid message, when placed on the T's at the
top of the graph, satisfy the graphical constraints.
Specifically, all lines connecting to a variable node
(box with an '=' sign) have the same value, and all
values connecting to a factor node (box with a '+'
sign) must sum, modulo two, to zero (in other words,
they must sum to an even number).
4.
bit rate is very high in MIMO-OFDM because of the
unique features of OFDM. In this paper, we are
proposing the new technique to reduce bit error rate
in MIMO-OFDM. The LDPC is used at the receiver
side with the multiple signals are merged together to
form the complete signal. The simulation results
show that bit rate error is reduced when LDPC filter
is used with the MIMO-OFDM as compared to
traditional MIMO-OFDM. The proposed technique is
shown in the form of block diagram in figure 1
Fig1: Block Diagram Of proposed Technique
5.
CONCLUSION
In this paper, we conclude that MIMO-OFDM is the
efficient approach for fast data transmission. Due to
the unique features of MIMO-OFDM bit error rate is
very high. To reduce the bit error rate we use LDPC.
The simulation results show that more bit error rate
will be reduced by using proposed technique as
compared to the previous techniques. In figure 2,
figure 3 and figure 4 Design to OFDM with BER,
OFDM with LDPC andcompression is shown.
NEW PROPOSED TECHNIQUE
The new proposed technique is based on the LDPC
filter. The multiple spit signals received by receiver
are joined together and form the original signal. The
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
Fig4: Compression
Fig2: Design of OFDM with BER
REFERENCES
[1]TheodoreS.Rappaport,“Wireless
nd
Fig3: OFDM with LDPC
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Communication:PrinciplesandPractice”,2 Edition,PrenticeHall,India.
[2]R. Malik, Student Member IEEE”Spread Spectrum −Secret
Military Technology to 3G”
[3]Moe Z. Win, Senior Member, IEEE, and Robert A. Scholtz,
Fellow, IEEE”Ultra-Wide Bandwidth Time-HoppingSpreadSpectrum
Impulse
Radio
for
WirelessMultiple-Access
Communications“
[4]Christian R. Berger, Shengli Zhou, ZhiTian, Peter
Willett”Precise Timing for Multiband OFDM in a UWB
System”
[5]MalharChauhan,
Saurabhpatel,
Hardikpatel”
Different
Techniques to Reduce the PAPR in OFDM System”
[6] Y. Awad, L. H. Crockett and R. W. Stewart” OFDM
TRANSCEIVER FOR IEEE 802.20 STANDARDS”
[7] Suchita Varade,Kishore Kulat,” Performance Analysis of
MIMO-OFDM System using
Space-time Turbo Codes and Adaptive Beamforming”,
International Journal of Computer Applications (0975 – 8887)
Volume 22– No.3, May 2011
[8] Debasis S.N Das, Sarmistha Satrusallya
, Mihir N. Mohanty,” Reduction of PAPR in MIMO-OFDM
Broadcasting System”, Special Issue of International Journal of
Computer Applications (0975 – 8887) International Conference on
Computing, Communication and Sensor Network (CCSN) 2012
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