Orthogonal frequency division multiplexing (OFDM) has become a prevalent and widespread technique for the broadcast and transmission of signals over wireless channels and has been adopted in many wireless standards. OFDM may be united with antenna arrays at the transmitter and receiver side to improve the diversity gain and to improve the system competence on time-variant along with the frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) composition.
Trang 1Performance Analysis for PAPR Reduction using SLM technique in 2 x 1 and 2 x 2 Differential STBC MIMO RS OFDM Systems in Rayleigh Fading
Channel
P.Sunil Kumar1, M.G.Sumithra2, M.Sarumathi3, E.Praveen Kumar4
1,4P.G.Scholars, Department of ECE, Bannari Amman Institute of Technology, Sathyamangalam
2Professor, Department of ECE, Bannari Amman Institute of Technology, Sathyamangalam
3Assistant Professor, Department of ECE, Bannari Amman Institute of Technology, Sathyamangalam
1sunilprabhakar22@gmail.com
Abstract— Orthogonal frequency division multiplexing (OFDM)
has become a prevalent and widespread technique for the
broadcast and transmission of signals over wireless channels and
has been adopted in many wireless standards OFDM may be
united with antenna arrays at the transmitter and receiver side to
improve the diversity gain and to improve the system competence
on time-variant along with the frequency-selective channels,
resulting in a multiple-input multiple-output (MIMO)
composition The space-time block coding (STBC) incorporated
for OFDM systems with multiple transmit antennas is actually a
type where coding is implemented in the time domain, that is,
OFDM symbols To be more particular and specific, Alamouti’s
code is taken into consideration, which is shown to be the most
favourable block code for time domain coding and two transmit
antennas The Differential Space time block codes (DSTBC) are
ways of transmitting data in wireless communications and they
are forms of space time code that do not need to know the
channel impairments at the receiver in order to be able to decode
the signal When Reed Solomon (RS) codes are used at the
demodulator side the system becomes a DSTBC RS OFDM
system and a performance analysis study is investigated based on
Peak to Average Power Ratio (PAPR) using Partial Transmit
Sequence (PTS) for different DSTBC RS OFDM Systems under
Quadrature Phase Shift Keying (QPSK) modulation scheme and
its performance is evaluated in terms of Bit Error Rate (BER)
under Rayleigh Multipath channel
Keywords— OFDM, MIMO, STBC, SLM, Alamouti, Diversity
1 INTRODUCTION TO MIMO-OFDM SYSTEMS
Wireless channels suffer from time-varying impairments
such as multipath fading, interference, and noise Diversity,
such as time, frequency, space, polarization, or angle diversity,
is typically used to mitigate these impairments [3] Diversity
gain is achieved by receiving independent-fading replicas of
the signal The multiple antenna system employs multiple
antennas at either the transmitter or the receiver, and it can be
either multiple-input single-output (MISO) for beam forming
or transmit diversity at the transmitter, single-input
multiple-output (SIMO) for diversity combining at the receiver, or
MIMO, depending on the number of transmit and receive
antennas The MISO, SIMO and MIMO channel models can
be generated by using the angle-delay scattering function
A MIMO system consists of multiple antennas at both the transmitter and receiver They are typically used for transmit diversity and spatial multiplexing Spatial multiplexing can maximize the system capacity by transmitting at each transmit antenna a different bit stream MISO, SIMO and MIMO can
be collectively treated as MIMO, and thus the smart antenna system can be regarded as a special case of the MIMO system [4]
There is always a great assurance that MIMO technology brings out a significant improvement and increase in the system capacity A MIMO system takes good advantage of the spatial diversity scheme that is obtained by spatially separated antennas in a dense multipath scattering environment In a number of different ways, MIMO systems may be successfully implemented in reality to obtain either a diversity gain to combat signal fading or to obtain either a capacity gain In a much generalized manner, MIMO technique aims to improve the power efficiency by maximizing spatial diversity Such techniques include delay diversity, space-time block codes [4], [5] and space-time trellis codes [6]
According to the analysis and simulation performed in [7], MIMO can provide a spectral efficiency as high as 20-40 bits/s/Hz MIMO and OFDM is absolutely the key technique for next-generation wireless LAN and 4G mobile communications MIMO-OFDM is used in IEEE 802.11n, IEEE 802.16m, and LTE OFDM is being engaged for very keen and committed short-range communications for road side
to vehicle communications and interactions and it is widely considered as a potential technique for fourth-generation (4G) mobile wireless systems OFDM generally converts a frequency-selective channel into a parallel gathering of frequency flat sub channels The subcarriers have the least amount of frequency separation which is absolutely compulsory to maintain the orthogonality so that the overlap
in frequency can be easily avoided Hence, the available bandwidth is used in a very economic and resourceful manner
287 978-1-4673-6126-2/13/$31.00 c2013 IEEE
Trang 2If the necessary knowledge of the channel is accessible at the
transmitter, then the OFDM transmitter can familiarize or
adapt its signalling scheme so that it could match easily with
the channel Due to the reality of fact that OFDM uses a
significant and large collection of narrowly spaced sub
channels, these adaptive strategies can come within the reach
of the ideal water pouring capacity of a frequency-selective
channel In practice this is accomplished by using adaptive bit
loading techniques, where altered or different sized signal
constellations are transmitted on the subcarriers
2 ALAMOUTI SPACE TIME CODES
The idea of space-frequency coding was bought into
existence in [1] The main concept behind the
Space-frequency coding is that it broadens the conjecture of
space-time coding incorporated for specific narrowband flat fading
channels to broadband frequency-selective channels which are
entirely time variant The most conventional and relevant
application of such classical space-time coding procedures for
Àat fading channels which are narrowband to OFDM seems
promising and straightforward, since each and every
individual subcarrier can be analysed as an independently Àat
fading channel Later in [1], it was proved that the intended
criteria for space-frequency codes which are operated in the
space-time and frequency domain are completely different
from those which are involved with the classical space-time
codes for narrowband fading channels as introduced in [2]
In this particular paper, the utility of the Alamouti
space-time block codes[3], which results in a space-space-time block coded
OFDM systems with a generic two transmit antennas and one
or two receive antennas is discussed and the Peak to Average
Power Ratio is reduced and its performance is evaluated in
terms of Bit Error Rate Generally when operating in
frequency selective fading channels, the common use of
conventional decoding algorithms always result in a
signi¿cant performance decrease [9] This is due to the fact
that the equivalent and corresponding channel matrix is no
longer orthogonal Subsequently when the two transmitted
symbols are independently decoded as done mostly in basic
and conventional decoding algorithms cannot be appropriate
anymore So for demodulation purposes Reed-Solomon Codes
can be used here In this particular paper, the utility of the
Alamouti time block codes[3], which results in a
space-time block coded OFDM systems with a generic two transmit
antennas and one or two receive antennas is discussed and the
Peak to Average Power Ratio is reduced and its performance
is evaluated in terms of Bit Error Rate
2.1 SYSTEM MODEL
An ordinary communication system incorporating the
differential space-time block coding technique with just two
transmit antennas and one or more receive antennas is
considered for the analysis The information blocks of
symbols in the transmitter side are passed to the next unit
called space-time block encoder, where two symbols are
embedded in each block The code words of length M = 2 is
generated by the differential space-time block encoder where
M signifies to the total number of transmit antennas The OFDM Modulator and the radio frequency (RF) front-ends obtains these code words and then it modulates the useful information onto the carrier frequency On the constructed receiver side, up to N receiver antennas can be efficiently made use of for reception probably The RF signals are completely down-converted and digitized in the RF front-ends and then finally passed to the OFDM demodulator unit and then the differential space-time block decoder unit [8][9] The interpretation of the received signals is done by the space-time block decoder and after that the received signals are obtained and generated for estimates as the transmitted information symbols, which are again provided simultaneously in blocks
of two symbols
Fig 1 Block Diagram for Transmitter side in Differential Space time Block Coded MIMO-OFDM System
Fig 2 Block Diagram for Receiver side in Differential Space Time Block Coded MIMO-OFDM System
3 PAPR REDUCTION TECHNIQUE USING SLM SCHEME
The following algorithm steps are followed completely in the paper
Step 1: The program is started and then the input bits are generated randomly
Step 2: The serial data is converted into parallel data and then the sparse H matrix for Differential Space Time Encoder is calculated The Differential Space time codes are ways of transmitting data in wireless communications They are forms
288 2013 International Conference on Green Computing, Communication and Conservation of Energy (ICGCE)
Trang 3of space time code that do not need to know the channel
impairments at the receiver in order to be able to decode the
signal
Fig 3 Block Diagram for incorporating the PAPR reduction
technique using SLM scheme in DSTBC MIMO-OFDM
system
Step 3: The input signals is then multiplied with different
phase sequences and kept ready for the computation of the
IFFT operation [1]
Step 4: Meanwhile, the sparse matrix H is shifted and then the
input bits are encoded
Step 5: The input signals are modulated using QPSK
modulation
Step 6: The mapped sequences are computed using Inverse
Fast Fourier Transforms
Step 7: The signal with the lowest Peak to Average Power
Ratio is selected and then proceeded to the next step
Step 6: Then the parallel data is then converted into serial bits
and then the PAPR value is calculated
Step 7: The threshold value is calculated and then it is checked
that whether PAPR>threshold value
Step 8: As a final step the CCDF plot Vs probability of PAPR
is computed and the plot is drawn
Step 9: The bit error rate is also computed
Step 10: A Performance Comparison is drawn between the Bit
Error Rate obtained for (2 x 1) and (2 x 2) DSTBC
MIMO-OFDM systems
Step 11: Stop the program
4 PERFORMANCE ANALYSIS OF DSTBC
MIMO-OFDM SYSTEMS USING RAYLEIGH CHANNELS
UNDER QPSK MODULATION
The channel is assumed to be a flat fading, in simple terms,
it means that the multipath channel has only one tap The type
of modulation engaged here is Quadrature Phase Shift Keying
So, the convolution operation reduces to a simple
multiplication Generally, the channel experienced by each
transmit antenna is independent from the channel experienced
by other transmit antennas For the ith antenna to jth receive antenna, each transmitted symbol gets multiplied by a randomly varying complex number As the channel under consideration is a Rayleigh channel, the real and imaginary parts are Gaussian distributed having a particular mean and variance The channel experienced between each transmitter to the receive antenna is independent and randomly varying in time The random binary sequence of +1’s and -1’s are generated Then it is grouped into pairs of 2 symbols and then the symbols are sent in one time slot The symbols are multiplied with the channel and then white Gaussian noise is added The minimum among the four possible transmit symbol combinations is found out Based on the minimum the estimate of the transmit symbol is chosen It is then repeated for multiple values of Eb/No and then the simulation and theoretical results are plotted
Table 1: Simulation Parameters for PAPR Reduction
2 x 1 MIMO-OFDM Number of parallel channels
to transmit
512
No of OFDM symbols/
frame
4
Fig 4 PAPR Reduction of STBC MIMO-OFDM using
QPSK Modulation in both (2 x 1) and (2 x 2) Systems
On the careful analysis of the figure 4, it is apparent that when QPSK modulation is engaged ,irrespective of the
2013 International Conference on Green Computing, Communication and Conservation of Energy (ICGCE) 289
Trang 4number of receive antennas the PAPR remained the same for
both the (2 x 1) and (2 x 2) DSTBC MIMO-OFDM Systems
From the Figure 4, when QPSK is employed and for U=2,
U=4, U=8, and U=16, when the SLM is incorporated in
DSTBC MIMO-OFDM System with side information scheme,
the PAPR remained the same for both the modulation schemes
and it provided a PAPR reduction of 1.4dB, 2.5dB, 3.15dB
and 3.8dB at CCDF=10-4 respectively
Fig 5 Performance Analysis of (2 x 1) and (2 x 2) DSTBC
MIMO OFDM Systems under QPSK Modulation
On the careful analysis of Figure 5, it is evident that when
QPSK modulation is engaged ,the Bit Error Rate produced is
high in (2 x1) DSTBC MIMO-OFDM system when compared
to that of (2 x 2) DSTBC MIMO-OFDM System
5 CONCLUSION
This paper gives a short introduction to MIMO–OFDM
Systems followed by a short survey on the Alamouti Space
Time Codes Further the performance analysis of PAPR and
the Bit Error Rate using the Selected Mapping Technique for
different Differential Space Time Block Coded OFDM is
analysed for two transmit antennas and one or more receive
antennas and the performance is evaluated Future works may
incorporate the use of a different oversampling rate,
modification of the selected mapping scheme, usage of
different modulation schemes under different channel
conditions to produce a low Bit Error Rate and Peak to
Average Power Ratio
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290 2013 International Conference on Green Computing, Communication and Conservation of Energy (ICGCE)