International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number 10–Nov2014
An Efficient Channel Estimation of
Mimo_OFDM System LTE Network
K.Yogi Seshaiah
M.tech II year, Dr.SGIT, markapur
M.V.NaraSimha Reddy
Prof. P. Prasanna Murali Krishna
Asst.prof, Dr.SGIT,markapur
H.O.D of DECS ,Dr.SGIT.markapur
Abstract: Future wireless communication system have to be designed to integrate features such as high data rates,high
quality of service and multimedia in the existing communication framework. Increased demand inwireless communication
system has led to demand for higher network capacity and performance. Higherbandwidth, optimized modulation offer
practically limited potential to increase the spectral efficiency.Hence MIMO-OFDM systems utilizes space multiplex by
using array of antenna’s for enhancing the efficiencyat particular utilized bandwidth. MIMO-OFDM use multiple inputs
multiple outputs from single channel. Thesesystems defined by spectral diversity and spatial multiplexing. The aim of this
paper is to design andimplement of channel estimation method and modulation technique for MIMO system. The
designspecifications are obtained using MATLAB.In this paper we present an analysis of MIMO-OFDM system using MQAM (where M=16, 32, 64, 256). We compare the computer simulation results of OFDM system with OFDM system.
Keyword: PAPR, OFDMA, LSI, ICI,LTE, SNR and BER.
I.
INTRODUCTION
Mobile communications has become an everyday
commodity. In the last decades, it has evolved from being
an expensive technology for a few selected individuals to
today's ubiquitous systems used by a majority of the
world's population. From the first experiments with radio
communication by Guglielmo Marconi in the 1890s, the
road to truly mobile radio communication has been quite
long. [1]. Digital communication techniques appeared in
the Second Generation (2G) systems, and main access
schemes are Time Division Multiple Access (TDMA) and
Code Division Multiple Access (CDMA). The two most
commonly accepted 2G systems are Global System for
Mobile (GSM) and Interim Standard-95 (lS-95). These
systems mostly offer speech communication, but also data
communication limited to rather low transmission rates.
The concept of the Third Generation (3 G) system started
operations during October, 2002 in Japan [2]. The LongTerm Evolution (LTE) is often called "4G". The LTE
depends in its work on the principle of the multiple
accesses which is a radio transmission scheme that allows
many senders to transmit signals in the same time span
without interfering with each other [3, 4].
OFDM thwarts Inter Symbol Interference (ISI) by
inserting a Guard Interval (GI) using a Cyclic Prefix (CP)
and moderates the frequency selectivity of the Multi Path
(MP) channel with a simple equalizer. This leads to cheap
hardware implementation and makes simpler the design
of the receiver. OFDM is widely adopted in various
communication standards like Digital Audio Broadcasting
(DAB), Digital Video Broadcasting (DVB), Digital
Subscriber Lines (xDSL), Wireless Local Area Networks
(WLAN), Wireless Metropolitan Area Networks
(WMAN), Wireless Personal Area Networks (WPAN)
and even in the beyond 3G Wide Area Networks (WAN)
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etc. Additionally, OFDM is a strong candidate for
Wireless Asynchronous Transfer Mode (WATM).
However, among others, the Peak to Average Power Ratio
(PAPR) is still one of the major drawbacks in the
transmitted OFDM signal [5]. Therefore, for zero
distortion of theOFDM signal, the HPA must not only
operate in its linear region but also with sufficient backoff. Thus, the RF High Power Amplifier (HPA) with a
large dynamic range are required for OFDM system.
These amplifiers are very expensive and are major cost
component of the OFDMsystem. Thus, if we reduce the
PAPR it not only means that we are reducing the cost of
OFDM system and reducing the complexity of the
Analogue to Digital (A/D) and Digital to Analogue (D/A)
convertors, but also increasing the transmit power, thus,
for same range improving received Signal to Noise Ratio
(SNR), or for the same SNR improving range.
A large number of PAPR reduction techniques have been
proposed in the literature. Among them, schemes like
constellation shaping [6], coding schemes [7,8], phase
optimization [9], nonlinear commanding transforms [10],
Tone Reservation (TR) and Tone Injection (TI) [11,12],
clipping and filtering [13], Partial Transmit Sequence
(PTS) [14], Precoding based Selected Mapping (PSLM)
[15], precoding based techniques [16] and Selected
Mapping (SLM) [17] are
popular. In [10] Wang and Tellambura proposed a soft
clipping technique which preserves the phase and clips
only the amplitude. They also put a lot of effort to
characterize the performance and discover some
properties to simplify the job. However, the PAPR gain is
only estimated by simulations and is limited to a specific
class of modulation technique.In [8] Han and Lee
proposed a PAPR reduction technique based on Partial
Transmit Sequence technique in which they divide the
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International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number 10–Nov2014
frequency bins into sub blocks and then they multiply
each sub-block with a constant phase shift. Choosing the
appropriate phase shift values reduces PAPR. The most
critical part of this technique is to find out the optimal
phase value combination and in this regard they also
proposed a simplified search method and evaluated the
performance of the proposed technique
II.
illustrates the block diagram of an OFDM system.
Baseband modulated symbols are passed through serial to
parallel converter which generates complex vector of size
N. We can write the complex vector of size N as X = [X0,
X1,X2… X N-1]T. X is then passed through the IFFT block.
The complex baseband OFDM signal with N subcarriers
can be written as
SYSTEM DESIGN MODEL
A. OFDMA
The down link LTE air interface is based on Orthogonal
Frequency Division Multiple Access, which offers good
flexibility and performance for a reasonable complexity.
OFDMA is a multi-user version of a digital modulation
scheme OFDM. In OFDM the signal is first split into
independent sub-carriers and these closely-spaced
orthogonal subcarriers are used to carry the data. The data
is divided into several parallel data streams or channels,
one for each subcarrier. OFDMA is a modulation &
access technique that combines both Time Division
Multiple Access (TDMA) & Frequency Division Multiple
Access (FDMA) technologies. OFOMA improve the
spectral efficiency by saving (50%) of assigned
bandwidth. Among the bandwidths that will be supported
are 5, 10 and 20 MHz one beneficial feature of this
technique is the ease of adaptation to different
bandwidths. The smaller bandwidth unit can remain fixed,
even as the total bandwidth utilization is changed.
OFDMA allows fast allocation of radio resources and
orthogonal multiuser multiplexing in the frequency
domain. The wide band signal is generated from
modulated frequency domain subcarrier signals using a
scalable FFT transform. The subcarrier spacing was
selected to allow for Doppler tolerance over a large range
of UE velocities. This will yield a sufficiently high SNR
to modulate either by quadrature phase shift keying,
16QAM, or 64QAM or up to two independent symbol
streams in the presence of imperfect power control, phase
noise, and RF imperfections, while keeping the
computational complexity at a reasonable level.Two
frame structure types are defined for E-UTRA: frame
structure type 1 for FDD mode as shown in Fig. 3. For the
frame structure type 1, the 10 ms radio frame is divided
into 20 equally sized slots of 0.5 ms. a sub frame consists
of two consecutive slots, so one radio frame contains ten
sub frames. Tsis expressing the basic time unit
corresponding to 30.72 MHz.
Fig. 1.Block diagram of OFDM system
Here = √−1 and the PAPR of OFDM signal in (1) can
bewritten as
where E [.] denotes expectation and the Complementary
Cumulative Distribution Function (CCDF) for an OFDM
signal can be written as
where PAPR0 is the clipping level. This equation can be
read as the probability that the PAPR of a symbol block
exceeds some clip level PAPR0.
B. OFDM SYSTEM & PAPR REDUCTION
The OFDM system splits the high speed data stream into
a number of parallel low data rate streams and these low
rates data streams are transmitted simultaneously over a
number of orthogonal subcarriers.Fig. 1 and fig.1 (a)
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International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number 10–Nov2014
Fig.1(b).Block diagram of Pre IFFT based OFDM system
III.
SIMULATION RESULTS
We performed extensive simulations in MATLAB® in
order to evaluate the performance of OFDM system. To
show the PAPR analysis OFDM system, data is generated
randomly then modulated by M-QAM (where M=16, 32,
64, 256). We compared our simulation results with
OFDM system on one hand and also we compared our
simulation results with OFDM system, SLM-OFDM
system and OFDM-Original system. To show the overall
performance of the DHT-Precoder based OFDM system
for PAPR reduction in MATLAB® we considered MQAM for M=64. It is to be noted that M-QAM has itself
PAPR. Table 1 summarizes the PAPR of M-QAM
(M=16, 32, 64, 256).In this section, we demonstrate the
capacity lower bound of MIMO-OFDM systems using
pilot symbol based MMSE channel estimation with the
optimized PDR using Monte Carlo simulation over the
channel realizations, for each of the three considered pilot
symbol constructions.
Fig 2. Graph Between MSE And SNR.
Fig 3. Capacity Enhancement in MIMO
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International Journal of Engineering Trends and Technology (IJETT) – Volume17 Number 10–Nov2014
Fig.4. Capacity with optimal PDR versus SNR for 2£2
and 4£4 MIMO-OFDM systems.
An overall observation is that the capacity is not
especially sensitive to the PDR as long as it is in a certain
region. For example, a quasi-optimal region in 2X2
MIMO-OFDM systems with independent or orthogonal
pilot patterns. Fig. 4 shows the comparison of the capacity
of 2X2, 2X4, and 4X4 MIMO-OFDM systems with three
different pilot patterns versus the percentage of pilot
power when SNR is 10dB. The optimal PDR of scattered
pilot pattern is more sensitive to the number of transmit
antennas than that of independent pilot and orthogonal
pilot.
IV.
CONCLUSION
In this paper, we analysed the PAPR of OFDM system for
M-QAM (where M=16, 32, 64, 256). Matlab simulation
shows that OFDM System shows better PAPR gain as
compared to OFDM-Original system, OFDM system and
SLMOFDM (with V=2) system respectively. Thus, it is
concluded that OFDM System shows better PAPR
reduction then OFDM System, SLM-OFDM System and
OFDM-Original system for MQAM. Additionally, the
OFDM system does not require any power increase,
complex optimization and side information to be sent for
the receiver.
V.
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