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Volume 2, Issue 4, April 2013
ISSN 2319 - 4847
BER analysis of MIMO-OFDM system in
different fading channel
Niharika Sethy1 and Subhakanta Swain2
1
National Institute of Technology, Durgapur
2
Sikhsa ‘O’ Anusandhan University, Bhubaneswar
ABSTRACT
Today commercial wireless communication systems are required to provide higher data rates and reliable communication. Two
major challenges in system design are the limited spectrum and the fading caused by multipath components in the wireless
channel. Multiple transmit and receive can be used to form multiple input multiple-output (MIMO) channels to
increase the capacity and data rate. The advantage of employing multiple antennas is to get reliable performance
through diversity and the achievable higher data rate through spatial multiplexing. In MIMO system the
information can transmitted and received from multiple antennas simultaneously since the fading for each link
between a pair of transmit and receive antennas can usually be considered to be independent. The probability that
the information is detected accurately is higher. Fading of the signal can be mitigated by different diversity
techniques, where the signal is transmitted through multiple independent fading paths in terms of the time,
frequency or space and combined constructively at the receiver. In this paper the Bit Error Rate (BER) analysis of BPSK
signal in MIMO and Multiple Input Multiple Output Orthogonal Frequency Division multiplexing system (MIMO-OFDM) is
discussed. BER analysis is done using MATLAB SIMULINK. Here different fading channel is considered. MIMO System is
used to achieve full diversity using OSTBC encoder, to overcome fading effect of channel. By using OFDM, ISI can be reduced
with higher data rate and higher spectral efficiency.
Keywords: MIMO, OFDM, Rayleigh Distribution, Rician Distribution
1. INTRODUCTION
To obtain higher data rates, larger bandwidths are required [1]. The emerging multiple-input multiple-output (MIMO)
communication technologies have the potential to improve the performance without increasing the bandwidth or
transmitted power. MIMO systems exploit spatial diversity by employing multiple antennas at either side of the
communication. OFDM is becoming a very popular multi-carrier modulation technique for transmission of signals over
wireless channels [3]. It converts a frequency-selective fading channel into a collection of parallel at fading sub
channels, which greatly simplifies the structure of the receiver link. The time domain waveform of the subcarriers is
orthogonal. The signal spectral corresponding to different subcarriers overlap in frequency domain [9]. Hence, the
available bandwidth is utilized very efficiently in OFDM systems without causing the ICI. OFDM helps to eliminate the
inter-symbol interference(ISI).
2. METHODOLOGY
2.1 MIMO
Wireless communication using multiple-input multiple-output (MIMO) system enables increased spectral efficiency for
a given total transmit power [2]. The capacity is increased by introducing additional spatial channels that are exploited
by using space-time coding. Multiple antennas are used at both the source (transmitter) and the destination (receiver)
side. The antennas at each end of the communications system are combined to minimize errors and optimize data
speed. The radio wave propagating through the wireless channel undergo transmit power dissipation (path loss) and
shadowing caused by obstacles on the course from transmitter to receiver which attenuates signal power through
absorption, reflection, scattering and diffraction. Constructive and destructive addition of different multipath
components is even introduced by the wireless channel to cause the fading effect. To obtain these spectral efficiency
improvements, we would often need knowledge of the channel condition, which is represented by the channel matrix
[11]. In order to investigate the channel model we describe MIMO channel at certain time n, with Nt transmit and Nr
receive antenna. The transmitted symbol vector is given as x[n] = [X1.....XNt],and the received vector is
y[n] = Hx[n] + w[n]
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Volume 2, Issue 4, April 2013
ISSN 2319 - 4847
In, W[n] = [W1....WNr] T represents noise. The channel matrix is H. Here hi,j represents the complex gain from jth
transmitting antenna to the ith receiving antenna
H 11
.
H
. .
. .
.
H Nr ,1
H 1, Nt
.
. .
.
. . H Nr , Nt
The Alamouti space time coding scheme can be used to achieve diversity at the transmitter and receiver [4].The
information bits are first modulated using an M-ary modulation scheme. The encoder then takes a block of two
modulated symbols s1 and s2 in each encoding operation and gives it to the transmit antennas according to the code
matrix given bellow [12].
S1
S
S2
S2
S1
2.2 OFDM
The basic idea underlying OFDM systems is the division of the available frequency spectrum into several sub carriers.
To obtain a high spectral efficiency, the frequency responses of the sub carriers are overlapping and orthogonal [3]. The
binary information is first grouped, coded, and mapped according to the modulation in a “signal mapper”. After the
guard band is inserted, an N-point inverse discrete-time Fourier transform (IDFT) block transforms the data sequence
into time domain. Following the IDFT block, a cyclic extension of time length, chosen to be larger than the expected
delay spread, is inserted to avoid inter symbol and inter carrier interferences [10]. At the receiver side, after passing
through the analog-to-digital converter (ADC) and removing the CP, the DFT is used to transform the data back to
frequency domain. Lastly, the binary information data is obtained back after the demodulation and channel decoding.
2.3 MIMO and OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is used for high data rate in wireless communications [1].
OFDM can be used in conjunction with a Multiple-Input Multiple-Output (MIMO) transceiver to increase the diversity
gain and the system. The OFDM system effectively provides numerous parallel narrowband channels [4].The
Block diagram for MIMO-OFDM is shown in Fig. 1[13].
2.4 FADING
When signal travels from transmitter to receiver due to fading effect of channel, the envelope of received signal follows
Rayleigh or Rician distribution. When a there is relative motion between mobile user and base station, the frequency
of received signal changes and this phenomenon is called Doppler frequency shift.
(a) Doppler Frequency Shift
Doppler shift is the random changes that occur in a channel introduced as a result of a mobile user’s mobility or
movement [5]. It is the apparent difference in frequency of the received signals from that of the transmitted signals
when there is a relative motion between the transmitter and receiver. This Doppler frequency shift
is given in
d
F
equation (1), where is
is
and C is speed of light.
F
d
V
COS
C
(1)
(b) Rayleigh Distribution
This occurs when the envelope of the received signal follows a Rayleigh distribution. Rayleigh distribution is
statistically used to model a faded signal, when there is no dominant LOS path [6]. The envelope of the received signal
with Rayleigh distribution has the probability density function (pdf) given by as
p r
r
2
exp
r
2
(2)
(c) Rician Distribution
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Volume 2, Issue 4, April 2013
ISSN 2319 - 4847
The Rician distribution used to model a distribution when a strong line of sight component is present along with the
weaker components [6]. It has the probability density function (pdf) given by
r
P
s,
r
2
exp
r
2
2
s
2
2
2
2
I
s
0
2
2
(3)
Figure 1 Block diagram of MIMO OFDM system
3. RESULT
Figure 2 BER perforrmance of BPSK signal in MIMO OFDM system
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Volume 2, Issue 4, April 2013
ISSN 2319 - 4847
Figure 3 BER perforrmance of BPSK signal in MIMO system
Figure 4 BER perforrmance comparison of MIMO and MIMO OFDM system
4. DISCUSSION
In this paper simulation is done using MATLAB SIMULINK tools. In Fig.2 comparison is done between the BER
analysis of MIMOOFDM system in Rayleigh channel and Rician channel (2x2). The BER performance of Rician
channel is better than Rayleigh channel. At BER = 0.207 the Rician channel has Eb/No = 15dB but Rayleigh has
Eb/No= 25dB. So there is an improvement of 10dB in SNR of Rician channel than Rayleigh channel. In Fig3 BER
performance of Rayleigh channel in MIMO system is compared with Rician channel (3x2). Here there is an
improvement of of 8dB.In the SNR of Rayleigh channel than Rician channel. In Fig.4 the BER performance of both
Rayleigh and Rician channel is given.
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Volume 2, Issue 4, April 2013
ISSN 2319 - 4847
5. C ONCLUSION
The BER performance of Rayleigh channel and Rician channel in MIMOOFDM system are same in lower value of
Eb/No. But after 8dB there is large difference between BER of Rayleigh and Rician channel. The BER performance of
Rayleigh channel in MIMO system is much better than that of Rician channel. After 20dB there is no change in BER
performance of Rician channel even if Eb/No increases to 30dB. The BER performance of MIMO system is compared to
that of MIMO OFDM system. Here we can observe that the BER performance of MIMO system is better than that of
MIMO OFDM system. But as comparison to MIMO system the MIMO OFDM is more spectrally efficient.
Reference
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AUTHOR
Niharika Sethy obtained her B.E degree in Electronics and Telecommunication engineering from B.P.U.T,
ODISHA. She has 5 years of teaching experience in different Engineering Schools and Colleges. Now she
is continuing her Master of Technology in National Institute of Technology, Durgapur, West Bengal .Her
area of interest is performance of different Wireless system (MIMO, MIMOOFDM) in presence of Noise,
Interference and Fading.
Subhakanta Swain received B.Tech from BPUT, Orissa in 2010. And continuing his M.Tech in Sikhsa
‘O’ Anusandhan University, Bhubaneswar, Orissa, India. He has performed research in the areas of
Wireless multimedia sensor networks. His core area of research is Priority-based rate control for service
differentiation and congestion control in Wireless multimedia sensor networks.
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