Comparative evaluation of bit error rate for different ofdm subcarriers in rayleigh channel

International Journal of Mobile Network Communications & Telematics ( IJMNCT) Vol. 5, No.1,February 2015
DOI : 10.5121/ijmnct.2015.5101 1
COMPARATIVE EVALUATION OF BIT ERROR RATE FOR
DIFFERENT OFDM SUBCARRIERS IN RAYLEIGH
CHANNEL
Sutanu Ghosh
Department of Electronics and Communication Engineering,
Dr Sudhir Chandra Sur Degree Engineering College.
ABSTRACT
In the present situation, the expectation about the quality of signals in wireless communication is
as high as possible. This quality issue is dependent upon the different communication parameters.
One of the most important issues is to reduce the bit error rate (BER) to enhance the performance
of the system. This paper provides a comparative analysis on the basis of this bit error rate. I
have compared the BER for different number of subcarriers in OFDM system for BPSK
modulation scheme. I have taken 6 varieties of data subcarriers to analyze this comparison. Here
my target is to reach at the lowest level of BER for BPSK modulation. That is achieved at 2048
number of subcarriers.
KEYWORDS
Data subcarriers, OFDM, BPSK, BER, Rayleigh fading environment.
1.INTRODUCTION
Multicarrier modulation [1] like OFDM [2] has great applications in present day wireless
communication. Bandwidth scalability and higher spectrum efficiency is the most important
advantages of this OFDM. It has the ability to minimize Inter Symbol Interference and Inter
Channel Interference, where it doesn’t have required the equalizer [3]. This OFDM is a
multiplexing technique, where large information stream is subdivided into multiple number of
small information streams. It is most useful to reduce the bit rate on each of these streams. These
streams are modulated and mapped using orthogonal carriers (is known as subcarriers).
Effectively, low bit rate signals are transmitted instead of a large bit rate signal. The subcarriers
have large channel gains utilize higher order modulation to carry a large number of bits per
OFDM symbol. In OFDM, it is very easy to demodulate the received signals without any
crosstalk between information in subcarriers.
This OFDM has disadvantage of its sensitivity to non-linear distortion, which is the reason of
crosstalk between subcarriers. Finally if it is received at matched filter receiver, then this amount
of crosstalk can enhance the error rate. This error rate is deteriorating the quality of received
signal. So, first of all we the research workers have required finding and eliminating

2
that amount of error rate to boost up the quality of signal. Here I have worked on that issue to find
the BER for different number of subcarriers. On this occasion, I have taken BPSK modulation for
my experimental analysis. BPSK is lower order modulation, but it has a simple phase calculation
with respect to other higher order modulation.
Before this work there have a few works on BER in OFDM. Ref. [2] produced the result on BER
for different subcarrier allocation technique. This work is not sufficient for the analysis of
comparison between different subcarriers. Ref. [4] investigated BER for different modulation in
fading environment. Ref. [5] analyzed the BER for QR decomposition M algorithm. The Authors
produced results on the basis of different values of M. Ref. [6] produced another important result
of BER for maximum likelihood, coding, modulation etc for different antenna configuration.
These works are not sufficient to compare the BER for different values of subcarriers. To the best
of my knowledge there have no such work on that issue. So, I have done this work to show the
BER for different values of subcarriers.
The rest of the sections of this paper are arranged as follows: Sections 2 describes OFDM
technology with its transmission process and resource allocation procedure. Section 3 gives the
idea about the modulation schemes under OFDM technology. Simulation and experimental
results is showed in 4th
Section of this paper. Finally, the whole idea of this paper is concluded in
Section 5.
2.OFDM TECHNOLOGY WITH ITS TRANSMISSION PROCESS AND RESOURCE
ALLOCATION
In this section I have used to describe the basic OFDM technology and its transmission technique.
In serial data transmission, symbols are transmitted sequentially and the frequency spectrums of
each data symbol are allowed to occupy in the whole available bandwidth [7].
A parallel data transmission system provides the different possibilities to resolve many of the
problems came across the serial data transmission systems. Several sequential data streams are
transmitted simultaneously in parallel system. Therefore, large numbers of data elements are
being transmitted at a particular instant of time.
OFDM signals are generated at baseband by acquiring the Inverse Fast Fourier Transform (IFFT)
of different modulated subsymbols. Any of the transmitted symbols xi is taken from multi-
amplitude signal constellation. D to A and A to D converters include ideal low pass filters with
bandwidth 1/Tb, where, Tb is the sampling interval. A guard time of length Tg is used to eliminate
the inter block interference in OFDM and it preserves the orthogonality of tones [8].

3
Figure 1: Block diagram of OFDM transceiver system [3]
Tsym is the symbol duration, which is mathematically defined as –
Tsym = (Number of samples can be chosen to be the power of 2/Total Bandwidth) + Guard
interval;
This guard interval duration is cyclic prefix(CP) duration. CP is applied to provide a periodic
extension to OFDM signal through which a “linear convolution” operation can be executed on the
transmitted signal by the channel, can be estimated by “circular convolution” [9].
Figure 2: OFDM symbol with CP [3]
Figure 1 describes the overall modulation-demodulation techniques in OFDM system. At the end
of the transmitter, block of information-carrying symbols are converted into N number of
substreams by using serial to parallel converter [3]. These streams are modulated using N number
of orthogonal waveforms with frequency fl, where, l= 0,…, N-1. Output of IFFT in Figure 1 is
considered as an overall sum of complex exponential basis functions, complex sinusoids, the
harmonics, or tones of multitone signal [10]. If, I consider that one of these tones or harmonics,
which is complex exponential function related to a particular subcarrier. Then, it is defined as:

4
N
l
N
n
lj
lfl eanx
1
2
|)( ….(1)
where, fl = l/Ts = lΔf [Δf is amount of subcarrier spacing and Ts is symbol duration]; here, ω is
equivalent with fl.
If the channel response Hi (there are total L+1 number of channels) is operated on input
transmitted signal then final output can be defined as –
)()(
0
i
L
i
i dnxHny ….(2)
where, di is multipath delay of a particular assigned channel.
If linearity issue of OFDM signal is considered for a multipath fading channel, then each of its
complex exponential components is also focused to the identical channel model. Consequently, it
can be computed that each received subcarrier component of the OFDM signal (y(n)|ω=lΔf) as
convolution between transmitted signal and channel impulse response. That can be showed by eq.
3 -
fl
L
i
ifl nxHny
0
|)(|)( ….(3)
After the addition of CP, we can replace it with an expression of x(n) | ω=kΔf = akej2πkn/N
, if and
only if the multipath propagation delay is less or equal to the length of CP. Otherwise, a single
delay value outside range of CP, we cross OFDM symbol boundary and the orthogonality
between subcarrier components can be lost. First of all, it is assumed that delay spread is lying
within a range of the length of CP. So, overall received subcarrier component can be expressed as
a function of transmitted subcarrier –
L
i
N
dn
lj
lifl
i
eaHny
0
)(
2
|)( ….(4)
At the receiver end reverse operation of transmitter can be performed using a FFT operation.
Sampled signals of receiver are processed to find their origin point of a block and proper
demodulation window. At the next level, it has required to remove the CP and N′ (N′ = N) point
sequence is to be converted from serial to parallel form and supplied it to the FFT. The outputs of
this FFT are symbols modulated on N subcarriers, each multiplied by complex channel gain.
Depending upon availability of channel information, different types of demodulation or decoding
can be used to pick up the information bits. The output of the multipliers is then integrated over
the interval of 0 to T to recover the estimated signals A′k,0 … A′k,N′-1, which are then converted
from parallel to serial form of data and binary form of transmitting signal is obtained after
decoding.

5
Due to cause of time-varying nature of wireless channel, dynamic resource allocation makes full
utilization of multiuser diversity in OFDM to achieve higher performance [11]. In the case of
access mechanism of OFDM, subset of subcarriers is allocated to each user and thus the
allocation of each user must be pre-scheduled by the system. A basic unit of resource allocation
in OFDM access mechanism is subchannel. This subchannel is a group of subcarriers. There is
different length of these subchannels. On the basis of the allocation of subchannel, all the
mechanisms of resource allocation in OFDM is categorized as – Block, Comb and Random type
allocation method. Generally, it is assumed that block type is used in the environment of slow
fading channel. Comb Type is utilized to satisfy the requirement for equalization, when the
variation of channel is too fast. In case of Random type, the whole channel environment is
utilized for fast fading channel. Block type arrangement is utilized to map the pilot subcarriers on
all the different subcarriers. Comb type is utilized to map on specific number of subcarriers. In
case of Random type, pilot subcarrier indexes can be modified periodically [3, 12-13].
3. MODULATION SCHEMES UNDER OFDM TECHNOLOGY
OFDM have different modulation schemes are different on the basis of channel conditions. These
modulation schemes provide certain interaction between spectrum efficiency and bit error rate.
OFDM transmitter maps message information into a chain of PSK symbol or QAM symbols
which will be consequently converted into N parallel streams. This process is implemented by
applying IFFT operator to data stream. Each of N symbols from serial-to-parallel conversion is
passed through different subcarriers [14 - 17]. So, one of the most important thing is modulation
to describe the performance in my work. At receiver side output of FFT is passed through parallel
to serial converter and at that time output of this converter can be detected by the QAM
demodulator to detect the exact data. In the next section, I have described the simulation method
using BPSK modulation.
4. SIMULATION AND EXPERIMENTAL RESULTS
This work is based on a comparison between BER of different number of subcarriers in OFDM
technology. I have used a set of parameters to execute the experimental simulation. Everything of
this research work has been done through the simulation in MATLAB. 6 different subcarriers
have been taken to perform this work. I have chosen the following simulation parameters and set
their specific value as given in Table I. This work has been simulated in MATLAB and validated
through Qualnet 6.1 simulation software [18].
Table I. Set up Values of Experimental parameters
Parameters Value
Number of Symbols 1000
FFT size 64
SNR Variation (For this issue) 0 - 35 dB
Modulation Type BPSK
Channel type 3 Tap Rayleigh

6
Figure 3: BER for BPSK using OFDM Subcarriers 64, 128, 256 in 3 tap Rayleigh channel
Figure 3 shows the BER for modulation type of BPSK using different types of OFDM subcarriers
of 64, 128, and 256. The graphical comparison of figure 3 and 4 proves that BER is lowest for
2048 number of subcarriers. BER is almost fixed after SNR of 15 dB in OFDM technology under
2048 number of subcarriers. In case of 64 numbers of subcarriers, simulated result of BER and
theoretical values of BER is almost same upto 10 dB.

7
Figure 4: BER for BPSK using OFDM Subcarriers 512, 1024, 2048 in 3 tap Rayleigh channel
Here, it is observed that flat nature of BER curve increases with the increasing order of
subcarriers.
5. CONCLUSIONS
This paper explores an idea about the comparative evaluation on the basis of BER for different
number of subcarriers in Rayleigh fading model. I have taken the results for 6 standard
subcarriers, which is mostly used in latest wireless technology. From output it is clear that
simulated result is too poor with respect to theoretical value at higher value of SNR. This total
work is verified through the compatible value of Qualnet Software. Finally it is concluded that,
better performance with less amount of error in OFDM system is possible to achieve through the
higher number of subcarrier size.

8
REFERENCE
[1] Bingham, J. A., “Multicarrier modulation for data transmission: An idea whose time has come”, IEEE
Communications Magazine, vol. 28, no. 5, pp. 5-14, May 1990.
[2] Wong, C. Y., Cheng, R. S., Lataief, K. B., and Murch, R. D., “Multiuser OFDM with adaptive
subcarrier, bit, and power allocation”, IEEE Journal on Selected Areas in Communications, vol. 17,
no. 10, pp. 1747-1758, Oct. 1999.
[3] Ghosh, S., “Performance evaluation on the basis of Bit error rate for different order of Modulation
and different length of Subchannels in ofdm system”, International Journal of Mobile Network
Communications & Telematics, vol. 4, no.3, pp. 33-41, Jun. 2014.
[4] Rugini, L., and Banelli, P., “BER of OFDM systems impaired by carrier frequency offset in multipath
fading channels”, IEEE Transactions on Wireless Communications, vol. 4, no. 5, pp. 2279-2288, Sep.
2005.
[5] Kim, K. J., Yue, J., Iltis, R. A., and Gibson, J. D., “A QRD-M/Kalman filter-based detection and
channel estimation algorithm for MIMO-OFDM systems”, IEEE Transactions on Wireless
Communications, vol. 4, no. 2, pp. 710-721, Mar. 2005.
[6] Van Zelst, A., Van Nee, R., and Awater, G. A., “Space division multiplexing (SDM) for OFDM
systems”, IEEE Vehicular Technology Conference, VTC 2000, vol. 2, pp. 1070-1074, Tokyo, DOI:
10.1109/VETECS.2000.851289, May 2000.
[7] Zou, W. Y., and Wu, Y., “COFDM: An overview”, IEEE Transactions on Broadcasting, vol. 41, no.
1, pp. 1-8, Mar. 1995.
[8] Van de Beek, J. J., Edfors, O., Sandell, M., Wilson, S. K., and Ola Borjesson, P., “On channel
estimation in OFDM systems”, IEEE Vehicular Technology Conference, VTC 1995, vol. 2, pp. 815-
819, Chicago IL, DOI: 10.1109/VETEC.1995.504981, Jul. 1995.
[9] Dahlman, E., Parkvall, S. and Sköld, J., “4G LTE/LTE-Advanced for Mobile Broadband”, Elsevier,
2011.
[10] Zarrinkoub, H., “Understanding LTE with MATLAB: From Mathematical Modeling to Simulation
and Prototyping”, John Wiley & Sons Pub., 2014.
[11] Shen, Z., Andrews, J. G., and Evans, B. L., “Adaptive resource allocation in multiuser OFDM
systems with proportional rate constraints”, IEEE Transactions on Wireless Communications, vol. 4,
no. 6, pp. 2726-2737, Nov. 2005.
[12] Socheleau, F. X., Ciblat, P., and Houcke, S., “OFDM system identification for cognitive radio based
on pilot-induced cyclostationarity”, IEEE Wireless Communications and Networking Conference,
WCNC 2009, pp. 1-6, Budapest, DOI: 10.1109/WCNC.2009.4917840, Apr. 2009.
[13] Asadi, A., and Tazehkand, B. M., “A New Method to Channel Estimation in OFDM Systems Based
on Wavelet Transform”, International Journal of Digital Information and Wireless Communications
(IJDIWC), vol. 3, no. 1, pp. 1-9, 2013.
[14] Cho, Y. S., Kim, J., Yang, W. Y., and Kang, C. G., “MIMO-OFDM wireless communications with
MATLAB”, John Wiley & Sons Pub., 2010.
[15] Wang, F., and He, X., “The Performance Analysis of AFH_OFDM System Based on Simulink”,
IEEE 7th International Conference on Wireless Communications, Networking and Mobile Computing
(WiCOM 2011), pp. 1-4, Wuhan, DOI: 10.1109/wicom.2011.6040057, Sept. 2011.
[16] Goldsmith, A., “Wireless communications”, Cambridge university press, 2005.
[17] Armstrong, J., “Analysis of new and existing methods of reducing intercarrier interference due to
carrier frequency offset in OFDM”, IEEE Transactions on Communications, vol. 47 no. 3, pp. 365-
369, Mar. 1999.
[18] Qulanet Simulator, Scalable Network Technologies, available at: www. qualnet.com, 2012.

Comparative evaluation of bit error rate for different ofdm subcarriers in rayleigh channel

More Related Content

What's hot (19)

Viewers also liked (12)

Similar to Comparative evaluation of bit error rate for different ofdm subcarriers in rayleigh channel (20)

Recently uploaded (20)

Comparative evaluation of bit error rate for different ofdm subcarriers in rayleigh channel