This document describes a simulation of an optical orthogonal frequency division multiplexing (OFDM) system using different M-ary quadrature amplitude modulation (QAM) techniques. The performance of the system was evaluated in terms of bit error rate (BER) and signal-to-noise ratio (SNR) by varying the modulation order M to 16, 64, and 256. Simulation results showed that higher modulation orders require higher SNR values to achieve the same BER as lower modulation orders. The simulated results closely matched theoretical predictions, though there was a deviation of 3-3.5 dB, likely due to additional noise and losses not accounted for in the theoretical model.

1. Performance Evaluation using M-QAM
Modulated Optical OFDM Signals
Harsimran Jit Kaur1
and Dr.M. L. Singh2
1
Chitkara University/ School of electronics and Electrical Engineering, Punjab, India.
Email: harsimran.kaur@chitkara.edu.in.
2
Guru Nanak Dev University, Amritsar,India
Email: mlsingh7@gmail.com
Abstract— OFDM (Optical Orthogonal Frequency Division Multiplexing) in the optical
domain is an emerging technology for fulfilling increased bit rate demand of real time
sophisticated applications. In this work an optical OFDM system is designed using different
M-ary QAM (Quadrature amplitude Modulation) techniques by varying M to 16, 65, 256.
Performance evaluation has been carried in terms of SNR (Signal to noise ratio) and BER
(bit error rate).In present research OPTSIM and MATLAB has been used as Simulation
tools.
Index Terms— OFDM, OSNR, BER, QAM
I. INTRODUCTION
Orthogonal Frequency Division Multiplexing (OFDM) has emerged as leading technology for high data rates
due to its robustness and flexibility in resource allocation. [1,2]. OFDM is based on Multi carrier
transmission which transforms a high data rate signals into a group of parallel low data rate signals which are
transmitted over subcarriers. The spectrum has been utilized much more efficiently by spacing the channels
much closer together. OFDM offers robustness against channel dispersion and ease of phase and channel
estimation in a time-varying environment. OFDM uses digital signal processing of fast-Fourier transforms
(FFTs) to obtain high sub-carrier density and computationally-efficient phase and amplitude equalization [3].
Optical OFDM (OOFDM) technology has witnessed increase of interest in recent years [4]. OOFDM
technique could be classified into two categories based on detection techniques - direct detection and
coherent detection. A dramatic growth has been observed in number of publications in OOFDM as it has
been evolved to be vibrant and fast progressing for long-haul transmission in coherent detection or in direct
detection [1]. In this work a system is demonstrated by using the simulative environment for optical fiber in
OPTSIM software, which allows the design of many configurations regarding optical communications. In
this work, an optical OFDM transmission is simulated based on direct detection. The programming of the
OFDM coder and decoder has been done with MATLAB software. It has been proposed that the concept of
software defined radio (SDR) is a practical solution, for software implementation of the radio transceiver
which is capable of adapting itself dynamically to the user environment instead of relying on dedicated
hardware [5, 6].Performance analysis could be carried over by monitoring various signal conditioning
parameters like OSNR, chromatic dispersion, and electrical Signal to Noise Ratio (SNR) Bit error Rate
(BER) [7]. We can use these parameters for identifying the fault [8]. On the basis of conclusion drawn for
values of these parameters system may dynamically adapts and reconfigure itself for better transmission.
DOI: 02.ITC.2014.5.558
© Association of Computer Electronics and Electrical Engineers, 2014
Proc. of Int. Conf. on Recent Trends in Information, Telecommunication and Computing, ITC

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Figure 1. Optical OFDM System Block Diagram[8]
In this work OOFDM transmission has been carried over by using M- Quadrature Amplitude Modulation
(QAM) by varying M to 16, 64, and 256. Performance has been monitored in terms of BER and SNR.
A block diagram of optical OFDM transmitter diagram has been shown in Fig. 1. Fourier transform
techniques have been used for encoding the data. Information has been carried over many lower rate parallel
subcarriers [6]. System transmitting OFDM modulated data over optical channel is divided into three parts;
transmitter, channel, receiver. Transmitter section involves presenting data into N parallel paths where data at
each path could be modulated using various digital modulation techniques including QPSK (Quadrature
phase shift keying), 16 QPSK, M-QAM etc. In this work M-QAM has been implemented by varying M to16,
64,256 [8]. Various sub-carriers frequencies required for parallel transmission could be fulfilled by using an
inverse-FFT (IFFT) which generates dense comb of OFDM sub-carrier frequencies [7].
II. SIMULATION MODEL
For analysing and performance monitoring the optical OFDM transmission system is modelled using
OPTSIM. OFDM modulated data from MATLAB has been interfaced with OPTSIM to carry the
transmission over fiber. Random generator has been used for generating OFDM signal binary data. M-ary
QAM technique has been used for mapping of binary data values to symbols. Before sending information a
training frame of pilot sub-carriers of length 96 samples is inserted which could be used at receiver for
carrying out channel estimation for compensating the effect of channel on transmitted signal. 64- bit IFFT
has been used for generating OFDM symbol data containing complex words frame and pilots frame.
Extension of the frame is carried over by insertion of zeros to reduce the effect of Inter-carrier Interference.
Cyclic prefix of 16 bits has been added which consists of the end of the OFDM symbol to be copied into the
guard interval. OFDM data after being conversion from parallel to serial by using Parallel-to-Serial (P/S)
converter has been interfaced with OPTSIM to carry over optical transmission [8].
Figure 2. System Model for OOFDM transmission
Figure 2 represents simulation model of an optical communication system working at 10 Gb/s around 1550
nm central wavelength. Standard single mode (SM) fiber of length 100 km with attenuation of 0.2 dB/km,
dispersion 16 ps/nm/km at reference frequency, zero dispersion at 1391.53354633 nm wavelength, average
beat length 5 m. has been used. CW Lorentzian Laser having center emission wavelength 1550 nm, CW
power 1mW and FWHM linewidth 10 MHz as main characteristics has been used as the optical source.

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TABLE I. OPTICAL OFDM SIMULATION PARAMETERS
Parameter Value
Data Rate 10Gbps
Wavelength 1550nm
Fiber length 100km
CW laser frequency 193.1THz
Gain 35dB
Cyclic prefix 16 bit
FFT 64 bits
To obtain modulation of optical signal amplitude dual-arm Mach Zehnder modulator has been used. Mach
Zehnder modulator have excess loss 0 dB offset voltage corresponding to the phase retardation in the absence
of any (on both arms) electric field 0.5 V, extinction ratio 20 dB, chirp factor 0 and average power reduction
due to modulation 3 dB. Reverse operations are employed at receiver side by doing serial to parallel
conversion, removing cyclic pre-fix. For recovering sub-carriers FFT operations has been performed. To
obtain binary values de-mapping of M-ary QAM has been carried over. Various important parameters values
used during simulation are listed in Table I [8].
III. SIMULATION RESULTS
SNR is very important figure of merit for monitoring the performance of transmission quality. In this work
performance analysis has been reported in terms of SNR by varying M to 16, 64 and 256 QAM. Analytical
expression of Eq. (1) [9] has been used to evaluate theoretical BER for M-QAM transmission.
BER=1 − 1 − 1 −
√
Q SNR (1)
Where M represents modulation order; Q is quality factor; SNR is Signal to Noise Ratio. Figure 3(a), 3(b),
3(c) is showing performance comparison curve for BER and signal to noise ratio for M-QAM optical OFDM
system with M to be 16, 64 and 256.
Figure 3(a).BER Vs SNR for Theoretical and
Simulated 16-QAM
Figure 3(b).BER Vs SNR for Theoretical and Simulated 64-QAM

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Fig.3(c).BER Vs SNR for Theoretical and Simulated 256-QAM
TABLE II: COMPARISON OF 16 QAM, 64 QAM, 256 QAM OOFDM
Simulation has been conducted for a QAM modulated OFDM system and comparison of simulated result
with that obtained theoretically using the analytical expression of Equation (1) [9] has been represented. It
has been observed that simulated results match closely to the theoretical performance with small deviation at
very low BER values. Figure 3(a) represents that the theoretical BER predicts a SNR of almost 24.9 dB at a
BER of 10-10
for 16-QAM.
At the same BER of 10-10
the simulated results suggest a SNR of about 27.9 dB, which is not far from the
theoretical prediction, but may suggest additional noises and losses in fiber channel during transmission,
inadequacy in the statistical sampling of the error process at extremely low error rates. From figure 3(b) for
BER 10-10
theoretical SNR has been reported to 27 dB whereas simulated SNR is 30 dB. From figure 3(c) for
BER 10-10
theoretical SNR has been reported to 30 dB whereas simulated SNR is 32 dB. Simulated Optical
OFDM system is performing 3-3.5 dB worse than the theoretical formula. Theoretical formulas do not
consider synchronization errors and quantization of received samples which degrade the performance of
system [10].
Table II. Summarizes simulation results for 16, 64, 256 QAM OOFDM system. Comparison of BER Vs SNR
for different modulation order depicts that a change in modulation order M from 16 to 64 QAM results
approximately a change from 3 dB to 3.5 dB for SNR, whereas change of M from 64 to 256 QAM make
SNR to change between 2 dB to 2.5 dB. The results clearly depicts that selecting higher modulation index
requires high value of SNR to achieve the same BER as compared to lower modulation index. Selecting high
modulation index definitely improve spectral efficiency but require higher values of SNR to achieve same
BER as compared to lower modulation index
IV. CONCLUSIONS
OOFDM system has been simulated and compared over M-ary QAM by varying M to 16, 64, 256.
Comparison of simulated and theoretical results find deviation of 3-3dB from theoretical results, which is not
far from the theoretical prediction, but implies additional noises and losses in fiber channel during
transmission, inadequacy in the statistical sampling of the error process at extremely low error rates. Higher
SNR( dB)
BER 16 QAM 64 QAM 256 QAM
10-10
27.9 30 32
10-12
29 30.5 32.5
10-14
30 33 34
10-16
32 34 36
10-18
33 36 39
10-20
34 38 -

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modulation order requires higher value of SNR to achieve a target BER as compared to lower modulation
index.
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