International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
Comparison of ACO-OFDM and DCO-OFDM in
IM/DD Systems
Sanjana C Saju Mr. Agi Joseph George
Department of Electronics and Communication Department of Electronics and Communication
AmalJyothi College Of Engineering AmalJyothi College Of Engineering
Koovapally-Kerala Koovapally-Kerala
Abstract— In this paper the two Optical Wireless orthogonal negative values (bipolar). At the receiver a local oscillator
frequency division multiplexing (OFDM) techniques in intensity and coherent detector is used. In contrast in a typical
modulated/direct detection (IM/DD) optical systems are intensity-modulated direct-detection optical system, the
compared. These are asymmetrically clipped optical OFDM information is carried on the intensity of the optical signal
(ACO-OFDM) and DC biased optical OFDM (DCO-OFDM). and therefore can only be positive (unipolar). There is no
The comparison is done by the analysis of BER versus SNR of
laser at the receiver acting as a local oscillator and direct
ACO-OFDM and DCO-OFDM for intensity-modulated direct-
detection systems. DCO-OFDM is less efficient in terms of detection rather coherent detection is used. OFDM is now
optical power than ACO-OFDM for lower SNR value. But for increasingly being considered as a modulation technique for
higher SNR values it is power efficient. This is because the DC optical wireless systems. Many optical wireless systems use
bias used in DCO-OFDM is inefficient in terms of optical power, intensity modulated/direct detection (IM/DD). In IM/DD
while the use of only half of the subcarriers to carry data in systems the transmitted electrical signal is modulated onto the
ACO-OFDM is inefficient in terms of bandwidth. intensity of the optical carrier. Therefore, only real and non-
negative signals can be transmitted.
Keywords—ACO-OFDM, DCO-OFDM, IM/DD, optical systems.
Optical OFDM Using Intensity Modulation have many
I. INTRODUCTION optical modes that are present at the receiver result in optical
Orthogonal frequency division multiplexing (OFDM) allows wireless systems being linear in intensity. So, for optical
high-speed data transmission across a dispersive channel, so wireless systems and other systems where many modes are
is used in many new and emerging high-speed wired and received, the OFDM signal must be represented as intensity.
wireless communication systems. However, OFDM is not This means that the modulating signal must be both real and
used in commercial optical communication systems [1]. This positive, whereas baseband OFDM signals are generally
is because OFDM signals are bipolar, while in optical complex and bipolar. A real baseband OFDM signal can be
systems that use intensity modulation (IM), only unipolar generated by constraining the input signal X to have
signals can be transmitted. Despite the many advantages of Hermitian symmetry. Two forms of unipolar OFDM have
OFDM, and its widespread use in wireless communications, been used in this paper. They are DC-biased optical OFDM
OFDM has only recently been applied to optical (DCO-OFDM) as in [2] and asymmetrically clipped OFDM
communications. This is partly because of the recent demand (ACO-OFDM) from [4]. In DCO-OFDM the signal is made
for increased data rates across dispersive optical media and positive by adding a DC bias. Because OFDM signals have a
partly because developments in digital signal processing very high peak-to average power ratio, a very high bias
(DSP) technology make processing at optical data rates would be required to eliminate all negative peaks. Instead, a
feasible. However another important obstacle has been the moderate bias is normally used and the remaining negative
fundamental differences between conventional OFDM peaks are clipped, resulting in clipping noise. Normally in
systems and conventional optical systems. In typical DCO-OFDM both even and odd subcarriers are modulated
(nonoptical) OFDM systems, the information is carried on the and clipping noise affects all subcarriers. In ACO-OFDM,
electrical field and the signal can have both positive and data is carried only on the odd subcarriers.
IJERTV4IS041422 www.ijert.org 1315
(This work is licensed under a Creative Commons Attribution 4.0 International License.)
International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
Figure 1: DCO-OFDM system
The resulting bipolar signal at the output of the IFFT is remaining negative peaks are clipped, resulting in clipping
clipped at zero to give a non-negative signal [3]. The noise. In typical DCO-OFDM systems both odd and even
resulting clipping noise in ACO-OFDM affects only the subcarriers carry data symbols and the clipping noise affects
all the subcarriers. Any negative peak which remains after the
unused even subcarriers and no clipping noise is present in
addition of DC bias level is clipped at zero. The clipped
the data carrying odd subcarriers. signal is then input to an optical modulator. Here an ideal
optical modulator is used; therefore the intensity of the output
II. DESCRIPTION OF OFDM SCHEMES optical signal is directly proportional to the input electrical
Two forms of unipolar OFDM have been used in this paper. current. The resulting signal is transmitted across a flat
DC-biased optical OFDM (DCO-OFDM) and asymmetrically channel. Shot noise which affects the signal is modeled as
clipped OFDM (ACO-OFDM). They are described below. additive white Gaussian noise (AWGN), is added in the
electrical domain. At the receiver, the received signal is first
converted from an optical signal to an electrical signal using a
A. DCO-OFDM
photodiode. The processing after this point is the same as a
A DCO-OFDM system is shown in figure 1. In DCO-OFDM conventional OFDM receiver. i.e. the output of photodiode is
all the subcarriers carry data symbols. The complex data then filtered and the resulting signal is then analog to digital
signal, is input into the inverse fast Fourier transform (IFFT). converted. Then the appended cyclic prefix is removed and
The input signal is constrained to have Hermitian symmetry. the signal is converted from serial to parallel. The signal is
Because of the Hermitian symmetry of the input, the output given as the input of FFT. Then it is decoded and converted
signal of the IFFT is real not complex. Signal is then from parallel to serial and finally data signal is retained.
converted from parallel to serial (P/S), a cyclic prefix (CP) is
appended, the resulting signal is digital to analog (D/A) B. ACO-OFDM
converted and low pass filtered resulting in x(t). In this paper, In ACO-OFDM, only the odd subcarriers carry data symbols,
an ideal low pass filter (LPF) is assumed. while the even subcarriers form a bias signal which ensures
that the transmitted OFDM signal meets the non-negativity
For large subcarriers, the signal can be modeled as a requirement. Figure 2 shows an ACO-OFDM system. The
Gaussian random variable. Next a suitable DC bias is added input signal to the IFFT consists of only odd components.
and then the remaining negative peaks are clipped. Because Also, the elements of the vector have Hermitian symmetry.
OFDM signals have a very high peak-to-average power ratio, The front-end of the ACO-OFDM transmitter is similar to a
so a very high bias is required to eliminate all negative peaks. DCO-OFDM transmitter where is first serialized and a CP is
If a large DC bias is used, the optical energy per- bit to single appended to it. Then signal is D/A converted and sent across
sided noise power spectral density, becomes very large, an ideal LPF. As negative samples cannot be transmitted in
thereby making the scheme inefficient in terms of optical an IM/DD system, signal is clipped at zero
power. Instead, a moderate bias is normally used, and the
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
Figure 2: ACO-OFDM system
which results in the ACO-OFDM signal. As a result of the transferred bits during a studied time interval. BER has been
anti-symmetry of signal, clipping does not result in any loss measured by comparing the transmitted signal with the
of information. The ACO-OFDM signal is then given as input received signal and computing the error count over the total
number of bits. For any given modulation, the BER is
to an ideal optical modulator and the resulting signal
normally expressed in terms of signal to noise ratio (SNR).
transmitted across a at AWGN channel. The processing in SNR is defined as the ratio between signal power to noise
the receiver is similar to a DCO-OFDM receiver, except that power and it is normally expressed in decibel (dB).
in ACO-OFDM only the odd subcarriers are demodulated, as The figure 3 shows the BER versus SNR graph for ACO-
only they carry the data symbols. OFDM and DCO-OFDM.From the figure it can be observed
that there is a clear difference between two OFDMs in terms
III. EXPERIMENTAL RESULTS of BER and SNR. In the initial stage BER for ACO-OFDM is
higher than DCO-OFDM. As SNR value increases the BER
The comparison of ACO-OFDM and DCO-OFDM are
will decrease suddenly for ACO-OFDM and in DCO-OFDM
discussed here. when SNR value increases the BER will be constant up to
30dB and then it will decrease. But for larger SNR the value
of BER for DCO-OFDM is less. Because a larger DC-bias is
used in DCO-OFDM, the nonlinear distortion is mitigated,
but more power is sacrificed. Since there is no DC-bias for
ACO-OFDM, it has significant advantages in terms of power
efficiency. i.e. DCO-OFDM is less efficient in terms of
average optical power in lower SNR values but for larger
values it is power efficient. This is because the DC bias used
in DCO-OFDM is inefficient in terms of optical power, while
the use of only half of the subcarriers to carry data in ACO-
OFDM is inefficient in terms of bandwidth.
IV. CONCLUSION
In this paper two forms of orthogonal frequency division
multiplexing (OFDM) in intensity modulated/direct detection
Figure 3: BER versus SNR for ACO-OFDM and DCO-OFDM (IM/DD) optical systems are compared. They are asymmetri-
cally clipped optical OFDM (ACO-OFDM), DC biased
The graph of bit error rate (BER) versus SNR is used for the optical OFDM (DCO-OFDM). The comparison is done by
performance analysis. Bit error rate (BER) of a the analysis of BER versus SNR of ACO-OFDM and DCO-
communication system is defined as the ratio of number of OFDM for intensity-modulated direct-detection systems
error bits and total number of bits transmitted during a DCO -OFDM is less efficient in terms of optical power than
specific period. It is the likelihood that a single error bit will ACO-OFDM for lower SNR value. But for higher SNR
occur within received bits, independent of rate of values it is power efficient.
transmission. The bit error rate or bit error ratio (BER) is the
number of bit errors divided by the total number of
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International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181
Vol. 4 Issue 04, April-2015
ACKNOWLEDGMENT REFERENCES
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