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SEMINAR ECE4130
OFDM for Next Generation Optical
Access Networks.
Submitted by
Muhammad Ashiqul Islam
Roll: 0909018
Dept: Electronics and communication engineering
Khulna University Of Engineering And Technology10/30/2013
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Abstract:
This report contains a general overview on paper OFDM for NextGeneration Optical Access Networks written by Neva Cvijetic, Member,
IEEE. It will focus on the principles, advantages, challenges, and practical
requirements of optical orthogonal frequency division multiplexing
(OFDM)-based optical access with an emphasis on orthogonal frequency
division multiple access (OFDMA) for application in next-generation
passive optical networks (PON). It also contain various optical OFDM(A)
transceiver architectures for next-generation PON and functional
requirements are outlined for high-speed digital signal processors (DSP)
and data converters in OFDMA-PON. These all topics were discussed by theauthor Neva Cvijetic and here it is written from my perspective of
understanding the paper.
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Fig. 1. OFDM(A) spectrum with four orthogonal subcarriers.
The reason why OFDM boasts a spectral efficiency advantage over conventional FDM is
precisely that it eliminates the spectral guard bands f by invoking the principle of
orthogonality.(Since multilevel modulation, such as M -ary QAM, can be used in SC, FDM,
and OFDM, it is not in itself the reason behind OFDM spectral efficiency.) Orthogonality,
in turn, can be achieved by judicious selection of the subcarrier frequencies, fn ,
n=0,1,2,N-1. For example, let us assume that f1 is a sinusoidal carrier that hasbeen modulated with a complex QAM symbol,A1-jB1, where the exact values of A1 and
B1will depend on the selected QAM constellation. For 16-QAM, for example,A1 {1,3}and B1{1j,3j}. As such, s1(t) can be expressed as
.(1)
Where in the first and second terms, respectively, denote the in-phase and quadrature
portions of the signal, produced by the complex nature of M -ary QAM signaling. To now
ensure orthogonality between s1(t) and s1(t) , where is the QAM-modulated signal on
subcarrier f2, it must be the case that
.(2)An elegant way to meet this requirement of orthogonality is to define the subcarrier
frequencies as integer multiples (i.e., harmonics) over the symbol time as
(3)
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Fig.2: Block diagram of a generic OFDM communication system.
Combining (1) and (3), the complete electrical OFDM signal may be expressed as
(4)
where g(t) denotes the impulse response of any baseband pulse shaping filter that
might be used; the simplest choice is the rectangular pulse given by g(t)=1,0tT, andzero elsewhere, which produces the aggregate spectrum of Fig. 2. However, from (4), a
practical OFDM implementation difficulty becomes apparent: for an OFDM signal with
256 subcarriers, for example, directly implementing (4) would require an array of 255
synchronized analog oscillators at both the transmitter and receiver sides. Fortunately,
most of (4) can in fact be done digitally. To observe this, we rewrite (4) as
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KHULNA UNIVERSITY OF ENGINEERING AND TECHNOLOGY
From (5) and (6), we note that if (6) can be generated efficiently, producing (5) becomes
a simple task, requiring just one oscillator operating at fRF. Converting (6) to the digital
domain by sampling (6) at times t=kT/N , where the discrete time index is defined by
k=1,2,3,.., we obtain
We now observe that, by definition, at each discrete time, ,(7) is in fact the inverse
discrete Fourier transform of the complex QAM symbols, , over the OFDM subcarriers,
which can be implemented using the highly efficient inverse fast Fourier transform
(IFFT) algorithm. The IFFT and the FFT, therefore, become the baseband OFDM
modulator and demodulator, respectively. In other words, (7) states that if we pick any
frequency-domain complex QAM data symbols and take their -point IFFT, we will get
the sampled time-domain version of the corresponding -subcarrier OFDM signal. Digital
to-analog conversion and up conversion to using a single analog oscillator then
complete the RF OFDM signal generation in (5).
Cyclic prefix and single tap equalization:
Fig 2 also illustrates the use of the cyclic prefix (CP) to combat ISI in OFDM and
enable efficient -subcarrier FDE.CP insertion consists of prepending some predefined
tail-end portion of an OFDM data frame to its beginning, as shown in Fig.
4.Consequently, the frame begins and ends the same way, acquiring a cyclic quality. Aslong as the CP is at least as long as the dispersive delay of the channel, the CP, rather
than the front-end data symbols, will absorb any residual symbol spreading (i.e., ISI).
From this perspective, it is only the CP length that matters: the CP content could even bea silent interval. However, the beauty of the CP content as illustrated inFig. 4 is that it
turns the channels time-domain dispersive effect from a linear convolution into a cyclic
convolution, such that no matter how long the impulse response becomes, as long as the
CP is as long, data symbols can still be recovered via single-tap equalization in the
frequency domain.
To summarize, we can say that the key idea of OFDM is to realize high aggregate data
rates through parallel symbol transmission on many narrowband orthogonal
subcarriers. By orthogonality, the OFDM subcarrier spectra can partially overlap
without interfering, which increases the spectral efficiency compared to conventional
FDM. Moreover, the longer time-domain symbol durations and the use of the CP enable
high resistance to linear dispersion, while an efficient DSP-based implementation can be
realized with the FFT/IFFT. Finally, a natural extension to a multiuser access
environment can be made through the OFDMA concept. These key advantages have
propelled OFDM(A) into an array of high-speed transmission applications, and can all be
exploited in fiber-optic communication as well.
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Fig.3: OFDM symbol transmission in (a) ideal, non-dispersive channel, (b)
dispersive channel without CP insertion, resulting in ISI, and (c) dispersive channel
with CP insertion used to eliminate ISI.
Why Optical OFDM in Access?:
1.Fiber-to-the-home emerging as future-proof access solution.
i)Bandwidth driver: digital video.
2.Point-to-multipoint passive optical network (PON) expected to play leading role in
next-generation access.
i)Global deployment of Gigabit/Gigabit Ethernet PON (G/GE PON)
ii)Ratification of 10G/GE PON standards.
3. Bandwidth flexibility between multiple users/applications of premium value in future
access.
i) Services include mix of digital, analog, circuit and packet-switched, legacy andemerging applications.
ii)Orthogonal Frequency Division Multiple Access (OFDMA).
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Modulations for optical OFDM:
An un-modulated, continuous wave (CW) optical carrier signal
offers several options, or dimensions, for data modulation: its amplitude, phase,
frequency, polarization, intensity, or a combination thereof can be modulated.
Depending on the choice of the modulation dimension(s) at the transmitter, different
receiver side detection schemes become possible as well. Bringing OFDM(A) into the
optical domain thus generates several new transmitter and receiver architectures
compared to purely electronic and/or RF OFDM(A). In this section, three prominent
modulation/detection combinations for O-OFDM will be overviewed, with a focus on the
resulting transmitter and receiver side architectures. These include optical (intensity or
field) modulation with IM/DD, which we will refer to as optical OFDM (O-OFDM),
optical modulation with coherent detection, referred to as CO-OFDM, and all-optical
field modulation with coherent detection, termed AO-OFDM. Particular emphasis will be
placed on implementation aspects that are of unique importance to next-generation
optical access. The different flavors of optical OFDMA will also be discussed andclassified according to specific bandwidth sharing mechanisms.
Fig.4: a)O-OFDM b)CO-OFDMA c)AO-OFDM
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Passive optical network:
A passive optical network (PON) is a telecommunications network that uses
point-to-multipointfiber to the premises in which unpoweredoptical splitters are used
to enable a singleoptical fiber to serve multiple premises. A PON consists of anoptical
line terminal (OLT) at the service provider's central office and a number of optical
network units (ONUs) near end users. A PON reduces the amount of fiber and central
office equipment required compared withpoint-to-point architectures. A passive optical
network is a form of fiber-opticaccess network.
OFDMA-PON Technology:
Several candidates for future passive optical network (PON)l OFDMA, TDM, WDM, hybrid TDM/WDM
OFDMA-PON differentiator: tackle key challenges in electronic domain through
digital signal processing (DSP) Leverage advanced DSP to achieve superior performance, rapid and robust networkre-configurability, cost reduction OFDMA-PON: novel DSP-based platform for speed, flexibility and cost-efficiency infuture high-speed PON access systems
Critical that future PON technologies be highly cost-efficient to remain attractive andpractical
i)Re-use existing optical distribution network (ODN)
ii)Upgrade with advanced modulation and digital signal processing (DSP)
OFDM-Based PON for Next Generation Optical Access:
Fig.5: Single-wavelength optical OFDMA-PON architecture for heterogeneous service
delivery.
http://en.wikipedia.org/wiki/Point-to-multipointhttp://en.wikipedia.org/wiki/Fiber_to_the_premiseshttp://en.wikipedia.org/wiki/Optical_splitterhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Optical_line_terminalhttp://en.wikipedia.org/wiki/Optical_line_terminalhttp://en.wikipedia.org/wiki/Optical_network_unithttp://en.wikipedia.org/wiki/Optical_network_unithttp://en.wikipedia.org/wiki/Point-to-point_%28network_topology%29http://en.wikipedia.org/wiki/Access_networkhttp://en.wikipedia.org/wiki/Access_networkhttp://en.wikipedia.org/wiki/Point-to-point_%28network_topology%29http://en.wikipedia.org/wiki/Optical_network_unithttp://en.wikipedia.org/wiki/Optical_network_unithttp://en.wikipedia.org/wiki/Optical_line_terminalhttp://en.wikipedia.org/wiki/Optical_line_terminalhttp://en.wikipedia.org/wiki/Optical_fiberhttp://en.wikipedia.org/wiki/Optical_splitterhttp://en.wikipedia.org/wiki/Fiber_to_the_premiseshttp://en.wikipedia.org/wiki/Point-to-multipoint8/13/2019 Seminar 1 hjgfjhghfghdfgdgfsdfsyutiutiu
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Fig.5 depicts a single-wavelength OFDMA-PON architecture that exploits the
principles described earlier; a WDM extension can readily be made by launching
multiple wavelengths from the OLT and adopting the single-wavelength architecture of
previous pages fig on each of the launched wavelengths. As shown in Fig. 8, at the OLT,
a bandwidth-sharing schedule is formed according to ONU-side demand, and is
distributed to all ONUs over pre-reserved subcarriers and/or timeslots. Different OFDMsubcarriers can thus be assigned to different ONUs; each OFDM subcarrier can also be
time shared to realize 2-D multiuser bandwidth partitioning. One and 2-D bandwidth
provisioning can also be combined. For example, while ONU-2 in Fig. 8 maintains a fixed
subcarrier assignment over several frames, ONU-1 and ONU-3 engage in time-domain
sharing of the same OFDM subcarriers. Since traffic is aggregated and de-aggregated
electronically, the architecture also features the important advantage of a passive last-
mile optical splitter, such that the legacy fiber distribution network, which accounts for
the majority of PON investment cost, can be reused. At the ONUs, each ONU recovers its
pre assigned OFDM subcarriers and/or time slots in DSP. An orthogonal OFDMA-based
schedule for upstream transmission is likewise generated by the OLT and distributed tothe ONUs. At the OLT, a complete OFDMA frame is assembled from the incoming sub
frames originating at different ONUs. Consequently, for both downstream and upstream
OFDMA-PON accurate synchronization is very important to enable multiuser access.
The OFDMA-PON platform of Fig. 8 can support heterogeneous services because OFDM
subcarriers essentially become transparent pipes for the delivery of arbitrary signals
(e.g., legacy T1/E1 lines, Ethernet packets, both analog and digitized mobile backhaul,
IPTV, VPN, etc.) In terms of implementation, the integrated DSP-based transmission and
control planes mean that the MAC protocols can be regarded as simply another
functional block of the digital OFDM(A) transceivers. This software-defined approach
features high re-configurability, such that the system can be extended to new and
emerging applications in a non-disruptive fashion. Consequently, OFDMA-PON (see Fig.8) can cost-efficiently coexist with legacy systems and devices on the same fiber ODN by
operating in an orthogonal wavelength or frequency band. For example, 15101540 nm
downstream and 13401360 nm upstream OFDMA-PON operation can enablecoexistence with G-PON, XG/10 G-EPON, and legacy RF video overlays. For backward
compatible networks, optimal spectral mapping and the required degree of OLT and
ONU-side change thus emerge as important challenges. In green-field deployments,
which would not involve backward compatibility considerations, a wavelength plan that
maximizes mature optical component reuse would be preferred. Because of a
directional asymmetry in the PON topology i.e., a point-to-multipoint downstreamand multipoint-to-point in the upstreamdifferent issues exist for downstream versus
upstream transmission in OFDMA-PON. These are surveyed next, along with some
potential techniques for mitigating these challenges.
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OFDM-based PON Flavors for Multi-User Access:
DSP Requirements for OFDMA-PON:
The contention that OFDM is a great technology is generally not viewed ascontroversial. In the context of PON-based access, optical OFDMA has been shown to be
advantageously flexible, non-disruptive to the legacy ODN, and capable of record
transmission rates and distances. However, the implicit reliance of a practical OFDMA
PON implementation on advanced ADC, DAC, and DSP technologies has often been
regarded as a liability. The primary reason for this is that an optical OFDMA-based
system would require DSP-based components that are at least one order of magnitude
faster than those of any other successfully commercialized OFDM(A)-based application.
The decisive question is thus whether VLSI technologies could keep up with optical
OFDMA. To address this, the key DSP requirements and technology trends related to a
practical OFDMA-PON implementation are overviewed next.
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DSP-based Transmitter/Receiver Architecture:
Fig.6: Digital transmitter and receiver architectures for OFDMA-PON.
i)Keygoal:real-time, high-speed (multi Gb/s) operation
ii)KeyDSP-basedcomponents:
Digital to Analog Converter (DAC),
Analog to Digital Converter (ADC)
DSP processor
iii)Digital processor implemented either through
FieldProgrammableGateArrays(FPGAs)or
ApplicationSpecificIntegratedCircuits(ASIC)
DSPComponentsforOFDM-PONImplementation:There are mainly three components. They are
1. ADC (Analog to digital converter)2. DAC (Digital to analog converter)
3. DSP processors.Due to following characteristics of DSP components those are used in OFDM-PON
implementation----------
Silicon platformADC/DAC + DSP integration, mass production, cost-efficiency. Low power consumption.
65nm, 40nm, 28nm CMOS processes Cost-efficient packaging options
Component cost profile driven by volume DSP complexity: IFFT/FFT dominates, ~log(N) scaling
Optimized, readily available algorithms
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Fig.7: Summary of recent high-speed ADC/DAC sampling rate trends.
Summary and Conclusions:
A comprehensive overview of OFDM-based optical access has been
presented in this report on basis of the paper OFDM for next generation optical accessnetworks written by Neva Cevijetic, Member IEEE, covering technology principles,
practical advantages and challenges, as well as recent progress and application
scenarios in future PON. The techno-economic prospects for DSP-based enabling
technologies, including high speed digital processors and data converters, have also
been discussed.
In summary, the fundamental motivation for using optical OFDM(A) in optical access
can be regarded as threefold: 1)OFDM enables multilevel modulation and efficient
dispersion compensation to achieve spectrally efficient, high-speed, long reach access
over a legacy PON fiber plant; 2) OFDMA subcarriers can be used as finely granular
bandwidth resources for highly dynamic multiuser traffic aggregation in point-to-multipoint optical access networks; and 3) OFDM(A) implementation is largely DSP
based and can thus be realized in silicon to achieve a cost-efficient, volume-driven cost
profile. Moreover, with the advent of efficient high-speed DSP and data converters in
recent years, and an increasing momentum of R&D activity in the field, the progress in
this area is expected to continue.
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