Seminar 1 hjgfjhghfghdfgdgfsdfsyutiutiu

8/13/2019 Seminar 1 hjgfjhghfghdfgdgfsdfsyutiutiu

1/14

0

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


2/14

KHULNA UNIVERSITY OF ENGINEERING AND TECHNOLOGY

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.


3/14


4/14


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)


5/14


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


6/14


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.


7/14


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).


8/14


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


9/14


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-multipoint


10/14


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.


11/14


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.


12/14


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


13/14


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.


14/14


Seminar 1 hjgfjhghfghdfgdgfsdfsyutiutiu

Documents

Transcript of Seminar 1 hjgfjhghfghdfgdgfsdfsyutiutiu