coherent optical communication systems in digital signal ... · international journal of advanced...

International Journal of Advanced Engineering Research and Technology (IJAERT) Volume 2 Issue 7, October 2014, ISSN No.: 2348 – 8190

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COHERENT OPTICAL COMMUNICATION SYSTEMS IN DIGITAL

SIGNAL PROCESSING

K. Nordin1, N. M. Z. Hashim

2, S. Idris

3, A. M. Darsono

4, A. Salleh

5, N. R. Mohamad

6

1-6(Faculty of Electronic and Computer Engineering

UniversitiTeknikal Malaysia Melaka (UTeM), Hang Tuah Jaya, Melaka, Malaysia)

ABSTRACT For the future of optical communication systems, Digital

signal processing (DSP) is an enabling technology. This

system is the most promising widely viewed in the next

generation for long-haul transport systems. In past, the

digital carrier phase estimation demonstration in

coherent receivers has stimulated an extensive attention

in coherent optical communications again. In this paper,

the emergence of DSP tutorial will discussed before

surveying in the algorithms required in digital coherent

transceivers for optical communication systems.

Keywords–Coherent, Detection, Digital Signal

Processing (DSP), Signal

I. INTRODUCTION Digital Signal Processing (DSP) emerged as an

enabling technology for high capacity optical

transmission systems since optical communication has

undergone the evolution over the last decade. As a

promising technique, digital signal processing is under

consideration for optical signal modulation, fiber

transmission, and signal detection and dispersion

compensation. Coherent detection and DSP combination

is expected to become part of optical communication

systems generation and provide new capabilities that

were not possible without the phase of the optical signal

detection. Lately, digital coherent optical communication

is the main technology for optical transport networks [1].

In future of optical networks, capacity increase

will need a flexible bandwidth transmitter which is

dynamically allocates bandwidth by user demand. The

function is to further increase the network’s spectral

efficiency [2]. DSP has increase become embedded into

optical transceivers when the first application specific

integrated circuit (ASIC) implementing advanced

algorithms appeared. ASIC are designed for 11.5-

Gsymbol/s polarization-multiplexed QPSK signal has

been developed, and digital coherent receiver real time

operation at 46 Gbit/s bit rate has been demonstration

striated by using an ASIC. In modern coherent optical

communications history, the achievement is really

milestone [3]. Nascent technology culmination is the

digital coherent receivers for DSP essential to operate

with allow 40Gbits/s, 100Gbit/s and to deployed in core

networks over fiber. It is due to polarization-mode

dispersion (PMD) not support 10Gbit/s by using

conventional technique.

The basic of the principle coherent optical

detection is coherent measure the complex amplitude of

the optical signal with the shot noise limited sensitivity.

This is how information on the state of polarization can

be extracted by polarization diversity uses. Coherent

detection associated of DSP can be very advantageous.

To increase optical receiver sensitivity and permitting a

greater span loss to be tolerated, coherent detection is a

promising technology. Quadrature amplitude modulation

(QAM) and quadrature phase shift keying (QPSK) are an

example of supported of more spectrally efficient

modulations format by coherent detection. Coherent

detection also allows digital signal processing for

transmission impairments compensation such as

chromatic dispersion (CD), PMD, signal carrier offset

and spectrum restricting instead of requesting expensive

physical impairments compensation links.

Next generation of optical transmission systems

need adaptive fitting for time varying transmission

impairments. For example, channel spectrum narrowing

and random phase noise. DSP become more powerful

solution for the future optical transmission links. For this

tutorial, the aim is to outline the development and issues

associate details with realizing a digital coherent

transceiver.

Fig. 1: Generic coherent system [8]

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In Figure 1, in a real arrangement, this black box

embodies a number of complex functions. As well the

purposes that are additionally performed in a classical

arrangement, such as data aggregation, coding, and

framing, supplementary steps demand to be gave in a

transmitter for convoluted modulation formats, as it is

normally utilized in a consistent system. Early of all, the

data have to be mapped to constellation points and, in

case of several messenger systems, to frequencies.

Often, the data are additionally differentially preceded to

cope alongside period slips across receiver-side

messenger synchronization. In a subsequent pace, the

mapped data could be processed digitally, for instance, it

could be pre-distorted to compensate for the nonlinearity

of amplifiers and modulators, or it might be pre-

compensated for deterministic fiber results, for instance,

chromatic dispersion (CD).

The processing of the preceding delineated data

aftermath in four digital data streams those afterward

demand to be modified into analog data. In case of

single-carrier QPSK indicating, every single data stream

carries merely a solitary bit each signal, and

consequently, does not need a DAC. This reduces the

intricacy, and therefore, power consumption of the

transmitter significantly. But even for multicarrier

arrangements [6] or modulation formats alongside higher

order than QPSK, the presentation necessities alongside

respect to resolution and conversion speed are normally

less restrictive for the send DAC than for the accord AD

converter (ADC).

For instance, for a 16-QAM transmitter lacking

preprocessing, only 2 bits (four levels) at a conversion

speed equivalent to the symbol rate are needed, as at the

receiver side, typically 6–8 bits at a sampling rate of

twice the data rate are needed [8].

II. INTERFACING ANALOG AND

DIGITAL DOMAINS To apply DSP, the key element has been the

availability of high speed, time interleaved CMOS

digital to analog and the important is analog to digital

converters (ADC). The real-time sampling oscilloscope

is currently test equipment that led the way in the ADC

development technology. The first generation 20GSa/s 8

bit ADCs in 2003 were emerged in test equipment by

using time-interleaving in 180nm CMOS. Test

equipment has move to 160GSa/s in 2012, increase ADC

rate sampling at steady rate of 25% per annum over the

same period.

60GSa/s is common place for 100GbE

transceivers using PDM-QPSK while ADCs sampling

rates lag test equipment. Test equipment with rates of

50GSa/s is only emerging even though 40GSa/s and

60GSa/s digital to analog converters (DAC) s already

exist in commercial products in contrast for DAC [4].

III. DIRECT DETECTION SYSTEMS IN

DSP The detect detection systems is a first

application of digital signal processing. The primary

application increases the range of uncompensated

transmission. Maximum likelihood sequence estimators

(MLSE) include at the receiver to pre-compensation of

chromatic dispersion by coherently modulating both the

phase and amplitude at the transmitter in addition. The

exponential increase in complexity with transmission

distance resulting in commercial MLSE is one of the

limitations of MLSE. It is being unable to achieve

uncompensated transmission over more than

metropolitan distances. With transmission distance since

it acts on the coherent field, the complexity of pre-

compensation at the transmitter.

Coherent detection is the most advanced

detection method. The receiver computes decision

variables based on the recovery of the full electric field.

It’s contains amplitude and phase information. In

modulation formats, coherent detection allows the

greatest flexibility that information can be encoded in

amplitude and phase or in both in-phase (I) and

quadrature (Q) components of a carrier. Knowledge of

the carrier phase need in the coherent receiver detection

because of received signal is demodulated by a local

oscillator (LO). LO function to serves as an absolute

phase reference. Usually, carrier synchronization has

been performed by a phase-locked loop (PLL). In optical

systems, it can used an optical PLL (OPLL) which

synchronizes the frequency and phase of the LO laser

with the TX laser. The system also can used an electrical

PLL where the function to down conversion using a

free-running LO laser. It followed by a second-stage

demodulation by an analog or digital electrical VCO

whose frequency and phase is synchronized.

Duplex systems can have an advantage by using

an electrical PLL which the transceiver can use one laser

as both TX and LO. Delay requirement is difficult to

satisfy due to PLLs are sensitive to propagation delay in

the feedback path. To fix this problem, the feed forward

(FF) carrier synchronization has the solution. In

addition, , FF can achieve better performance than a PLL

as a FF synchronizer uses both past and future symbols

to estimate the carrier phase. FF can only employ past

symbols due to it is function as a feedback system.

Normally, to perform in software, DSP has enabled

polarization alignment and carrier synchronization. A



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coherent transmission system and the model system are

shown in Figure 1 and Figure 2.

With both quadrature’s detected employing full

coherent detection that shown in Figure3, to gain 3 dB in

receiver sensitivity is possible. Polarization multiplexing

is available for additional spectral efficiency and

compensates ability to an arbitrarily high degree from

linear impairments. The most attractive possibility for

full coherent detection is about 100 Gb/s and above.

Fig. 2 Coherent transmission system implements

Fig. 3 System model

Fig. 4 Quadrature coherent detection

IV. COHERENT TRANSMITTERS IN DSP Coherent transmitters allow a conventional

direct detection receiver to be employed at coherent

system in DSP. Modulated complex optical fields are

used in this situation. It was shown in a Cartesian Mach-

Zehnder modulator. An electrical drive signal modulates

the real part of the field and another electrical drive

signal modulates the imaginary part of the field. By

using finite impulse response (FIR) filter, it can achieve

chromatic dispersion pre-compensation.

This purpose can implement the complex

transfer function with the inverse chromatic dispersion.

The directly achievement for FIR filter implementation

in the time domain or the frequency domain are using

overlap methods. The time domain implementation can

be more efficient for the length of filter typically

required for core if the networks at the transmitter are

also more efficient. At the transmitter, in contrast to the

general case, it occurs since the inputs are binary

removing the need for multiplication. With merely

summing a suitable combination of the tap weights,

convolution can be achieved. To control the optical

spectrum for non-binary modulation formats something

like 16- QAM, transmitter DSP has primarily focused on

pulse shaping. It implement by using FIR filters.

V. COHERENT RECEIVERS IN DSP Coherent detection of optical communication

signals has long been recognized to propose several

performances advantages over direct detection [5], but to

date it has not been used in fiber optic networks.

Coherent detection is sensitive to the amplitude and

phase of an optical signal and it can be used to notice

phase-encoded modulation formats like quadrature phase

shift keying (QPSK), binary phase shift keying (BPSK)

and quadrature amplitude modulation (QAM). These

formats give better receiver sensitivity than easy on-off

modulation.

QPSK and QAM permit many bits each signal to

be transmitted without a substantial degradation in

receiver sensitivity, by transmitting the information in

both quadrature components. Coherent detection replies

only to the optical spectrum in the instant area of the

local oscillator, so it is equivalent to encompassing a

narrow optical filter in the receiver. In fact coherent

detection is the only method that can notice information

spectral densities approaching the Shannon limit.

One more advantage of coherent over direct detection is

the potential of correcting fiber propagation impairments

like chromatic dispersion (CD) in the mechanical

domain. These advantages are all priceless in today’s

fiber optic transmission systems, which use high channel

count wavelength division multiplexing above multi-

1000km distances. The outstanding disadvantage of

coherent detection is the complexity, and consequently

the price, of the receiver.

Fig 5 Typical DSP subsystems in a digital coherent



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Based on Figure 5, inside a digital coherent

receiver there are several key subsystems, that could be

loosely tear in to those concerned alongside equalization

and those concerned alongside synchronization more

details of that could be discovered in. Equalization

algorithms are primarily concerned alongside

compensating for the imperfections in the optical

channel in supplement to those present in the physical

transmitter and receiver, fluctuating from timing and

amplitude correction for the high speed time interleaved

analog to digital converter and also to digital

compensation of residual chromatic dispersion.

After the signal has been equalized it is probable

for the synchronization algorithms to align the

oscillators, both mechanical and optical, thereby

mitigating the encounter of the difference in phase and

frequency amid the transmitter and receiver. Naturally

there are numerous probable variations of the structural

design of the DSP, for example one could select to

digitally compensate for the frequency offset, compare

than estimate the frequency offset to provide a control

signal for the local oscillator to hold the frequency offset

to within the range of the carrier phase estimation and

correction algorithm. As the last way, simplifies the DSP

the feedback induces coupled dynamics and

consequently care have to be seized to ensure that time

constants for every single subsystem are optimized to

ensure stable operation.

Fig. 6 DSP signal flow model

Hence for fast acquisition a feed onward

structure is needed, albeit at the price of higher DSP

complexity [6]. In the end the demodulated signal is

optimally decoded to produce the best estimate of the

sequence of bits encoded by the transmitter alongside the

coding overhead optimized for the working condition of

the system [7].

Estimation of power consumption is challenging

due to the largely parallel nature of the DSP that

frequently pushes the design instruments to their limits.

One of the challenges in approximating power

consumption is the paucity of data considering the

parameters for the power consumption and the

algorithms utilized in business digital coherent receivers.

The notable exclusion is the early commercially

obtainable 40Gbit/s application specific integrated

circuit (ASIC) projected by Nortel (now Ciena), for that

a significant amount of detail has been published.

Instituted on a preceding scrutiny [8] for the ASIC

projected in 90nm CMOS, the manipulation

consumption was manipulated by multiplications

alongside 2.7pJ utilized each real multiplication.

VI. OPTICAL COMMUNICATION

SYSTEMS IN DSP Applying DSP to optical transceivers has

revolutionized optical fiber communication systems

enabling transceivers to evolve to become software

defined. With this new possibilities such as cognitive

transceivers emerge, permitting for both software

describes webs and cognitive webs both in the core and

the admission networks. As we have concentrated

primarily on core webs going onward, DSP is probable

to be a key technology for optical admission webs, just

as it has completed for wireless admission networks [9-

18].

VII. CONCLUSION Digital signal processing (DSP) is an enabling

technology for future optical communication system.

Over the last decade optical communications has

undergone a revolution, as digital signal processing

(DSP) emerged as an enabling technology for high-

capacity optical transmission systems when the first

application specific integrated circuit (ASIC)

implementing advanced algorithms appeared, DSP has

increasingly become embedded into optical transceivers.

The culmination of this nascent technology is the digital

coherent receiver for which DSP is essential to its

operation.

ACKNOWLEGMENT

We are grateful to Centre for

Telecommunication Research and Innovation (CeTRI)

and UniversitiTeknikal Malaysia Melaka (UTeM)

through PJP/2013/FKEKK (29C)/S01215 for their kind

and help for supporting financially and supplying the

electronic components and giving their laboratory

facility to complete this study.

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