keysight technologies characterizing high-speed coherent...

Keysight Technologies Characterizing High-Speed Coherent Optical Transmission Systems

Application Brief

M8195A 65 GSa/s Arbitrary Waveform Generator

N4391A Optical Modulation Analyzer & N4392A Integrated Optical Modulation Analyzer

02 | Keysight | Characterizing High-Speed Coherent Optical Transmission Systems - Application Brief

Overview

The needs of Big Data usage, Cloud applications, the Internet of Things (IoT) and a huge number of mobile devices are challenging the existing communication networks for wireline and wireless voice and data traffic.

The industry has adopted coherent optical transmission communication systems, which use advanced modulation schemes, to cope with the explosive demand for data transportation. This solution comes with some new requirements, which need to be understood along the whole communication link from the transmitter, through fiber cable and network elements to the receiver. The network design needs to be robust against signal distortions and link impairments.

Keysight Technologies, Inc. explains in a series of application notes:

– how to generate clean modulated signals using the M8195A 65 GSa/s Arbitrary Waveform Generator

– how to generate signal distortions and link impairments using the M8195A 65 GSa/s Arbitrary Waveform Generator and the Optical Modulation Generator Tool

– how to emulate link impairments – how to characterize and analyze link performance with the N4391A and N4392A

Optical Modulation Analyzers. – how to test the robustness of compensation algorithms in the receiver’s digital signal

processor (DSP)

Motivation for higher order modulation

Optical transmission links are the backbone for all data traffic over the internet – wired and wireless.

The Bandwidth ChallengeIf we look at the global mobile traffic over the next couple of years the Compound Annual Growth Rate (CAGR) from 2013 to 2018 is 61%; it is estimated that by 2016 the internet traffic will reach 1 zettabyte (1021 bytes). Global IP traffic has increased eightfold over the past 5 years, and will increase fourfold over the next 5 years. Today, the gigabyte equivalent of all movies ever made is crossing global IP networks every 5 minutes. The number of devices connected to IP networks is already twice as high as the global population.

Figure 1. Drivers for higher bandwidth are still there.

1.5 2.6

4.4

7.0

10.8

15.9

2011 2012 2013 2014 2015 2016 2017 2018

Exabytes per month (1 Exabyte is 1,000,000,000 GigaBytes)

Source: Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2013–2018

Monthly mobile traffic will grow 11-fold over the

next 5 years

Average mobile connection speed

has more than doubled in 2013

Mobile video grows disproportionately

(69% CAGR)

Global mobile traffic 2013 - 2018

Online gaming

Mobile services

Cloud computing

Smart home

61% CAGR 2013 - 2018

Internet of things


The BottleneckThere is no doubt that the growing amount of data generated by the Internet of Things (IoT), Big Data and Cloud usage will fill the existing communication pipes. The more critical question is if the telecommunication infrastructure can keep pace.

The telecommunication fiber infrastructure worldwide uses dense wavelength division multiplexing (DWDM) for long-haul transmission, usually with carrier channels spaced on a 50 GHz grid defined by the ITU-T. This will remain dominant for the foreseeable future. So how can service providers still increase the transmission capacity, if all the channels are fully occupied by the signal spectrum?

– One way is lighting unused DWDM transmission channels as already done in most cases.

– A second way is the deployment of new fibers, which is the most expensive and time-consuming alternative.

– The third way would be to increase the transmission speed, at the cost of higher bandwidth consumption in the fiber. This can be done as long as the signal bandwidth stays within the 50 GHz ITU grids. Currently we are also at this limit in many core transmission systems using 10 G transmission. Simply increasing the speed of NRZ transmissions does not help here as its channels start to interfere with their neighboring channels.

Figure 2(a). With OOK, we face channel interference or degradation at 100 Gb/s and beyond.

The SolutionThe RF community in the mobile industry has solved similar problems in the last two decades. So the optical community is able to leverage these technologies from the RF world and adopt them to the special optical requirements. The solution is to use modulation schemes that transmit more information per bandwidth than the traditional on and off switching of the light (RZ or NRZ modulation). This way, the spectral efficiency and the transmission rate can be increased at the cost of more complex hardware and additional digital signal processing.

Figure 2(b). Complex modulation schemes can solve this problem.


Table 1 gives a selection of commonly used modulation formats. With the number of points, the number of bits transmitted per symbol increases but the occupied bandwidth stays the same.

Bits/ symbol

Symbol

1 BPSK

2 QPSK

3 8-PSK, 8-QAM

4 16-QAM, 16-APSK, 16-Star QAM

5 32-QAM, 32 Star QAM, 32-APSK

6 64-QAM

7 128-QAM

8 256-QAM

9 512-QAM

10 1024-QAM

11 ... ...

Table 1. Overview of commonly used modulation formats and their constellation diagrams

Using advanced modulation formats and dual polarization allows to transmit on-off keying more than 100 Gb/s in a 50 GHz wide DWDM channel. Figure 3 shows at the bottom a 112 Gb/s NRZ on-off keying (OOK) signal that occupies more than 100 GHz of bandwidth. Moving upwards in the figure, it is illustrated how the spectral efficiency can be increased by using an advanced modulation format, polarization multiplexing and pulse shaping. This way, a signal of the same data rate occupies in the end only 50 GHz of bandwidth as defined by the OIF for 100 Gb/s optical transmission.

Figure 3. Increasing spectral efficiency: from NRZ OOK to DP-QPSK

QPSK - 2 bits/symbol

64 QAM - 6 bits/symbol




The horizontal axis shows the optical frequency (not wavelength) covering several ITU channels each with 50 GHz bandwidth.

– The lowest spectrum shows a 112 Gb/s signal that is modulated with classical on-off keying as has been done for the last 20 years. This would occupy much more than one ITU channel.

– When switching from on-off keying to complex modulation (here QPSK) as proposed by the OIF, we gain a factor of 2 in efficiency since we now transmit 2 bits per symbol compared to 1 bit per symbol for OOK. The effect on the spectral occupancy is shown in the middle trace named “NRZ-QPSK”.

– An additional method to gain further spectral efficiency is called polarization-divi-sion or dual polarization multiplex, often marked with a DP- or PDM- prefix in the modulation format like DP-QPSK or PDM-QPSK.

Polarization multiplexing launches two independent signals in two orthogonal polarization planes. These polarization planes carry two independent data streams that can be recovered at the receiver side. This results in another reduction in spec-tral bandwidth by a factor of two, shown in the trace “PDM NRZ QPSK”

– With the use of both methods, complex modulation and polarization multiplexing as defined by the OIF, the efficiency increases by a factor of 4, leading to a clock rate of 28 GHz instead of 112 GHz in the case of on-off keying (112 GHz / 4 = 28 GHz).

This concept not only uses much less bandwidth, but also relaxes the bandwidth requirements for the used electronics.

Using advanced modulation schemes is the industry-chosen way to address the explosive growth of data in telecommunication networks for the next years. The use of this technique will lead us to next-generation speed classes in the 400 Gb/s and 1Tb/s arena.

A thorough understanding of all effects in the end-to-end coherent transmission link is needed to overcome the challenges of adjusting an already existing 10 Gb/s-40 Gb/s telecommunication infrastructure.

Keysight is committed to working closely with and supporting the industry with high-speed test and analysis tools and solutions.


What’s next?

The chart below shows a block diagram describing a typical high-speed transmission line with network components, distortions, and impairments.

Optical Transmitter Fiber Connectors Splicing

Optical Amplifier (EDFA)Reconfigurable Optical Add Drop Multiplexer (ROADM)

Optical Receiver

– IQ Gain Imbalance, Offset

– XY Imbalance – Common Mode – Skew – Phase Noise – ENOB – Carrier Feed-Through

– Polarization Mode Dispersion (PMD)

– Chromatic Dispersion (CD)

– Polarization Dependent Loss (PDL)

– Cross-Phase Modulation (XPM)

– Self-Phase Modulation (SPM)

– Four-Wave Mixing (FWM)

– Reflections

– Crosstalk – Spontaneous Emission – Amplified

Spontaneous Emission (ASE)

– Phase Noise

– Frequency Offset – Constant Phase Offset – IQ Gain Imbalance,

Phase Angle – XY Imbalance – Common Mode – Skew – LO Phase Noise,

Frequency Offset – Bandwidth – ENOB

Figure 4. Typical transmission network elements – distortions & impairments

– The impact of distortions and impairments in each network element needs to be understood to assure proper high-speed transmission.

– The transmission distance for advanced modulation schemes are limited not only by the available optical signal-noise ratio (OSNR) but also by fiber non-linearities. Increasing signal power generates unwanted non-linearities. How can non-linearity effects be avoided or compensated?

– How does one link impairment or signal distortion interact with others? – How can high-capacity optical transmission links be developed that minimize

electrical power consumption? – How can the distortions and impairments mentioned above be emulated to develop

more robust digital signal processing for the receivers? – How can the tolerance of the transmission link for the above mentioned impairments

and distortions be increased?

Follow Keysight’s series of application notes, which describe in depth Keysight’s product offering to answer these questions.

Application Note Part 1: Generating clean modulated signals using the M8195A 65 GSa/s AWG.

Application Note Part 2: Generating distorted test signals and link impairments using the M8195A 65 GSa/s AWG.

Application Note Part 3: Keysight analysis tools to support the verification and design of coherent transmission links.


Outlook

The described developments in coherent optical transmission systems and sub-systems require more and more flexibility to generate clean modulated signals as well as distorted test signals. The Keysight M8195A 65 GSa/s Arbitrary Waveform Generator provides the versatility to create the signals needed for dual polarization digital coherent transmis-sion, orthogonal frequency division multiplexing (OFDM), time domain pulse shaping and more.

Furthermore you can add linear and non-linear impairments to your signal or compensate for distortions between the AWG and the system under test or even components inside your system.

The Keysight M8195A offers sample rates of up to 65 Gsa/s with 20 GHz bandwidth and up to four channels for generating > 32 GBaud complex modulated signals – simultaneously in one single AXIe module.

M8195A 65 GSa/s Arbitrary Waveform Generator

To characterize and analyze the impact of versatile signal and impairment generation, a high-performance analysis tool is needed.

The N4391A and N4392A Optical Modulation Analyzers offer comprehensive characterization of amplitude and phase modulated optical signals for 400 Gb/s to 1 Tb/s transmission systems and advanced research for terabit transmission.

The Optical Modulation Analyzer provides:

– Standalone software to extend analysis capabilities offline – Wide-bandwidth polarization-diverse coherent optical receiver technology with

real-time detection – Novel signal processing algorithms combined with Keysight 89600 vector signal

analysis software – Seamless integration of Keysight’s high-speed real-time data acquisition unit, the

Infiniium 90000-Q Series Oscilloscope

N4391A Optical Modulation Analyzer & N4392A Integrated Optical Modulation Analyzer


This information is subject to change without notice.© Keysight Technologies, 2017Published in USA, December 1, 20175992-0022ENwww.keysight.com

www.keysight.com/find/M8195Awww.keysight.com/find/oma

Related Literature

Title Publication number

Keysight M8195A 65 GSa/s Arbitrary Waveform Generator – Data Sheet 5992-0014EN

N4391A Optical Modulation Analyzer – Data Sheet 5990-3509EN

N4392A Optical Modulation Analyzer – Data Sheet 5990-9863EN

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