Measurements on Complex Optical Modulation formats for Efficient Data Transfer enabling 100G Networks
Over the last decade, the communications world has changed much more than it did in the previous century.We can get services via internet or mobile phone that we couldn’t imagine even 20 years ago.These services are available nearly everywhere and anytime in the world. The services are still growing and more advanced media like audio and video will dominate internet traffic in the future. Smartphones bring the wired world to the mobile world. Thinking about the fact that Youtube now generates more internet traffic than the whole internet in the US in 2000 gives an idea about the network speed requirements for the present and future.By 2015 the internet traffic will reach 1 zettabyte (1021 bytes)
Data centers are being built across the globe to store and analyze big collections of structured and unstructured data mentioned above in the cloud. In addition to the storage and analysis capacities Data Centers need fast network infrastructure to transfer data faster and faster. This explosively growing amount of data is becoming an enormous challenge for our backbone networks. The spectral efficiency of the fiber optical networks have to be increased as fiber optical infrastructure and signal concepts need to support data rates of 100 Gbit/s now, soon 400 Gbit/s and even higher in future.
Complex Modulation Schemes
Optical data transport started with the simplest and cheapest digital on-off-keying (OOK). Coding schemes. The signal here is ideally a rectangular sequence of ones (power-on) and zeros (power-off). This concept faced a limit when transfer rates reached for 40 Gb/s. Due to the high data rate, the bandwidth occupied by the signal gets larger than the channel bandwidth of a 50 GHz ITU channel and adjacent channels start to overlap in spectrum resulting in crosstalk and degradation of the modulated information. Thus it is not practical to increase data rate beyond 40 Gb/s with OOK.
Thus Conventional OOK is being overtaken by complex modulations of light due to their superiority in terms of bit transfer efficiency and spectral efficiency. Complex modulation reduces the required bandwidthand higher data rates can be transmitted in the 50 GHz- ITU channel. Complex Modulations modulate Phase of lightwave in addition to amplitude and have been used extensively in RF Communication. The optical bandwidth required by complex modulated signals does not depend on the data rate but only on the symbol rate.
Modulation formats include QPSK, QAM etc. QPSK can transmit 2 bits per symbol. Further if the signal is also polarization multiplexed, the spectral efficiency further increases by a factor of 2. This is called as DP-QPSK (Dual Polarization QPSK). Thus another factor of 2 can be gained through polarization division multiplexing. QPSK plus PDM enables transfer of 2 x 2 = 4 times more bits at the same time which means at the same clock rate. In the end, after further narrowing the occupied spectrum with a pulse shaping filter, one can transmit 100 Gb/s in a 50 GHz wide channel. TAs bit rates increase beyond 10G to 40G/100G, fiber chromatic dispersion (CD), polarization mode dispersion (PMD) and non linear effects emerge as two main limitation factors in how far a signal can be transmitted without electronic regeneration. In Complex modulation schemes increase the bit rates without increasing CD/PMD effects.
As the Modulation Schemes become complex so are the Transmitters ,Receivers, Signals and their measurements.
Signal Quality Measurements – Error Vector Magnitude
In conventional OOK only the amplitude of light is used to code the information onto the carrier signal. Thus typical parameters can be derived from an eye-diagram of on-off keyed signals. An eye mask is used to test the quality of the signal or system. Performance standards specify performance masks and the Eye Signals are not allowed to hit the mask points. The estimate of the bit error ratio (BER) can be derived from the noise distribution during the, 1‘and, 0‘amplitude segments of the signal.
In Complex Modulation Schemes like QPSK (Quadrature Phase Shift Keying) the information is coded only in the phase of the carrier signal, whereas in QAM (Quadrature Amplitude Modulation) the information is coded in both phase and amplitude. The complex modulations consists of an I (In-phase) and Q (Quadrature-Phase) components. In this case the Eye Mask does not provide any insight about the phase of light. Instead I-Q Diagrams and Constellation diagrams provide the insight into the complex modulated signals. The I-Q diagram (also called a polar or vector diagram) displays demodulated data, traced as the in-phase signal (I) on the x-axis versus the quadrature-phase signal (Q) on the y-axis.
In a constellation diagram, information is shown only at specified time intervals. The constellation diagram shows the I-Q positions that correspond to the symbol clock times. These points are commonly referred to as detection decision-points, and are interpreted as the digital symbols. Constellation diagrams help identify such things as amplitude imbalance, quadrature error, or phase noise and most important provide the measure of EVM (Error Vector Magnitude).
EVM is the deviation of my received signal’s I-Q vector from the ideal I-Q vector. This concept of error vector magnitude (EVM) is common in standards like WLAN which also use complex modulations. Work is on by different committees to standardize EVM as a performance measure for complex modulated light
EVM is calculated as an RMS value over a statistically significant number of vectors. EVM has a direct relation to the BER. EVM masks are also being standardized on the lines of the eye diagrams masks in OOK. Thus here is exactly a new test need and challenge, known as Optical Modulation Analysis to analyze the transmission signals where parameters like EVM (Error Vector Magnitude), Phase Error, Gain Imbalance etc is to be measured. Also components working at these data rates have to characterized by measuring the linear electro-optic transmission and electrical reflection characteristics as a function of modulation frequency for reliability.
A New Set of Test Instruments
For analyzing physical layer complex modulated optical signals and for measuring there EVM, optical modulation analyzers are used. Optical Modulation Analyzers (OMA)are test instruments with integrated coherent receiver and digitizer with all required optics and the vector analysis software.
Keysight optical modulation analyzers support upto 60Gbaud of Baud rates which translate to 240 Gbit/sec for DP-QPSK (Dual Polarization – QPSK) which is the most commonly used complex modulation format.Keysight OMAs offer most sophisticated signal processing algorithms with highest flexibility with extensions for the optical requirements like dual polarization data processing.
Keysight OMAs measure EVM, Phase Error, BER, Constellations etc for a variety of modulation formats. Keysight OMAs also have a unique self calibration capability for optical inputs and do not need complex calibrations.
For testing Coherent Receivers multichannel Arbitrary Waveform Generators (AWG) are needed for creating complex signals. For example for DP-QPSK at 25 GBaud (100 Gb/s) a 4 channel arbitrary waveform generator with 25 GBps bit rate per channel is needed. The Arbitrary waveform generators must generate distortions such as IQ Gain Imbalance, Offset, Frequency Offset, Phase Offset etc.
Keysight AWGs provide flexibility to generate clean modulated signals as well as distorted test signals with theversatility to create multiple modulation formats. 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. Keysight AWG offers 20 GHz bandwidth on four channels for generating > 32 GBaud complex modulated signals – simultaneously within one single module.
Author profile
Mr. Sanchit Bhatia, Application Engineer, Keysight Technologies India Pvt. Ltd.
