Advanced modulation format with Symbol Mapper

Coherent detection in combination with digital signal processing (DSP) provides new capabilities such as enabling the use of highly spectrally efficient modulation formats and compensating a wide variety of transmission impairments. However, coherent transceivers are extremely complex and contain many interconnected optical and electrical components. Photonic integrated circuits (PICs) are a key technology to not only reduce the complexity, cost, and footprint of the transceivers but enable future scaling. Proponents of silicon photonics usually focus on high-volume, low-cost applications, such as short-reach interconnects, because photonic integrated circuits can be manufactured in large-scale. However, silicon photonics is also suited for high-end telecommunications applications such as advanced modulation formats, because of its high index contrast and high element yield.

This page provides an overview of the methodology for simulating advanced modulation format transceivers, and a number of helpful tips, together with a few examples of the advanced modulation format with the Modulation Symbol Mapper element.

Methodology

Work flow of design and simulate transceivers

As the modulation format varies, the transceiver systems keep some general common work flow procedures to construct. The work flow of INTERCONNECT advanced modulation transceivers can be summarized as below:

1. Bit stream generation and pulse shaping. A PRBS generator is usually used for digital signal sequence generation and a pulse generator (e.g. RZ, NRZ) is used for pulse shaping.
2. Modulation. A laser source (usually CW laser) is needed as carrier and different types of modulators (e.g. MZM, AM, PM) can be used to do the modulation with different modulation formats.
3. Signal detection and de-modulation. Photodiodes can be used for signal detection and corresponding receivers (e.g. direct, coherent) can be used for de-modulation.
4. Measurements and visualization. Different analyzers can be used for variant types of measurements and system qualification.  

What do we want to measure with our simulations?

There are a number of factors we want to measure for advanced modulation transceivers.

• Bit/Symbol Error Rate (BER/SER): BER is the number of error bits divided by the total number of transmitted bits in a studied time interval it is a unitless performance measurement. The BER/SER can be affected by a number of elements in the system, but the key factors are the received signal power and the noise level in the channel.    
• Eye Diagram: eye diagram is an oscilloscope display formed by horizontally sweep the repeatedly sampled received digital data signal. Several system performance measurements, such as Q-factor and time jitter, can be derived by analyzing the eye diagram.
• Q-factor: Q-factor is a dimensionless parameter that describes the eye opening in optical communication systems, it can be derived from the eye diagram and measures the system's quality.  
• Error Vector Magnitude (EVM): EVM is a measurement used to qualify the performance of the receiver or transmitter in communication systems. Informally, it is a measure of the variance of the constellations from their ideal positions. Noise, phase noise and distortion can all degrade EVM, hence EVM provides a comprehensive measure of the quality of the receiver or transmitter.

Modulation Symbol Mapper

The Modulation Symbol Mapper element maps the binary bits to symbols according to the modulation formats. The following table gives an example of how the bits and symbols map to each other for a 16-QAM modulation format.

Visualize measurements

It is relatively straightforward to visualize and analyze the performance and quality of the system. The approach is summarized below:

Step 1: select and connect analyzers to the systems at different points and simulation stages.

Analyzers connection in INTERCONNECT systems are indicated by dashed lines. Please note that the connection cannot be made if the input data to the analyzer is not as the supposed type.

Step 2: select the analyzer, go to the results window and right-click on the result and select "Visualize".

Simulation tips

1. When run the simulation, we often set the PRBS "automatic seed" to be "true". We created several "seeds" in our server and the sequence generator will grab one of them the first time the simulation is run, then if we re-run the simulation, the seed is locked and the bit sequence will be kept the same. Otherwise when "automatic seed" is set to "false" and "seed" is set to "0", a new initial condition is chosen every time the simulation is run.
2. In "Root Element", we often set the "simulation input" to be "sequence length", then the "samples per bit" is automatically calculated and guaranteed to be integer.
3. Add "monitor ports" to key elements that may be re-measured several times, then the analyzer can be added, validated and visualized post simulation process. To validate the analyzer, please see using the schematic editor.

Application Examples

Pulse-amplitude modulation (PAM) modulation formats

Pulse-amplitude modulation (PAM) is a form of signal modulation where the information is encoded in the amplitude of a series of carrier pulses. It is a pulse modulation scheme in which the amplitudes of a sequence of carrier pulses are varied according to the sample value of the driving signal. Demodulation is performed by directly detecting the power level of the carrier at every symbol period.

In this example, we use INTERCONNECT solutions to study the 2-PAM format. Other PAM modulation formats can be simulated using the same method.

In this example, you will learn how to:

• Generate 2PAM signals
• Measure signal qualities
• Map signals

Problem Definition: More Details

The system in this example contains the following elements:

• 1 Pseudo-random Bit Stream (PRBS) block
• 1 Modulation Symbol Mapper (MAP)
• 1 Electrical Amplifier (AMP)
• 1 Low Pass Filter (LPF)
• 1 Eye Diagram Analyzer (EYE), and
• 1 Vector Signal Analyzer (VSA)

Set up Model

• Start a new INTERCONNECT project. You can start a new project by pressing Ctrl+N, or by selecting New in the File menu.
• To generate 2-PAM modulated signal, from the Element Library drag and drop a PRBS Generator (Element Library\ Sequence Generator) and a Modulation Symbol Mapper (MAP, Element Library\ Pulse Generators\ Electrical), then connect the elements as follows:

• Click on the schematic background and set the global properties according to the following table:

• Click on the MAP and set the "modulation type" to be 2PAM with the "symbol map table" as following:

• Drag and drop an Electrical Amplifier (Element Library\ Amplifiers\ Electrical) and a LPF (Element Library\ Filters\ Electrical), connect the elements as follows:

• Set the gain of the AMP to be 3 dB and the cutoff frequency of the LPF to be 1.625 GHz.
• To measure and visualize the signal, from the Element Library drag and drop an Eye Diagram Analyzer (Element Library\ Analyzers\ Electrical) and a VSA (Element Library\ Analyzers\ Electrical), connect the elements as follows:

Set the VSA "modulation type" to be 2PAM with the same build in "symbol map table" as MAP.

Run Simulation, Visualize Results

• To run the simulation, click on the run button

  

  on the tool bar. When a simulation is running, the calculation process will appear at the top of the analyzer.

• When the simulation finishes running, the Results Window of the Analyzers will be populated with results. Users can simply right-click on each result to visualize this in the Visualizer window. For example, to look at the eye diagram, select the Eye Diagram analyzer, go to the results window and right-click on the "eye diagram" result and select "Visualize".

Discussion and results

Signal integrity analysis is done by special elements, the analyzers. Analyzers allows for post-processing of data stored in monitors. Analyzers can be inserted at different points of the circuit for detailed analysis of the signal evolution from the transmitter to the receiver.

At the 16QAM signal generation stage, the 16QAM MAP modulates the signals according to the mapping shown below, then the separated in-phase part and quadrature-phase part are added to a DC source to shift up the electrical signal voltage to the [0, 2V] MZM driving voltage range.

Signal waveforms at different transmission stages can also be visualized and measured in the system.

At the receiver, to map the symbols and measure the signal quality, both the build in 16QAM mapping table and customized mapping table can be used. When using the 16QAM "symbol map table", set the "normalize IQ values" to be true, the symbol mapping and measurements are shown as following:

When using the user defined "symbol map table", disable the "normalize IQ values", the symbol mapping and measurements are shown as following:

Even though the two mapping tables both give 0 symbol error rates (SER), the lower error vector magnitude (EVM) indicates that the customized mapping table fits the signals better.

Quadrature amplitude modulation (QAM) modulation formats

Quadrature amplitude modulation (QAM) is a modulation scheme which conveys two analog message signals, or two digital bit streams, by modulating the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) modulation. The two carrier waves are out of phase with each other by 90° and are thus called quadrature carriers. The modulated waves are summed and then transmitted, and the final waveform is a combination of both phase-shift keying (PSK) and ASK. In the digital QAM cases, a finite number of at least two phases and at least two amplitudes are required. 

In this example, we use INTERCONNECT solutions to study the 16 QAM modulation format and corresponding quality measurements. Other QAM modulation formats can be simulated using the same method.

In this example, you will learn how to:

• Generate and map 16 QAM signals
• Measure signal qualities
• Symbol map for decision making

Set up Model

• Start a new INTERCONNECT project. You can start a new project by pressing Ctrl+N, or by selecting New in the File menu.
• To generate 16-QAM modulated signal, from the Element Library drag and drop a PRBS Generator (Element Library\ Sequence Generator) and a Modulation Symbol Mapper (MAP, Element Library\ Pulse Generators\ Electrical), then connect the elements as follows:

Click on the schematic background and set the global properties according to the following table:

Click on the MAP and set the "modulation type" to be 16-QAM with the "symbol map table" as following:

• Drag and drop an Electrical Amplifier (Element Library\ Amplifiers\ Electrical) and a LPF (Element Library\ Filters\ Electrical), connect the elements as follows:

• Drag and drop another AMP and LPF and connect them to the other branch output of the MAP. Set the gain of the AMP to be 3 dB and the cutoff frequency of the LPF to be 1.625 GHz.

• To measure and visualize the signal, from the Element Library drag and drop an Eye Diagram Analyzer (Element Library\ Analyzers\ Electrical) and a VSA (Element Library\ Analyzers\ Electrical), connect the elements as follows:

Set the VSA "modulation type" to be 16QAM with the same build in "symbol map table" as MAP.

Run Simulation, Visualize Results

• To run the simulation, click on the run button

  

  on the tool bar. When a simulation is running, the calculation process will appear at the top of the analyzer.

• When the simulation finishes running, the Results Window of the Analyzers will be populated with results. Users can simply right-click on each result to visualize this in the Visualizer window. For example, to look at the eye diagram, select the Eye Diagram analyzer, go to the results window and right-click on the "eye diagram" result and select "Visualize".

Discussion and results

Signal integrity analysis is done by special elements, the analyzers. Analyzers allows for post-processing of data stored in monitors. The analyzers we used in this 16-QAM generator system is the Eye Analyzer and the VSA.

The MAP modulates the pseudo-random bit sequence (which is a random bit stream with combination of 1s and 0s) to the pulse amplitude and phase. The reference inputs to VSA generates the decision points of the data and then the received data can be mapped according to the decision regions.

All the points falling into the specific circle will be recognized as the corresponding symbol. The time domain signal and constellation are as following:

The Eye Diagram analyzer creates eye plots from the signal at the Bessel filter output port. It uses the original signal from the MAP as a reference signal to estimate and compensate for propagation delays (clock recovery) between the transmitter output and receiver input. 

Analyzers can be inserted at different points of the circuit for detailed analysis of the signal evolution from the transmitter to the receiver:

See also

system script command