Modulation Symbol Mapper (MAP) - INTERCONNECT Element – Ansys Optics

INTERCONNECT

The symbol mapper takes consecutive bits and maps them to appropriate constellation points

Keywords

digital, electrical, unidirectional

Ports

Name	Type
modulation	Digital Signal
output I	Electrical Signal
output Q	Electrical Signal

Properties

General Properties

Name	Default value	Default unit	Range
name Defines the name of the element.	Modulation Symbol Mapper	-	-
annotate Defines whether or not to display annotations on the schematic editor.	true	-	[true, false]
enabled Defines whether or not the element is enabled.	true	-	[true, false]
type Defines the element unique type (read only).	Modulation Symbol Mapper	-	-
description A brief description of the elements functionality.	The symbol mapper takes consecutive bits and maps them to appropriate constellation points	-	-
prefix Defines the element name prefix.	MAP	-	-
model Defines the element model name.	-	-	-
library Defines the element location or source in the library (custom or design kit).	-	-	-
local path Defines the local path or working folder $LOCAL for the element.	-	-	-
url An optional URL address pointing to the element online help.	-	-	-

Standard Properties

Name	Default value	Default unit	Range
configuration Defines whether or not to enable I and Q ports.	dual output	-	[dual output, single output
load map from file Defines whether or not to load IQ values from an input file or to use the currently stored values.	false	-	[true, false]
symbol map filename The file containing user defined IQ values. Refer to the Implementation Details section for the format expected.	-	-	-
bitrate mode Controls whether the output signal bitrate varies with modulation order (variable) or remains constant (fixed). Variable mode maintains constant symbol period; fixed mode maintains constant bitrate.	variable	-	[variable, fixed
modulation type Defines the input signal modulation scheme. The user can choose between different standard modulation schemes.	2PAM	-	[user defined, 2PAM, 4PAM, 8PAM, 16PAM, BPSK, QPSK, 8PSK, 8QAM, 16QAM, 32QAM, 64QAM, 128QAM, 256QAM
symbol map table The table allows the user to read or modified a symbol map by changing the position of one or more symbols.	<2,2> [-1, 1, 0,...]	-	-
scale factor I The output signal scale factor.	1	-	(0, +∞)
scale factor Q The output signal scale factor.	1	-	(0, +∞)
bias The DC offset added to the amplitude of the output signal.	0	a.u.	(-∞, +∞)
rotation The rotation angle rotates the output IQ values	0	rad	(-∞, +∞)
rise period The ratio of the bit period required for the response to rise from 10% to 90% of its final value.	0.05	-	(0, 1]
fall period The ratio of the bit period required for the response to fall from 90% to 10% of its final value.	0.05	-	(0, 1]

Simulation Properties

Name

Default value

Default unit

Range

output signal mode

The output signal mode.

%output signal mode%

[sample, block

sample rate

The sample rate of the generated signal. This is typically set by the global properties in the root (top-most) element.

%sample rate%

[0, +∞)

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Implementation Details

The Symbol Mapper converts a sequence of input bits into output symbols according to the selected modulation type and constellation mapping. For accurate signal generation and recovery, both the modulation type and bitrate mode must match between the mapper and demapper in the same signal path.

The mapper supports two operating modes, variable and fixed, via the bitrate mode property. The default mode is the variable bitrate mode, and both modes are fully supported in both sample mode and block mode simulations.

In the variable bitrate mode, the mapper maintains a constant symbol rate. As the number of bits per symbol increases (for example, from NRZ to PAM-4), the bit period decreases proportionally to fit more bits into the same symbol period. This approach is ideal for applications where the symbol rate must be fixed.

For applications that require a constant bit period regardless of modulation type, the mapper offers fixed bitrate mode. In this mode, the bit period remains constant while the symbol period adjusts based on the number of bits per symbol in the modulation type. When switching to a higher-order modulation (QPSK to PAM-4), the symbol period becomes longer. This mode is useful if you want to maintain a constant bitrate regardless of the modulation type.

Example: PAM-4 Symbol Mapping

Please see also the example file 4PAM_Symbol_Map.icp attached to this article.

In this example, the mapper is used to generate a PAM-4 signal by mapping input bits to the PAM-4 constellation. The Symbol Map Table for PAM-4 is preloaded in the element, but you can modify this table to customize the bit-to-symbol mapping if needed. The table below shows the default PAM-4 symbol map:

Index	I (in-phase)	Q (quadrature)
1	-3	0
2	-1	0
3	3	0
4	1	0

Note: This table is preloaded for all modulation types (PAM, QAM, PSK, etc.), but you can customize the entries to implement alternative constellation mappings.

Example: Variable and fixed bitrate modes

Please see the example file 4PAM_variable_fixed_bitrate_modes.icp attached to this article

The figures below illustrate how the symbol period changes between the two bitrate modes, given the same random bit sequence input.

In the variable bitrate mode, the symbol period remains constant as the modulation type changes. The bit period would scale inversely with the number of bits per symbol, resulting in the mapped PAM4 signal sequence lasting only half as long as the input PRBS signal.

The figure below shows the first few cycles of a different PRBS and mapped PAM4 signals. Each period in the PAM4 signal encodes two bits, and hence the bit period is lowered.

In the fixed bitrate mode, the bit period stays constant, while the symbol period scales proportionally with the modulation order, resulting in the modulated PAM4 signal being the same length as the input PRBS signal.

The figure below shows the first few cycles of a different PRBS and mapped PAM4 signals. The symbol period scales up as each symbol encodes two bits.

These two modes enable flexibility in designing communication systems with different timing constraints.

Modulation Symbol Mapper (MAP) - INTERCONNECT Element