Causes of Processing Delay

Introduction

Processing delays occur depending on the computational complexity of the signal processing algorithms in your communication system. As data rates increase and modulation schemes become more sophisticated, processing requirements escalate, potentially introducing significant delays in signal processing operations. When designing your system, you must carefully consider these delays to ensure that the end-to-end latency remains within acceptable limits, especially for time-sensitive applications. For each delay source that you introduce, account for the type and amount of delay that it produces in the output.

Sources of Processing Delays

Processing delays come from sources with functions or operations that introduce latency during the processing of data. For example:

Digital demodulators
Filters
Equalizers
Encoders and decoders
Explicit delay blocks, such as the Delay (Simulink) or Variable Integer Delay (Simulink) block
Convolutional interleavers or deinterleavers
Blocks with buffering, downsampling, derepeating, and similar signal operations

While some blocks can determine the current output value by using only the current input value, other blocks need input values from multiple time steps to compute the current output value.

Types of Processing Delays

The major processing delays are:

Demodulation Delay — Demodulation delay is the time it takes to extract the baseband or low frequency signal from the modulated carrier wave.

Technique	Situation in Which Delay Occurs	Amount of Delay
FM demodulators listed in Frequency Modulation	Sample-based processing	delay = One output period
All demodulator objects and blocks listed in Continuous-Phase Modulation	Single-rate processing, D = traceback depth value	delay = D output periods
	Blocks configured for multirate processing and if the model uses a variable-step solver or a fixed-step solver with the Tasking mode parameter set to `SingleTasking` D = Traceback length value	delay = D+1 output periods
OQPSK demodulator listed in Phase Modulation	Single-rate processing	OQPSK demodulation delay varies depending on the pulse shaping filter and the input/output settings. For more information, see `comm.OQPSKDemodulator` and OQPSK Demodulator Baseband.
	Blocks configured for multirate processing, and the model uses a fixed-step solver with the Tasking Mode parameter set to `Auto` or `MultiTasking`
	Blocks configured for multirate processing, and the model uses a variable-step solver or the Tasking Mode parameter is set to `Single Tasking`
All demodulator objects and blocks listed in Trellis-Coded Modulation	Configured for continuous operation with Tr equal to the traceback depth value, and code rate k/n	delay = Tr × k output bits

Filtering Delay — A filter introduces latency as it processes and shapes the frequency characteristics of a signal under these circumstances:

Technique Situation in Which Delay Occurs Amount of Delay

Technique	Situation in Which Delay Occurs	Amount of Delay
System object™: `comm.RaisedCosineTransmitFilter`, `comm.RaisedCosineReceiveFilter` Block: Raised Cosine Transmit Filter, Raised Cosine Receive Filter	Filtering of signal for pulse shaping	delay = (filter span in symbols * samples per symbol) / 2. The delay is half the product of the filter span (in symbols) and the number of samples per symbol.
System object: `comm.IntegrateAndDumpFilter` Block: Integrate and Dump	Integration period over each symbol duration	See Transients and Delays and Example of Transient and Delay.
System object: `comm.MultibandCombiner` Block: Multiband Combiner	Reducing the sampling rate during decimation and increasing the sample rate during interpolation.	See Multiband Combining Delay

System object™: comm.RaisedCosineTransmitFilter, comm.RaisedCosineReceiveFilter

Block: Raised Cosine Transmit Filter, Raised Cosine Receive Filter

Filtering of signal for pulse shaping

delay = (filter span in symbols * samples per symbol) / 2. The delay is half the product of the filter span (in symbols) and the number of samples per symbol.

System object: comm.IntegrateAndDumpFilter

Block: Integrate and Dump

Integration period over each symbol duration

See Transients and Delays and Example of Transient and Delay.

System object: comm.MultibandCombiner

Block: Multiband Combiner

Reducing the sampling rate during decimation and increasing the sample rate during interpolation.

See Multiband Combining Delay

Equalization Delay — An equalizer introduces latency as the equalizer algorithm adjusts the signal to mitigate channel-induced distortions and intersymbol interference under these circumstances:

Technique Situation in Which Delay Occurs Amount of Delay

Technique	Situation in Which Delay Occurs	Amount of Delay
System object : `comm.LinearEqualizer` Block: Linear Equalizer	Filter adaptation and symbol-to-symbol processing	For more information see, Linear Equalizers and Linearly Equalize Delayed Signal
System object: `comm.DecisionFeedbackEqualizer` Block: Decision Feedback Equalizer	Filter adaptation, feedback processing and decision making	For more information see, Decision Feedback Equalizers
System object: `comm.MLSEEqualizer` Block: MLSE Equalizer	Viterbi algorithm traceback operation	delay = traceback depth. Traceback depth of the Viterbi algorithm, defines the number of trellis stages used to determine the most likely path.

System object : comm.LinearEqualizer

Block: Linear Equalizer

Filter adaptation and symbol-to-symbol processing

For more information see, Linear Equalizers and Linearly Equalize Delayed Signal

System object: comm.DecisionFeedbackEqualizer

Block: Decision Feedback Equalizer

Filter adaptation, feedback processing and decision making

For more information see, Decision Feedback Equalizers

System object: comm.MLSEEqualizer

Block: MLSE Equalizer

Viterbi algorithm traceback operation

delay = traceback depth. Traceback depth of the Viterbi algorithm, defines the number of trellis stages used to determine the most likely path.

Decoding Delay — This delay comes from the time takes to decode the data upon reception, which includes error detection and correction processes.

Technique Situation in Which Delay Occurs Amount of Delay

Technique	Situation in Which Delay Occurs	Amount of Delay
System object: `comm.ViterbiDecoder` Block: Viterbi Decoder	During the decoding process	decoding delay = traceback depth.

System object: comm.ViterbiDecoder

Block: Viterbi Decoder

During the decoding process

decoding delay = traceback depth.

Queuing and Buffering Delay — Queuing delay is the time data packets spend waiting in queue before being processed and transmitted over the network. Buffering delay is the time incurred when data is temporarily held in a buffer, a storage area to accommodate a difference in the rate of data flow between devices or processes.