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
  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
  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
  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.

Related Examples

• Calculate and Specify Receive Delay for Error Rate Block
• Set Receive Delay Using Find Delay Block

More About

• Delays in Communication Systems
• Manipulate Delays