Communications Blockset

Delays in Digital Modulation

Digital modulation and demodulation blocks sometimes incur delays between their inputs and outputs, depending on their configuration and on properties of their signals. The following table lists sources of delay and the situations in which they occur.

Note    The situations in the table are not mutually exclusive. If more than one situation applies to a given block or model, then the separate delays are additive. For example, if a passband demodulator in the AM sublibrary processes a sample-based signal and has a Samples per symbol parameter of 8, then the block's total delay is two output periods. As another example, if a passband OQPSK modulator-demodulator pair has a Baseband samples per symbol parameter of 7, then the two blocks together have a total delay of three output periods from the demodulator.

Table 1-3: Delays Resulting from Digital Modulation or Demodulation

Modulation or Demodulation Type	Situation in Which Delay Occurs	Amount of Delay
All demodulators in AM, PM, and FM sublibraries except OQPSK	Sample-based input, and Samples per symbol or Baseband samples per symbol parameter is greater than 1	One output period
All demodulators in CPM sublibrary	Sample-based input, D = Traceback length parameter	D+1 output periods
	Frame-based input, D = Traceback length parameter	D output periods
All passband demodulators except OQPSK	Always	One output period
OQPSK modulator- demodulator baseband pair	Frame-based input	One output period
	Sample-based input, Samples per symbol parameter is greater than 1	Two output periods
	Sample-based input, Samples per symbol parameter is equal to 1, and the model uses a fixed-step solver with Mode parameter set to Auto or MultiTasking	Two output periods
OQPSK modulator- demodulator baseband pair	Sample-based input, Samples per symbol parameter is equal to 1, and the model uses a variable-step solver or the Mode parameter is not set to Auto or MultiTasking	One output period
OQPSK modulator- demodulator passband pair	Sample-based input, and Baseband samples per symbol parameter is equal to 1	One output period
	Sample-based input, and Baseband samples per symbol parameter is greater than 1	Two output periods

As a result of delays, data that enters a modulation or demodulation block at time T appears in the output at time T+delay. In particular, if your simulation computes error statistics or compares transmitted with received data, then it must take the delay into account when performing such computations or comparisons.

First Output Sample in DPSK Demodulation.   In addition to the delays mentioned above, the DPSK, DQPSK, and DBPSK demodulators produce output whose first sample is unrelated to the input. This is related to the differential modulation technique, not the particular implementation of it.

Example: Delays from Demodulation

Demodulation in the model below causes the demodulated signal to lag, compared to the unmodulated signal. This delay is typical for sample-based data that the modulator upsamples. When computing error statistics, the model accounts for the delay by setting the Error Rate Calculation block's Receive delay parameter to 1. If the Receive delay parameter had a different value, then the error rate showing at the top of the Display block would be close to 1/2.

Note    If this model used the OQPSK method instead of DBPSK, then the proper Receive delay parameter would be 2 instead of 1.

To open the completed model, click here in the MATLAB Help browser. To build the model, gather and configure these blocks:

• Random Integer Generator, in the Data Sources sublibrary of the Comm Sources library
• Set M-ary number to 2.
• Set Initial seed to any positive integer scalar, preferably the output of the randseed function.
• DBPSK Modulator Baseband, in the PM sublibrary of the Digital Baseband sublibrary of Modulation
• Set Samples per symbol to 8.
• AWGN Channel, in the Channels library
• Set Es/No to 4.
• DBPSK Demodulator Baseband, in the PM sublibrary of the Digital Baseband sublibrary of Modulation
• Set Samples per symbol to 8.
• Error Rate Calculation, in the Comm Sinks library
• Set Receive delay to 1.
• Set Computation delay to 1.
• Set Output data to Port
• Display, in the Simulink Sinks library.
• Drag the bottom edge of the icon to make the display big enough for three entries.

Connect the blocks as shown above. Also, from the model window's Simulation menu, choose Simulation parameters; then in the Simulation Parameters dialog box, set Stop time to 100. Then run the model and observe the error rate at the top of the Display block's icon. Your error rate will vary depending on your Initial seed value in the Random Integer Generator block.

Representing Signals for Digital Modulation

Upsampled Signals and Rate Changes