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 table below lists sources of delay and the situations in which they occur. If more than one situation applies to a given model, then the separate delays are additive.

Table 2-3: 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	Samples per symbol parameter is greater than 1, and the input is sample-based.	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	Always	One output period
OQPSK Modulator Baseband, OQPSK Modulator Passband	Always	One output symbol, which is half the input period
OQPSK Demodulator Baseband, OQPSK Demodulator Passband	Always	Three input symbols, which is 3/2 times the output period

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 Comm Sources library
• Set M-ary number to 2
• Set Initial seed to any positive integer scalar
• 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