Communications Blockset

Comparing Baseband and Passband Signals

This section discusses the relationship between the baseband and passband models pbmoddoc and bbmoddoc. In particular, it shows that even though the baseband modulated signal is a complex-valued signal and not a real-valued sinusoid, it is equivalent to the real sinusoid that the passband model processes. While the section Baseband Modulated Signals Defined gives a theoretical description of the equivalence between baseband and passband signals, this discussion uses a more hands-on approach; using the example model, this discussion shows how the baseband algorithm forms part of the passband algorithm and how the conversion from baseband to passband representation occurs.

Baseband Algorithm Within the Passband Algorithm

One way to see how the passband and baseband modulator blocks are related to each other is to look inside the passband algorithm. Open the passband model by typing pbmoddoc in the MATLAB Command Window and then follow these instructions to open the subsystem that the passband modulator block represents:

1. In the top of the model window, click the M-PSK Modulator Passband block.
2. From the model window's Edit menu, choose Look under mask.

   

If you double-click the M-PSK icon in the subsystem and look in the title bar of the dialog box, then you can see that the block is the M-PSK Modulator Baseband block from the digital baseband phase modulation library of the blockset. This indicates that the baseband algorithm forms a part of the passband algorithm.

Conversion from Baseband to Passband Representation

The Frequency Up-Converter block represents another part of the passband modulation algorithm. This block converts a baseband modulated signal into an equivalent passband modulated signal. To see the conversion in more detail, follow these instructions:

1. In the subsystem window, click the Frequency Up-Converter block.
2. From the model window's Edit menu, choose Look under mask.

   

Studying this block diagram from left to right, you can see that it multiplies the real and imaginary parts of the baseband modulated signal by

and

respectively, and then adds the results together. Here and are the Carrier frequency and Carrier initial phase parameters, respectively, in the M-PSK Modulator Passband block. The result of this process is the real-valued sinusoidal signal that you would expect from a passband modulator.

Speed of Baseband Versus Passband Models

Troubleshooting a Passband Simulation