Communications Toolbox

Customizing the Modulation Process

Recall from Digital Modulation Overview that the modulation and demodulation processes each consist of two steps. You can tell the toolbox functions to carry out only selected steps in the processes. For example, this might be useful if you want to use standard mapping and demapping techniques along with unusual or proprietary modulation and demodulation techniques.

Mapping Without Modulating and Demapping Without Demodulating

To map the digital signal to an analog signal without modulating the analog signal, use the modmap function instead of the dmodce function. To demap the analog signal to a digital signal without demodulating the analog signal, use the demodmap function instead of the ddemodce function.

To alter the basic example so that it does not modulate or demodulate the analog signals at all, replace the "old commands" listed in the first column of the table below with the "new commands" listed in the second column.

Changes in "Simple Digital Modulation Example" to Avoid Modulating

Old Command	New Command
y = dmodce(x,Fd,Fs,'psk',M);	y = modmap(x,Fd,Fs,'psk',M);
ynoisy = y + .04*randn(300,1) + .04*j*randn(300,1);	ynoisy = y + .04*randn(300,2) + .04*j*randn(300,2);
z = ddemodce(y,Fd,Fs,'psk',M);	z = demodmap(y,Fd,Fs,'psk',M);

Modulating Without Mapping and Demodulating Without Demapping

To carry out the analog modulation step on a signal that has already been mapped from a digital signal to an analog signal, use the dmodce function with the extra word /nomap appended to the method string. To carry out the analog demodulation step but avoid demapping the resulting signal to a digital signal, use the ddemodce function with the extra word /nomap appended to the method string.

If you substituted your own mapping and demapping steps into the basic example then it would look something like the code below. The lines in the second grouping differ from the original example.

• M = 16; % Use 16-ary modulation.
  Fd = 1; % Assume the original message is sampled
  % at a rate of 1 sample per second.
  Fs = 3; % The modulated signal will be sampled
  % at a rate of 3 samples per second.
  x = randint(100,1,M); % Random digital message
  
  % Important changes are below.
  mapx = mymappingfunction(x); % Use your own function here.
  y = dmodce(mapx,Fd,Fs,'psk/nomap',M); % Modulate without mapping.
  % Demodulate y without demapping.
  demody = ddemodce(y,Fd,Fs,'psk/nomap',M);
  % Now demap.
  z = mydemappingfunction(demody); % Use your own function here.
  

Simple Digital Modulation Example

Other Options in Digital Modulation