Communications Toolbox

Using a Preamble or Postamble

Some systems include a sequence of known symbols at the beginning or end of a set of data. The known sequence at the beginning or end is called a preamble or postamble, respectively. The mlseeq function can accommodate a preamble and postamble that are already incorporated into its input signal. When you invoke the function, you specify the preamble and postamble as integer vectors that represent the sequence of known symbols by indexing into the signal constellation vector. For example, a preamble vector of [1 4 4] and a 4-PSK signal constellation of [1 j -1 -j] indicate that the modulated signal begins with [1 -j -j].

If your system uses a preamble without a postamble, then use a postamble vector of [] when invoking mlseeq. Similarly, if your system uses a postamble without a preamble, then use a preamble vector of [].

Example: Using a Preamble

The example below illustrates how to accommodate a preamble when using mlseeq. Notice that the same preamble symbols appear at the beginning of the message vector and in the syntax for mlseeq. If you wanted to use a postamble, then you could append it to the message vector and also include it as the last input argument for mlseeq. In this example, however, the postamble input in the mlseeq syntax is an empty vector because the system uses no postamble.

M = 4; % Use 4-PSK modulation.
const = pskmod(0:3,4); % PSK constellation
tblen = 16; % Traceback depth for equalizer

preamble = [3; 1]; % Expected preamble, as integers
msgIdx = randint(98,1,M); % Random symbols
msgIdx = [preamble; msgIdx]; % Include preamble at the beginning.
msg = pskmod(msgIdx,M); % Modulated message
chcoeffs = [.623; .489+.234i; .398i; .21]; % Channel coefficients
chanest = chcoeffs; % Channel estimate
filtmsg = filter(chcoeffs,1,msg); % Introduce channel distortion.
d = mlseeq(filtmsg,chanest,const,tblen,'rst',1,preamble,[]);

[nsymerrs ser] = symerr(msg,d) % Symbol error rate

The output is below.

nsymerrs =

     0


ser =

     0

Equalizing in Continuous Operation Mode

Selected Bibliography for Equalizers

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