GMSK Demodulator Baseband

Integer-Valued Signals and Binary-Valued Signals

This block accepts a scalar-valued or column vector input signal with a data type of single or double. If you set the Output type parameter to Integer, then the block produces values of 1 and -1. If you set the Output type parameter to Bit, then the block produces values of 0 and 1.

Single-Rate Processing

In single-rate processing mode, the input and output signals have the same port sample time. The block implicitly implements the rate change by making a size change at the output when compared to the input. The input width must be an integer multiple of the Samples per symbol parameter value, and the input can be a column vector.

For a column vector input signal, the width of the input equals the product of the number of symbols and the value for the Samples per symbol parameter.

Multirate Processing

In multirate processing mode, the input and output signals have different port sample times. The input must be a scalar. The output symbol time is the product of the input sample time and the Samples per symbol parameter value.

Traceback Depth and Output Delays

Internally, this block creates a trellis description of the modulation scheme and uses the Viterbi algorithm. The Traceback depth parameter, D, in this block is the number of trellis branches used to construct each traceback path. D influences the output delay, which is the number of zero symbols that precede the first meaningful demodulated value in the output.

The optimal Traceback depth parameter value is dependent on minimum squared Euclidean distance calculations. Alternatively, a typical value, dependent on the number of states, can be chosen using the five-times-the-constraint-length rule, which corresponds to 5log₂(numStates. The number of states is determined by the following equation:

$$numStates=\left\{\begin{array}{l}p\cdot 2^{(L-1)},forevenm\\ 2p\cdot 2^{(L-1)},foroddm\end{array}\right\}$$

where: