M-FSK Demodulator Baseband

Integer-Valued Signals and Binary-Valued Signals

When you set the Output type parameter to Integer, the block outputs integer values in the range [0, (M – 1)]. M represents the M-ary number block parameter.

When you set the Output type parameter to Bit, the block outputs binary-valued signals that represent integers. The block represents each integer using a group of K = log₂(M) bits, where K represents the number of bits per symbol. The output vector length must be an integer multiple of K.

The Symbol set ordering parameter indicates how the block maps a symbol to a group of K output bits. When you set the parameter to Binary, the block maps the integer, I, to [u(1) u(2) ... u(K)] bits, where the individual u(i) are given by

u(1) is the most significant bit.

For example, if M = 8, you set Symbol set ordering to Binary, and the demodulated integer symbol value is 6, then the binary output word is [1 1 0].

When you set Symbol set ordering to Gray, the block assigns bit outputs from points of a predefined Gray-coded signal constellation. The predefined M-ary Gray-coded signal constellation assigns the bit representation in the pth row of bit matrix b to the pth integer, where the left-most bit is the most significant bit (MSB).

M = 8; P = [0:M-1]';
nBits = log2(M);
b = int2bit(bitxor(P,floor(P/2)),nBits);
b = reshape(b,[],8)';

The typical Binary to Gray mapping for M = 8 is shown in the following tables.

Whether the output is an integer or a binary representation of an integer, the block maps the highest frequency to the integer 0 and maps the lowest frequency to the integer M-1. In baseband simulation, the lowest frequency is the negative frequency with the largest absolute value.

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.

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.

To run the M-FSK Demodulator block in multirate mode, clear the Treat each discrete rate as a separate task checkbox (in Simulation > Configuration Parameters > Solver).