Could someone explain to me BFSK Modulation on the Simulink model below?

The simulation above simulates a binary FSK system with a correlation (or matched filter) receiver.

I have to understand the general idea about it.

• Could someone explain this?
• Moreover I cannot understand the meaning of correlator and tone frequency fn the above simulator?
• Is the signal to the multiplier complex or real? I have seen FSK demodulators with phase rotators and can explain that case if that is indeed what is going on (that would not be clear from the block diagram as shown). If the signal is real and the two references are indeed just combined can you then comment on what frequency each reference is and what frequency deviation of the BFSK signal is for the two FSK symbols? – Dan Boschen Mar 28 '17 at 20:55
• I wish people would at least explain why they down-vote a question when it is down-voted. Not sure what was wrong with this question?? – Dan Boschen Mar 28 '17 at 23:32

So, ignoring noise, the input into the receiver multiplier coming from the channel is either $\cos(2\pi f_0t)$ or $\cos(2\pi f_1t)$. The other multiplier input is $\cos(2\pi f_0t)-\cos(2\pi f_1t)$. All signals are defined from 0 to $T$. Let's assume the transmitted signal has frequency $f_0$. Then you have an integrator:
The frequencies $f_0$ and $f_1$ are chosen so that the cosines are orthogonal for $0<t<T$; that means that $$\int_0^T \cos(2\pi f_0t)\cos(2\pi f_1t)dt =0.$$ The integrate and dump block outputs the result of the definite integral at $t=T$. You can see that the result is positive if frequency $f_0$ was transmitted and negative if $f_1$ was transmitted.