You can basically apply the rake receiver whenever.
However, OFDM was invented especially to make that unnecessary.
The point of a rake receiver is to demodulate the different paths independently and sum up the observed information. It is a time-domain decomposition of the channel impulse response, if you will.
OFDM considers the effects of the multipath channel, but in frequency domain, and converts the complicated time-domain equalization problem (which the Rake, partially, solves) to a frequency-domain problem, where the channel properties are independent between OFDM subcarriers and trivial to correct. It's the frequency-domain decomposition of the channel, if you will.
So, if you do OFDM, and your channel delay spread is within what your OFDM system can deal with, adding a rake in before seems redundant.
Your "pick the stronges rake path" is simply like a bad OFDM synchronizer. So, this is worse than just doing the very standard Schmidl&Cox synchronization for packetized OFDM, or to fixed-lag correlation methods used in streaming OFDM applications.
regarding how can I get the BER, I can do it similarly to this paper journals.sagepub.com/doi/pdf/10.1155/2016/3137014 .. But anyway, don't care of that method, let's consider the method of selecting the strongest signals. It means, "the signal can reach the receiver via multiple distinct pathways. The rake receiver is used to correct this effect, selecting the stronger signals.Each of these multipath components in rake receiver is called finger" so then, we can demodulate the different taps in different demodulators
Yeah, no. An OFDM synchronization would already do that for you. Rake and OFDM solve the same problem: Multipath == frequency selective channels. But OFDM inherently solves the problems of single carrier systems; applying Rake before doing OFDM is IMHO a dead end.
You need to read up on both when to use rake, and when to use OFDM. Recent papers aren't the way here; they are either low-quality (explaining concepts that have been standard for decades), or extremely special in application. Get a good communications textbook!