multiplying the received signal by carrier, in OFDM system
That is just a simulation of a direct conversion downmixer.

You're probably looking at a simulation of OFDM in e.g. Matlab. In a real OFDM system, the signal at carrier frequency is analog, and has to be mixed down to complex baseband with an analog mixer. Of course, in an analog mixer, there's no discrete time $t$, but just an oscillator of the right frequency with which the I and Q output branches of the mixer are generated.

Since complex baseband is so fundamentally important to how OFDM works, I strongly recommend refreshing your knowledge of it and of direct conversion mixers, if this sounds new to you.