Equation for Baseband OFDM

I am simulating OFDM using Matlab.

from a source, i was told the baseband OFDM signal, s(t) is given by equation below, where

N is number of subcarriers

Sk is symbol(for eg QAM symbols)

fn is subcarrier frequency

T is symbol period

However, from another source, I was told s(t) is envelope of baseband OFDM signal instead of the baseband OFDM signal itself. So, now actually s(t) is envelope of baseband OFDM signal or the baseband OFDM signal itself? anybody knows the answer?

Thanks.