I have a question regarding the spreading sequence duration using Walsh matrix. I've read a paper related to that, they consider the signal $S = R(S) + I(S)$, where $R(S), I(S)$ are the real and imaginary parts of the signal $S$ resulted from the modulation $QAM$.
what is done, briefly, a Walsh-Hadmard matrix is built with dimension $4$x$4$, then the real are imaginary parts are multiplied by the spreading code taken from the Walsh matrix and then transmitted into receiver. So the resulted signal before transmitting is $S = W_1R(S) + W_2I(S)$ where $W_1, W_2$ are two different spreading code taken from the Walsh matrix.
So, without talking about why that process was done, My question is related into the signal after spreading itself:
Will the duration of transmitted signal "after spreading" is similar to duration of signal $S$ before spreading?I think yes, because the duration of chip of spreading sequence is shorter by $n$ times of duration of signal $S$, it's similar to this question here.. If that right, what's the downside of using spreading sequence? I mean when using it, what's the disadvantages we face compared with any other technique. Finally, does using spreading sequence as above process mean that we are using CDMA? ..