What is the duration of the spreading sequence 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? ..  

thank you
When you multiply the real and imaginary parts by $\mathbf{W}_1$ and $\mathbf{W}_2$, respectively, the resulting signal is no longer $S$. It will be a matrix.  

Regarding your questions:

1- Yes, the signal after spreading has the same duration. 

2- One of the disadvantages of Walsh-Hadamard codes is that they don't have clear auto-correlation peak, which makes synchronization at the receiver side challenging. Also, they must have a dimension of $2^k$, i.e., you cannot have arbitrary dimension to support an arbitrary number of users. In your example, to make the system efficient, you need 4 users, otherwise, one user will occupy 4 times the minimum required bandwidth, which isn't spectrally efficient. 

3- Yes. This is called direct sequence (DS) CDMA.