What the disadvantages of up-sampling Let's say we have a signal $x[n]$ upsampled at the transmitter by $4$ and then downsampled at the receiver by $4$ too.   As known, the performance will be much better compared with transmitting the signal without upsampling.  but what I need to understand, what is the disadvantage of using up-sampling ? Does that effect the data rate of the transmitted signal since we transmit $4$ times the signal $x[n]$ instead of $x[n]$?
When you upsample a signal by a factor of 4, then, you have to churn out 4 times the number of samples in the same time in order to maintain the same Symbol Rate. Otherwise your symbol rate will be $\frac{1}{4}$ times the original symbol rate. This means your DAC needs to operate at least at 4 times the sampling frequency now. This can be costly. Also faster clocks need better synchronization techniques.