# Aliasing in digital communication system

In a digital communication system, is it possible to counteract the aliasing problem by properly tuning the constellation order $M$ and/or the carrier frequency?

Here is my train of thought, please correct me if I'm wrong:

The sampling rate needs to be 2 times the signal bandwidth. This is because we need to cover all the frequencies in which the signal operates. If the signal breaks the Nyquist criterion we'll only get a part of the signal. So if I look at a fix sampling rate and try to tune the constellation order $M$ up and down it won't help, because the signal will still have the same bandwidth. The only thing that will change is the transfer rate $r_b = \log_2(M)/T_s$, where $T_s$ is the sampling rate. Also tinkering around with carrier frequency will only move the signal up or down the frequency. So neither of these solutions will counteract the aliasing problem. The only solution is to somehow reduce the signal bandwidth. I'm thinking of using an RRC filter on the transmitter (to shrink the bandwidth) and then using another RRC filter on the receiver to counteract the ISI.

• Hi! What do you (spceifically) mean by counteract ? – Fat32 Aug 21 '17 at 15:14
• Getting the most out of your signal. From my understanding aliasing happens when we go below the Nyquist criterion ( the sampling rate goes below the 2*bandwidth of the signal). Also I have updated my questing with a possible solution. – Tilen Kavčič Aug 21 '17 at 15:29
• ok so you want to prevent aliasing, bu tuning some parameters of the signal... under a given sampling rate. – Fat32 Aug 21 '17 at 15:47
• This question in a "theoretical" one to see if we understand things in dsp. I want to prevent aliasing. – Tilen Kavčič Aug 21 '17 at 15:50
• One thing: the rate is $r_b = \log_2 M / T_p$, where $T_p$ is the symbol rate (how many pulses/symbols per second are transmitted). Sampling ocurrs at the boudary between the analog front end and the digital back end; it is at this point that Nyquist must be obeyed. – MBaz Aug 21 '17 at 21:51

You're correct that this can't be solved by modifying the constellation order $M$ or the carrier frequency. As you correctly assume, you have two options. You can reduce the bandwidth through pulse shaping; if the symbol rate is $R$, then the minimum bandwidth $B=R/2$ is achieved with sinc pulses. The second option is to reduce the rate.