My understanding of sampling frequency is when we have an analog signal x(t) we convert it to a digital signal. Assume the sampling frequency is denoted by $f_s$ then the sampling time $T_s$. In my understading this is denoted the number of samples per second.

In MATLAB programs, I have come across generation of base band signal. The authors have upsampled the base band signal and the sampling rate was stated to be 3 samples per symbol.

My question is what does samples per symbol mean.Assuming we have QAM symbols are we repeating it three times?



1 Answer 1


A digital communications signal such as QAM can be generally written as $$s(t) = \sum_{n=0}^{N}a_n p(t-nT),$$ where $a_n$ are (possibly complex) amplitudes taken from a finite alphabet, and the symbol rate is $1/T$. The pulses $p(t-nT)$ are often called "symbols". Matlab commands that ask you to specify "samples per symbol" are asking how many samples you want to use to define $p(t)$.

Of course, since you know the pulse duration, specifying the samples per symbol is an indirect way of specifying the sampling rate.

  • $\begingroup$ Thank you very much. Is the pulse duration the inverse of symbol rate? i.e the symbol time? How is it related to the transmissiob bandwidth. Thanks again. $\endgroup$
    – Henry
    Sep 16, 2015 at 3:15
  • $\begingroup$ The pulse duration is not, in general, the inverse of the symbol rate, because the pulses may overlap. In the equation in my answer, one pulse is transmitted every $T$ seconds, so the symbol rate is $R_s=1/T$. The bandwidth is determined by the spectrum of $p(t)$, but it cannot be less than $1/(2T)$. The pulse with smallest bandwidth for a given symbol rate is the sinc pulse. $\endgroup$
    – MBaz
    Sep 16, 2015 at 13:19

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