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I have some confusion regarding the impact of SNR on the bit rate. For example, if I have a QPSK-OFDM system, then my bit rate will be 2bits mutiplied by the number of sub-carriers of the OFDM modulation over the symbol time. How can I account for the SNR in this case?

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    $\begingroup$ This is the transmit bit rate. At receiver, the better SNR, the lower reception error probability, thus the expected correctly received bit rate does depend on SNR. $\endgroup$ – AlexTP Oct 18 '18 at 8:23
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Yes, try to understand what is the concept of SNR and it depends on what. Then, it will be easier to understand why SNR and BER increase and decrease in all cases.

In your case, OFDM, the relationship between the SNR and BER is given:

$\textrm{BER} = \frac{1}{2} \left(1 - \sqrt{ \frac{L/N \times \textrm{SNR}}{2 + L/N \times \textrm{SNR}}} \right)$

Where $N$ is the number of subcarrier and $L$ is the channel diversity. $\textrm{SNR} = p/\sigma^2$ .. where $p$ is the transmitted signal power, and $\sigma$ is the variance.

so now, you check the relationship between the type of used modulation with the above equation, (QPSK and N) , it's easier to explore the behavior of BER. remember also that probability of error when using QPSK is different about when using for example QAM and so on

good luck

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