BER Vs SNR/Eb/No for QPSK demodulator

[asraf]

hai there,
im verifiying my QPSK architecture and quit confuse on whether to choose SNR or Eb/No in the AWGN block. is there any difference between SNR and Eb/No to obtain BER? what is the difference if there are?

 

[pstuckey]

I usually see Eb/No used for BER curves.

Fundamentally, SNR refers to bandwidth (signal power vs noise power in a certain bandwidth) and can be measured for both analog or digital modulations. Eb/No refers to bits (energy per bit vs noise per bit) and is only relevant to digital data streams. SNR is commonly used when the data is modulated because it can be measured without knowledge of the underlying modulation. For instance, if you received an unknown signal, you could observe it's spectrum and estimate the SNR. Without demodulating (or at least knowing the modulation) you could not calculate Eb/No.

 

[asraf]

I usually see Eb/No used for BER curves.

Fundamentally, SNR refers to bandwidth (signal power vs noise power in a certain bandwidth) and can be measured for both analog or digital modulations. Eb/No refers to bits (energy per bit vs noise per bit) and is only relevant to digital data streams. SNR is commonly used when the data is modulated because it can be measured without knowledge of the underlying modulation. For instance, if you received an unknown signal, you could observe it's spectrum and estimate the SNR. Without demodulating (or at least knowing the modulation) you could not calculate Eb/No.

hai there,
thank you so much for your feedback. i would like to know whether its appropriate if i do plot my BER Vs SNR graph instead of BER Vs Eb/No for my QPSK demodulation? is there going to be any difference?

 

[pstuckey]

The two are related by:

Eb/No = (S/N)*BW*Tb

where BW = bandwidth and Tb = bit duration

I think Eb/No is normally used since Eb/No shows up in the probability of bit error equations directly, but they are related by constants for a given system.

 

[asraf]

The two are related by:

Eb/No = (S/N)*BW*Tb

where BW = bandwidth and Tb = bit duration

I think Eb/No is normally used since Eb/No shows up in the probability of bit error equations directly, but they are related by constants for a given system.

if in Matlab simulink simulation, i have to place the AWGN block as noise source in the channel.this mean that the AWGN add noise to the analog waveform. so its that mean the setting inside the AWGN block should be Eb/No? the symbols of the QPSK signal in the waveform are affected by the noise, so is it appropriate to use Eb/No instead of SNR? if i have to use Eb/No to evaluate my system, where i should place the AWGn block?

 

[pstuckey]

I'll defer to a simulink expert on this, but I think you can use whichever definition is most convenient and let Matlab worry about keeping track of the constants.

If the result of your simulation is a BER curve, it seems that Eb/No would be the easiest, if the output is used for link analysis, then maybe SNR would be easiest.

 

[asraf]

what do you mean by link analysis?please explain to me.thank you

 

[pstuckey]

Link analysis means looking at the whole comms link from transmitter to receiver and determining what range the communications link will work over. It's usually an iterative process where you'll trade-off between transmit power, antenna gain, receive sensitivity, maybe modulation type, etc. to arrive at a system that will perform reliably over the specified distance.