The bit error probability of QPSK over an AWGN channel is given by:
${p}_{b}=\frac{1}{2}erfc\left(\sqrt{\frac{{E}_{b}}{{N}_{0}}}\right)$ ${p}_{b}=Q\left(\sqrt{\frac{2{E}_{b}}{{N}_{0}}}\right)$ 

See Error function and Qfunction.
$\frac{{E}_{b}}{{N}_{0}}$  Energy per bit to noise power spectral density ratio  
${N}_{0}$  Noise power spectral density  ${N}_{0}=\frac{N}{B}$ 
$N$  Noise power  
$B$  Bandwidth 
In this experiment the bit error rate (BER) vs ${E}_{b}/{N}_{0}$ of QPSK over an AWGN channel is analyzed.
Start
The simulation starts with a setting of ${E}_{b}/{N}_{0}=0dB$. This yields to this bit error probability:
${p}_{b}=\frac{1}{2}erfc\left(\sqrt{\frac{{E}_{b}}{{N}_{0}}}\right)=\frac{1}{2}erfc\left(1\right)=\mathrm{7,865}E02$ 
$\frac{{E}_{b}}{{N}_{0}}$
[dB] 
Bit error probability ${p}_{b}$ 

2  1,306E01 
0  7,865E02 
2  3,751E02 
4  1,250E02 
6  2,388E03 
8  1,909E04 
10  3,872E06 
Experiment
Now adjust ${E}_{b}/{N}_{0}$ . Measure the corresponding BER and compare it to the analytical bit error probability.
Note
 Adjusting
${E}_{b}/{N}_{0}$
adapts the noise power spectral density. These settings are not modified:
Transmitting power $1{V}^{2}$ Bit duration ${T}_{bit}$ $1\mu s$
Energy per bit ${E}_{b}$ $1\mu V{s}^{2}$ The initial noise powerspectraldensity of ${N}_{0}=1\mu {V}^{2}/Hz$ results in:
$\frac{{E}_{b}}{{N}_{0}}=\frac{1\mu V{s}^{2}}{1\mu {V}^{2}/Hz}=1=0\text{\hspace{0.17em}}dB$  This BER simulation is quite slow as it implements quadrature modulation and pulse shaping. To boost simulation speed you might change to QPSK BER  equivalent baseband without pulse shaping.
Next steps
 Vary the pulse shape. Note that the pulse shape has no effect on the BER but the transmit spectrum.
 Switch to equivalent baseband. Note that the modem doesn’t affect the BER performance but the shifts the transmit spectrum to the base band.
 Switch to modem none. Note that the modem doesn’t affect the BER performance. There’s only one sample per symbol and no pulse shaping, so the simulation is much faster.
Adjust ${E}_{b}/{N}_{0}$ and measure the corresponding BER.