# QPSK MMW Wireless Communication System Based On p-i-n InGaAs Photomixer
## 4 Experimental Results and Discussion  
The generated MMW was first analyzed using KSA to measure the magnitude and frequency of the new test set-up, see Figure 3. The VSA demodulator was configured with the center frequency and symbol rate using the parameters acquired from the KSA and the input data rate. As described in the above section, after analyzing the CW signal, the PRBS signals were added to the RF inputs of the IQ-MZM. The modulator can tolerate RF input signals of 6 V peak to peak. In this experiment, PRBS signals with peak to peak voltages of 1 V, 1.5 V, and 3.5 V were applied to the modulator RF inputs. A real-time received QPSK signal reconstruction for the different peak to peak voltage values of the PRBS signals is presented in the following pictures. The IQ constellations of the received signal for the 1 Vpp PRBS input signal are more scattered and non-symmetrical. As can be seen in Figure 4, the opening of the eye diagram does not appear to be broad and clear. The symbols are not fully detected due to low bit energy at the transmitter. When the signal power was increased by 0.5 Vpp, as shown in Figure 5, the IQ constellations and the eye diagram improved compared to the 1 V pp PRBS signals. The symbols are close to the ideal IQ symbol placements and the symmetry of the symbols improved. The eye diagram opening is broad and clear without a symbol crossing. By further increasing the bit energy of the transmitted signal, the constellation and the eye diagram measurements can show an improvement. Figure 6 shows an open and clean eye diagram with a better symmetry of the symbols for 3.5 Vpp PRBS signal input. As shown in the above figures, the IQ constellation and the eye pattern improved as the data voltage increased; thus, the QPSK signal was detected more precisely. The estimations of the BER and Signal-to-Noise Ratio (SNR) can be done using the Q-factor equations found in the works in [16] or [17], based on the given eye diagrams. Table 1 presents the Q-factor calculated values, BER, \(\frac{F_{b}}{N_{0}}\), bit rate, bit period, and timing jitter depending on the peak to peak voltage values of the input data. Figure 4: Real-time received QPSK signal reconstruction using VSA code for transmitted 1 Vpp PRBS signal: (**a**) IQ constellation and (**b**) eye pattern. The calculated BER value for 3.5 Vpp PRBS input data, as given in Table 1, is in the order of one magnitude better than the Hard-Decision Forward Error Correction Threshold (HD-FEC) of \(3.8\times 10^{-3}\). For the same test, an Error Vector Magnitude (EVM) value of 20% rms was achieved. By applying different modern error-correcting algorithms, the BER value can further improved. The state-of-the-art technologies for MMW communication are summarized in Table 2. The table compares researches based on the carrier wave frequency, data rate, achieved BER or EVM value, demodulation method, and the maximum transmission distance. Even though other reported researches are available, only the researches with a carrier frequency of less than 200 GHz presented. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Input Data** & **Q-Factor** & **BER** & \(\mathbf{E_{b}}\)/\(\mathbf{N_{o}}\)**(dB)** & **Bit Rate** & **Bit Period (ns)** & **Timing** \\
**Voltage (Vpp)** & **Q-Factor** & **BER** & \(\mathbf{E_{b}}\)/\(\mathbf{N_{o}}\)**(dB)** & **(Mbps)** & **Bit Period (ns)** & **Jitter (ns)** \\ \hline \(1\) & \(1.115\) & \(1.32\times 10^{-1}\) & \(-2.05\) & 100 & 10 & 6.27 (62.72\%) \\ \(1.5\) & \(1.411\) & \(7.91\times 10^{-2}\) & 0 & 100 & 10 & 5.25 (52.57\%) \\ \(3.5\) & \(3.667\) & \(1.23\times 10^{-4}\) & 8.286 & 100 & 10 & 4.15 (41.5\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Calculated values of the Q-factor and bit error rate (BER) for deferent peak to peak voltage values. Figure 5: Real-time received QPSK signal reconstruction using VSA code for transmitted 1.5 Vpp PRBS signal. (**a**) IQ constellation and (**b**) eye pattern.