# QPSK MMW Wireless Communication System Based On p-i-n InGaAs Photomixer
## 3 Experimental Setup
### MMW Generator and Transmitter: InGaAs p-i-n Photomixer  
As mentioned in the above section, the THz wave generation is implemented using different techniques and components. An antenna-integrated Low-Temperature Grown (LTG)-InGaAs photomixer, as used in this experiment Figure 1a, provides tunability, high output power, and wide bandwidth, i.e., above 2 THz, see Figure 1b. The photomixer is fiber-pigtailed and reverse-bias voltage-controlled with voltages between \(-\)2 V to 0.6 V, see Figure 1a. The DC voltage set point of \(-\)1.9 V corresponds to \(-\)11 mA DC photocurrent which gives maximum output power at 32.9 mW optical input power based on the device datasheet. The performance of the photomixer investigated using the two independent laser sources was configured with 90 GHz frequency difference. The combined optical power of the two lasers further amplified to 15 dBm and fed into the photomixer optical input. The photomixer antenna focused onto MMW horn antenna at a minimum distance. The received signal from the antenna was downconverted to low IF signal to analyze it using spectrum analyzer. Figure 2a shows the simplified schematic diagram used for evaluation of the photomixer. The photomixer can radiate 65 \(\upmu\)W output power at an input power of 32.9 mW, which means it has a conversion loss of \(\sim\)27 dB around 100 GHz. We used the LO power, IF power, cable losses, and Friis transmission equation during the experiment to calculate the photomixer output power, which was found to be in good agreement with the manufacturer datasheet. As shown in Figure 2c, the photomixer generates a single CW frequency free of harmonic signals and spurious signals. After testing the performance of the photomixer as standalone, the proposed MMW wireless communication link built by including additional components. Figure 3 shows the proposed schematic diagram of the overall experimental set-up for QPSK MMW wireless communication link based on the photomixer. As shown in Figure 3, the output of the MX10B laser fed into MXIQ-LN-30, an IQ-MZM modulator from iXblue Photonics, which operates at wavelengths of 1530-1580 nm with a maximum insertion loss of 7 dB at its maximum operating wavelength. It has a typical electro-optical bandwidth of 25 GHz and high input and output optical power capabilities. The optical output of the IQ-MZM connected to the modulator bias voltage and gain controller, where the dc bias voltage, gain, and dither signal adjustments were performed. The MBC enables configuring different operating points of the modulator by setting the bias voltages manually or automatically. A \(2^{7}-1\) length Pseudorandom Binary Sequence (PRBS) signal with 50 Mbps data rate generated by Keysight Arbitrary Signal Generator (KAWG) was fed into the two IQ-MZM RF inputs. As a result, a QPSK modulated signal of 100 Mbps bitrate was achieved at the output of the MBC. As a result, a QPSK modulated signal of 100 Mbps bitrate was achieved at the output of the MBC. The modulated optical signal output at the MBC and the SFL were  
Figure 1: The millimeter-wave (MMW) generator and transmitter. (**a**) An InGaAs p-i-n photomixer module. (**b**) Output power spectrum of the photomixer [15]. combined using PFC1550F, and amplified using EDFA100P to 15 dBm, which is required at the input of the photomixer to get maximum output power. Figure 3: Experimental set-up. (**a**) Proposed MMW wireless communication link based on InGaAs p-i-n photomixer and receiver. (**b**) IQ-MZM modulator and the Modulator Bias Control (MBC). (**c**) The free space transmission overview. Figure 2: Photomixer evaluation test set-up. (**a**) A schematic diagram of the test set-up with the optical link and receiver channels. (**b**) Two optical signals at the photomixer input with 90 GHz frequency difference. (**c**) Downconverted infrared (IF) signal measured by Keysight Spectrum Analyzer (KSA). The radiated signal from the photomixer focused to the direction of the receiver antenna by two identical LAT115 lenses.