# QPSK MMW Wireless Communication System Based On p-i-n InGaAs Photomixer
## 1 Introduction  
In recent years, the study of MMW and THz frequency bands has attracted significant attention due to the fast growth of wireless data communication technology [1]. Within ten years, the required data rate of wireless communication expected to be 100 GB/s [2]. As the demand for high data rate increased due to the emergence of mobile devices such as multifunction mobile phones and tablets, UHD TV, HD graphic video games, and real-time data transmissions, lower frequency bands (below 60 GHz) are extremely limited [3]. Therefore, moving to the MMW and THz frequency bands is essential to exploit the available wide bandwidth. However, the lack of availability of high-frequency components and the high cost and complexity of designing these components makes the construction of MMW wireless communication link very challenging. In order to close the \"THz gap\" between the RF and the optical spectrum, new techniques and components combining optical and RF components are required [4]. These techniques include Resonant Tunneling Diodes (RTD), electronic sources (frequency multipliers), and the generation of MMW and THz frequencies using the optical photomixing process [4, 5]. Different techniques to realize the optical photomixing have been suggested [6]. Each technique is characterized by the improvement it offers regarding bandwidth, maximum output power, frequency tuning, noise, and stability. Antenna-based and large area emitters for Continuous-Wave (CW) or quasi-CW THz generation using two 1.55 \(\upmu\)m lasers have been reported [6, 7]. A Uni-Traveling Carrier Photodetector (UTC-PD), heterogeneously integrated on silicon, can provide a very high 3 dB bandwidth of more than 67 GHz with the responsivity of 0.7 A/W at 1.55 \(\upmu\)m wavelength [8]. Recently, THz photonics wireless transmission link at 400 GHz was reported using On-Off Keying (OOK), Quadrature Phase-Shift Keying (QPSK), 16-Quadrature Amplitude Modulation (16-QAM), and 32-QAM based on photonics for THz generation [9]. A 64-QAM modulation format at 300 GHzwith a data rate of 0.096 Gb/s reported using UTC-PD. A 100 Gb/s transmission was demonstrated using a dual-polarization 16-QAM method in the W-band [10]. UTC-PD-based photonics at the transmitter and active Microwave Monolithic Integrated Circuits (MMICs) at the receiver was demonstrated experimentally at 237.5 GHz [11]. The 8-PSK modulation format was successfully transmitted at 240 GHz enabling a 30 Gb/s data rate capability [12]. Higher digital modulation formats are possible by integrating Mach-Zehnder Modulator (MZM) to one of the laser sources. A modified dual-drive In-phase and Quadrature (IQ) modulator driven by equal amplitude binary signals for generation of offset-free QPSK and 16-QAM constellations has been reported [13]. The MMW and THz coherent receivers rely on the availability of carrier phase-locking to the transmitter. Thus, after signal detection, digital synchronization (off-line demodulation) is required in order to construct the constellations (IQ map) and eye diagrams [11; 12]. In this paper, we experimentally demonstrated an MMW wireless coherent communication link. The MMW link is based on an InGaAs p-i-n photomixer transmitter module and receiver. The modulated signal was generated using IQ-MZM with the required bitrate and output power. The proposed transmission link uses digital modulation formats and standard detection methods for W-band, which include horn antenna, W-band mixer, and Local Oscillator (LO) signal to downconvert the carrier wave from W-band to low Intermediate Frequency (IF) signal. The signal analysis of the IF signal was performed using a vector signal analyzer (VSA) software. Recently, we demonstrated On-Off Keying (OOK) modulation using photomixer and Schottky Barrier Diode (SBD) at the detection [14]. The remainder of the content for this paper arranged with sections and subsections, where Section 2 presents a general approach of the THz wave generation using photomixing.