# Transmitter and Receiver Circuits for a High-Speed Polymer Fiber-Based PAM-4 Communication Link
## 1 Introduction  
On the emergence of the internet of things (IoT), autonomous driving, and virtual reality applications, the number of interconnected devices and the throughput of connections between such devices are increasing exponentially. This is especially true for real-time imaging and video capture devices, for which several to tens of Gbps throughput is required. For example, current cars with autonomous driving features are already equipped with multiple high-definition cameras, and the video has to be aggregated and processed centrally [1]. Currently, fiber optic solution is the main high data rate connectivity solution for cars; however, high data rate opto-electronics are expensive and sensitive to high temperature. Another use example lies in the telecommunication field, where distributed antennas in the 5G base stations require cost-effective connectivity solutions to processor units with above 40 Gbps data rates. Millimeter-wave modulated signals transferring over polymer microwave fibers (PMF) is a promising solution for high-speed connectivity. The PMFs are flexible and made of low-cost plastics. Compared to optical fibers, the alignment to the PMF is much simpler, because of the larger size of the PMF. Furthermore, Millimeter-wave semiconductor processes such as SiGe BiCMOS and InP DHBT can be certified for automotive or even space temperature grade, which makes the PMF a more robust solution in comparison with fiber optic. The modulated signals are mostly confined in the PMF; therefore, there are no legal frequency restrictions, which either limit the data rate or require high modulation schemes, thus requiring high complexity, and therefore high cost and high power consumption. Several PMF links have already been demonstrated: A QPSK transmission over 4 m with a data rate of 4 Gbps was presented at 60 GHz [2]. Using continuous-phasefrequency-shift keying (CP-FSK) at 120 GHz center frequency, a high data rate of 17.7 Gbps was demonstrated over one meter [3]. In this work, a PAM-4 modulated PMF link is demonstrated. It is measured to support data rates up to 30 Gbps over one-meter PMF. It covers large parts of D-band (110-170 GHz) and requires no carrier recovery, lowering the complexity of the transmitter and receiver. Demodulation of an amplitude-modulated signal is performed with a power detector (PD). The design of the PD in this work was based on insights gained from previous work [4], which can support demodulation of an amplitude-modulated signal up to 14 Gbps. The design topology contained two passive baluns, which are made of coupled transmission lines. A side effect, noticed in the previous work, of using coupled transmission lines together with existing resistance in the circuit is that it can increase response time and result in distortion of the shape of the output waveform (for example overshoot). In this work, the use of a passive balun is eliminated to avoid these issues and improve the speed of the PD. The PMF link consists of three different parts: a radio frequency digital-to-analog converter (RF-DAC), a rectangular high-density (with a density above 0.930 g/cm\({}^{3}\)) polyethylene (HD-PE) fiber acting as a flexible dielectric waveguide, and a power detector (PD), which receives the signal and demodulates it. A simplified illustration of the PMF link can be seen in Figure 1. An LO at desired output frequency is applied to the RF-DAC, where the data modulate the signal into an amplitude modulated signal. The signal is then transmitted over the PMF to a power detector (PD), where the signal is demodulated. To realize the transmitter and receiver, high data rate monolithic microwave integrated circuits (MMICs) are designed and fabricated using indium phosphide (InP) double heterojunction bipolar transistor (DHBT) technology with 250 nm emitter width developed by Teledyne Scientific Company.