# A Back-to-Back Gap Waveguide-Based Packaging Structure for E-Band Radio Frequency Front-End
## 1 Introduction  
With the rapid development of communication technology, the conventional microwave band has failed to meet the escalating demand for high-speed transmission. In recent years, the E-band (60-90 GHz) has emerged as a prominent frequency range usable for the enabling of high-speed wireless communication systems due to its abundant bandwidth resources compared to the microwave band and relatively lower atmospheric loss in comparison with the terahertz band. This frequency range finds extensive applications in 5 G fronthaul and backhaul links, as well as Space-Earth and Earth-Space communication links [1,2]. Moreover, owing to advancements in processing and packaging technology, E-band communication products are progressively being integrated and commercialized. Currently, low-power components such as frequency multipliers, mixers, and couplers can be seamlessly integrated into a single system-on-a-chip (SoC). However, integrating high-power components like power amplifiers and diplexers into chips re-mains challenging; hence, it becomes crucial to package diplexers and amplifiers within a module. A typical E-band frequency division duplex (FDD) communication transceiver topology is shown in Figure 1; it consists of a diplexer, a low-noise amplifier (LNA), a power amplifier (PA), and two modulation links. Generally, in the E-band system, the modulationlink takes the form of a chip. Therefore, this paper primarily focuses on integrating the LNA, PA, and diplexer components (as indicated by the red dash-dot circle in Figure 1). In this context, particular emphasis is placed on optimizing both the diplexer structure and the packaging configuration. The diplexer plays an indispensable role in separating \"receive\" (RX) and \"transmit\" (TX) signals in the communication system. Typically, it consists of two bandpass filters (BPFs) and a power distribution unit, such as T-junctions [3; 4], a manifold [5; 6; 7], hybrid couplers [8; 9], circulator couplers [10], and so on. Among them, the T-junction and manifold structure are relatively simple and compact, resulting in lower insertion losses as well as improved amplitudes and group delays. However, the interactions between multiple branches' BPFs can lead to design complexity and challenges. On the other hand, although hybrid-coupler diplexers and circulator-coupler diplexers have larger volumes, they offer a high level of isolation between each channel's BPF, without any interaction, simplifying the design process [11]. Moreover, due to the power division effect of the hybrid coupler, the input power can be equally divided, which enables an increase in saturated output power for the module. As depicted in Figure 2, two PA chips can be separately packaged between the hybrid coupler and two BPFs to achieve power combination. In recent years, there have been numerous advancements in chip and system packaging technology. Due to its advantages of low insertion loss and high-power capacity, the waveguide probe has always been the mainstream method of RF chip or system packaging. It can be categorized into H-plane probe structures [12; 13; 14] and E-plane probe structures [15; 16]. Additionally, the gap waveguide (GW) structure has gained significant attention in packaging design owing to its ability to accommodate a certain air gap between the periodic metal structure and the top metal plate, for components not requiring complete electrical contact [17; 18]. Components utilizing the gap waveguide structure have also been the subject of extensive research [19; 20; 21]. This research effectively addresses issues arising from air gaps and generated during the packaging process. Although the E-plane probe has the advantages of easy design and low insertion loss, its input/output ports are perpendicular to each other, which means that additional waveguide bending or other conversion structures are required, so the E-plane probe is not suitable for packaging, given the structure of the GW. Figure 1: Topology of a typical FDD transceiver.