# A 245 GHz Real-Time Wideband Wireless Communication Link with 30 Gbps Data Rate
## 1 Introduction  
The ever-increasing demands for high-speed wireless communications keep pushing wireless systems toward a higher frequency regime [1, 2]. The research in wireless communications has now moved from mm-wave towards sub-terahertz (THz)/THz frequencies, where there are abundant spectral vacancies [3]. It is anticipated that THz communications will be the choice of future space communication technology because there is no or very little atmospheric attenuation in space, potentially facilitating long-range communication between satellites, satellites to constellations, or high-altitude aircrafts/airbuses [4, 5, 6, 7, 8]. Early progress in and developments of THz wireless communications have been reported in [9, 10, 11, 12, 13] based on electronics and optical front-end hardware. In [12], a 240 GHz optical wireless communication system is demonstrated with a maximum data rate of 100 Gbps, and a 300-GHz-band 120-Gb/s wireless link is reported in [13] with InP-based high-electron-mobility-transistor (InP-HEMT) front-ends. However, these high-speed base-band signals are generated from benchtop instruments such as an arbitrary waveform generator (AWG) and received by oscilloscopes for data analysis or offline processing on the receiver side while the data generating and processing tasks in the real communication systems are performed by digital circuit modules such as FPGAs. In recent years, several real-time THz wireless systems achieving multi-Gbps data rates have been published [14, 15, 16, 17, 18, 19, 20], which is more suitable for practical application scenarios compared with instrument-based demonstrations. One major solution of the real-time communication system is a simple modulation scheme, typically on-off keying (OOK) or binary phase-shift keying (BPSK), over a large frequency bandwidth [14, 15, 16]. These systems demonstrate a high data rate with simple base-band configuration, but their spectrum efficiency is relatively low compared with higher order modulation schemes, namely quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM). Another approach that hasbeen reported is the use of modulations of high spectral efficiency within a narrow bandwidth [18; 19; 20]. In particular, a real-time THz wireless transmission at 300 GHz is reported in [18], with QPSK modulation and digital beamforming implemented, featuring a 3 Giga bits per second (Gbps) data rate within the 1.5 GHz bandwidth. In [19; 20], a 220 GHz solid-state dual-carrier wireless link was demonstrated based on 16 QAM and 64 QAM across a frequency range of 5.2 GHz, demonstrating a communication speed at 20.8 and 31.2 Gbps, respectively. These systems, however, have relative operational bandwidths lower than 3%, and hence the full potential of the large THz frequency spectrum capacity has not been explored. To achieve high-order modulation in a wide frequency range, on the other hand, major challenges need to be tackled on the base-band and THz front-end modules. First, low-complexity algorithms and implementation in the base-band module are essential with system resource and timing optimization. Additionally, an intermediate frequency (IF) stage is required to accommodate multiple channels of base-band signals to increase the spectral capacity. Furthermore, high-frequency selectivity is also desirable to suppress background noise and mirror image interferences. In this paper, we present the development of a high-speed, real-time wideband wireless communication system operating at 245 GHz over a distance of 1.2 m. The 16 QAM modulation scheme is used in four base-band channels, which occupy the 10 GHz frequency band in total, to achieve an overall 30 Gbps data rate. Low-complexity implementation with low resource usage and a high frequency of the system clock on the field programmable gate array (FPGA) is applied in the base-band development. Real-time experimental test results are also given.