Long-Distance 20.1 km THz Wireless Transmission Using CVMSO NN Equalizer by Photonics-Aided Technology

Recently, the continuous advancement of sixth-generation (6G) mobile communication technology has positioned the space-air-ground-sea integrated network (SAGSIN) as a highly promising architecture for 6G networks. However, emerging applications and intelligent access devices have significantly strained network capacity, making it increasingly difficult to meet the ever-growing demand. To address these challenges, high-frequency terahertz-waveband has been proposed as an attractive solution for next-generation communication networks. Terahertz (THz) technology, enables data rates from tens to hundreds of gigabits per second, thus making it suitable for ultrahigh-speed wireless communication. Our study explores integrating photonics-aided signal generation with THz transmission technologies in a long-haul wireless communication system with a partial over-the-sea transmission. However, nonlinear distortions often occur for the THz-band system, the traditional Volterra-series nonlinear equalizers (VNEs) do not perform well in our experiment since the Volterra series cannot give an accurate polynomial approximation of the nonlinear channel response for the complicated over-the-sea channel conditions. A complex-valued multiple-symbol output (CVMSO) neural network (NN) equalizer is proposed to solve this problem. In this article, we achieved the transmission of a 2 GBaud THz-band quadrature phase shift keying (QPSK) signal over a 20.1 km wireless link at 125 GHz using photonics-assisted technology. Meanwhile, we establish a 20.1 km THz wireless communication model over the sea, revealing that elevated temperature and humidity markedly enhance air attenuation loss, adversely affecting the THz communication system. With the CVMSO equalization method, we achieved a bit error rate (BER) below 1.56 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-2}$</tex-math> </inline-formula> and a 1.8 dB receiver gain compared with VNE. Furthermore, the CVMSO NN equalizer demonstrated a 5.7% reduction in computational complexity and a reduction of 2000 training lengths because it can utilize the network information trained by the current input symbols for equalizing following multiple symbols, thereby minimizing unnecessary computations. Moreover, we extend our research on the complex climate environment in the over-the-sea channel, instructive for long-haul THz over-the-sea communications.