Photonics-aided terahertz (THz) technologies in capable of supporting large signal bandwidths are promising in future ultrahigh-speed wireless communication systems, especially the THz-over-fiber (ToF) indoor applications. $2 \times 2$ multiple-input–multiple-output (MIMO) architecture for polarization-division multiplexing (PDM) can further double the system capacity with polarization dimension, while bringing more crosstalk and impairments to signal recovery. In this article, we propose to apply a MIMO equalizer with embedded adaptive phase recovery (eAPR) and independent channel processing (ICP) (MIMO-ICP-eAPR) digital signal processing (DSP) to better compensate for the crosstalk and impairments in a $2 \times 2$ MIMO PDM THz transmission system. With the novel MIMO-ICP-eAPR and photonics-aided THz-wave technologies, we experimentally demonstrate 60-GBaud probabilistically shaped (PS)-64QAM signal transmission over 20-km fiber and 3-m THz wave at 322 GHz in the air, empowered by a $2 \times 2$ MIMO antenna array for PDM technologies. The adaptive phase recovery (APR) integrated with MIMO equalizers can provide a better performance compared to separately applying MIMO equalizers followed by APR. Compared to the APR-separated counterpart, the proposed MIMO-ICP-eAPR can improve the input power sensitivity by 0.6 dB. Furthermore, even without ICP for mitigating IQ crosstalk, the MIMO-eAPR approach still shows performance enhancement compared to MIMO followed by APR. To the best of our knowledge, a record-breaking net rate of 510 Gbps per wavelength for a THz-wave transmission system with PS signals is reached. This work can provide fiber-THz architecture as well as a solution for future THz applications.

We proposed a low-complexity multi-input multi-output neural network integrated with a maximum likelihood phase recovery algorithm (MIMO-NN-BMLPR), which is adopted in long-haul coherent optical communication. Neural network based equalization can effectively overcome the nonlinear distortions induced during long-haul transmission. However, the forward neural network equalizer is difficult to track the phase of coherent optical signals. We integrate an additional maximum likelihood phase recovery layer with the MIMO-NN structure so that the phase noise can be compensated. In this work, 8-channel 100-GBaud dual-polarization (DP) PS-16QAM signals are successfully transmitted over 6400km and satisfy the normalized generalized mutual information (NGMI) threshold at 0.86. The proposed MIMO-NN-BMLPR integrates the low-complexity phase estimation with the NNE to improve the performance. Compared with the traditional DSP structure, the proposal prolongs the distance by 48.84% with a 39.80% complexity reduction.

In this work, we propose a novel design of a hybrid transmission integrated system for radio frequency (RF) and free-space optical (FSO) communications, in which the RF and FSO links are able to share the transmitter and the transmission link. In this system, the RF link can usually be considered as a complement to the FSO link, and the hybridization of the two transmissions allows the system to adapt to a variety of complex environments while maintaining a high rate of transmission, improves system stability, and greatly reduces system complexity through the design of a shared transmitter. An experimental demonstration of the system has been carried out, and the results show that the hybrid RF/FSO system supports 50 Gbit/s transmission, satisfying the 20% soft-decision forward error correction (SD-FEC) threshold of 2.4 × 10⁻². Additionally, for the FSO link, the system supports 100 GBaud QPSK and 32 GBaud 16QAM transmissions individually. The proposed structure combines the advantages of RF and FSO, providing a foundation for future high-speed, broadband, all-environment communication.

This Letter proposes a scheme for optimizing the signal-to-noise ratio (SNR) of signal to improve the system performance by a 1 bit delta–sigma modulation (DSM) in a four-mode MDM system for mobile fronthaul. A 1 bit digitalized signal with an SNR of 60 dB from transmitter digital signal processing (Tx DSP) can be achieved. Based on this system, an experimental demonstration of the ultrahigh-order 1048576-QAM signal transmission over a 50 km strong-coupling few-mode fiber (FMF) is successfully realized. With DSP, the bit error rate (BER) of the received 1048576-QAM signals over four modes transmission is below the 20% soft-decision forward error correction (20% SD-FEC) threshold of 2.4 × 10⁻². To the best of our knowledge, this is the first time that the combination of DSM technology and strong-coupling MDM system is achieved and that the highest-modulation order with DSM reported in MDM system is reached. This experimental demonstration of the proposed novel scheme in MDM system can provide an effective solution for ultra-large-capacity mobile fronthaul in the future.

The Terahertz (THz) band as an underdeveloped frequency range, possesses a significant bandwidth and holds great potential for valuable applications. Photonics-aided THz wireless transmission techniques that can generate and detect broadband signal have bright application prospects in the future. While the larger bandwidth usually refers to higher sampling rates of digital-to-analog converter (DAC) and analog-to-digital converter (ADC), which are more difficult to keep the actual sampling frequency align with the nominal frequency. The sampling frequency offset (SFO) between DAC and ADC in the transceiver will seriously degrade the performance of these asynchronous optical communication systems. The SFO-induced impairments and their compensation methods have been widely studied in optical multi-carrier systems, while rarely investigated in signal-carrier systems. In this paper, we successfully realize a 60-GBaud 64-QAM THz-wave signal delivery at 322 GHz in a 2×2 MIMO dual-polarization system, with 20-km wired and 3-m wireless distance, achieving a record-breaking single-wavelength net rate of 562.5 Gbps. To achieve this, digital signal processing (DSP) based digital-interpolation (DI) SFO method and other advanced DSP methods are applied. The SFO between DAC and ADC in the transceiver reaching up to $\sim$20 parts per million (ppm) in nominal case can be compensated to as if case with SFO of $< $ 0.2 ppm and satisfy the bit error rate (BER) threshold. To the best of our knowledge, this currently represents the highest achievable single-lane net rate in short-range single-wavelength THz wireless transmission systems.