Adaptive feedback-driven probabilistic shaping for high-order modulation formats in a fiber-THz seamless integration system at 320 GHz.

In this Letter, we propose a novel, to the best of our knowledge, adaptive feedback-driven probabilistic constellation-shaping (FBD-PCS) method based on the robustness evaluation criteria and employ variational autoencoder (VAE)-based equalizers to implement polarization demultiplexing and nonlinear equalization for the recovery of high-order PCS-QAM signals. We experimentally demonstrate the fiber-THz 2 times 2 MIMO system with a net rate of 366.4 Gbit/s using dual-polarization 40 Gbaud PCS-64QAM signal over a 20 km SSMF and 6 m wireless link. Specifically, the feedback mechanism drives the fiber-THz system to solve optimization problems, adaptively matching the optimized distribution of transmitted symbols that maximizes normalized generalized mutual information (NGMI). We also examine six scenarios to explore nonlinear resistances of FBD-PCS symbols and the robustness of VAE-based equalizers. The results demonstrate the superiority of FBD-PCS over the Maxwell-Boltzmann (M-B) distributions in practical nonlinear-dominant systems. Additionally, the FBD-PCS signals can break limitations for ultrahigh rate transmission with the help of advanced equalizers.

Spectrally efficient multi-service fiber-wireless access in low-cost direct-detection THz system at 300†GHz.

This Letter demonstrates a novel, to the best of knowledge, overlapping single-sideband (OSSB) transmission scheme for spectrally efficient multi-service fiber-wireless (FiWi) access in a low-cost direct-detection (DD) THz system. Utilizing the proposed OSSB scheme, user data from different services can share the same spectrum resource yet can be successfully demodulated via one cost-effective DD THz receiver in conjunction with the Kramers-Kronig (KK) based SSB field reconstruction and look-up table (LUT) enabled signal separation algorithms. A proof-of-principle experiment is conducted. Based on an IQ modulator and a single THz zero-bias diode (ZBD), two independent 10-GBd quadrature phase shift keying (QPSK) signals with an overlapped spectrum are successfully demodulated after 20-km fiber and up to 3-m wireless transmission at the 300-GHz band. To the best of our knowledge, this is the first demonstration of multi-service FiWi access with an OSSB format in a 300-GHz DD THz system.

Nonlinearity mitigation in a fiber-wireless integrated system based on low-complexity autoencoder and BiLSTM-ANN equalizer.

We propose and experimentally demonstrate an intelligent nonlinear compensation method using a stacked autoencoder (SAE) model in conjunction with principal component analysis (PCA) technology and a bidirectional long-short-term memory coupled with ANN (BiLSTM-ANN) nonlinear equalizer for an end-to-end (E2E) fiber-wireless integrated system. The SAE-optimized nonlinear constellation is utilized to mitigate nonlinearity during the optical and electrical conversion process. Our proposed BiLSTM-ANN equalizer is primarily based on time memory and information extraction characteristics, which can compensate for the remaining nonlinear redundancy. A low-complexity 50 Gbps E2E-optimized nonlinear 32 QAM signal is successfully transmitted over a span of 20 km standard single-mode fiber (SSMF) and 6 m wireless link at 92.5 GHz. The extended experimental results indicate that the proposed E2E system can achieve a reduction of up to 78% in BER and a gain in receiver sensitivity of over 0.7 dB at BER of 3.8 × 10-3. Moreover, computational complexity is reduced by more than 10 times compared to the classical training model.

Real-time 100-GbE fiber-wireless seamless integration system using an electromagnetic dual-polarized single-input single-output wireless link at the W band.

This Letter demonstrates a real-time 100-GbE fiber-wireless seamless integration system operating at the whole W band (75-110 GHz). Based on a pair of commercial digital coherent optical modules, the real-time transparent transmission of 125-Gb/s dual-polarized quadrature phase-shift keying signal has been successfully achieved over two-spans of 20-km fiber and up to 150-m electromagnetic dual-polarized single-input single-output wireless link. To the best of our knowledge, this is the first real-time demonstration of 100-GbE signal transmission over >100-m wireless distance at the millimeter-wave band based on photonics. We believed this real-time and high-speed fiber-wireless seamless integration system with a wireless coverage up to hundreds of meters can significantly accelerate the progress of upcoming 6G.

Photonics-aided integrated sensing and communications in mmW bands based on a DC-offset QPSK-encoded LFMCW.

The evolution of mobile communications towards millimeter-wave (mmW) bands provides a strong opportunity for the seamless integration of radar and wireless communications. We present a photonics-aided mmW integrated sensing and communications (ISAC) system constructed by photonic up-conversion using a coherent optical frequency comb, which facilitates zero frequency offset of the resulting mmW signal. The sensing and communications functions are enabled by a joint waveform that encodes a DC-offset QPSK signal on a linear frequency-modulated continuous wave (LFMCW) in baseband. The QPSK encoding ensures the constant envelope of the mmW ISAC signal for long-distance radar detection. The optimized DC offset preserves the distinctive chirp phase and good cross-correlation of the original LFMCW, which can achieve high-resolution sensing by radar de-chirping and assist in communication sequence synchronization by pulse compression, respectively. Experimental results show that the single-user detection with less than 20-mm sensing error and dual-user detection with a 10.4-cm ranging resolution are realized at 28-GHz band, respectively. The wireless communication with a 11.5-Gbit/s transmission rate also at 28-GHz band is successfully tested. Moreover, the proof-of-concept experiments demonstrate the good frequency tunability and wavelength tolerance of the proposed ISAC system.

Demonstration of 144-Gbps Photonics-Assisted THz Wireless Transmission at 500 GHz Enabled by Joint DBN Equalizer.

The THz wireless transmission system based on photonics has been a promising candidate for further 6G communication, which can provide hundreds of Gbps or even Tbps data capacity. In this paper, 144-Gbps dual polarization quadrature-phase-shift-keying (DP-QPSK) signal generation and transmission over a 20-km SSMF and 3-m wireless 2 × 2 multiple-input multiple-output (MIMO) link at 500 GHz have been demonstrated. To further compensate for the linear and nonlinear distortions during the fiber-wireless transmission, a novel joint Deep Belief Network (J-DBN) equalizer is proposed. Our proposed J-DBN-based schemes are mainly optimized based upon the constant modulus algorithm (CMA) and direct-detection least mean square (DD-LMS) equalization. The results indicate that the J-DBN equalizer has better bit error rate (BER) performance in receiver sensitivity. In addition, the computational complexity of the J-DBN-based equalizer can be approximately 46% lower than that of conventional equalizers with similar performance. To our knowledge, this is the first time that a novel joint DBN equalizer has been proposed based on classical algorithms. It is a promising scheme to meet the demands of future fiber-wireless integration communication for low power consumption, low cost, and high capacity.

Real-time demonstration of 103.125-Gbps fiber-THz-fiber 2 × 2 MIMO transparent transmission at 360-430 GHz based on photonics.

In this Letter, we experimentally demonstrate the first real-time transparent fiber-THz-fiber 2 × 2 multiple-input multiple-output (MIMO) transmission system with a record line rate of 125.516 Gbps at 360-430 GHz based on photonic remote heterodyning, hybrid optoelectronic down-conversion, and commercial digital coherent modules. The 103.125-Gbps net data rate using dual-polarization quadrature phase-shift keying (DP-QPSK) modulation is successfully transmitted over two spans of 20-km standard single-mode fiber (SSMF) and 60-cm wireless distance under 15% soft-decision forward error correction (SD-FEC) for a pre-FEC bit error ratio (BER) threshold of 1.56 × 10-2 (post-FEC BER < 10-15). The optical signal to noise ratio (OSNR) margin and the stability of the transmission system are extensively investigated. To the best of our knowledge, this is the first time to realize >100-Gbps real-time transparent fiber-THz-fiber link transmission at beyond the 350-GHz band, making it a promising scheme to pave the way towards a practical seamless integration of a fiber-THz-fiber link to the future 6G mobile communication system.

Experimental demonstration of single-sideband, four-level pulse amplitude modulation/direct-detection degenerate mode-division multiplexing transmission based on multiple-input, multiple-output nonlinear equalizer.

We experimentally demonstrate a degenerate mode-division multiplexing transmission system with single-sideband Nyquist pulse-shaped pulse amplitude modulation-4 and direct detection techniques. Hilbert superposition cancellation (HSC) processing is applied to cancel the first-order cross talk at the receiver side. A least mean square Volterra-based multiple-input, multiple-output (MIMO) nonlinear equalizer (NLE) is used to compensate the second-order distortion. Experimental results show that compared with the MIMO linear equalizer without HSC processing, the MIMO NLE with HSC processing can help improve the bit error rate performance by almost 1 order of magnitude at the received optical power of $ - {10}\;{\rm dBm}$-10dBm.