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We propose a non-uniform-digital-analog radio-over-fiber (NUDA-RoF) scheme based on a low-complexity 4-fold symmetric non-uniform quantization method and demonstrate it experimentally in a polarization-division-multiplexed (PDM) coherent transmission system. The 50-Gbaud PDM-NUDA-RoF signal with a net rate of 391 Gb/s/λ and a record single-channel common public radio interface equivalent data rate (CPRI-EDR) of 2 Tb/s/λ is successfully delivered in a 10-km standard single-mode fiber (SSMF) link. Results show that the NUDA-RoF scheme achieves a maximum signal-to-noise ratio (SNR) gain of ∼1.3 dB compared to digital-analog radio-over-fiber (DA-RoF) at an equivalent capacity. Significantly, the NUDA-RoF scheme demonstrates a ∼3.8-dB optical signal-to-noise ratio (OSNR) sensitivity improvement over DA-RoF at the 256-QAM error vector magnitude (EVM) threshold. The NUDA-RoF scheme has advantages over analog-radio-over-fiber (A-RoF) and DA-RoF in terms of SNR improvement, as well as advantages over digital- radio-over-fiber (D-RoF) in CPRI-EDR. Our work suggests many optimistic possibilities for the future radio access networks and mobile fronthaul.

We propose a truncated digital delta-sigma modulated radio-over-fiber (TDDSM-RoF) scheme and verify it experimentally in a 10-km standard single-mode fiber (SSMF) coherent system. The 40-Gbaud TDDSM-RoF signal with a common public radio interface equivalent data rate (CPRI-EDR) of 0.214 Tb/s is successfully transmitted in the system with excellent performance. The recovered error vector magnitude (EVM) of TDDSM-RoF signal is 1.27%, meeting the EVM requirement for 4096-quadrature amplitude modulation (4096-QAM) transmission in fronthaul. Compared with the analog RoF (A-RoF) and digital-analog RoF (DA-RoF) schemes, the TDDSM-RoF scheme with a 6-times oversampling rate (6-OSR) DSM achieves signal-to-noise ratio (SNR) gains of 15.7 and 10.2 dB, respectively, at the cost of extra bandwidth. We also demonstrate a truncated-DA-RoF (TDA-RoF) scheme with analog quantization error. Although the TDA-RoF consumes less bandwidth than TDDSM-RoF, it has limited improvement in SNR. Compared with the commercial digital RoF (D-RoF) solution, the proposed TDA-RoF has an advantage in the required bandwidth and the TDDSM-RoF achieves a higher SNR gain and higher-QAM transmission. Both of them can serve as promising solutions for future 6G fronthaul.

Photonics-assisted generation of millimeter-wave (MMW) signals can support larger modulation bandwidths compared with electronic algorithms so that it is considered as a key technology for future Radio-over-Fiber (RoF) communications. Meanwhile, multi-input multi-output (MIMO) system can concentrate the signal energy in an extremely narrow beam in space and precisely directed to the downlink users, thereby maximizing the propagation distance in that direction. Therefore, we demonstrate a W-band photonics-assisted MMW 2 × 2 MIMO communication system. We employ a photo-generated millimeter wave approach to generate a 93.7 GHz W-band signal, which is received according to the maximum ratio combining (MRC) technique and constant modulus algorithm (CMA) equalization algorithm after 2 × 2 MIMO wireless transmission, and carrier recovery using advanced digital signal processing (DSP) algorithms. According to the experimental results, the maximum signal-to-noise ratio (SNR) gain between 1 × 1 and 2 × 2 MIMO can up to 7.1 dB. To the best of our knowledge, this is the first experimental demonstration of high gain in a W-band MMW 2 × 2 MIMO system by using these technologies. For subsequent studies, we also experimentally explored the performance gain of the 2 × 2 dual-polarization MIMO system, the 2 × 2 dual polarized MIMO can achieve a SNR gain of 2.9 dB. These results will provide significant assistance in our future efforts to increase the distance of MMW or even terahertz-band RoF communications.

In this article, a support 65536 QAM signal mobile fronthaul (MFH) architecture based on the noise transfer function (NTF)-improved delta-sigma modulation (DSM) was proposed and demonstrated by experiment. In the NTF-improved DSM, the extended Lee's rule was used to optimize the NTF generated by the Closed-Loop Analysis of Noise-Shaping Coders (CLANS) method to achieve greater gain of signal-to-noise rate (SNR). The performance of traditional DSM and NTF-improved DSM under different oversampling ratios (OSRs) and different quantization levels were simulated and compared. The results show that the NTF-improved DSM can achieve about 7 dB SNR improvement at 8 times OSR compared with the traditional DSM. Based on NTF-improved DSM, the orthogonal frequency-division multiplexing (OFDM) signal with high-order quadrature amplitude modulation (QAM) is quantized into 4-level pulse amplitude modulation (PAM4) to evaluate the scheme performance. The quantized PAM4 signal was transmitted over 25-km single-mode fiber (SMF) in C-band 25G baud intensity modulation direct detection (IM/DD) system. At the receiver, the SNR of the recovered OFDM signal after filtering reaches 51.7 dB. In the case of transmitting 65536 QAM OFDM signals, the error vector magnitude (EVM) of the recovered signal is 0.25%, which is below the 0.34% EVM threshold of 65536 QAM MFH. The experimental results show that the NFT-improved DSM can achieve 65536 QAM MFH when the OSR is 8. Compared with the traditional DSM that requires an OSR of 10, the spectral efficiency (SE) has been greatly improved.

The Carrier-less Amplitude and Phase (CAP) modulation is an advanced modulation technology with the advantages of high spectrum utilization, low cost, and low complexity. In this paper, based on photonics-aided technology and envelope detection, we experimentally demonstrate the wireless transmission of 4.6 km of 8Gbaud CAP-4QAM signal and 1.2 km of 5Gbaud CAP-16QAM signal over D-band at 125GHz, respectively. At the same time, combined with antenna polarization multiplexing technology, the maximum bit rate can reach 32 Gb/s and 40 Gb/s, respectively. Both BERs are below the 25% overhead soft-decision forward error correction (SD-FEC) threshold of 4.2×10−2. In addition, in the back-end equalization algorithm processing, we compared the time domain (TD) constant module algorithm (CMA), the Decision Directed Least Mean Squares (DDLMS) algorithm of the time domain multiple input multiple output (MIMO) structure, the frequency domain (FD) CMA algorithm, and the FD MIMO-DDLMS algorithm. The results show that the FD MIMO DDLMS algorithm can effectively compensate for the I/Q imbalance problem of CAP signals while maintaining low computational complexity. As far as we know, this is the first experimental demonstration of a rate exceeding 30 Gb/s in a D-band long-distance transmission system with envelope detection, which provides an effective solution for low-cost millimeter wave transmission.