Recently, remarkable efforts have been made in developing wireless communication systems at ultrahigh data rates, with radio frequency (RF) carriers in the millimeter wave (30–300 GHz) and/or in the terahertz (THz, >300 GHz) bands. Converged technologies combining both the electronics and the photonics show great potential to provide feasible solutions with superior performance compared to conventional RF technologies. However, technical challenges remain to be overcome in order to support high data rates with considerably feasible wireless distances for practical applications, particularly in the THz region. In this work, we present an experimental demonstration of a single-channel THz radio-over-fiber (RoF) system operating at 350 GHz, achieving beyond 100 Gbit/s data rate over a 10-km fiber plus a >20-m wireless link, without using any THz amplifiers. This achievement is enabled by using an orthogonal frequency division multiplexing signal with a probabilistic-shaped 16-ary quadrature amplitude modulation format, a pair of highly directive Cassegrain antennas, and advanced digital signal processing techniques. This work pushes the THz RoF technology one step closer to ultrahigh-speed indoor wireless applications and serves as an essential segment of the converged fiber-wireless access networks in the beyond 5G era.

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26.8-m THz wireless transmission of probabilistic shaping 16-QAM-OFDM signals

In this paper, we proposed an improved mobile fronthaul architecture employing the SNR-improved delta-sigma digitization scheme and 4-level pulse amplitude modulation (PAM-4) format. Different from traditional 2-bit quantization, this scheme deploys two-fold delta-sigma quantization, which uses 1-bit delta-sigma modulator to quantize the signal, and another 1-bit delta-sigma modulator to quantize the in-band noise. A significant reduction of the in-band noise can be achieved only applying a differentiator, bringing about a much better noise shaping performance. Meanwhile, the two 1-bit streams can be combined and transmitted in a PAM-4 manner through an intensity modulation direct detection (IM-DD) channel. We have successfully experimentally demonstrated the digitization and transmission of 65536 quadrature amplitude modulation (QAM) baseband orthogonal frequency division multiplexing (OFDM) signal with sampling rate of 1.25 GSa/s over 20-km fiber with standard 10-Gbaud PAM-4 signal, with a signal to noise ratio (SNR) of 57.7 dB. Compared to the conventional 2-bit quantized OFDM scheme, experimental results show that a significant SNR improvement of 17.7 dB has been achieved by the proposed scheme. In addition, we also realized a transmission of 16384-QAM intermediate frequency (IF) signal at the center frequency of 3.5 GHz (with an SNR of 49.6 dB). An improvement of 15.9 dB can also be maintained.

An SNR-improved Transmitter of Delta-sigma Modulation Supported Ultra-High-Order QAM Signal for Fronthaul/WiFi Applications

We experimentally demonstrate 2-Gbaud 4096-ary quadrature amplitude modulation (4096QAM) signal transmission over 2-m wireless distance at 370 GHz in a photonics-aided terahertz (THz)-wave communication system. By using one-bit quantized delta-sigma modulation (DSM) and photonics-aided technology, we successfully achieve the transmission of high-order QAM signals over THz multiple-input multiple-output (MIMO) links. After 2 m of free space transmission, its bit error rate (BER) 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 to apply DSM technology in a THz MIMO link transmission.

Demonstration of 4096QAM THz MIMO wireless delivery employing one-bit delta-sigma modulation.

Wired-wireless convergence is considered one of the most promising concepts for the access networks in the upcoming beyond-5G era, a notable example being the radio-over-fiber (RoF) technology that delivers broadband wireless data from/to the edge cloud to/from the antenna sites seamlessly. For future radio access, the capacity, latency, and fidelity requirements pose great challenges to the RoF schemes. With the advances of field-programmable gate arrays, application specific integrated circuits, and high-speed data converters, in recent years, digital signal processing (DSP) has been able to play an increasingly important role in RoF system/processing. Such an analog-digital coordinated scheme, namely, DSP-enhanced RoF, has merits in flexibility, parallelized processing capability, and robustness. This paper aims to provide comprehensive analysis of three major DSP-enhanced RoF techniques, namely, DSP-based intermediate-frequency over fiber, analog-to-digital compression RoF, and delta-sigma RoF. Besides performance, quantitative and qualitative analysis will be presented on latency, power consumption, and system complexity, attributes that are important in practice. In addition, we will discuss the challenges to and opportunities for future research. 

DSP-enhanced radio-over-fiber technologies for 5G-and-beyond wired-wireless convergence

High-Speed Optical Fiber Communication in China

A novel topology multi-stage noise-shaping (MASH) delta-sigma modulator is proposed for 20-km digital mobile fronthaul (MFH) in this article. In the proposed MASH structure, a newly designed feedback unit is combined with a traditional fourth-order sturdy MASH structure to enhance the noise-shaping capacity. The detailed comparison between the conventional fourth-order single delta-sigma modulator (SDSM) and the proposed new topology MASH is presented in a 512/1024 quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) transmission system with the bandwidth of 1.125 GHz. The OFDM signal is quantized to two bits by SDSM/MASH analog-to-digital conversion (ADC), and this digitized signal is transmitted over 20-km single mode fiber (SMF) in 20-Gbaud 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IM/DD) system. The signal to noise ratios (SNRs) of the retrieved OFDM signal utilizing the proposed new topology MASH and the fourth-order SDSM ADCs are 38.7dB and 34.5dB, respectively. In the case of 1024-QAM PAM4 system, the error vector magnitude (EVM) floors of the proposed new topology MASH and the conventional fourth-order SDSM schemes are 1.64% and 1.96% over 20-km SMF transmission at off-line digital signal processing (DSP) reception, and 1.2 dB receiver sensitivity improvement is achieved.

Digital Mobile Fronthaul Based on Performance Enhanced Multi-Stage Noise-Shaping Delta-Sigma Modulator

In order to reduce the quantization noise of low-resolution DAC, we study a noise shaping technique that can reshape the white quantization noise to an irregular spectrum, in which the noise in the data subcarrier band is almost transformed into the null subcarrier band. Both in simulation and experiment, we verify the effectiveness of the investigated scheme in 25 GHz 16-QAM/32-QAM/64-QAM DMT transmission systems. The experimental results show that the noise shaping technique can enhance the receiver sensitivity of 4-bit quantized 16-QAM DMT and 5-bit quantized 32-QAM DMT by 4.3 dB and 2.5 dB respectively, at BER of hard decision forward error correction (HD-FEC) threshold (3.8 × 10−3). In addition, with the aid of noise shaping, the performance of 4-bit quantization 16-QAM DMT system is the same as the 8-bit quantization 16-QAM DMT system at the HD-FEC threshold. The experimental results reveal that the proposed noise shaping scheme is a good solution for high-speed, low-cost short reach intra-DCI with low-resolution DAC in future 6G networks evolved from 5G mobile networks.

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Noise Shaping Enhanced DMT Signal Transmission Utilizing Low-Resolution DAC

Digital-to-analog converters (DAC) with a high sample rate and large bandwidth are preferred to achieve high data-rate signal generation in an optical discrete multi-tone (DMT) system. Clock leakage is one of the imperfections in high-speed DACs. One or more data subcarriers of the generated DMT signal may be interfered by the additional clock tone, which may cause error floor in bit error rate (BER) performance. In this work, the pre-equalized 16/32QAM-DMT signals with the net data rate up to 121 Gbit/s are generated by an 80 GSa/s DAC with a clock leakage at 20 GHz. Two simple DAC clock leakage compensation (CLC) methods are employed and investigated in the DMT receiver. The results exhibit the two CLC methods have a similar performance. After 2 km of standard single-mode fiber (SSMF) transmission, by using the two CLC methods, the BER performance can be improved by up to about an order of magnitude and below 3.8e-3 for 16QAM-DMT. The receiver sensitivity in terms of the received optical power can be improved by 2-dB in 32QAM case at the BER of 2e-2. Furthermore, the BER performance with two CLC methods has good stability over the measurement period.

Ke Bai

Papers

Digital Mobile Fronthaul Based on Performance Enhanced Multi-Stage Noise-Shaping Delta-Sigma Modulator

Noise Shaping Enhanced DMT Signal Transmission Utilizing Low-Resolution DAC

100-Gb/s DMT Transmission for Short-Reach Optical Interconnect With DAC Clock Leakage Compensation

Compensation of DAC clock leakage in short-reach DMT system in the presence of sampling frequency offset