Mobile Fronthaul Research Articles

A new full-duplex analog RoF transmission system for 5G/5G+ broadband mobile communication

Abstract A new cost-effective mobile fronthaul architecture has been proposed by designing promising analog radio over fiber (ARoF) transmission system compliant with the next generation ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON). It is conceived for standalone 5G/5G+ mobile communication systems driven by broadband multi-input multi-output (MIMO) millimeter wave data signals using two high spectral efficient modulations. The feasibility of the proposed system was firstly proven in terms of spectral tracing supported by theoretical analysis that disclosed good agreement. Furthermore, the obtained simulation results regarding the system performance evaluation after 22 km bidirectional optical fiber links were revealed high reliable characteristics including error-free transmissions at low receiver sensitivities, and measured power penalties less than 1 dB. In addition, the recovered eye diagrams maintained wide openings with high-quality patterns.

Machine learning enhanced next-generation optical access networks—challenges and emerging solutions [Invited Tutorial

Optical access networks are envisioned to become increasingly complex as they support more and more diverse and immersive services, each with a different capacity, latency, and reliability need. While machine learning has been touted as a silver bullet that will intelligently manage network operations and resources to meet these demands, as it had been anticipated for core and metro networks, there exist various challenges that need to be addressed to progress machine learning models from research to production. In this tutorial, we first aim to motivate the continued push to advance optical access networks and rationalize the use of machine learning in these networks. We then highlight the challenges that are especially amplified due to the traffic dynamicity and heterogeneity, data scarcity, and computation-resource constraints of optical access networks. We discuss emerging machine learning approaches that are being explored to address these challenges. Finally, we consider a fast and self-adaptive machine learning enhanced dynamic bandwidth allocation scheme in an illustrative future use case of supporting immersive human-to-machine communications over the mobile fronthaul of next-generation mobile networks.

245Gb/s P2MP Mobile Fronthaul Downstream Transmission Using Code-Division Multiplexing and Self-Homodyne Coherent Technologies

We have proposed and experimentally demonstrated a high-speed and point-to-multipoint (P2MP) mobile fronthaul (MFH) downstream system based on the code-division multiplexing (CDM) and self-homodyne coherent (SHC) technologies. We find that compared with the traditional 16-ary quadrature amplitude modulation (16-QAM) signal transmission, the change of phase noise between two demultiplexed symbols is much larger in the CDM-16QAM signal transmission because it is the accumulation of phase noise change of n multiplexed symbols. Therefore, a low-complexity pilot sequence-assisted phase noise estimation algorithm is proposed and designed to compensate for the phase noise caused by fiber mismatch length, which makes it possible to use a low-cost distributed feedback (DFB) laser. Besides, the spread signal's distribution in the frequency domain of each code sequence is different, leading to different transmission performances between different remote radio units (RRUs). A power optimization scheme is proposed to balance the transmission performances of different RRUs by multiplying each channel's signal by a normalized weight factor at the transmitter. The experimental results show that the proposed system could successfully support the transmission of 245 Gb/s 16QAM signal from baseband unit (BBU) to 7 RRUs (each RRU could have 2 antennas to transmit signals at 2 polarizations) over 10 km standard single-mode fiber (SSMF). The measured worst bit error rate (BER) value of 14 channels is 1.77e-3 which is below the hard-decision forward error correction (HD-FEC) threshold (3.8e-3).

Bidirectional WDM Multi-Nodes Analog Radio-Over-Fiber Mobile Fronthaul Link Enhanced by Photonic Integrated Devices

A bidirectional wavelength division multiplexing (WDM) analog radio-over-fiber (A-RoF) mobile fronthaul (MFH) link is enhanced using photonic integrated devices. Two key photonic integrated devices are combined in the A-RoF link: an 8-channel InP directly modulated laser (DML) transmitter and a 32×100-GHz silicon array waveguide grating (AWG). The DML transmitter has 8 parallel monolithically integrated distributed feedback lasers, enabling cooperative and reconfigurable downlink analog transmission. Moreover, the 32×100-GHz AWG is featured by low insertion loss (<4.5-dB) and low crosstalk (<−20.4-dB), to achieve a high-density WDM system. In the distributed field experiments, we have successfully demonstrated a bidirectional A-RoF MFH over 10-km standard single mode fiber, providing an 8×5-Gbit/s 4-quadrature amplitude modulation (QAM) orthogonal frequency division modulation (OFDM) downlink transmission and a 3×12-Gbit/s 16-QAM uplink transmission.

Comparative study for evaluating the cost efficiency of 5G Ethernet mobile fronthaul networks

With the advances in mobile network technologies, the fronthaul is being evolved to centralized and cloud networks, leading to the emergence of various optical multiplexing architectures. Among others, the Ethernet-based packet switching network is considered one of the most promising concepts due to the inherent merits of Ethernet such as backward compatibility, high reliability, and cost reduction. This work aims at evaluating the Ethernet-based mobile fronthaul network by comparing it with other configuration options. In particular, we calculate and compare the total cost of ownership (TCO) in order to emphasize the cost efficiency of the 5G Ethernet fronthaul network. For this, we analyze the packet aggregation capabilities of the Ethernet fronthaul through queueing latency modeling. Hence, we show that the statistical multiplexing enabled by packet switching reduces the cost related to optical transceiver modules and subsequently fiber optic cabling, which eventually allows for the TCO to be less than 70% of the active transparent system.

A Novel Structure Optimizer Based on Heuristic Search for Delta-Sigma Modulator in Mobile Fronthaul

In this letter, a novel delta-sigma modulator structure design method based on the genetic algorithm is proposed. Compared to the widely adopted R.Schreier’s method, the proposed approach can significantly reduce the in-band quantization noise. Performance comparisons are carried out on 4th-order baseband delta-sigma modulators (DSMs) and 6th-order bandpass DSMs with center frequency at 2GHz. OSR of these delta-sigma modulators are set at 8. Numerical simulations, as well as experimental demonstrations, are conducted to show a 10-Gbaud intensity modulation direct detection transmission with the 64-QAM signal. Experimental results show that compared to R. Schreier’s method, the proposed GA method can improve the SNR by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 5$ </tex-math></inline-formula> dB and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 3$ </tex-math></inline-formula> dB for baseband DSM and bandpass DSM, respectively.

Feasibility of rate-adjustment function in redundant mobile fronthaul system using short-reach transceiver

Feasibility of rate-adjustment function in redundant mobile fronthaul system using short-reach transceiver

Analog Mobile Fronthaul Supporting 128 RF Chain With Nonlinear Compensator for B5G and 6G Ma-MIMO Beamforming

These paper details a fronthaul optical link using delay-division-multiplexing (DDM) scheme with Volterra based nonlinear compensator for beyond fifth generation (B5G) and 6G massive multiple-input multiple-output (Ma-MIMO) beamforming. Based on pre-allocated relative time delays, pre-processed signals can be deaggregated to different RF chain signals following spectral aliasing caused by the digital signal processing (DSP) at remote radio units. The Volterra based nonlinear compensator is used to compensate nonlinear distortion and IQ imbalance of the fronthaul link. By combined the DDM scheme and Volterra technique, we can easily deaggregate signals and mitigate the interference caused by nonlinear distortion and IQ imbalance at the same time. In experiments, the DDM scheme support 128 RF chain signals is demonstrated. With the Volterra compensator, the signal input power of Mach-Zehnder modulator (MZM) has 18 dB margin. IQ imbalance caused by phase misalignment can also be compensated by the IQ Volterra. It can keep the acceptable error vector magnitude (EVM) performance within a ±10° range. After 25-km optical link, the EVM of each RF Chain is underneath 8%. And the total received 128 RF chain signals will require CPRI data rate as high as 840 Gb/s.

Fifth-generation new radio millimeter-wave radio signal transmission over a seamless fiber-terahertz mobile fronthaul system for an ultra-dense small cell network.

This Letter demonstrates the transmission of fifth-generation new radio (5G NR) millimeter-wave signals over a seamless fiber-terahertz-wave mobile fronthaul system in the 350 GHz band for an ultra-dense small cell network. The system utilizes a simple optical heterodyne method at the transmitter and direct detection at the receiver. As a proof-of-concept demonstration, we successfully transmitted 256- and 64-quadrature amplitude modulation 5G-NR-compliant signals at 24.2 and 38 GHz over a seamless fiber-terahertz system in the 350 GHz band. The proposed system can provide a simple solution to facilitate the deployment of ultra-dense small cells in high-frequency bands in 5G networks and beyond.

Full-duplex transmission without an uplink light source for a millimeter-wave radio over a free-space optical system.

The radio over free-space optical (RoFSO) system is a promising alternative technique for mobile fronthaul networks such as aerial base stations where fiber link is unavailable to deploy. However, the full-duplex transmission imposes challenges on the design of the RoFSO system due to the additional structural complexity when ease-of-installation is a mandatory request. The interplay between the uplink and downlink for a simplified structure has been less investigated in related works. In this paper, we propose a full-duplex RoFSO transmission system structure in which the uplink light source is physically saved. The optical center carrier of the double-sideband (DSB) modulated signal in the downlink is extracted as the light source for the uplink, and thus the system complexity is reduced. In order to mitigate the permanence degradation of uplink transmission due to the attenuated light source, channel estimation and power pre-compensation are implemented without additional training symbols by utilizing the characteristics of the channel correlation between uplink and downlink and the information independence of optical center carrier in the DSB. The proposed channel estimation algorithm is verified theoretically and experimentally. The results show that the difference between uplink and downlink channel attenuation is -0.7dB. In case of the excessive power compensation and the resulting effects, a power control mechanism is introduced to the power pre-compensation. By using the power pre-compensation mechanism, the bit error rate (BER) can reach 1.58×10-3 and 3.8×10-3, respectively, under the channel conditions of medium turbulence and weak turbulence. Owing to the power control mechanism, the power saving rates of downlink and uplink are up to 78% and 67%.

Power consumption analysis of an optical modulator based on different amounts of graphene

Energy-efficient devices will play a key role in the continued performance scaling of next-generation information and communications technology systems. Graphene has emerged as a key optoelectronic material with unique energy-like properties. But to the best of our knowledge, these advantages have not yet been fully exploited in optical modulators design. In this work, we design and analyze an optical modulator which is composed of two graphene layers and a ring resonator made with different amount of graphene. For performance analysis, the ring resonator’s amount of graphene is varied from 25 to 100% with four discrete steps. The critical coupling condition representing the OFF-state, and the 3-dB transmission level representing the ON-state of the device are obtained. Numerical results show this new optical modulator consumes as little energy as 4.6 fJ/bit whilst achieving a high-speed operation with a bandwidth up to 42.6 GHz when employing surprisingly only 25% of graphene. The 42.6 GHz modulator has a footprint as small as 22.1 µm2 with an active area of 1.68 µm2 only, the smallest active area to date. Alternatively, the optical modulator achieves up to ∼88.5 GHz at the expense of consuming 17.5 fJ/bit when using 100% of graphene. The proposed graphene-based modulator proved to be a compact, energy-efficient, high-speed device, useful for a myriad of applications including mobile fronthaul, telecom, and datacom.

CNN-Based Inter-Band Interference Cancellation for 100Gbit/s/λ Non-Orthogonal m-CAP in Fronthaul

To improve the spectral efficiency (SE) of next-generation-fronthaul-interface-based (NGFI) mobile fronthaul (FH) network, we present a novel inter-band interference (IBI) cancellation method based on convolutional neural network (CNN) in non-orthogonal multiband CAP (NO-m-CAP) system. As a proof of concept demonstration, a NO-4-CAP 100Gb/s/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> NGFI-II data transmission experiment is carried out, which shows 15% SE improvement with the proposed CNN-based IBI canceller assuming a 7% HD-FEC limit. Being more straightforward and practical, the proposed CNN-based IBI canceller with lower complexity could offer more power budget and promise more compression ratio compared to our previously proposed ICA-based (Independent Component Analysis) method.

Real-Time DSP-Free 100 Gbit/s/λ PAM-4 Fiber Access Link Using EML and Direct Detection

A 100 Gbit/s/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> PAM-4 fiber link with an optical budget of 30 dB and 20 km fiber reach is achieved in real time experiments. This is compliant with class A (20 dB) point to point (PtP) applications as mobile fronthaul for example, and with class N1 (29 dB) point to multipoint (PtMP) for residential market. We used an integrated externally modulated laser, an analog pre-equalizer, an optical booster amplifier and/or non-filtered preamplifier and direct detection without any digital signal processing (whether real-time or offline).

Performance Comparison of Photonic Downconversion for Uplink Mobile Fronthaul

This study compares the performance of photonic downconversion technology using single and dual wavelength modulation methods using an optical phase modulator. We compared the system characteristics, including the conversion gain, phase noise, and spurious-free dynamic range, of the two approaches. To demonstrate the proof-of-concept, we transmitted high-order quadrature amplitude modulation orthogonal frequency-division multiplexing signals in the 43 and 52 GHz bands over the system. The results confirm the potential of the system as a promising solution for uplink mobile fronthaul in high-frequency bands, such as in 5G-Advanced networks.

Hybrid fiber–THz fronthaul supporting up to 16384-QAM-OFDM with the delta-sigma modulation

With the progress of high-capacity radio access networks, ultra-dense small cells are rapidly being deployed in urban areas. As a result, the deployment of a large number of optical fibers in urban areas becomes a severe issue. In this Letter, we propose a hybrid fiber–terahertz (THz) mobile fronthaul system supporting flexible and high-order wireless signal transmission with the delta-sigma modulation. The photonic THz transmission is used as the seamless extension of fiber-based fronthaul in small cells. A 20-Gbit/s digital fiber–THz fronthaul system is experimentally demonstrated to validate the proposed scheme, with 10-km optical fiber transmission and 300-GHz wireless relay. Carrier aggregation of up to 10 40-MHz and 60-MHz 5G-new radio (5G-NR) channels at the radio carrier frequency of 3.9 GHz is reported. The design of quantization noise suppressed delta-sigma modulation enables the system to transmit orthogonal frequency division multiplexing (OFDM) modulation up to 16384 order quadrate amplitude modulation (QAM) mapping with the error vector magnitude (EVM) below 0.5%.

AMCC nonlinear baseband superimposition and extraction aided by proposed interference cancellation for WDM-PON used in 5G mobile fronthaul.

As the demand for mobile Internet capacity explodes, the research on fifth-generation (5G) mobile communication systems has gradually deepened, which has stringent requirements for latency and transmission capacity, especially in the section called mobile fronthaul (MFH). Wavelength division multiplexing passive optical network (WDM-PON) is an attractive technology for 5G MFH which requires an auxiliary management and control channel (AMCC) to achieve efficient network deployment. In previous studies, more research has been done on AMCC superimposition methods and transmission performance on 10 Gbps WDN-PON systems, and the role of AMCC in wavelength management has been studied at the system level. In this paper, we realize non-linear baseband modulation of AMCC signals up to 20 Mbps in a single wavelength 25-Gbps PON system through the distributed feed-back laser and Mach-Zehnder modulator. At the receiving end, we propose a low-complexity interference cancellation method to suppress the interference caused by WDM-PON signals and significantly reduce the bit error rate of AMCC signals. The method realizes a simplified scheme to reconstruct the PON signals by analyzing the characteristics of nonlinear modulation and the reconstructed signals can be applied to the original AMCC signals through filtering and subtraction to eliminate interference. These operations have low complexity and can be easily implemented by analog circuits, thus making it an effective way to improve the quality of AMCC signals. With the help of the proposed method, AMCC transmission at 20 Mbps can be achieved with different modulation depths, which is very promising for 5G MFH.

Reconfigurable Fiber Wireless Fronthaul With A-RoF and D-RoF Co-Existence Through a Si3N4 ROADM for Heterogeneous Mmwave 5G C-RANs

Towards enabling bandwidth-hungry 5G applications, a Fiber-Wireless (FiWi) mobile fronthaul architecture is experimentally presented supporting the co-existence of spectrally efficient analog transport formats with Digital Radio over Fiber transport schemes, combined with millimeter wave (mmWave) high-capacity wireless channels. Flexible transport network reconfiguration is proposed by means of a low-loss 1 × 4 Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> Reconfigurable Optical Add/Drop Multiplexer (ROADM) on TriPleX platform, selectively dropping four FiWi mmWave fronthaul links to four different antenna location. Detailed investigations of the proposed system are reported, including frequency characterization, multi-stage EVM penalty study and analog/digital traffic coexistence PHY layer feasibility. Following, four 10 Gb/s multi-band 16-QAM WDM FiWi analog IFoF links are transmitted through 10km fiber and 1-m directional V-band antennas and flexibly reconfigured by a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ROADM, achieving a record 40 Gb/s wavelength-routed fronthaul capacity for mmWave 5G, while simultaneously meeting multiple 5G Use Cases within 3GPP requirements.

Digital mobile fronthaul based on delta-sigma modulation employing a simple self-coherent receiver

The coherent digital radio-over-fiber (DRoF) system is a promising candidate for future mobile fronthaul networks (MFNs) due to its high receiver sensitivity and excellent robustness against nonlinearities. However, conventional coherent receivers with complicated structure and heavy algorithms are too expensive and power-hungry for cost-sensitive MFN applications. In addition, currently deployed digital MFNs based on common public radio interface (CPRI) suffer from low spectral efficiency and high data rate. Towards these issues we propose a novel DRoF downlink scheme employing a simple self-coherent receiver. In baseband unit (BBU), the radio signal is converted to a digital bit stream by a band-pass delta-sigma modulator (BP-DSM), which can be simply recovered with the utilization of a band-pass filter at the receiver. In remote radio unit (RRU), an electro-absorption modulated laser (EML) acts as a low-cost coherent homodyne receiver in virtue of injection locking technique. In the experiment, the injection-locked operation of the DSM signal is successfully achieved, and two modified schemes are proposed for the DSM signal to increase the locking range with a tolerable sensitivity penalty. The experimental results demonstrate the superiority of our approach in two aspects: 1) the EML-based coherent receiver outperforms a PIN photodiode in terms of receiver sensitivity; 2) compared to the analog RoF system, a 5-dB improvement in loss budget is obtained when DSM is employed with the aid of a simple equalizer.

Cascaded digital-analog radio-over-fiber for efficient SNR scaling at >10 dB per extra bandwidth.

Efficient signal-to-noise ratio (SNR) and spectral efficiency (SE) trade-off can offer fundamental guiding law for future large-capacity and high-fidelity mobile fronthaul. In this Letter, we propose and experimentally demonstrate a cascaded digital-analog radio-over-fiber (CDA-RoF) scheme, which transmits digitized and analog segments using time-division-multiplexing aggregation. Specifically, multiple digital parts generated by a rounding function describe the main features of the original waveform, while the residual error is magnified and delivered as analog RoF with SE advantage. Based on a 20-GHz O-band directly modulated laser (DML), an SNR enhancement of >10 dB is observed for each extra bandwidth. Up to 70.4-dB SNR is also achieved for a 3.5-GHz 1048576-ary quadrature amplitude modulation (1048576-QAM) signal after 20-km standard single-mode fiber (SSMF) transmission.

Coherent EML+TIA Detector for DSP-Free 1 Gb/s/λ Reception Over Extended Budget

The continuing drive towards broadband access necessitates cost-effective optical access and mobile fronthaul solutions. While the fixed-mobile convergence motivates high-speeds for the antenna remoting through broadband digitized radio-over-fiber, the access segment still uses time division multiplexed access equipment. Although the steadily increasing line rates call for 50 Gb/s solutions, the unshared per-user rate remains in the 1-Gb/s range. It is a question whether such data rate can be facilitated without high-speed components, at the same time supporting higher passive splits for the optical distribution networks. We experimentally demonstrate a simplified receiver based on an electro-absorption modulated laser, co-integrated on a die-level with a transimpedance amplifier. We investigate its sensitivity for the phase-agnostic coherent heterodyne downstream detection of data signals in a passive optical network scenario. We will show that despite the omission of a high-performance local oscillator, balanced detection and digital signal processing, the proposed receiver accomplishes reception of 1 Gb/s phase-modulated signals over a link reach of 57 km and an optical budget of 48 dB, thus enabling high passive splits of 1:128. We study the implications raised by this single-ended coherent receiver architecture under multi-channel transmission and investigate the susceptibility of its integrated local oscillator to optical feedback.