Mobile Fronthaul System Research Articles

Upgraded architecture of TWDM-PON with type B protection for capacity and budget improvement

The next-generation mobile fronthaul system based on technologies beyond 5G requires optical access networks with excellent disaster resistance. For passive optical networks (PONs), also known as economical optical access networks, the Type B protection method that considers disaster resistance is a practical conventional approach to address transceiver and link failure. However, the transmission capacity upgrade and effective utilization of optical power of time wavelength division multiplexing (TWDM)-based-PON with Type B protection have not been extensively studied. In this study, we intend to upgrade the architecture by applying Type B protection to a 40 Gbit/s-class TWDM-PON that uses the uplink C band/downlink L band specified by NG-PON2. To improve the downlink capacity, we have considered a bidirectional asymmetric modulation transmission system to introduce a dual-polarization quadrature phase-shift keying system for the downlink C band. The downlink coherent signal is transmitted by selecting either the primary or secondary link using the optical switch. There are no major drawbacks with either the primary or secondary link. Moreover, we have numerically and experimentally clarified the impact of improving the reception power by receiving a burst single-input multi-output in the uplink under the condition of no link failure. Additionally, we describe the bidirectional transmission feasibility of C-band asymmetric modulation under link failure and reflection. We have found that bidirectional transmission at the same wavelength is possible by allowing a specific penalty even for asymmetric symbol rate/polarization/modulation formats. A principle verification experiment revealed that SIMO reception could be improved by a 1.5 dB budget on the uplink. The method of upgrading the TWDM-PON system can be changed while effectively using NG-PON2 technology until the future coherent PON method is completely introduced.

Space-time domain equalization algorithm based on complex-valued neural network in a long-haul photonic-aided MIMO THz system.

The urgent demand for high-bandwidth wireless services in enhanced mobile broadband networks needs innovative solutions for mobile front-haul systems. The terahertz (THz) band offers a promising candidate for ultrahigh-capacity data transmission. This study investigates the integration of photonics-aided THz signal generation with MIMO and PDM technologies. We proposed a novel, to the best of our knowledge, space-time domain equalization algorithm based on MIMO-complex-valued neural networks (CVNN), which can preserve the signal phase and the relation between the X- and Y-polarization. We experimentally demonstrate the transmission of 60-GBaud PDM-QPSK and 30-GBaud PDM-16QAM signals over a 100-m 2 × 2 wireless MIMO link at 320 GHz with BER below 3.8 × 10-3 and 1.56 × 10-2 for QPSK and 16QAM signals, respectively. Compared with the MIMO-Volterra, our MIMO-CVNN has an advantage in terms of calculation complexity and decision accuracy due to its effective handling of phase information and inter-polarization relationships simultaneously.

One-Shot Blind Frequency-Domain Nonlinear Equalization for Millimeter-Wave Fiber-Wireless IFoF Mobile Fronthaul System

Nonlinear equalization (NE) plays a pivotal role in the analog intermediate-frequency over fiber (IFoF) mobile fronthaul (MFH) system demanding a high signal fidelity. This work proposes and experimentally validates a novel blind frequency-domain NE (FD-NE) approach for millimeter-wave (MMW) fiber-wireless (FiWi) IFoF MFH systems. This proposed blind FD-NE scheme relies on the FD two-block cascade Hammerstein NE structure and particular pairs of M-ary phase shift keying (MPSK) and M-ary amplitude shift keying (MASK) subcarriers. It features lower computational complexity, higher transmission capacity, and comparable equalization performances to the widely-employed training-assisted memory polynomial (MP) NE schemes. Moreover, it only requires a small fraction of subcarriers in a single received signal block to be modulation-format constrained, which renders one-shot blind learning of NE coefficients and thus achieves symbol-by-symbol channel tracking. The performance of the proposed NE method is experimentally evaluated in MMW FiWi IFoF MFH systems operating at 5G NR frequency range 2-1 (FR2-1) n258 band (24.25 GHz to 27.50 GHz) and FR2-2 (52.6 GHz to 71 GHz). The obtained experimental results show that our proposal can effectively improve the nonlinearity tolerances of coherent 25-GHz and non-coherent 60-GHz FiWi IFoF MFH systems transmitting multiple 64QAM/256QAM IF OFDM signals.

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

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.

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%.

Deep-learning-enabled high-performance full-field direct detection with dispersion diversity.

Data center interconnects require cost-effective photonic integrated optical transceivers to meet the ever-increasing capacity demands. Compared with a coherent transmission system, a complex-valued double-sideband (CV-DSB) direct detection (DD) system can minimize the cost of the photonic circuit, since it replaces two stable narrow-linewidth lasers with only a low-cost un-cooled laser in the transmitter while maintaining a similar spectral efficiency. In the carrier-assisted DD system, the carrier power accounts for a large proportion of the total optical signal power. Reducing the carrier to signal power ratio (CSPR) can improve the information-bearing signal power and thus the achievable system performance. To date, the minimum required CSPR is ∼7 dB for all the reported CV-DSB DD systems having electrical bandwidths of approximately half of baud rates. In this paper, we propose a deep-learning-enabled DD (DLEDD) scheme to recover the full optical field of the transmitted signal at a low CSPR of 2 dB in experiment. Our proposal is based on a dispersion-diversity receiver with an electrical bandwidth of ∼61.0% baud rate and a high tolerance to laser wavelength drift. A deep convolutional neural network enables accurate signal recovery in the presence of a strong signal-signal beat interference. Compared with the conventional method, the proposed DLEDD scheme can reduce the optimum CSPR by ∼8 dB, leading to a significant signal-to-noise ratio improvement of ∼5.8 dB according to simulation results. We experimentally demonstrate the optical field reconstruction for a 28-GBaud 16-ary quadrature amplitude modulation signal after 80-km single-mode fiber transmission based on the proposed DLEDD scheme with a 2-dB optimum CSPR. The results show that the proposed DLEDD scheme could offer a high-performance solution for cost-sensitive applications such as data center interconnects, metro networks, and mobile fronthaul systems.

Channel Estimation Based on Complex-Valued Neural Networks in IM/DD FBMC/OQAM Transmission System

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) is a promising candidate for 5G mobile fronthaul system. Due to the inherent imaginary interference of FBMC/OQAM signals, an accurate channel estimation is particularly indispensable. In this paper, an efficient method is proposed based on complex-valued neural networks (CVNN) for an accurate channel estimation of FBMC/OQAM signals with low computational complexity and pilot overhead. Here, the channel frequency response (CFR) is first calculated for training the CVNN. Next, the CFR estimated from the extracted pilots is exploited as the input of the trained CVNN to yield the CFR of data symbols. We experimentally demonstrate a 12.5-GBd intensity modulation direct detection (IM/DD) FBMC/OQAM transmission system over 30-km and 50-km standard single mode fibers (SSMF). The experimental results show that the proposed method achieves 3-dB and 1-dB receiver sensitivity improvement at the bit error ratio (BER) of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> with only 5% pilot overhead respectively, compared to the conventional least square (LS) and linear minimum mean error (LMMSE) methods. When the pilot overhead decreases from 10% to 1%, its BER performance is always better than LS and LMMSE. For the computational complexity, its complexity is the same order of magnitude as LS and lower than LMMSE. On the other hand, the CVNN requires less hidden neurons to keep the similar BER performance as real-valued neural networks (RVNN). Meanwhile, it has excellent resilience to fiber chromatic dispersion over 30-km and 50-km SSMF.

800-MHz Bandwidth Signal Transmission with Radio over Multi-Mode-Fiber for Cascaded IFoF-Based C-RAN Mobile Fronthaul

Millimeter-wave band signals have the advantage of a high capacity and have been used in fifth-generation (5G) mobile services. An analog radio over multi-mode fiber (A-RoMMF) employing a directly modulated vertical cavity surface emitting laser is a promising technology for relaying millimeter-wave band signals to its dead area because of the advantages of low latency, low cost, and low power consumption. One of the applications of A-RoMMF is a cascaded intermediate-frequency-over-fiber (IFoF)-based centralized radio access network (C-RAN) mobile fronthaul (MFH) system. We conducted two studies on A-RoMMF to extend the transmission distance and bandwidth to adapt it to the system. First, we evaluated the effect of employing a single-mode vertical cavity surface emitting laser (VCSEL) and a multi-mode VCSEL on the transmission characteristics of A-RoMMF. Next, we evaluated the transmission characteristics of A-RoMMF using a cost-efficient bias-tee consisting of an electrical circuit pattern, the frequency response of which has a slope inverse to that of A-RoMMF at the 28-GHz band, to suppress the channel power difference of two component carrier signals for a wide-bandwidth signal transmission. We conducted these evaluations using A-RoMMF itself and herein demonstrate its adaption to a cascaded IFoF-based C-RAN MFH system at the 28-GHz band. When employing it with a 200-m long multi-mode fiber, the system achieves 400-MHz/CC and two component carrier signal transmissions.

Transition technologies towards 6G networks

The sixth generation (6G) mobile systems will create new markets, services, and industries making possible a plethora of new opportunities and solutions. Commercially successful rollouts will involve scaling enabling technologies, such as cloud radio access networks, virtualization, and artificial intelligence. This paper addresses the principal technologies in the transition towards next generation mobile networks. The convergence of 6G key-performance indicators along with evaluation methodologies and use cases are also addressed. Free-space optics, Terahertz systems, photonic integrated circuits, softwarization, massive multiple-input multiple-output signaling, and multi-core fibers, are among the technologies identified and discussed. Finally, some of these technologies are showcased in an experimental demonstration of a mobile fronthaul system based on millimeter 5G NR OFDM signaling compliant with 3GPP Rel. 15. The signals are generated by a bespoke 5G baseband unit and transmitted through both a 10 km prototype multi-core fiber and 4 m wireless V-band link using a pair of directional 60 GHz antennas with 10° beamwidth. Results shown that the 5G and beyond fronthaul system can successfully transmit signals with both wide bandwidth (up to 800 MHz) and fully centralized signal processing. As a result, this system can support large capacity and accommodate several simultaneous users as a key candidate for next generation mobile networks. Thus, these technologies will be needed for fully integrated, heterogeneous solutions to benefit from hardware commoditization and softwarization. They will ensure the ultimate user experience, while also anticipating the quality-of-service demands that future applications and services will put on 6G networks.

Cascaded IF-Over-Fiber Links With Hybrid Signal Processing for Analog Mobile Fronthaul

We present an intermediate frequency-over-fiber (IFoF) transmission system that uses cascaded connections between a broadband link and multiple narrowband links for future mobile fronthaul (MFH). The downlink MFH system employs analog and digital signal processing after broadband and narrowband IFoF transmissions, respectively, for extractions and frequency conversions of IF signals. The bandwidth of the optical components for the downlink MFH system is investigated and compared with that for the subcarrier multiplexed passive optical network (SCM-PON). The results support the satisfactory performance of the MFH system for bandwidth reduction. Two types of our developed digital signal processors for the MFH system are also described in detail: One is for an output of a single radio frequency (RF) stream, whereas the other is for simultaneous outputs of multiple RF streams. In addition, using either of the digital signal processors, we experimentally demonstrate the downlink MFH system's transmission of 64-QAM filtered OFDM (f-OFDM) signals with 360-MHz signal bandwidth and 3.6-MHz guard bands between adjacent IF signals and 64-QAM OFDM signals with 380.16-MHz signal bandwidth and 19.84-MHz guard bands between IF signals. In both experiments, eighteen IF signals are successfully transmitted over 20-km and 1-km fibers, which includes analog and real-time digital signal processing for IF channel extractions and frequency conversions. The results obtained in this study demonstrate the potential of cascaded IFoF links using hybrid signal processing for 5G and future MFH systems.

Proactive real-time interference avoidance in a 5G millimeter-wave over fiber mobile fronthaul using SARSA reinforcement learning

We propose and experimentally demonstrate a proactive real-time interference avoidance scheme using SARSA reinforcement learning (RL) in a millimeter-wave over a fiber mobile fronthaul system. The RL consists of three core factors, including state, action, and reward. The state is defined as a discretized value from the center frequency, the left, right, and center sub-EVM of the signal. We use five actions to shift the signal frequency in the proposed scheme, which is -20, -10, 0, 10, and 20MHz, for the RL agent to choose the action to avoid the dynamic interference. For the agent to learn from the experience, the reward is defined as the log value of BER difference between the past and the present state. The RL-based approach is an online learning algorithm, which can learn in real time based on environmental feedback. Besides, the agent can learn from past experience owing to the Q-value table, which makes it act intelligently when facing a similar situation again. We verify the capability of the proposed scheme under both fixed and dynamic interference scenarios. The agent demonstrates an efficient intelligent mechanism to avoid the interference, which provides a promising solution for proactive interference mitigation in the 5G mobile fronthaul network.

Non-iterative blind linearization algorithm for DML-based multi-IF-over-fiber mobile fronthaul systems.

We propose and experimentally demonstrate a novel digital post-linearization algorithm for the mobile fronthaul system employing bandwidth-efficient multiple intermediate frequency-over-fiber technology and a low-cost directly-modulated laser (DML). The proposed linearization algorithm is distinguished for blind adaption, the absence of training signal, or prior knowledge of the transmission channel. Particularly, it provides a closed-form solution for the estimation of optimal linearizer coefficients, leading to a non-iterative mode without convergence restrictions. In our experiments, one and nine 200MHz 64-QAM OFDM IF signals (or channels) are transmitted over a 10km single-mode fiber link using a DML. In both cases (one and nine IF channels), the linearization performance of the proposed algorithm is experimentally validated in terms of the reduction (>8 dB) of the adjacent channel leakage ratio, decrease (>10 dB) of error vector magnitude (EVM), and an increase (>9 dB) of EVM-compliant input RF power range.

Multi-IF-Over-Fiber Based Mobile Fronthaul With Blind Linearization and Flexible Dispersion Induced Bandwidth Penalty Mitigation

The nonlinear degradations and chromatic dispersion induced bandwidth penalty are critical obstacles for implementing the analog radio-over-fiber (A-RoF) technique in future mobile fronthaul (MFH). In this paper, we propose an analog multiple intermediate-frequency-over-fiber (multi-IFoF) MFH system with blind linearity improvement and flexible dispersion-induced bandwidth penalty mitigation. The bias adjustment of a single-electrode driven dual-drive Mach–Zehnder modulator is used to compensate the bandwidth penalty arising from the dispersion-induced power fading of fiber links in different lengths. This operation can increase the available bandwidth for the MFH system free of even-order intermodulation distortions (IMDs). At the receiver, a self-adaptive digital post-processing algorithm is introduced to blindly improve the linearity of the IFoF MFH system by suppressing the residual odd-order IMDs, with no need of training signals or prior knowledge of the transmission channel. In the proof-of-concept experiments, the transmission of 64-QAM-OFDM signals in 1 to 12 IF channels with a 500-MHz bandwidth has been demonstrated over standard single-mode fibers (SSMFs) of 20, 50, and 100 km. For the 50-km (or 100-km) SSMF, the bandwidth penalty for a broadband signal with a bandwidth of 4.5 GHz (or 2.5 GHz) is fully compensated. In addition, the proposed post-processing algorithm can significantly suppress the odd-order IMDs and thus improve the linearity by greater than 5.4 dB in terms of the input RF power range which retains an acceptable error-vector-magnitude (EVM) performance for the 64-QAM-OFDM signal.

Seamless Convergence of Fiber and Wireless Systems for 5G and Beyond Networks

We propose different fronthaul systems for facilitating future mobile networks based on the seamless convergence of fiber-optic and wireless systems in the millimeter-wave (mmWave) bands. First, a flexible and high-performance wireless fronthaul system is proposed through an encapsulation of radio signals onto a converged fiber–mmWave system. A simultaneous transmission of three radio signals over the system is successfully demonstrated. Second, a high-performance optical self-heterodyne system is proposed and demonstrated for the generation and transmission of radio access signals in high-frequency bands. Third, a high-spectral-efficiency optical fronthaul system for the simultaneous transmission of multiple radio signals in different frequency bands is proposed using a subcarrier-multiplexing intermediate-frequency-over-fiber system. Satisfactory performance is experimentally confirmed for the transmission of three different radio signals in the microwave and low- and high-mmWave bands. The proposed systems can overcome the challenges and bottlenecks of the current mobile fronthaul systems and can be useful in different usage scenarios of 5G and beyond networks.

A Novel ANN Equalizer to Mitigate Nonlinear Interference in Analog-RoF Mobile Fronthaul

We first analyze the nonlinearities in the analog radio-over-fiber (A-RoF)-based mobile fronthaul systems with a multi-user uplink load. The inter-band cross modulations (XMs) present in the common fiber channel cause severe inter-user interference. Then, we propose a novel complex-valued multi-level artificial neural network nonlinear equalizer (ANN-NLE) for single-carrier frequency division multiple access signal uplink transmissions. The inter-user interference can be mitigated by employing a multi-user ANN-NLE. Finally, we design an experimental test bed comprising a 60-GHz wireless link and an A-RoF link in order to verify the model predictions and thereby demonstrate the ability of the ANN-NLE to overcome the nonlinear intra-band and inter-band XMs in the case of simultaneous joint equalizations of all users (multi-point-to-single-point) as well as in the case of individual user equalizations (point-to-point). From the experimental and simulation results, we demonstrate that the ANN-NLE can substantially reduce the inter-user interference and communication bit error.

High-fidelity and low-latency mobile fronthaul based on segment-wise TDM and MIMO-interleaved arraying.

In this paper, we firstly demonstrate an advanced arraying scheme in the TDM-based analog mobile fronthaul system to enhance the signal fidelity, in which the segment of the antenna carrier signal (AxC) with an appropriate length is served as the granularity for TDM aggregation. Without introducing extra processing, the entire system can be realized by simple DSP. The theoretical analysis is presented to verify the feasibility of this scheme, and to evaluate its effectiveness, the experiment with ~7-GHz bandwidth and 20 8 × 8 MIMO group signals are conducted. Results show that the segment-wise TDM is completely compatible with the MIMO-interleaved arraying, which is employed in an existing TDM scheme to improve the bandwidth efficiency. Moreover, compared to the existing TDM schemes, our scheme can not only satisfy the latency requirement of 5G but also significantly reduce the multiplexed signal bandwidth, hence providing higher signal fidelity in the bandwidth-limited fronthaul system. The experimental result of EVM verifies that 256-QAM is supportable using the segment-wise TDM arraying with only 250-ns latency, while with the ordinary TDM arraying, only 64-QAM is bearable.

Bit-based support vector machine nonlinear detector for millimeter-wave radio-over-fiber mobile fronthaul systems.

An effective bit-based support vector machine (SVM) is proposed as a non-parameter nonlinear mitigation approach in the millimeter-wave radio-over-fiber (RoF) mobile fronthaul (MFH) system for various modulation formats. First, we analyze the impairments originated from nonlinearities in the millimeter-wave RoF system. Then we introduce the operation principle of the bit-based SVM detector. As a classifier, the SVM can create nonlinear decision boundaries by kernel function to mitigate the distortions caused by both linear and nonlinear noise. In our design, SVM can learn and capture the link characteristics from only a few training data without requiring the prior estimation of the system link. The bit-based SVM only needs log2M SVMs to detect the signal of M-order modulation format. Experimental results have been obtained to verify the feasibility of the proposed method. The sensitivities are improved by 1.2-dB for 16-QAM, 1.3-dB for 64-QAM, 1.8-dB for 16-APSK and 1.3-dB for 32-APSK at BER = 1E-3 with SVM detector, respectively. The proposed bit-based SVM gains a large improvement in the nonlinear system tolerance and outperforms the system employing k-means algorithm.

Polarization-Tracking-Free IFoF Mobile Fronthaul With Adaptively Modulated PDM Multiband DDO-OFDM

This letter proposes a polarization-tracking-free polarization division multiplexing (PDM) in an intermediate frequency over fiber mobile fronthaul system based on multiband direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) with adaptive modulation. The use of two orthogonally polarized optical carriers with a simple optical filter at each radio access unit is sufficient to perform passive demultiplexing of the corresponding multiband signals in PDM schemes without transmission efficiency loss in training signal, which is commonly found in conventional PDM demultiplexing schemes. We experimentally achieved a multiband OFDM signal with adaptive modulation, which maximizes signal capacity, in a PDM scheme with the data rate of 49.64 Gb/s, which doubles that of corresponding single polarization schemes, over a 25-km single mode fiber. We also achieved a 6-dB power margin which is sufficient for the wireless transmission part.

Bidirectional Fiber-Wireless Access Technology for 5G Mobile Spectral Aggregation and Cell Densification

Empowered by spectral aggregation and cell densification, future 5G mobile data networks pose a huge challenge to building next-generation mobile fronthaul systems with higher capacity, scalability, and energy efficiency. Under this circumstance, traditional solutions based on the Common Public Radio Interface or channel aggregation with digital signal processing (DSP) are not sufficient to support heterogeneous ubiquitous wireless access. In this paper, we demonstrate a point-to-multipoint bidirectional mobile fronthaul system. Wavelength division multiplexing plus frequency division multiplexing is applied to support independent asynchronous small cells. Intensity-modulation and direct-detection downlink (DL) as well as field-modulation and heterodyne-detection uplink (UL) are proposed. Combined with efficient virtual tone-based DSP for phase recovery and carrier-frequency-offset estimation, signal degradations from beating among incoherent asynchronous UL signals are mitigated. Proof-of-concept experiments are demonstrated where 20 and 16 80-MHz component carriers are transmitted over 25-km standard single-mode fiber for DL and UL, respectively. Less than 6% error vector magnitudes with 64-, 16-, and 4-ary quadrature amplitude modulation are obtained.