Structured light, especially beams carrying orbital angular momentum (OAM), has gained much interest due to its unique amplitude and phase structures. In terms of communication systems, multiple orthogonal OAM beams can be potentially utilized for increasing link capacity in different scenarios. This review describes challenges, advances, and perspectives on different aspects of the OAM-based optical communications, including (a) OAM generation/detection and (de)multiplexing, (b) classical free-space optical communication links, (c) fiber-based communication links, (d) quantum communication links, (e) OAM-based communications in different frequency ranges, (f) OAM-based communications using integrated devices, and (g) novel structured beams for communications.

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Orbital angular momentum of light for communications

We report high-capacity intermediate-frequency-over-fiber (IFoF) transmission using parallel intensity/phase modulators (IM/PM). Thanks to the parallel IM/PM transmitter, we can avoid all the null frequencies caused by dispersion-induced RF power fading. Compared to other techniques to avoid RF power fading, the parallel IM/PM transmitter offers a simple solution because it does not rely on any sophisticated digital signal processing. We transmitted 14 $\times$ 1.2-GHz orthogonal-frequency-division-multiplexed signals over a 20-km single-mode fiber using the parallel IM/PM transmitter and achieved a Common Public Radio Interface equivalent data rate of 1.032 Tb/s. Thus, an IFoF system with the parallel IM/PM transmitter shows great potential to be applied to high-capacity mobile fronthaul links in the upcoming fifth-generation mobile system and beyond.

1.032-Tb/s CPRI-Equivalent Rate IF-Over-Fiber Transmission Using a Parallel IM/PM Transmitter for High-Capacity Mobile Fronthaul Links

mmWave radio, although instrumental for achieving the required 5G capacity KPIs, necessitates the need for a very large number of access points, which places an immense strain on the current network infrastructure. In this article, we try to identify the major challenges that inhibit the design of the Next Generation Fronthaul Interface in two upcoming distinctively highly dense environments: in Urban 5G deployments in metropolitan areas, and in ultra-dense Hotspot scenarios. Second, we propose a novel centralized and converged analog Fiber-Wireless Fronthaul architecture, specifically designed to facilitate mmWave access in the above scenarios. The proposed architecture leverages optical transceivers, optical add/drop multiplexers and optical beamforming integrated photonics towards a Digital Signal Processing analog fronthaul. The functional administration of the fronthaul infrastructure is achieved by means of a packetized Medium Transparent Dynamic Bandwidth Allocation protocol. Preliminary results show that the protocol can facilitate Gb/s-enabled data transport while abiding to the 5G low-latency KPIs in various network traffic conditions.

Next Generation Fiber-Wireless Fronthaul for 5G mmWave Networks

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Laser Diode Modulation And Noise

In this  study, we demonstrate the intermediate-frequency-over-fiber (IFoF)  based mobile fronthaul technology implemented in a 28-GHz millimeter-wave (mmWave) based 5G prototype supporting Giga-bit mobile broadband services. In order to confirm the technical feasibility of the proposed IFoF-based mobile fronthaul, the clock signal for frequency synchronization and mobile payload data are simultaneously transmitted in the form of IF carriers. Phase noise and error vector magnitude measurements were performed on separate and simultaneous clock transmission paths, respectively. The results confirm an acceptable impact on performance, and the simultaneous clock transmission method reduces system costs and complexity, and improves the flexibility and efficiency of wavelength operation. We also successfully demonstrate the operation of real-time Giga-bit mobile services such as 4K video streaming over IFoF-based mobile fronthaul for 5G prototypes with a 28-GHz mmWave. The achieved peak data-rate per user is up to 1.5 Gb/s, which complies with the requirements of the 5G vision of IMT-2020.

Demonstration of IFoF-Based Mobile Fronthaul in 5G Prototype With 28-GHz Millimeter wave

We investigate the feasibility of implementing a mobile fronthaul network (MFN) based on the radio-over-fiber (RoF) technology for the fifth generation wireless communication systems cost effectively by using either directly modulated lasers (DMLs) or electroabsorption modulated lasers (EMLs) operating in the 1.55- μ m window. The results show that the performance of the RoF-based MFN implemented by using DMLs is primarily limited by the composite second-order (CSO) distortions arising from the interplay between the DML's adiabatic chirp and fiber's chromatic dispersion. Thus, when we implement the RoF-based MFN by using the currently available commercial DML, its reach could be limited to ∼5 km due to the CSO distortions. To increase this reach, we can utilize the DML having a very small adiabatic chirp (or the optical dispersion compensation or CSO cancelation technique). On the other hand, when we implement the RoF-based MFN by using EML, its performance is limited mostly by the relatively poor linearity and low output power of EML rather than the chirp. For example, if we increase the optical modulation index to compensate for the low output power of EML, the distortions caused by the EML's nonlinear transfer curve can also be increased. Thus, for the use in the RoF-based MFN, it would be highly desirable to increase the output power of EML as much as possible. For a demonstration, we successfully transmit 24 100-MHz-bandwidth filtered orthogonal-frequency-division-multiplexing signals over 20 km of the standard single-mode fiber by using an EML transmitter with 7-dBm output power.

RoF-Based Mobile Fronthaul Networks Implemented by Using DML and EML for 5G Wireless Communication Systems

We explore the possibility of utilizing 1.55- $\mu \text{m}$ directly modulated lasers (DMLs) for optical access networks operating at 28 Gb/s per wavelength. For this purpose, we attempt to maximize the power budgets of 28-Gb/s duobinary and four-level pulse amplitude modulation (PAM-4) signals generated from a DML without using optical amplifiers or optical dispersion compensation modules. For experimental investigation, we set the bias current of the DML far above the threshold current, optimize the extinction ratio of the signals, and utilize the electrical equalization at the receiver. The experimental results show that we can secure power margins of 9.6 and 10 dB after transmission over 40-km long standard single-mode fiber for the duobinary and PAM-4 signals, respectively.

Transmission of 28-Gb/s Duobinary and PAM-4 Signals Using DML for Optical Access Network

We demonstrate the transmission of 51.56-Gb/s on-off keying (OOK) signals generated by using a 1.55-μm directly modulated laser (DML) over 15-km long standard single-mode fiber. In this experiment, a duobinary electrical equalizer based on a finite-impulse-response filter is used at the receiver to increase the dispersion-limited transmission distance. We evaluate the performances of the 51.56-Gb/s OOK signals with respect to the transmission distance by using the frequency response analysis of the proposed system. This result is used to explain why it is effective to utilize the duobinary equalization (instead of binary equalization) for increasing the transmission distance.

Transmission of 51.56-Gb/s OOK signal using 1.55-μm directly modulated laser and duobinary electrical equalizer.

We propose and demonstrate a simple composite second-order (CSO) cancellation technique based on the digital signal processing (DSP) for the radio-over-fiber (RoF) transmission system implemented by using directly modulated lasers (DMLs). When the RoF transmission system is implemented by using DMLs, its performance could be limited by the CSO distortions caused by the interplay between the DML’s chirp and fiber’s chromatic dispersion. We present the theoretical analysis of these nonlinear distortions and show that they can be suppressed at the receiver by using a simple DSP. To verify the effectiveness of the proposed technique, we demonstrate the transmission of twenty-four 100-MHz filtered orthogonal frequency-division multiplexing (f-OFDM) signals in 64 quadrature amplitude modulation (QAM) format over 20 km of the standard single-mode fiber (SSMF). The results show that, by using the proposed technique, we can suppress the CSO distortion components by >10 dB and achieve the error-vector magnitude performance better than 6% even after the 20-km long SSMF transmission.

DSP-based CSO cancellation technique for RoF transmission system implemented by using directly modulated laser.

We demonstrate the transmission of 56-Gb/s PAM-4 signal over 20-km long SSMF by using a 1.55-μm DML without optical dispersion compensation. Instead, a linear electric equalizer is used for the compensation of dispersion-induced waveform distortions.

Sung Hyun Bae

Papers

RoF-Based Mobile Fronthaul Networks Implemented by Using DML and EML for 5G Wireless Communication Systems

Transmission of 28-Gb/s Duobinary and PAM-4 Signals Using DML for Optical Access Network

Transmission of 51.56-Gb/s OOK signal using 1.55-μm directly modulated laser and duobinary electrical equalizer.

DSP-based CSO cancellation technique for RoF transmission system implemented by using directly modulated laser.

Transmission of 56-Gb/s PAM-4 signal over 20 km of SSMF using a 1.55-μm directly-modulated laser