Radio-over-fiber Signal Research Articles

Non-Orthogonal Uplink Services Through Co-Transport of D-RoF/A-RoF in Mobile Fronthaul

To efficiently support multiple services with diverse requirements in data capacity, latency, and connectivity in the fifth-generation (5G) communication system and beyond, various mobile fronthaul solutions, including digital and analog radio-over-fiber (RoF), will be considered. We propose a non-orthogonal multiplexing scheme for multiservice uplink transmissions in mobile fronthaul, where in-band analog RoF (A-RoF) signals can be transmitted with digital RoF (D-RoF) signals simultaneously, resulting in high spectrum resource utilization. The demultiplexing is fulfilled by exploiting different characteristics of the D-RoF and A-RoF signals in the time-domain and it can be implemented with analog signal processing, providing low processing complexity and processing delay. The proposed scheme is experimentally demonstrated to achieve the joint transmission of the D-RoF and A-RoF signals over a shared mobile fronthaul through 20-km standard single mode fiber (SSMF). Experimental results show that in-band A-RoF signals from 2-10 GHz can be co-transmitted with D-RoF 10 GBaud NRZ-OOK signals, with the signal-to-interference-plus-noise ratio (SINR) enhanced by the demultiplexing module. Both received D-RoF and A-RoF signals can achieve bit error ratio (BER) lower than the 7% hard-decision forward-error correction (FEC) threshold.

Integrated Silicon Photonics Remote Radio Frontend (RRF) for Single-Sideband (SSB) Millimeter-Wave Radio-Over-Fiber (ROF) Systems

Radio-over-fiber (ROF)-based mobile fronthaul network supporting the baseband unit and simple remote radio head is a promising network architecture for future wireless networks. In this paper, we propose and demonstrate a silicon photonics (SiPh)-based remote radio frontend (RRF) for the mm-wave ROF systems. The proposed SiPh-based RRF consisted of an integrated Mach-Zehnder modulator and a micro-ring modulator, producing a single-sideband 40 GHz millimeter-wave orthogonal-frequency-division-multiplexing ROF signal, which is robust against fiber chromatic dispersion in the mobile fronthaul transmission. The RRF is fabricated using SiPh platform and could be potentially low cost. High split-ratios can be potentially achieved. Data rate of 7.813 Gbit/s per wavelength channel is achieved in the experiment. Numerical analysis is also performed; there are good matches of the simulation and experimental results.

Multi-dimensional resources allocation based on reconfigurable radio-wavelength selective switch in cloud radio over fiber networks.

To support the bandwidth-hungry applications and ubiquitous services, cloud radio access network (C-RAN) provides the large bandwidth connecting hundreds of terminals for different demands. Applying the technology of radio over fiber (RoF), cloud radio over fiber network (C-RoFN) migrates the processing and calculation functions of the base station to processing unit (PU) which simplifies the system complexity and reduces the data flood caused by digitization in 5G and beyond. The flexible allocation of multi-dimensional resources including radio and optical resources is necessary for C-RoFN to enhance the resource utilization and the quality of services. However, there is a lack of effective method to adjust spectral resources occupied by the RoF signal in C-RoFN. In our previous work, we realized multi-stratum resources optimization between processing and optical resources for C-RoFN from the perspective of network layer. In view of this, this work extends to consider the elastic switching architecture in the physical layer of C-RoFN and the scheduling of radio and optical spectral resources based on the proposed architecture. We propose a novel flexible switching and scheduling mechanism for multi-dimensional resources allocation in software defined C-RoFN. The main contributions on this work are twofold. 1) According to the flexible control demand of multi-dimensional resources, a reconfigurable radio-wavelength selective switch (RWSS) architecture is designed in a C-RoFN scenario to realize the tunability of spectral resources occupied by the RoF signal in optical network layer, which aims to break the restrictions of wavelength consistency and wavelength conflict. 2) Based on the proposed RWSS architecture, a routing, radio and wavelength assignment (RRWA) algorithm considering wavelength-frequency unsaturation is provided to implement the flexibility and allocation of radio and optical spectral resources. The feasibility and efficiency of the architecture are demonstrated on the testbed including date plane using software defined optical and wireless nodes experimentally. The performances of RRWA algorithm under heavy traffic load scenario are also quantitatively evaluated compared with the other algorithm in terms of resource utilization rate, blocking probability, and provisioning latency.

Evaluation of Radio Resource Management Impact on RoF Signal Transmission for Downlink LTE

Analog radio-over-fiber (RoF) is a suitable technology for efficiently developing the cloud-RAN concept in fifth generation deployments based on long-term evolution advanced (LTE-A). Distortion and radiofrequency (RF) power amplifier gain should be taken into account in order to achieve the desired transmission power at the base station. Both mainly depend on several parameters that characterize the optical link, such as the RF input power and bias current. This paper analyzes the link performance due to variations on the traffic load as expected under real operation. In this paper, we show that it is possible in this scenario to adaptively choose the optimum RF input power to minimize the measured error vector magnitude and reduce the RF subsystem gain. Results show that this adaptation allows to relax the RF amplifier gain requirements up to 3-4 dB for medium load conditions.

Photonics-Based Millimeter-Wave Band Remote Beamforming of Array-Antenna Integrated With Photodiode Using Variable Optical Delay Line and Attenuator

For simplification of mobile base station architecture, we present remote beamforming of an array antenna integrated with photodiode based on photonics. Variable optical delay lines (VDLs) and variable optical attenuators (VOAs) are utilized for tailoring the RF phases and amplitudes and radio-over-fiber (RoF) technique is adopted for RF signal transmission. First, we show a simple demonstration of 60-GHz antenna beamforming using VDLs and VOAs with the aim of simplification of RoF signal generation at a control site. Next, we present experimental results on 60-GHz band digital signal transmission at 7 and 14 Gbit/s with beamforming function, in which eight compact antenna modules are arrayed in a row. Finally, we investigate RoF signal generation at 40 GHz utilizing the 1.3- μ m band electroabsorption modulator-integrated laser diode, where an optical filter and an optical amplifier are used to boost the modulation component and the successful formation of various beam patterns by tuning VDL and VOA is shown.

Upgradable radio-over-fiber transport system

An upgradable radio-over-fiber (RoF) transport system is proposed based on a vertical-cavity surface-emitting laser (VCSEL)-composed wavelength selector. The passband window of the wavelength selector can be electrically adjusted to select a specific lightwave from multiple injected optical carriers and attenuate the others. By adding the VCSEL-composed wavelength selector installed inside each base station (BS), the proposed transport system can remain as the original tree topology, but its overall network capacity can be flexibly extended (reduced) by adding (removing) an additional optical carrier into (out of) the optical network. Each BS can dynamically receive data from one of the multiple injected optical carriers. This method is useful because the bandwidth requirement of each BS, which is serviced by an RoF transport system, may significantly change with the visitor flow rate. Experimental results prove that the BSs in the proposed upgradable RoF transport system can be reallocated to multiple logical RoF links and can be dynamically switched to receive data from any one of the logical RoF signals without modifying the original network structure. The transmission performances of these optical carriers are ensured by low bit error rate, clear eye, and optical/electrical spectra diagrams.

Simple and reconfigured single-sideband OFDM RoF system.

We propose a simple and reconfigured dispersion-tolerant single sideband (SSB) orthogonal frequency division multiplexing (OFDM) radio over fiber (RoF) system enabled by digital signal processing (DSP), one in-phase/quadrature (I/Q) modulator and direct-detection. The generated radio frequency (RF) is based on DSP and the frequency can be flexibly adjusted, which can be employed in the future software-defined radio access network (RAN). Based on our proposed system, we have experimentally demonstrated 16-ary quadrature amplitude modulation (16QAM) 21.87-Gb/s 21-GHz and 38-GHz SSB-OFDM RoF signal generation and transmission over 80-km single-mode fiber (SMF), respectively.

Method of joint bit rate/modulation format identification and optical performance monitoring using asynchronous delay-tap sampling for radio-over-fiber systems

In the context of carrying a wide variety of modulation formats and data rates for home networks, the study covers the radio-over-fiber (RoF) technology, where the need for an alternative way of management, automated fault diagnosis, and formats identification is expressed. Also, RoF signals in an optical link are impaired by various linear and nonlinear effects including chromatic dispersion, polarization mode dispersion, amplified spontaneous emission noise, and so on. Hence, for this purpose, we investigated the sampling method based on asynchronous delay-tap sampling in conjunction with a cross-correlation function for the joint bit rate/modulation format identification and optical performance monitoring. Three modulation formats with different data rates are used to demonstrate the validity of this technique, where the identification accuracy and the monitoring ranges reached high values.

A High Spectral Efficiency Coherent RoF System Based on OSSB Modulation With Low-Cost Free-Running Laser Sources for UDWDM-PONs

A coherent radio over fiber (RoF) system based on optical single-sideband modulation with no optical carrier (OSSB) for ultradense wavelength division multiplexing passive optical networks (UDWDM-PONs) is proposed and experimentally demonstrated. For one channel of the UDWDM-PONs, at the transmitter, the OSSB RoF signal is generated using a dual parallel Mach–Zehnder modulator (DP-MZM) with a real-valued precoded microwave vector signal which is applied to the DP-MZM via the two electrodes with one having a 90° phase shift. Then, the OSSB RoF signal is sent to a coherent receiver over a single-mode fiber (SMF). The coherent receiver with a free-running laser source as a local oscillator (LO) is used to perform the coherent detection. To recover the standard microwave vector signals from the real-valued precoded microwave vector signal that is embedded in a strong phase noise introduced by both the transmitter laser source and the LO laser source, a digital signal processing algorithm is developed to perform phase noise cancellation. An experiment is performed. The transmission of a 1.25-Gbps quadrature phase shift keying, a 1.875-Gbps eight-phase shift keying (8-PSK), and a 2.5-Gbps 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a channel spacing as narrow as 3 GHz over a 25-km SMF is experimentally demonstrated. For the transmission of the 2.5-Gbps 16-QAM microwave vector signal, the received optical sensitivity at forward error correction level over a 25-km SMF link is −24.8 dBm, while for the 1.875-Gbps 8-PSK microwave vector signal, it can reach −29.8 dBm which leaves an enough margin to accommodate the splitting losses in the optical distribution networks.

FTTH and Two-Band RoF Transport Systems Based on an Optical Carrier and Colorless Wavelength Separators

A hybrid fiber-to-the-home (FTTH) and radio-over-fiber (RoF) transport system is proposed to transmit one wired signal and two wireless signals simultaneously through an optical lightwave. By properly utilizing the polarization characteristic of a phase-remodulated and polarization-remodulated optical lightwave, the first- and second-order sidebands of the lightwave can be colorlessly separated for optical double- and quadruple-frequency RoF transmissions. Experimental results prove that the independent FTTH and two-band RoF signals can be simultaneously generated and separated by relatively low-frequency radio-frequency devices, optical modulators, and polarization beam splitters. The transmission performances are proven by proper bit error rate (BER) performance and clear eye and constellation diagrams. The interference among the transmitted wired signal and double- and quadruple-frequency RoF signals is acceptable. In this case, the wavelength of the optical carrier can be flexibly assigned, and the frequency of the transmitted wireless signals can be easily adjusted without affecting the functionality of the developed optical fiber transport system. The presented architecture can be flexibly employed to support various types of FTTH and microwave and millimeter-wave over fiber transmissions.

Generación de señales para sistemas radio sobre fibra basados en combinación óptica

This paper presents a comparative study of the signal generation techniques for the transport of information in Radio over Fiber (RoF) systems by using optical combination techniques. In these systems, the radio frequency carriers are conveyed as optical subcarriers over a link in a point-to-point or point-to-multipoint connectivity paradigm. This study is focused on three different generation techniques: double arm, parallel and serial generation. An analytical model to describe the elements, the configuration and the processes involved in the RoF signal generation is defined for each one of them. Subsequently, a modeling of the obtained electrical field functions allows assessing the quality of the generated signals based on the modulation depth. Results show that while there is a trade-off between the modulation depths of both signals for the double arm and serial configuration, the parallel technique relaxes the establishment of different modulation depths. An optimum configuration that guarantees a good quality for the generated signals is found for each generation technique.

Laser-phase-fluctuation-insensitive offset-frequency-spaced two-tone optical coherent detection scheme and its transmission for radio-over-fiber systems

We propose a new optical coherent detection scheme with a "two-tone" local light, which is also in principle insensitive to the laser phase fluctuation and is assisted by the digital signal processing technique for radio-over-fiber (RoF) systems. The main feature of our proposal is that the frequency separation of the two-tone local light is different from that of the RoF signal, which is called "offset-frequency-spaced" in this paper. First, we explain the principle of our new proposal and experimentally demonstrate the data recovery. Then, the influence of the frequency detuning between photo-detected modulated and unmodulated signals is discussed. Moreover, the transmission performance with an error vector magnitude (EVM) is evaluated for the optical coherent detection of a 10-Gbaud quadrature-phase-shift-keying RoF signal after a 20-km-long standard single-mode fiber transmission.

Polarization multiplexing of two MIMO RoF signals and one baseband signal over a single wavelength

Next-generation (NG) access networks require simultaneous provision of wired and wireless services and high data rates to meet the large demands of mobility and multiple services. In this paper, we propose a novel spectral efficient radio over fiber (RoF) scheme to simultaneously provide two spatially multiplexed multiple input multiple output (MIMO) wireless signals with a baseband (BB) wired signal in one wavelength using a centralized light source. The proposed scheme can be applicable to wavelength division multiplexed passive optical networks (WDM-PONs). The BB signal is modulated at a low extinction ratio (ER). The modulated light is re-used to modulate two MIMO signals that have the same carrier frequency that is combined optically using polarization-division-multiplexing (PDM). The data rate for each MIMO stream was 1.25Gb/s, and the data rate was 2.5Gb/s for the BB signal. Error free performance with a bit error rate (BER) of 10−9 was achieved for all three signals after 20km and 60km through single mode fiber (SMF) for 16-QAM and 4-QAM for the MIMO signals, respectively.

Simultaneous generation of 3G and millimeter-wave signals using a dual-electrode MZM in ROF systems

A novel radio-over-fiber (ROF) scheme to simultaneously generate and transmit the 3rd generation telecommunication (3G) and millimeter-wave (MMW) signals by using a single dual-electrode Mach-Zehnder modulator (MZM) is proposed. There is no apparent nonlinearity induced by the ROF system. By employing this analog ROF signal transmission technique, highly transparent fiber-wireless convergence networks can be realized, which are ideal for multi-standard wireless system operation.

MMW Signal Transmission in Coherent MIMO RoF System With Adaptive-Coded Spreading

A $2\times 2$ multiple-input multiple-output (MIMO) coherent radio over fiber (RoF) system with adaptive-coded spreading (ACS) is proposed and experimentally demonstrated. The spreading gain can be dynamically adjusted according to the cell configuration and channel condition. It can provide a flexible power budget and a signal to noise adaption for the system. In the experiment, the $2\times 2$ MIMO RoF signal with ACS is demonstrated to have good adaption in different wireless distances and transmission fiber lengths.

Simultaneous generation of wavelength division multiplexing PON and RoF signals using a hybrid mode-locked laser

The use of millimeter-wave (mm-wave) frequencies has been proposed to overcome the imminent saturation of the ultra high frequency band, justifying research on radio over fiber (RoF) networks as an inexpensive and green solution to distribute multi-Gbps signals. Coincidently, telecommunication operators are investing a significant effort to deploy their passive optical network (PON) infrastructure closer to the users. In this work, we present a novel cost-efficient architecture based on a hybrid mode locked laser capable to simultaneously generate up-to 5 wavelength division multiplexing PON and RoF channels, being compatible with the 50-GHz ITU frequency grid. We analyze the limits of operation of our proposed architecture considering the high modal relative intensity noise induced by mode partition noise, as well as fiber impairments, such as chromatic dispersion and nonlinearities. The feasibility of generation and transmission of 5×10-Gbps PON and 5×5-Gbps RoF using orthogonal frequency division multiplexing up to 50km has been demonstrated through realistic numerical simulations.

Passive Optical Network Supporting Seamless Integration of RoF and OFDMA Signals

Radio over Fiber (RoF) Technology is integration of radio emission in optical fibre transmission at intervals network infrastructures that's thought of to be cost-efficient, sensible and comparatively versatile system configuration for long-haul transport wireless signals. This project propose a Next generation PON design supports RoF and OFDMA signal integration while not WDM lasers, and demonstrate that 10-Gb/s OFDMA and 3 RF signals at two.1GHz area unit with success transmitted over 20km SMF during a 32-ONU in each upstream and downstream direction.

Laser-Phase-Fluctuation-Insensitive Optical Coherent Detection Scheme for Radio-over-Fiber System

Laser-phase-fluctuation-insensitive optical coherent detection scheme with two-tone local light and digital signal processing technique for a radio-over-fiber (RoF) signal is proposed and experimentally demonstrated. Data recovery with the error vector magnitude of less than 14% rms is successfully performed for 10- and 2.5-Gbaud 25-GHz-band QPSK RoF signals. In addition, bit error rate of less than 10$^{-5}$ is also verified for 10-Gbaud 25-GHz-band QPSK RoF signal after 20-km fiber-optic transmission even if the light source with the linewidth of more than 10 MHz in the transmitter is used.

Bidirectional hybrid PM-based RoF and VCSEL-based VLLC system.

A bidirectional hybrid phase modulation (PM)-based radio-over-fiber (RoF) and vertical cavity surface emitting laser (VCSEL)-based visible laser light communication (VLLC) systems employing injection-locked VCSEL-based PM-to-intensity modulation (IM) converters and optical interleavers (ILs) is proposed and demonstrated. To be the first one of using injection-locked VCSEL-based PM-to-IM converters and optical ILs in such bidirectional hybrid RoF and VLLC systems, the downstream light is successfully phase-remodulated with RoF signal for up-link transmission. Through a serious investigation in systems, bit error rate (BER) and eye diagram perform brilliantly over a 40-km single-mode fiber (SMF) transport and a 12-m free-space transmission. Such a bidirectional hybrid RoF and VLLC system would be very attractive for the integration of fiber backbone and in-door networks to provide broadband integrated services, including Internet and telecommunication services.

Simultaneous generation of independent WCDMA and millimeter-wave signals using a dual-electrode MZM in ROF systems

In this paper, a novel radio-over-fiber (ROF) scheme to simultaneously generate and transmit wideband code division multiple access (WCDMA) and millimeter-wave (MMW) signals by using a single dual-electrode Mach-Zehnder modulator (MZM) is proposed. There is no apparent nonlinearity induced by the ROF system. By employing this analog ROF signal transmission technique, the highly transparent fiber-wireless networks, which are ideal for multi-standard wireless system operation, can be realized.