Intermediate-frequency-over-fiber Research Articles

Outdoor mm-wave 5G/6G transmission with adaptive analog beamforming and IFoF fronthaul

Adaptive analog beamforming is a key technology to enable spatial control of millimeter-wave wireless signals radiated from phased array antennas (PAAs) which is essential to maximize the capacity of future mobile networks and to ensure efficient usage of scarce spectrum. Intermediate frequency-over-fiber (IFoF), on the other hand, is a promising technology for the millimeter-wave (mm-wave) mobile fronthaul due to its low complexity, high optical spectral efficiency, and low latency. The combination of IFoF and PAA is key to implement mm-wave mobile communications in a scalable, centralized, efficient, and reliable manner. This work presents, for the first time to the best of the authors’ knowledge, an extensive outdoor measurement campaign where an experimental IFoF mm-wave wireless setup is evaluated by using PAAs with adaptive beamforming on the transmitter and receiver sides. The configuration of the experimental setup is according to 5G standards, transmitting signals wirelessly at 27 GHz central frequency in the n258 band. The employed PAAs are composed of 8-by-8 patch antenna arrays, allowing beam steering in the azimuth and elevation angles. Furthermore, different end-user locations, antenna configurations, and wireless scenarios are tested in the outdoor experiment, showing excellent EVM performance and achieving 64-QAM transmission over up to 165.5 m at up to 1.88 Gbit/s. The experimental results enable optimization of the experimental setup for different scenarios and prove the system’s reliability in different wireless conditions. In addition, the results of this work prove the viability and potential of IFoF combined with PAA to be part of the future 5G/6G structure.

CMOS Linear Laser Driver for Intermediate Frequency over Fiber (IFoF) Links

The main objective of the proposed linear laser driver (LLD) is to reduce signal distortion in an analog direct modulation laser configuration used for intermediate frequency over fiber links. This work draws on an open-loop configuration featuring two differential pair blocks in a cascade arrangement to achieve a bandwidth measurement of 415 MHz at the half-power point, a total harmonic distortion of 4.57% for a fundamental frequency of 100 MHz, and an amplitude of 100 mVpp. The LLD provides a gain of 12.3 dB for a differential output and an output impedance of 46 Ω. The design, layout, and integration correspond to the process design kit for TSMC 65-nm CMOS technology. Experimental results show the advantage over other previously reported laser drivers.

10.51-Tbit/s multi-channel IF-over-fiber transmission with SDM/WDM/SCM for beyond-5G mobile fronthaul accommodating ultra-high-density antennas

In beyond-5G and 6G mobile communication systems, it is expected that massive numbers of antennas are densely deployed to provide a stable and high-speed communication environment. To efficiently accommodate high-density antennas, high-capacity mobile fronthaul (MFH) is also required. Intermediate-frequency-over-fiber (IFoF) is considered as one of the attractive solutions for realizing high-capacity MFH due to its high spectral efficiency and multi-channel transmission capability. In this paper, we demonstrate the highest-capacity IFoF transmission with combinations of space division multiplexing, wavelength division multiplexing, and subcarrier multiplexing; 4608 channels of 64-quadrature amplitude modulation orthogonal frequency division multiplexing signals with a total net bit rate of 10.51 Tbit/s (4608×380.16MHz/ch×6bit/s/Hz) are successfully transmitted over a 10.1-km uncoupled 12-core multi-core fiber.

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.

A 262-GHz Wireless IFoF Uplink With Remote Down-Conversion Using Optically Generated Sub-THz LO

We experimentally demonstrate the feasibility of a 262-GHz wireless intermediate frequency (IF)-over-fiber (IFoF) uplink with frequency down-conversion at a remote antenna unit (RAU). To distribute many RAUs for the coverage of terahertz (THz) wireless communications, it is important to simplify the RAU for the THz uplink. The remote down-conversion with a subharmonic mixer and a uni-traveling carrier-photodiode (UTC-PD) is driven by two optical tone signals from the central office (CO). This can reduce the high-frequency and wideband devices of the networks. Instead of coherent detection, the RAU can be simplified with optical transmission and direct detection (DD) by using a conventional optical modulator and a photodiode (PD). We measured the phase noise of the system and compensated for it with blind phase recovery. Then, the error vector magnitudes (EVMs) of the received signals were measured as a function of THz Tx power, optically generated local oscillator (LO) power, and THz wireless link distance. Single-channel 45-and 30-Gb/s data transmission was achieved over a 5-km fiber and 1-m THz wireless link based on soft-(2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-2}$</tex-math> </inline-formula> ) and hard-decision (3.8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-3}$</tex-math> </inline-formula> ) forward error correction (FEC) thresholds, respectively.

End-to-End Real-Time Service Provisioning Over a SDN-Controllable Analog mmWave Fiber -Wireless 5G X-Haul Network

The current work presents the first experimental demonstration of real-time Ethernet (ETH) trial services and 4K-Ultra High Definition (UHD) video application transmission over a 2λ Wavelength Division Multiplexing (WDM) analog Fiber-Wireless (FiWi) mmWave X-haul network, supporting dynamic flexible capacity allocation through an integrated silicon photonic 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) and 60 GHz wireless transmission across a 7 m V-band link distance, while controlled by a Software Defined Network (SDN) controller based on Open Daylight. Analog X-haul transport of the radio signals is based on an Intermediate Frequency over Fiber (IFoF) scheme centered at 1.5 GHz using an Orthogonal Frequency Division Multiplexing (OFDM) radio-waveform with a bandwidth of 204 MHz, generated by a Field Programmable Gated Array (FPGA)-based RF System-on-Chip (SoC) processor that converts on-the-fly the real-time ETH downlink traffic to analog radio and vice-versa for the uplink. Dynamic allocation of the X-haul traffic capacity is handled through the use of the 802.1Q Virtual Local Area Network (VLAN) tag-mechanism, which controls the forwarding operation to the proper DAC and InP EML for optical modulation using Intensity Modulation/Direct Detection (IM/DD) schemes, while the wavelength routing operation is handled by the low-loss four-port 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 featuring only 2.5 dB fiber-to-fiber losses based on a cascaded MZI-interleaver design on the TriPleX platform, routing the real-time traffic to a second mmWave antenna site. Detailed measurements and traffic statistics indicate end-to-end latency of less than 260 μs and a packet loss less than 0.0054% across a dynamic range of at least 6.5 dB in the optical domain, while the high user bandwidth and signal quality are validated by an uninterrupted 4K-UHD video transmission across the FiWi X-haul mmWave transport network. The current work aims to shape a complete technology roadmap for Point-to-Multipoint FiWi transport network architectures with high spectral efficiency X-haul transport for dense areas and 5G/6G hotspots of future mmWave Centralized Radio Access Networks (C-RANs).

Multiple beam-steering for 5G multi-user MIMO mobile fronthaul based on IFoF and RoF transmission

This paper describes the potential of analog radio-over-fiber (RoF) and intermediate-frequency-over-fiber (IFoF) transmission technology for the future mobile fronthaul (MFH). The background and approach of the analog RoF-based MFH combined with the beamforming technique are presented. We developed an antenna module by integrating sixteen sets of cascaded photodiode, RF amplifier and a patch antenna as a compact wireless transmitter. Three millimeter-wave (MMW) beams were successfully transmitted from the antenna module with beam separation of 30° by photonic controlled beam steering. The experimental results are presented to show the feasibility of a multi-user multiple-input multiple-output (MIMO) scenario.

IF-Over-Fiber Transmission of OFDM Quantum-Noise Randomized PSK Cipher for Physical Layer Encryption of Wireless Signals

This study proposes an intensity modulation/direct detection intermediate frequency-over-fiber (IFoF) system that generates a microwave orthogonal frequency-division multiplexing (OFDM) quantum-noise randomized phase-shift-keying (PSK) cipher. The system directly prevents wireless interception by an eavesdropper without a private key. We theoretically analyze the system and derive a fundamental tradeoff between the security and signal quality at an intermediate frequency (IF). The system achieves balanced security and signal quality. We experimentally demonstrate 4.09-Gbit/s PSK cipher generation at an IF of 3.6 GHz. The cipher has 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> phase levels, and more than 100 adjacent signals are masked by quantum noise. The quantum-noise signal masking imposes irreducible uncertainty on illegitimate reception of the cipher, and a symbol error ratio (SER) that an eavesdropper can reach approaches 1. A lower bound of the SER is promised because truly random signal masking by quantum noise cannot be reduced. The error vector magnitude of the IF signals after decryption is maintained at a low value for wireless transmission. The cipher directly and effectively protects the wireless transmission of personal data from interception.

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.

Reconfigurable Fiber Wireless IFoF Fronthaul With 60 GHz Phased Array Antenna and Silicon Photonic ROADM for 5G mmWave C-RANs

We demonstrate experimentally a bandwidth-reconfigurable mmWave Fiber Wireless (FiWi) fronthaul bus topology for spectrally efficient and flexibly reconfigurable 5G Centralized-Radio Access Networks (C-RAN). The proposed fronthaul architecture includes four 1 Gb/s Intermediate Frequency over Fiber (IFoF) channels that can be flexibly allocated among two in-series Reconfigurable Optical Add/Drop Multiplexer (ROADM) integrated nodes, supporting in total 8 V-band 32-element Phased Array Antenna (PAA) terminals. The ROADM was fabricated as an integrated photonic device exploiting the ultra-low loss 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> TriPleX waveguide integration platform and an architectural layout based on cascaded MZI interleavers. The device has flat top response of 32.5 GHz with a Free Spectral Range (FSR) of 100 GHz and fiber-to-fiber losses of 5 dB, while the V-band PAA supports analog RF beamsteering capabilities within a 90°-sector and 1m wireless distance. Each of the FiWi links carries a 250 MBd QAM16 waveform enabling a total of 1 Gb/s rate per end user beam, complying with the 5G Key Performance Indicator (KPI) user-rate requirement. Bandwidth-reconfigurability is experimentally demonstrated by selectively dropping channels either at the first or at the second ROADM node, allowing in this way the bandwidth allocation to be flexibly defined between two different network segments. Both uplink and downlink performance are experimentally validated for different ratios of bandwidth allocation among the two nodes, revealing Error Vector Magnitude (EVM) values that meet the respective 3GPP signal quality specifications. The two-stage FiWi IFoF/mmWave fronthaul bus topology, based on a miniaturized, integrated, low loss 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 and supporting high-capacity wireless beamsteering capability can form a promising roadmap towards flexible and reconfigurable 5G C-RAN architectures.

Radio over Fiber: An Alternative Broadband Network Technology for IoT

Wireless broadband access networks have been positioning themselves as a good solution for manufacturers and users of IoT (internet of things) devices, due mainly to the high data transfer rate required over terminal devices without restriction of information format. In this work, a review of two Radio over Fiber strategies is presented. Both have excellent performance and even offer the possibility to extend wireless area coverage where mobile networks do not reach, or the 802.11 network presents issues. Radio Frequency over Fiber (RFoF) and intermediate Frequency over Fiber (IFoF) are two transmission strategies compatible with the required new broadband services and both play a key role in the design of the next generation integrated optical–wireless networks, such as 5G and Satcom networks, including on RAU (Remote Antenna Unit) new functionalities to improve their physical dimensions, employing a microelectronic layout over nanometric technologies.

Nonlinear Distortion Mitigation in Multi-IF over Fiber Transmission Using Modulation-Based Adaptive Power Allocation

We propose a modulation-based adaptive power allocation (MBAPA) technique for nonlinear distortion mitigation in intermediate frequency over fiber (IFoF) systems. The technique allocates the spectral power of each IF band according to the required signal-to-noise power ratio (SNR) of the modulation format. To demonstrate the performance of the technique, transmission experiments were performed in 10 km and 20 km with 24-IF bands using OFDM signals. The feasibility of the proposed MBAPA technique was experimentally verified by reducing inter-modulation distortion (IMD) power and enhancing channel linearity.

Radio over Fiber: An Alternative Broadband Network Technology for IoT

Wireless broadband access networks have been positioning themselves as a good solution for manufacturers and users of IoT (internet of things) devices, due mainly to the high data transfer rate required over terminal devices without restriction of information format. In this work, a review of two Radio over Fiber strategies is presented. Both have excellent performance and even offer the possibility to extend wireless area coverage where mobile networks do not reach or the 802.11 network presents issues. Radio Frequency over Fiber (RFoF) and intermediate Frequency over Fiber (IFoF) are two transmission strategies compatible with the required new broadband services and both play a key role in the design of the next generation integrated optical–wireless networks, such as 5G and Satcom networks, including on RAU (Remote Antenna Unit) new functionalities to improve their physical dimensions, employing a microelectronic layout over nanometric technologies.

A 5G Fiber Wireless 4Gb/s WDM Fronthaul for Flexible 360° Coverage in V-Band massive MIMO Small Cells

Towards relieving the bandwidth limitations in 5G mobile fronthaul networks, a Fiber Wireless (FiWi) small cell architecture is presented, relying on spectrally efficient analog transport on intermediate frequency over fiber (IFoF) and a millimeter wave (mmWave) phased array antenna (PAA) interfaced with a low-loss optical add/drop multiplexer (OADM) on 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> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> TriPleX platform with 5 dB fiber-to-fiber losses. Specifically, four 1 Gb/s FiWi links, each carrying a 250 MBd 16-QAM signal on 5.8 GHz IFoF, are wavelength division multiplexed (WDM) and transported across a 10 km Single Mode Fiber (SMF), before being demultiplexed into the constituent channels in the OADM device and subsequently wirelessly transmitted by the PAA over a 1 m-long V-band link with 90° degree beamsteering capability. Featuring 4x 1Gb/s user rate with beamsteering to meet the respective 5G key performance indicator (KPI) for the peak user rate and an EVM within the acceptable 3GPP limit of 12.5%, the current work forms the first centralized FiWi point-to-multipoint small cell architecture with efficient transport scheme on IFoF and ubiquitous 360°-degree coverage for emerging 5G mmWave networks.

Massively Distributed Antenna Systems With Nonideal Optical Fiber Fronthauls: A Promising Technology for 6G Wireless Communication Systems

Employing massively distributed antennas brings radio access points (RAPs) closer to users, enabling aggressive spectrum reuse that can bridge gaps between the scarce spectrum resource and extremely high connection densities in future wireless systems. Examples include the cloud radio access network (C-RAN), ultradense network (UDN), and cell-free massive multiple-input, multiple-output (CF-mMIMO) systems. These systems are usually designed in the form of fiber wireless communications (FWC), where distributed antennas or RAPs are connected to a central unit (CU) through optical fronthauls. A large number of densely deployed antennas or RAPs require an extensive infrastructure of optical fronthauls. Consequently, the cost, complexity, and power consumption of the network of optical fronthauls may dominate the performance of the entire system. This article provides an overview and outlook on the architecture, modeling, design, and performance of massively distributed antenna systems (DAS) with nonideal optical fronthauls. Complex interactions between optical fronthauls and wireless access links require optimum designs across the optical and wireless domains by jointly exploiting their unique characteristics. It is demonstrated that systems with analog radio-frequency-overfiber (RFoF) links outperform their baseband-overfiber (BBoF) or intermediate-frequency-overfiber (IFoF) counterparts for systems with shorte fiber length and more RAPs, which are all desired properties for future wireless communication systems.

Multi-Carrier 5G-Compliant DML-Based Transmission Enhanced by Bit and Power Loading

The introduction of 5G enhanced mobile broadband (eMBB) services has brought unprecedented bit-rate demands to the optical transport infrastructure, which can hardly be supported through cost-effective digital fronthauling solutions. This opens up a whole new opportunity for bandwidth-efficient radio-over-fiber (RoF) analog fronthaul transmission, which avoids the well-known bandwidth multiplication issue associated with the digitization of radio signals. Using a low-cost directly-modulated laser (DML) with less than 3 GHz bandwidth and a standard PIN photodetector, we demonstrate the transmission of a carrier-aggregated 5G downlink signal over up to 25 km of single-mode fiber (SMF). Resorting to the use of intermediate-frequency-over-fiber (IFoF), we enable the transmission of 5G-compatible signals composed of up to 12 aggregated 400 MHz component carriers (CCs), resulting in a total radio bandwidth of 4.8 GHz, corresponding to >300 Gbps CPRI-equivalent downlink data-rate. 5G-compliant EVM performance across all CCs is achieved through the optimization of power and bit loading between component carriers, enabling an aggregated end user data-rate of 15.6 Gbps.

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.

Coverage Extension of Indoor 5G Network Using RoF-Based Distributed Antenna System

We propose and demonstrate a cascaded distributed antenna system (DAS) for efficiently extending the coverage of the mmWave-based 5G indoor network. For this, we exploit the radio-over-fiber (RoF) system based on the intermediate-frequency-over-fiber (IFoF) transmission technique that is enabled to add/drop the specific wavelength to the designated remote antenna unit (RAU) with using optical splitters and coarse wavelength division multiplexing (CWDM) filters. Moreover, the IFoF transceivers (TRx) perform the subcarrier multiplexing (SCM) in order to transmit 2 × 8 frequency allocation (FA) 5G signals per a single optical carrier, where each FA has 100 MHz bandwidth, leading each RAU to support 2 × 2 MIMO operation. Consequently, the cascaded structure allows for the adaptive and flexible configuration of the order of MIMO in accordance of the required data throughput at the specific indoor area. We introduce the cascaded IFoF link structure that can support up-to 13.5 dB optical power budget with following errorvector-magnitude (EVM) performance characterizations. And then we experimentally demonstrate the RoFbased cascaded DAS network, showing that more than 1 Gb/s total throughput can be achieved per a single antenna. Furthermore, we examine the use of avalanche photodiode (APD) to further increase the optical power budget (i.e., the coverage) based on experiment as well as simulation.

DAPSK-OFDM-based coherent PON and IFoF heterogeneous access network with PDM

Heterogeneous access networks with wired/wireless convergence comprise an important trend in the development of access networks because they reduce network investment and operating costs. In this paper, we propose a heterogeneous access network with a coherent passive optical network (PON) and intermediate frequency over fiber (IFoF) within a single wavelength. The coherent PON and IFoF are both DAPSK-OFDM-based and do not involve noise-compensation digital signal processing (DSP), which lowers bandwidth efficiency. DAPSK-OFDM is a differential modulation scheme that eliminates channel and phase noise during the demodulation process. The coherent PON and IFoF with different transmission schemes are combined using polarization division multiplexing (PDM) and a complete heterogeneous network without crosstalk through polarization optimization and maintenance at a transmission distance less than 20 km. The proposed heterogeneous network is experimentally tested, which shows that its performance is similar to that yielded when operating each system independently.

A 5G C-RAN Optical Fronthaul Architecture for Hotspot Areas Using OFDM-Based Analog IFoF Waveforms

Analog fronthauling is currently promoted as a bandwidth and energy-efficient solution that can meet the requirements of the Fifth Generation (5G) vision for low latency, high data rates and energy efficiency. In this paper, we propose an analog optical fronthaul 5G architecture, fully aligned with the emerging Centralized-Radio Access Network (C-RAN) concept. The proposed architecture exploits the wavelength division multiplexing (WDM) technique and multicarrier intermediate-frequency-over-fiber (IFoF) signal generation per wavelength in order to satisfy the demanding needs of hotspot areas. Particularly, the fronthaul link employs photonic integrated circuit (PIC)-based WDM optical transmitters (Txs) at the baseband unit (BBU), while novel reconfigurable optical add-drop multiplexers (ROADMs) cascaded in an optical bus are used at the remote radio head (RRH) site, to facilitate reconfigurable wavelength switching functionalities up to 4 wavelengths. An aggregate capacity of 96 Gb/s has been reported by exploiting two WDM links carrying multi-IF band orthogonal frequency division multiplexing (OFDM) signals at a baud rate of 0.5 Gbd with sub-carrier (SC) modulation of 64-QAM. All signals exhibited error vector magnitude (EVM) values within the acceptable 3rd Generation Partnership Project (3GPP) limits of 8%. The longest reach to place the BBU away from the hotspot was also investigated, revealing acceptable EVM performance for fiber lengths up to 4.8 km.