Intermediate-frequency-over-fiber Research Articles

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.

RoF-Based Radio Access Network for 5G Mobile Communication Systems in 28 GHz Millimeter-Wave

In this study, we report the successful demonstration of an intermediate-frequency-over-fiber (IFoF)–based radio access network (RAN) for 28 GHz millimeter-wave (mmWave)-based 5G mobile communication. In order to increase the network coverage of the mmWave-based 5G networks, we propose a distributed antenna system (DAS) that uses the IFoF technology. An IFoF-based DAS with 2 × 2 multiple-input multiple-output (MIMO) configuration was deployed in the PyeongChang area to provide 5G trial demonstration during the Winter Olympics. 5G trial services such as high-speed data transfer and autonomous vehicle driving were offered to the public through the IFoF-based DAS. A downlink throughput of ∼1 Gb/s and uplink throughput of ∼200 Mb/s were achieved in the DAS-deployed area. We also present an IFoF-based 5G mobile fronthaul that can overcome the bandwidth bottleneck in RANs. We performed real-time transmission of mmWave-based 5G wireless access networks using the IFoF-based mobile fronthaul. The real-time downlink throughput achieved per 5G terminal was approximately 9 Gb/s, when using a 4 × 4 MIMO configuration. An outdoor demonstration was performed to verify the technical feasibility of the 5G fronthaul based on IFoF technology. When moving the 5G terminal between remote radio heads at a speed less than 60 km/h, 5G mobile broadband services could be provided with real-time throughput more than 5 Gb/s. Thus, we confirmed that the IFoF technology was capable of supporting RANs for mmWave-based 5G networks and providing real-time multi-Gb mobile services.

MIMO-Supporting Radio-Over-Fiber System and its Application in mmWave-Based Indoor 5G Mobile Network

We demonstrate 4 × 4 multiple-input and multiple-output (MIMO) supporting mmWave-based indoor 5G mobile network based on a distributed antenna system (DAS) that has over-the-air interface to receive the broadband wireless signal from, e.g., remote radio head (RRH) of mobile fronthaul. To realize the cost/bandwidth-efficient and low-latency-inducing indoor 5G DAS network, we exploit the radio-over-fiber (RoF) system that is based on the intermediate frequency-over-fiber (IFoF) technology. To be more specific, we make simultaneous use of wavelength- and frequency-division multiplexing in the IFoF link so as to transport 4 × 4 MIMO 5G signal of which the bandwidth is effectively 3.2 GHz (= 4 × 800 MHz, where each antenna deals with 800 MHz 5G signal). The 4 × 4 MIMO-supporting IFoF link has error vector magnitude (EVM) performance of <4.5% at all channels of downlink and uplink for up to 2 km. The whole RoF system, exploiting the IFoF link, mmWave frequency conversion module, and control /management plane, proves itself to meet the 3GPP-defined EVM requirement (i.e., 8%) at all channels. Based on the RoF system, we demonstrate the indoor 5G DAS network in conjunction with Korea Telecom 5G mobile network, i.e., 5G baseband unit and RRH, investigating the downlink and uplink throughputs. We achieve ∼4 Gb/s total throughput for end user, where the additional latency induced by the DAS network is only a few hundreds of nanoseconds other than the single mode fiber transmission delay.

Multi-IF-Over-Fiber System With Adaptive Frequency Transmit Diversity for High Capacity Mobile Fronthaul

In this paper, we present and experimentally demonstrate a broadband intermediate frequency-over-fiber (IFoF) system with adaptive frequency transmit diversity for the next generation mobile fronthaul (MFH) applications. We propose to employ an integrated dual-parallel Mach-Zehnder Modulator (DPMZM) with adaptive dc-bias voltage setting for a simplified, efficient, and flexible IFoF communication system. The DPMZM is utilized to compensate the chromatic dispersion (CD) induced deep RF power fading impact, by taking advantage of the frequency transmit diversity of the two parallel modulators and optical paths. Using probe signals and the supervised machine learning (ML) technique, an accurate frequency response of the system is generated and utilized to design a dynamic allocation of IF channels to each DPMZM port. This ML-based link frequency response can capture the accumulated CD-induced power fading frequency region, as well as the random degradation caused by the used RF and optical devices characteristics and bandwidth limitation. Applying our proposed system, we successfully transmitted 15 IF channels with 1.2 GHz bandwidth and 64-QAM-OFDM modulated signals over 20-km fiber length, leading to about 1.18 Tbps CPRI-equivalent data rate. If we assume the maximum spectral efficiency of the LTE-A systems (i.e., 3.75 bit/s/Hz), the user throughput can be higher than 67 Gb/s. The obtained results show the potentials of DPMZM-based multi-IFoF system to satisfy the high capacity requirement of 5G mobile network and beyond.

The Room Temperature Multi-Channel Heterodyne Receiver Section of the PHAROS2 Phased Array Feed

This paper describes the design, fabrication, and test results of a room temperature multi-channel heterodyne receiver operating across the 2.3–8.2 GHz radio frequency (RF) band. Such a “Warm Section” (WS) receiver is part of phased arrays for reflector observing systems 2 (PHAROS2), a C-band phased array feed (PAF) demonstrator with digital beamformer for radio astronomy application. The WS receiver is cascaded to the PHAROS2 cryostat, which includes an array of Vivaldi antennas with low noise pre-amplification stages. The WS can handle up to 32 RF signals and, for each of them, realizes the operations of filtering, RF amplification and down-conversion from the RF to the 375–650 MHz intermediate frequency (IF). Also, the WS incorporates an IF-to-optical signal conversion through analogue wavelength division multiplexing IF over fiber (IFoF) and fiber-optic transmitters (OTXs). The 32-channel WS receiver consists of four eight-channel WS RF/IF modules, one local oscillator (LO) splitter module and one monitoring and control module, all hosted in a standard 6U × 19-inch rack.

5G Trial Services Demonstration: IFoF-Based Distributed Antenna System in 28 GHz Millimeter-Wave Supporting Gigabit Mobile Services

In this paper, we demonstrate fifth-generation (5G) trial services using intermediate frequency-over-fiber (IFoF) based distributed antenna system (DAS) implemented in a 28-GHz millimeter-wave (mmWave) during Pyeongchang winter olympics. To extend mmWave-based 5G wireless coverage, IFoF-based DAS with 2 × 2 multiple-input multiple-output configuration is presented. Error vector magnitude (EVM) measurements of the 5G signal on the IF and mmWave bands are performed for both downlink and uplink transmissions over 5-km single mode fiber. The experimental results confirm that the minimum requirement for 5G signals, defined by the technical specification of the 3G partnership project, are satisfied by EVMs less than 9%. In addition, the developed IFoF-based DAS that supports 5G mobile service using 28-GHz mmWave was deployed in the city of Pyeongchang, during winter olympics. An ultra-high speed 5G data service was offered to the general public for 5G trial demonstration. The reference signals received power and data throughput of the 5G handset are measured while moving in a DAS section and 5G connected car section. We confirm that the downlink throughput of ∼1 Gb/s and uplink throughput of ∼200 Mb/s are achieved for the 5G trial service area in which an IFoF-based DAS is deployed. We also successfully demonstrate the 5G connected car service, which is one of the 5G key services, through deployed IFoF-based DAS. At the intersection of baseband unit and DAS deployed sections, a seamless handover is performed to smoothly support 5G-based autonomous driving service.

Delta-sigma-modulated IFoF transmission system assisted by a correlative-level encoding technique.

A delta-sigma-modulated intermediate-frequency-over-fiber (IFoF) transmission system assisted by a correlative-level coding technique is proposed and experimentally demonstrated. Unlike conventional delta-sigma IFoF systems with multiple output levels to achieve higher signal quality or larger capacity, a correlative-level encoder is exploited as a second modulator preceded by the delta-sigma modulator. The encoder compresses the bandwidth of the delta-sigma modulated signal by creating a correlation between adjacent signal symbols. As a result, the sampling frequency of the delta-sigma modulator in the proposed system can be increased beyond the transmission bandwidth of the IFoF system, considerably improving the in-band signal quality and the transmission capacity over the conventional multi-level approach. This is because the quantization noise from the delta-sigma modulation in the proposed scheme is more aggressively pushed away from the signal bandwidth with the high sampling frequency. According to experimental results, the proposed link provides at least a 40% larger transmission capacity for similar in-band signal quality or 2.1% better average EVM performance for the same capacity than the conventional four-level pulse-amplitude-modulation delta-sigma IFoF systems.

4 × 4 MIMO architecture supporting IFoF-based analog indoor distributed antenna system for 5G mobile communications.

We demonstrate the analog indoor distributed antenna system (DAS) for 5G mobile communications that supports 4 × 4 multiple-input multiple-output (MIMO) configuration. For this, we exploit a pair of intermediate frequency over fiber (IFoF)-based analog optical links, transporting 32 frequency allocation (FA) 5G mobile signals (effectively ~4 GHz bandwidth). The analog optical link manifests its high fidelity in the measured characteristics: small gain variation (< ± 1 dB for the entire transmission bandwidth), low noise (<-136 dBm/Hz), and large dynamic range (spurious free dynamic range of >106 dB∙Hz2/3), subsequently providing superior error vector magnitude (EVM) performance (~2%) for a wide range of ambient temperatures (-20 ~60°C). Consequently, the IFoF-based 4 × 4 MIMO supporting analog indoor DAS is capable of providing record high peak throughput of 5.3 Gb/s for millimeter wave based 5G mobile communication system.

Millimeter-Wave-Based Fiber-Wireless Bridge System for 8K UHD Video Streaming and Gigabit Ethernet Data Connectivity

In this paper, we introduce an end-to-end and real-time seamless fiber-wireless (Fi-Wi) bridge system, for gigabit Ethernet standard data connectivity and 8K standard video streaming. The proposed end-to-end Fi-Wi bridge system consists of seamless integration of 2 × 20-km intermediate frequency-over-fiber (IFoF) and 96-GHz millimeter-wave (mmWave) wireless links. Using our system, zero packet loss of 36 compressed 8K standard videos streaming is successfully achieved for different values of mmWave attenuation levels, ranging from 35 to 55 dB. We also demonstrate the transmission of Ethernet frames at 5-Gbps capacity on the Fi-Wi bridge with 20 meters mmWave wireless link, where 8% error vector magnitude can be attained. We then investigate and evaluate the degradation caused by the IFoF and mmWave links characteristics on the overall system performance. Furthermore, we estimate analytically the attainable mmWave transmission range for different weather conditions. The obtained results demonstrate the potentials of our proposed Fi-Wi bridge systems to provide high data and reliable transmission for areas lacking fiber infrastructure and for live broadcasting relay of 8K based TV programs.

Broadband IF-Over-Fiber Transmission With Parallel IM/PM Transmitter Overcoming Dispersion-Induced RF Power Fading for High-Capacity Mobile Fronthaul Links

We demonstrate a broadband and long-distance intermediate frequency-over-fiber (IFoF) transmission scheme employing a transmitter composed of parallel intensity/phase (IM/PM) modulators with appropriate bandwidth allocations to IM and PM. Due to the proposed scheme, we can eliminate all the null frequencies caused by dispersion-induced RF power fading, which, in turn, enables us to significantly increase the available bandwidth. In addition, our system does not require any synchronization between IM and PM, which reduces complexity compared to conventional parallel transmitter architecture. We successfully transmitted 20 × 360 MHz filtered orthogonal-frequency-division-multiplexed signals corresponding to a common public radio interface equivalent data rate of 524.28 Gbps over a 30- and 40-km single-mode fiber satisfying the 8% threshold for the error-vector magnitude values for all the subcarriers. These results show that our proposed IFoF transmission scheme is scalable to long-distance mobile fronthaul links for 5G 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

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.

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

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.

Investigation of transmission performance in multi-IFoF based mobile fronthaul with dispersion-induced intermixing noise mitigation.

We demonstrate the improvement of the transmission performance based on intermixing noise mitigation techniques in a multiple intermediate-frequency-over-fiber (IFoF) based mobile fronthaul. The interaction between fiber chromatic dispersion and frequency chirp of the directly modulated laser generates the second-order distortion that degrades the performance of multi-IFoF transmission system. To avoid second-order distortion, we use intermediate frequency (IF) spacing optimization and octave-confined frequency plan schemes in which intermixing noise would be generated in the out of signal band and would not affect the quality of transmitted signal. For bandwidth efficient transmission of radio signal over mobile fronthaul link, we employ the dispersion compensation technique to suppress the intermixing noise sufficiently. For realization of the multi-IFoF based mobile fronthaul, we experimentally investigate the transmission performances of 48-, 72- and 144-IF carriers of the long term evolution-advanced (LTE-A) signals mapped with 64-quadrature amplitude modulation (QAM). It is clearly observed that the intermixing noise is suppressed owing to dispersion compensation technique and overall system performances are improved by IF spacing optimization and octave-confined frequency plan. As a result, we successfully transmit 144-IF carriers of the LTE-A signal with less than 8% error vector magnitude (EVM) over 20-km single-mode fiber (SMF) within only 3 GHz bandwidth.

Improvement of the transmission performance in multi-IF-over-fiber mobile fronthaul by using tone-reservation technique

We demonstrate the improvement of the transmission performance based on tone-reservation technique in a multiple intermediate-frequency-over-fiber (IFoF) based mobile fronthaul. The tone-reservation technique can suppress nonlinear distortion by eliminating the high peak components of orthogonal frequency-division multiplexing (OFDM) signal. To prevent the regrowth of peak, we employ tone-reservation after multiplexing IF carriers. Furthermore, we use an out-of-band signal as the reserved tones to avoid any modification of a mobile signal. The impact of the number of IF carriers on peak-to-average power ratio (PAPR) characteristics is presented via numerical simulation. For the multi-IFoF based mobile fronthaul, we experimentally investigate the transmission performance of 36-IF carriers of the long term evolution-advanced (LTE-A) signals mapped with 64-quadrature amplitude modulation (QAM). It is clearly observed that the clipping-induced nonlinear distortion is dramatically suppressed by using tone-reservation. As a result, the transmission performance of 36-IF carriers of the LTE-A signal is improved by an error-vector-magnitude (EVM) of 4% (from 9.7% to 5.7%) after 20-km transmission.