Digital Optical Link Research Articles

Performance evaluation of a DVB‐S2 system with a digital optical feeder link

SummaryWe investigate the physical layer performance of a DVB‐S2 system with a digital optical feeder link. We consider three scenarios depending on how the DVB‐S2 modulator functionalities are distributed between the optical gateway and the satellite. Our main objective is to specify the residual bit error rate of the optical channel error correction scheme in order to ensure a quasierror free link to the users. To that end, we provide theoretical models and/or simulations results. In two scenarios, our results show that a residual bit error rate of 10−4 (assuming equally distributed errors) gives end‐to‐end physical layer performance compliant with the DVB‐S2 standard. This specification is much higher than typical values in practical systems.

Mitigating weak dispersion in affordable RF-over-fiber channels.

A dispersion-compensating filter for next-generation low-cost analog and digital optical links is developed. In these high-frequency optical channels, even weak dispersion can be detrimental to data encoding (e.g., radar detection). Dispersion-compensating filters must be affordable for these low-cost channels, and they must be reliable for weak dispersion channels. In this paper, we design a dispersion-compensating filter based on three properties of the analog channel: weak modulation depth, a spectrally bounded signal, and a low-dispersion channel. We calculate the fundamental performance limits of the channel with and without the filter and quantify the improvement. Furthermore, numerical simulations are taken to estimate the filter's performances for both pulse amplitude modulation digital channels and for analog ones. The simulations agree with the analytical derivation, and, in both cases, the filter's integration improves the channels' performances considerably.

Digital Radio Frequency Transport over Optical Fiber for 5G Fronthaul Links

Abstract Radio over fiber (RoF) is an empowering technology for the next-generation wireless networks including fifth generation cloud radio access networks. In this paper, we propose a new fronthaul interface which has been demonstrated for a 20-MHz LTE signal having 64-Quadrature amplitude modulation (QAM) modulation over 50 km of standard single-mode fiber. The proposed methodology potentially reduces the overall bandwidth requirement of the link. The performance analysis has been demonstrated for conventional analog RoF (A-RoF) and proposed digital RoF (D-RoF). As a figure of merit, the performance evaluation of the two architectures is reported by analyzing error vector magnitude (EVM), signal-to-noise ratio and eye-opening penalty. It is shown that digital optical links can mitigate the impairments present in A-RoF links and can support the transmission up to 50 km. Moreover, it is shown that efficient D-RoF links can be obtained with a relatively low amount of analog-to–digital-converter resolution bits. For the D-RoF link, the EVM was measured to be less than 2 % at 50 km. This is within the LTE specifications, proving it as a cost and power effective solution for next-generation wireless networks.

Optimized digital radio over fiber system for medium range communication

In recent years, the demand of efficient and broadband wireless access has escalated, for which Radio over Fiber (RoF) is considered as an appropriate solution. In this paper, we present a Digitized Transmission of 20 MHz LTE signal having 64 quadrature amplitude modulation over 70 Km of Standard Single Mode Fiber (SSMF) for broadband wireless signal transportation and distribution applications. The performance analysis has been evaluated for conventional Analog Radio over Fiber (A-RoF) and proposed Digital Radio over Fiber (D-RoF). The bandpass sampling technique is discussed and employed for digitizing the RF signal. As a figure of merit, the performance evaluation of two architectures is reported by analyzing the Error Vector Magnitude (EVM), Signal to Noise Ratio and Eye-Opening Penalty. It is shown that digital optical links can mitigate the impairments present in A-RoF links and can support the transmission up to 70 Km. Moreover, it is shown that efficient D-RoF links can be obtained with a relatively low amount of analog to digital converter (ADC) resolution bits. An experimental validation of the proposed analytical D-RoF model is done for 30 Km of SSMF, whose experimental results are in line with the analytical ones. The evaluated parameters are within the LTE specifications, proving this is a cost and power effective solution for next generation wireless networks.

Modulator material impact on chirp, DSP, and performance in coherent digital links: comparison of the lithium niobate, indium phosphide, and silicon platforms.

We characterize the impact of the modulator material on chirp, digital signal processing (DSP) algorithms and system-level performance in coherent digital optical links. We compare theoretically, in simulations and experimentally the lithium niobate (LiNbO3), indium phosphide (InP) and silicon (Si) integrated platforms. Distortions to vector diagrams are traced back to modulation physics, and are interpreted as quadrature crosstalk. In a back-to-back BPSK setup with an RF drive signal amplitude of 1.5Vπ, we measure chirp parameters α of ~0, 0.10 and 0.06 and error vector magnitude EVMRMS of 5.3%, 9.4% and 10.6% with the LiNbO3, InP and Si modulators respectively. Both α and EVMRMS are found to scale with the RF signal amplitude. In simulations, using a polynomial fit over a sinusoidal fit when pre-compensating the Si modulator transfer function slightly improves EVM (-0.6%). We also show that Si-related distortions can impact the efficiency of symbol timing recovery. In conclusion, phase and attenuation distortions in InP and Si modulators deteriorate the overall performance in coherent links, and cannot be neglected for large RF signal amplitudes. These results will benefit the optical communications community.

High-fidelity angle-modulated analog optical link.

There has long existed a debate over whether analog or digital optical link is more suitable for wireless convergence applications. Digital link achieves the highest fidelity, with the sacrifice of huge bandwidth due to the high resolution of digitization, and large power consumption due to the exhaustive digital data recovery. Analog link avoids these drawbacks, but it inevitably suffers from the SNR degradation. In this paper, we propose the angle modulation for analog optical link, which successfully breaks the SNR ceiling of amplitude modulation, and achieves ultrahigh link fidelity. Using the digital link (CPRI) equivalent bandwidth, angle modulation exhibits around 30-dB SNR advantage over the conventional amplitude modulation. Combined with its high tolerance on link nonlinearity, angle modulation has great potential in the future SNR-hungry analog optical applications.

Data regeneration for an all‐photonic digital radio over fibre

All-photonic digital radio over fibre (AP-DRoF) is a suitable candidate for a digital optical link. In this study, the system and simulation modelling of the proposed photonic data regeneration architecture for the generated return to zero (RZ) pulses of an AP-DRoF link is presented. The proposed data regeneration architecture is based on time stretching, optical sampling and optical hard limiting. Based on the system model, a comprehensive analysis and performance evaluation of the proposed system is presented. The results of simulations carried out to evaluate the performance of the system show that using the proposed regenerator in an AP-DRoF link for regeneration of the ultra-short RZ pulses at 65 Gbit/s reduced root-mean-squared timing jitter by about 90%, while the regenerated pulse eye opening height is improved by 65%.

A home-built digital optical MRI console using high-speed serial links.

To develop a high performance, cost-effective digital optical console for scalable multichannel MRI. The console system was implemented with flexibility and efficiency based on a modular architecture with distributed pulse sequencers. High-speed serial links were optimally utilized to interconnect the system, providing fast digital communication with a multi-gigabit data rate. The conventional analog radio frequency (RF) chain was replaced with a digital RF manipulation. The acquisition electronics were designed in close proximity to RF coils and preamplifiers, using a digital optical link to transmit the MR signal. A prototype of the console was constructed with a broad frequency range from direct current to 100 MHz. A temporal resolution of 1 μs was achieved for both the RF and gradient operations. The MR signal was digitized in the scanner room with an overall dynamic range between 16 and 24 bits and was transmitted to a master controller over a duplex optic fiber with a high data rate of 3.125 gigabits per second. High-quality phantom and human images were obtained using the prototype on both 0.36T and 1.5T clinical MRI scanners. A homemade digital optical MRI console with high-speed serial interconnection has been developed to better serve imaging research and clinical applications.

Investigation on Transport Schemes for Efficient High-Frequency Broadband OFDM Transmission in Fibre-Wireless Links

This paper investigates the different transport schemes for delivery of high-frequency broadband OFDM signals via fibre-wireless links. We have investigated analog RoF, digitized RoF, and sampled RoF transport schemes and experimentally demonstrated the transmission of broadband RF-OFDM signal using these schemes. Analog RoF results in the simplest BS/RRH configuration but it demands for large optical bandwidth. The performance of this transport scheme is limited by inter-modulation distortion (IMD) and RF power fading. For the digitized RoF transport scheme based on bandpass sampling, although digital optical link is used for signal delivery, the performance is limited by signal clipping and ADC resolution. The sampled RoF transport scheme requires only a narrow optical bandwidth, but the performance is limited by IMD and noise processes introduced by bandpass sampling. Based on our analysis and experimental results, a design guide of fibre-wireless integration system for broadband RF-OFDM transmission is provided. It is found that: analog RoF is best suitable for short-range transmission; digitized RoF has the best performance; and sampled RoF offers a low-cost solution.

Digitized RF-over-fiber as a cost-effective and energy-efficient backhaul option for wireless communications

Digitized RF-over-fiber technique based on bandpass sampling theory is studied as an alternative of analog radio-over-fiber to deliver wireless signals, taking advantage of digital optical link and realizing simpler base station architecture. In this paper, we present an overview of our recent research on digitized radio-over-fiber (RoF) technique, including discussions on the designing issues and possible solutions, proof-of-concept demonstrations and experimental results. Then, we examine the energy efficiency of the digitized RoF technique using a generic base station (BS) power model and compare with conventional BS designs. Based on the estimated energy consumption per square meter, digitized RoF technique has advantage for larger cell size.

Analysis and Control of the DC Drift in LiNbO3Based Mach–Zehnder Modulators

The drift issue induces slow drifting of the optimum operating point for high efficiency or large nonlinearities in analog optical links, and requires complex control of the offset bias voltage for achieving high extinction ratio in digital optical links. We discuss and analyze the different sources of the drift in commercially LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Mach-Zehnder modulators. The different extrinsic and intrinsic origins are compared in terms of phase shift and the different corresponding orders of magnitude are given, pointing out the predominant role of the intrinsic (dc) drift. We show the large role played by the electrical inhomogeneities at the surface of the LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate by highlighting the link between the time dependence of the dc drift and the electrical conductivity measured at the surface and in the volume of the LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. This allows to propose a solution to the drift issue which consists in the engineering of the electrical conductivity of the lithium niobate substrate.

Simulation of thermal effects in laser diode and its impact on high speed fiber optic link

A high speed, 1.25 Gb/s digital optical link with Distributed Feedback Laser (DFB) transmitter and single mode fiber is investigated under different operating temperatures of the laser. The combined effects of temperature-dependent variations in laser diode output and the fiber-induced dispersion on the performance of digital optical link are analyzed. As the operating temperature of laser diode is varied, the optical pulse characteristics are determined for 1.25 Gb/s data input. Simulations carried out at various operating temperature and link distances predict an increase in eye opening for some range of temperature and distance, whereas exhibiting a regular decrease in other ranges.

FPGA-based signal processing for the LHCb silicon strip detectors

We have developed an electronic board (TELL1) to interface the DAQ system of the LHCb experiment at CERN. Two hundred and eighty-nine TELL1 boards are needed to read out the different subdetectors including the silicon VEertex LOcator (VELO) (172 k strips), the Trigger Tracker (TT) (147 k strips) and the Inner Tracker (129 k strips). Each board can handle either 64 analog or 24 digital optical links. The TELL1 mother board provides common mode correction, zero suppression, data formatting, and a large network interface buffer. To satisfy the different requirements we have adopted a flexible FPGA design and made use of mezzanine cards. Mezzanines are used for data input from digital optical and analog copper links as well as for the Gigabit Ethernet interface to DAQ.

The LHCb DAQ interface board TELL1

We have developed an electronic board (TELL1) to interface the DAQ system of the LHCb experiment at CERN. 289 TELL1 boards are needed to read out the different subdetectors. Each board can handle either 64 analog or 24 digital optical links. The TELL1 mother board provides common mode correction, zero suppression, data formatting, and a large network interface buffer. To satisfy the different requirements we have adopted a flexible FPGA design and made use of mezzanine cards. Mezzanines are used for data input from digital optical and analog copper links as well as for the Gigabit Ethernet interface to DAQ. The LHCb timing and trigger control signals are transported by a dedicated optical link, while the board slow-control is provided by an embedded PC running a Linux kernel.

Radiation hardness assurance and reliability testing of InGaAs photodiodes for optical control links for the CMS experiment

A radiation hard 80 Mbit/s digital optical link system with 7200 fiber channels is being produced for the CMS Experiment at CERN. A series of tests including radiation damage and thermally accelerated aging have been made to qualify and assure the radiation hardness and reliability of InGaAs photodiodes intended for use in this system.

Design of the LHCb silicon tracker

The LHCb silicon tracker is a large-surface silicon micro-strip detector that will cover an important part of the LHCb tracking system. Its data will be used in the trigger to select high- p T daughter particles from B-meson decays, and in the offline analysis to reconstruct trajectories of charged particles in the very forward region of the detector. Readout strips of up to 38 cm in length with strip pitches of approximately 200 μ m will be employed. For the readout, a custom developed front-end chip and a Gbit digital optical link will be used. The design of detector, silicon sensors and readout link is presented.

Optical readout and control systems for the CMS tracker

The Compact Muon Solenoid (CMS) Experiment will be installed at the CERN Large Hadron Collider (LHC) in 2007. The readout system for the CMS Tracker consists of 10000000 individual detector channels that are time-multiplexed onto 40000 unidirectional analogue (40 MSample/s) optical links for transmission between the detector and the 65 m distant counting room. The corresponding control system consists of 2500 bi-directional digital (40 Mb/s) optical links based as far as possible upon the same components. The on-detector elements (lasers and photodiodes) of both readout and control links will be distributed throughout the detector volume in close proximity to the silicon detector elements. For this reason, strict requirements are placed on minimal package size, mass, power dissipation, immunity to magnetic field, and radiation hardness. It has been possible to meet the requirements with the extensive use of commercially available components with a minimum of customization. The project has now entered its volume production phase after successful completion of technical feasibility studies. Components have been identified that meet both the stringent analogue performance targets and are sufficiently radiation-hard for use in the CMS Tracker, where lifetime radiation exposure is expected to reach 3.4/spl times/10/sup 14//cm/sup 2/ fluence and 150 kGy dose. Analogue and digital system performance, as well as the component radiation hardness and quality assurance procedures, are reviewed in this paper.

The control system for the CMS tracker front end

The Compact Muon Solenoid (CMS) tracker uses complex, programmable embedded electronics for the readout of silicon sensors, for the control of the working point of optical transmitters, for the phase adjustment with respect to the 40-MHz Large Hadron Collider (LHC) clock, and for the monitoring of voltages, currents, and temperatures. To establish reliable low-noise communication with the outside world, the control chain has been designed to operate over a ribbon of digital optical fibers. A small-scale prototype of the control system for the CMS tracker has been recently developed; it is based on a front-end controller unit that distributes, via high-speed digital optical links, both timing signals and slow control data to the clocking and control units where they are fanned out to the front-end electronics. To operate these devices and finally perform the downloading of the settings needed to operate the system, a multilayered software architecture has been developed in such a way that the end user does not need to know any of the details concerning the hardware structure. The parameters relevant to the proper operation of the entire system are stored in an Oracle database; an interface between the slow control software and the database allows one to access and retrieve the values of the parameters that need to be downloaded. This paper describes the components, hardware and software, of the prototype control system developed for the CMS tracker.

Single event upset tests of an 80-Mb/s optical receiver

This paper studies the single event upset (SEU) sensitivity of a radiation-tolerant 80-Mb/s receiver developed for the CMS Tracker digital optical link. Bit error rate (BER) measurements were made while irradiating the receiver with protons and neutrons at different beam energies and incident angles and for a wide range of optical power levels in the link. Monte Carlo simulations have also been used to assist in the interpretation of the experimental results. As expected, the photodiode is the most sensitive element to SEU. The fake signal induced by direct ionization dominates the bit-error cross-section only for protons incident on the photodiode at large angles and low levels of optical power. Comparison of the neutron and proton bit-error cross-sections demonstrates that nuclear interactions contribute significantly to the proton-induced SEU errors and that they will dominate the radiation-induced error rate in the real Tracker application.

Single-event upset studies of a high-speed digital optical data link

The results from a series of neutron and photon irradiation tests of a high-speed digital optical data link based on a commercial serialiser and a vertical cavity surface emitting laser are described. The link was developed as a candidate for the front-end readout of the ATLAS electromagnetic calorimeter. The components at the emitting end of the link were unaffected by neutron and photon irradiation levels exceeding those expected during 10 years of LHC running. However, the link suffered from Single-Event Upsets (SEUs) when irradiated with energetic neutrons. A very general method based on the Burst Generation Rate (BGR) model has been developed and is used to extrapolate the error rate observed during tests to that expected at the LHC. A model-independent extrapolation was used to check the BGR approach and the results were consistent once systematic errors were taken into account.