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As fifth-generation (5G) mobile networks are being rolled out, the telecom industry and academia are now coordinating the 6G research effort towards defining the requirements and use cases for beyond 5G (B5G) or so-called sixth-generation (6G) mobile networks. 6G envisages an evolutionary communication platform based on complete network softwarisation, inclusive communications mediums including satellite, and ultra-dense networks to cater for the market demands that requires ultra-high speeds, tactile response time, and lower cost of network ownership by 2030. This chapter provides an overview of the use cases for B5G/6G systems, including holographic telepresence, digital twin, connected robotics, distributed artificial intelligence, and blockchain technologies. It further reviews the current standardisation and deployment status of 5G technology as a baseline and the drive towards 6G by identifying key enabling technologies, system requirements, and an overview on global B5G/6G activities.

This article demonstrates the design and development of WR-15 transition using an antipodal microstrip dipole antenna at a frequency of 60 GHz for space applications. An inline microstrip line to rectangular waveguide (MS-to-RWG) transition is proposed for the V-band (50–75 GHz) functioning. The RF energy is coupled and launched through an antipodal dipole microstrip antenna. Impedance matching and mode matching between the MS line and dipole are achieved by a quarter wave impedance transformer. This results in the better performance of transitions in terms of insertion loss (IL > −0.50 dB) and return loss (RL < −10 dB) for a 40.76% relative bandwidth from 55.57 GHz to 65.76 GHz. The lowest values of IL and RL at 60 GHz are −0.09 dB and −32.05 dB, respectively. A 50 μm thick double-sided etched InP substrate material is used for microstrip antipodal dipole antenna design. A back-to-back designed transition has IL > −0.70 dB and RL < −10 dB from 54.29 GHz to 64.07 GHz. The inline transition design is simple in structure, easy to fabricate, robust, compact, and economic; occupies less space because the transition size is exactly equal to the WR-15 length; and is prepared using an InP substrate with high permittivity of 12.4 and thickness of 50 μm. Thus, the devices have the lowest insertion loss value and lowest return loss (RL) value, of

5th Generation (5G) communication systems are expected to undergo significant advances, with optical-wireless technologies being at the forefront of system design addressing key requirements targeting high performance, versatility and cost-effectiveness. The EU-funded EXPLOR project aims to develop a comprehensive modular platform with experimentally validated state-of-the-art component and system-level numerical models, as well as cognitive and adaptive features. It is a collaborative industry-academic driven project committed to creating a scientifically and socio-economically relevant outcome, enabling further innovation beyond 5G networks. The software will include libraries with novel use cases and scenarios targeting next-generation converged optical-wireless networks, and enable the deep academic and techno-economic considerations of relevant features such as high-end mm-wave frequency, adaptive and virtualized optical front-haul, femtocell based communications, network cognition and dynamic cloud-radio-access-network environment functionalities. EXPLOR will provide a holistic insight into the field that will act as a tool for promoting and demonstrating future innovation in the field.

We present a characterization of power penalty using p-i-n (PIN) and avalanche photodiode (APD) receivers for different extinction ratios (ER) of the transmitted signal. A simple approach based on experimental fitting is presented to describe the power penalty. The model is in a good agreement with the experiment, predicting both PIN and APD results accurately, even considering that the characterized devices may vary considerably in their intrinsic parameters. The fitting parameter from the proposed model was extracted using a mean squared error (MSE) based fitting function. Theoretical curves were obtained by fitting the numerical results to the experimental data for MSE=10−3.

We investigate the feasibility of low-cost, compact optical filtering for NG-PON2 based on thermally-tuned thin film filters. By considering several contrasting designs in a comparative assessment of their optical performance, complexity, tuning time and power efficiency, we identify relevant technical tradeoffs towards their optimized and low-cost application in NG-PON2 systems.

In this paper, we firstly review the progress in ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON), by making use of the key attributes of this technology in the context of optical access and metro networks. Besides the inherit properties of coherent technology, we explore different modulation formats and pulse shaping. The performance is experimentally demonstrated through a 12 × 10 Gb/s bidirectional UDWDM-PON over hybrid 80 km standard single mode fiber (SSMF) and optical wireless link. High density, 6.25 GHz grid, Nyquist shaped 16-ary quadrature amplitude modulation (16QAM) and digital frequency shifting are some of the properties exploited together in the tests. Also, bidirectional transmission in fiber, relevant in the context, is analyzed in terms of nonlinear and back-reflection effects on receiver sensitivity. In addition, as a basis for the discussion on market readiness, we experimentally demonstrate real-time detection of a Nyquist-shaped quaternary phase-shift keying (QPSK) signal using simple 8-bit digital signal processing (DSP) on a field-programmable gate array (FPGA).

We investigate the power budget of a fully coherent reflective passive optical network, based on self-coherent upstream (US) and self-homodyne downstream (DS) detection schemes. We consider, for US, both binary phase-shift keyed (BPSK) and quadrature phase-shift keyed (QPSK) at 1 GBd generated by direct modulation of a low-cost reflective semiconductor optical amplifier having 1-GHz electrical bandwidth and using simplified digital signal processing (DSP). We find that the penalty of increasing the order of US modulation format from BPSK to QPSK for higher capacity can be reduced to 1 dB by means of DS digital spectral shifting and US static postequalization.

This work presents an effective solution to reduce the computational complexity order of behavioral model generation for reflective semiconductor optical amplifier (RSOA). The proposed model is based on a complex valued (CV) neural network (CVNN) structure, using polar CV basis functions architecture. The CV model parameters are extracted by means of nonlinear complex-domain Levenberg–Marquardt algorithm, from recorded experimental 20 Msymbol/s 64-quadrature amplitude modulation (QAM) input–output data. The evaluation results of polar CVNN model prove to be more adequate to accurately describe the nonlinear dynamic magnitude and phase distortions of RSOA, compared to double-input double-output real-valued neural network (RVNN) rectangular structure. Additionally, significant reduction of the computational cost is achieved in comparison to the RVNN approach.

In this paper, we will address the benefits of the coherent detection in future optical access networks. The scarcity of the optical spectrum, the required flexibility, and constant evolution of requirements highlight the effectiveness of coherent techniques toward the future passive optical networks (PON). A set of architectures for coherent optical access networks will be presented and the key attributes of each scenario will be investigated. In addition, as a basis to decrease the cost of the local oscillator (LO) at customer side, we experimentally investigate the possibility of using a low-cost laser as LO with real-time detection of a Nyquist-shaped differential quadrature phase-shift keying (DQPSK) signal using simple 8-bit digital signal processing (DSP) on a field-programmable gate array. Moreover, we experimentally derive a set of optimized parameters and their impact on the network operation for coherent ultradense wavelength-division multiplexing (UDWDM) systems. The balance between the number of channels, power budget, and dynamic power range will be evaluated. Furthermore, we demonstrate a reconfigurable real-time receiver DSP for future flexible UDWDM-PON systems applying the DQPSK and D8PSK modulation formats. By reviewing some of the motivations for this technology, such as flexibility, spectral efficiency, as well as compatibility with software-defined networking, we show that this technology is approaching the required maturity.

We investigate the role of a reflective semiconductor optical amplifier (RSOA) in a fully coherent wavelength-shared passive optical network architecture, based on self-coherent upstream (US) and self-homodyne downstream (DS) detection schemes. We present a proof of concept experimental study, demonstrating 32-dB loss budget over a 30-km fiber link, operating at 9.4-Gb/s DS and 750-Mb/s US effective rates with 7% forward error correction and 8b/10b US overheads, respectively. This was achieved using simplified digital signal processing and a low-cost, off-the-shelf RSOA with 18-dB gain and 1-GHz modulation bandwidth.

In this letter, we demonstrate a proof of concept fully loaded bidirectional ultrahigh capacity coherent passive optical network (2 × 1008 × 8.3 Gb/s). This was achieved using partial spectrum overlap, Nyquist shaping, digital frequency shifting, self-homodyne detection, and pilot tone remodulation. Upstream burst mode operation is also demonstrated.

“Copyright © [2013] IEEE. Reprinted from IEEE Journal of Selected Topics in Quantum Electronics. ISSN: 1932-4553. This material is posted here with permission of the IEEE. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.” RSOA is considered a strong candidate to play an important role in realizing the next generation WDM PON, based on the wavelength reuse concept. Therefore, accurate and efficient modeling of RSOA is of significant importance. We present a time-domain wideband model for simulation of spatial and temporal distribution of photons and carriers in a bulk RSOA. A trade-off between the accuracy and the computational efficiency of the model is studied. Multi-objective genetic algorithm is utilized for parameter extraction. Experimental validation has been performed for continuous wave input, NRZ and QPSK signaling pulses up to 40 Gb/s of bit rate, in both amplification and remodulation regimes. Saturation, noise, chirp and signal broadening are successfully predicted, while reducing the computational time compared to other wideband models.

An overview of several candidate architectures for 100 Gb/s capable optical access networks is presented. Considerations for their system requirements are outlined in regards to performance, cost and energy efficiency, towards addressing the design issues for the next generation EPON.

A detailed investigation of multigigabit wavelength reuse DWDM system tolerance to bidirectional crosstalk, conditioned by the maximum seed power budget and channel spacing, was preformed. Maximum tolerated seed powers were measured for DSF and SMF, for 10 Gb/s NRZ US signal. We demonstrated that high power regime reveals that the most dominant nonlinear impairment arises for the seed frequency shift coinciding with the SBS anti-Stokes. We assess the optimal seed frequency shift range, allowing a successful trade-off between the seed power budget and enabled spectral efficiency.

We experimentally characterize 2.5-Gb/s bidirectional UDWDM based on Nyquist shaped QPSK with DSP in real-time. A 41 dB ODN is achieved over 80-km SSMF. Additionally, the required guard-bands for a coexisting scenario with TWDM and video overlay are evaluated.

This work reviews the latest advancements in coherent self-homodyne detection (SHD) using signals with polarizationor space-multiplexed pilot tones (PTs) originating from the same light source, towards the implementation of low-cost coherent receivers. The coherency between signals and PTs drastically reduces laser linewidth requirements, enabling the use of high-order modulation formats with low-cost DFB lasers. In this work, we revise the application of SHD in high-capacity space-division multiplexed links using multi-core fibers, outlining optical signal-to-noise ratio, skew and phase noise requirements of such systems. Furthermore, we evaluate the application of SHD for the implementation of laser-less optical network units in passive optical networks, as well as recent developments in digital SHD techniques.

We assess the importance of Stimulated Brillouin scattering (SBS) anti-Stokes wave on reflection tolerance in remotely seeded wavelength-division multiplexing passive optical networks (WDM PON). Experimental validation of the extended model for the externally seeded SBS is presented towards assessing the conditions in which the anti-Stokes contribution becomes relevant in the variant scenario of remotely carrier-seeded PON. We identify relevant operating conditions in which the latter can no longer be reliably neglected, and analyze the remote seed power budget implications of such contribution. Considering typical launch powers in PON systems we found that, even for seed launch powers below SBS threshold, non-optimized upstream (US) power spectral density may lead to a considerable anti-Stokes contribution to the SBS gain. In effect, in order to maintain a reference optical return loss (ORL) of 32 dB in such scenario, anti-Stokes wave contribution imposed rigorous remote seed power budget restrictions depending on fiber parameters.

We present a coherent-PON employing partial overlap of downstream/upstream Nyquist spectra, allowing bi-directional operation and enhanced spectral efficiency (2x120Gb/s@50GHz). Additionally, frequency shifting of Nyquist bands from the carrier allowed reduced RBS and dynamic SRS.

The plant polyphenol quercetin was shown to induce a significant CI − secretion in intestinal epithelium. In order to elucidate the structural requirements of quercetin and related flavonoids for this activity, we tested the ability of further flavonols and other flavonoids found in edible plants to induce CI − secretion which was measured as an increase in short-circuit current ( I sc ) in rat colon. Whereas several flavonols and the flavon luteolin increased I sc , other flavonoids such as flavanones, flavans, flavanols, and anthocyanidins failed to do so. Two glycosides of quercetin, spiraeosid, and isoquercitrin, as well as two methoxylated quercetin metabolites, isorhamnetin and tamarixetin, were also able to increase I sc . We conclude that a 2,3-double bond in conjunction with the 4-oxo group in the C ring and a hydroxylated B ring are necessary for the secretory activity of flavonoids. This activity requires different structural features than those mandatory for the antioxidative properties of flavonoids. Glucosidation and methoxylation of several hydroxyl groups does not necessarily abolish the secretory potential.

The dietary flavonol quercetin induces chloride secretion in rat intestine. To clarify the underlying mechanisms, experiments were performed in Ussing chambers with tissue from rat proximal and distal colon. Quercetin induced an increase in short-circuit current (Isc), which was largely independent of submucosal neurons, as it was not affected by the neurotoxin tetrodotoxin. The effect of quercetin was blocked by the calmodulin antagonists trifluoperazine and ophiobolin A and was diminished by a blocker of Ca2+ release from intracellular stores (TMB-8), whereas the muscarinic receptor antagonist atropine was ineffective. The quercetin-induced Isc was abolished in Ca2+-free solution. The flavonol was able to further increase Isc after maximal stimulation of the cAMP pathway by forskolin. The Isc increase by the flavonol was differently affected by two analogous phosphodiesterase inhibitors. Whereas 3-isobutyl-1-methylxanthine (IBMX) antagonized the effect of quercetin, 8-methoxymethyl-IBMX had no effect. Both phosphodiesterase inhibitors similarly influenced the Isc increase induced by forskolin. These results indicate that the chloride secretion induced by quercetin in rat colon depends on Ca2+ and calmodulin. The cAMP pathway and inhibition of phosphodiesterase appear not to be responsible for the secretory activity of the flavonol.

We experimentally analyze the required guard band of coherent optical UDWDM networks transporting 16×1.25 Gb/s - QPSK at 3.125 GHz frequency grid to coexist with Video and 10G - NRZ transmissions. Using simple DSP based ONU, guard bands of only 0.8-1.6 nm are achieved.

A dynamic, wideband model for simulation of spatial-temporal distribution of photons and carriers in a bulk InP-InGaAsP reflective semiconductor optical amplifier (RSOA) is presented. Saturation and noise characteristics are successfully predicted and in good accordance with theoretical predictions found in literature. Based on RSOA model and a model for access network link, numerical optimization of multiplexing (MUX) and demultiplexing (DMUX) 3-dB filter bandwidths when transmitting non-return-to-zero (NRZ) format is preformed. The access network simulated scenario is based on high density (< 25 GHz channel spacing) arrayed waveguide grating (AWG) and remodulation of 10 Gbit/s NRZ signal by using RSOA.

We experimentally investigated the efficiency of the Brillouin induced FWM process, using two types of fibers. Even with the pump power levels above the SBS threshold, the power of the FWM signal was found to saturate at low power, inducing low crosstalk in bidirectional WDM transmission.

We develop a model for simulation of spatial-temporal distribution of photons and carriers in RSOA for downstream optical pulses at 40 and 100 Gb/s, which demonstrates that signal broadening generates CW component of output signal.

We present and experimentally characterize a spectrally efficient, bidirectional, ultradense wavelength-division multiplexing passive optical network (PON) (2 × 120 Gb/s at 50 GHz) over 30 km standard single-mode fiber. This is achieved using a 4 GHz grid with partial spectrum overlapping, Nyquist-shaped 16-ary quadrature amplitude modulation, and digital frequency shifting. This scheme is based on a laserless, self-homodyne PON with centralized wavelength control. We achieve reduced impact of the Rayleigh back-scattering effect due to Nyquist-pulse shaping and frequency upshifting. Self-homodyne detection relaxes the laser linewidth requirement and digital signal processing complexity at the optical network unit. To demonstrate the extra benefit of the proposed PON system, we also experimentally and theoretically evaluate the impact of nonlinear Raman crosstalk in video overlay. Compared to a baseband Nyquist signal and baud rate non-return-to-zero equivalent, the proposed system improves the carrier-to-Raman crosstalk ratio of video overlay by 4 and 6 dB, respectively.

We investigate the possibilities of controlling and exploiting the spatial evolution of the four-wave mixing (FWM) idler in a fiber optical parametric amplifier, considering standard single-mode fibers. We found that the use of high pump powers, for a given requirement on the signal extinction ratio and frequency separation between pump and signal, can considerably affect the optical signal-to-noise ratio on the idler frequency due to the residual modulation. We observe that by choosing an opportune length of the fiber, the residual idler modulation can be suppressed or counteracted by an increase of optical average power. Thus, a proper choice of the transmission fiber length can considerably improve the remodulated idler characteristics.

We investigate the feasibility of low-cost, compact optical filtering for NG-PON2 based on thermally-tuned thin film filters. By considering several contrasting designs in a comparative assessment of their optical performance, complexity, tuning time and power efficiency, we identify relevant technical tradeoffs towards their optimized and low-cost application in NG-PON2 systems.

We analyze the impact of different types of splitter structures on resource sharing and power consumption in long reach PON. Using Cascaded splitter and Extender Box, power efficiency increases especially in low take rate areas.

A computationally efficient, time-domain model for simulation of spatial-temporal distribution of photons and carriers in a bulk RSOA is presented. Saturation, noise, chirp and signal broadening are successfully predicted for NRZ and QPSK signals.