In the present work, we formulate a new method to represent a given Flame Describing Function by analytical expressions. The underlying idea is motivated by the observation that different types of perturbations in a burner travel with different speeds and that the arrival of a perturbation at the flame is spread out over time. We develop an analytical model for the Flame Describing Function, which consists of a superposition of several Gaussians, each characterised by three amplitude-dependent quantities: central time-lag, peak value and standard deviation. These quantities are treated as fitting parameters, and they are deduced from the original Flame Describing Function by using error minimisation and nonlinear optimisation techniques. The amplitude-dependence of the fitting parameters is also represented analytically (by linear or quadratic functions). We test our method by using it to make stability predictions for a burner with well-documented stability behaviour (Noiray's matrix burner). This is done in the time-domain with a tailored Green's function approach.

The evolution beyond 5G towards 6G will accelerate the transformations begun by 5G to deploy new approaches to define delivery services. The network should be accessible anywhere, always-on and with the guarantee performances. This poses new challenges to the transport network that should assure available connectivity among the functions at different levels of granularity. The transport network should be so flexible to guarantee on-demand the connectivity among the processing functions that automatically will be required and, at the same time, should assure extreme performance. Optical network and Dense Wavelength Division Multiplexing technology are relevant candidates to match high bandwidth and low latency, but new challenges are required to assure the suitable cost and size for the wireless access networks. The current optical technology, suitable used in aggregation and long-haul transport network, cannot be simply downsized to be used in the access area, but novel paradigm should be considered for the optics as well. Integrated photonics seems a promising direction to realize low cost and low size optical components for the transport network. Moreover, this technology enables a certain level of remote tunability and reconfiguration of optical equipment that allow to simplify and reduce cost of installation in field and operation of the transport network. In this paper relevant optical components for transport networks are analyzed using the integrated photonics to assess the technology challenges.

Reconfigurable radiofrequency (RF) signal generation in the 30 – 300 GHz range is attractive for many applications. W-band (75 – 110 GHz) is currently targeted by both wireless and satellite communications and similar frequency ranges are employed for on-board automotive radar systems. Distribution of high-precision and high-frequency synchronization signals in modern centralized radio access networks might be addressed by radio-over-fiber solutions, but the stringent phase noise (PN) requirements in case of broadband signals with high subcarriers density are not easily attainable through conventional electric-domain solutions. This article reports on the performance of a monolithically integrated silicon photonics (SiP) circuit employed for RF carrier synthesis. Up to sixfold frequency multiplication of an 18.5 GHz reference clock is demonstrated with low additional phase noise. The circuit includes a high-speed electro-optic phase modulator employed for optical frequency comb (OFC) generation and a tunable distributed feedback resonator (DFBR) filter for selecting the desired OFC harmonic. The beating of the OFC tone with the input laser mode in an off-chip photodiode generates the target RF carrier wave. By tuning the reference clock and the DFBR filter, reconfigurable frequency generation up to W-band and beyond is achieved. PN and time jitter (TJ) of the generated RF carriers are experimentally measured, demonstrating a similar performance as of an ideal frequency multiplier. The effect of the noise of DC sources employed for filter tuning is assessed and a counteracting solution is implemented. TJ robustness versus laser drifting is analyzed and the potential for up to elevenfold frequency multiplication (203.5 GHz) is shown.

Integrated photonics is essential in Radio Access Networks (RAN). For example, current pluggable optics and upcoming co-packaged optics (CPO) rely on it to increase capacity and energy efficiency. In addition, Photonic Integrated Circuits (PIC) may perform in future processing functions such as radiofrequency (RF) generation and mixing. This paper provides a comprehensive review of the current and future applications of integrated photonics to radio systems. It starts with the most mature technology (pluggable optics), discussing the challenges to meet the demand for the increase of bandwidth (BW) density expected with the Sixth mobile Generation (6G). Then, it moves to more advanced short reach interconnection technologies, based on CPO and optical Printed Circuit Boards (PCB). These technologies will allow to meet that demand, provided that certain RAN-specific developments, discussed in the paper, are undertaken. The potential of Artificial Intelligence (AI) to further improve the energy efficiency of short reach optical interconnects is also shortly introduced. Finally, Micro-Wave Photonics (MWP) techniques to process the RF signal in the optical domain are presented. The paper elaborates on their potential to improve the performance of radio systems and on the challenges remaining to move these techniques from lab to market. As a conclusion, integrated photonic technologies will expand their role in the evolution of radio systems, but with a variable timescale depending on their degree of compatibility with existing production processes.

A packaged silicon photonics radio-frequency (RF) synthesizer operating in the millimeter (mm-) wave band suitable for clock signal distribution in b5G/6G radio access networks is realized and experimentally characterized. The assembly include a photonic integrated circuit (PIC) acting as a frequency multiplier for a local oscillator (LO) reference at microwave frequencies and a printed circuit board (PCB) hosting a custom bias tee designed to provide a wideband matching condition over more than 6 GHz around 20 GHz for the input LO signal and supporting high power levels for efficient frequency multiplication operation. Phase noise (PN) measurements performed on a 100 GHz generated RF carrier via five-fold multiplication of a LO wave at 20 GHz indicate a low PN of -97 dBc/Hz at 10 kHz from the carrier with a small excess timing jitter of less than 2 fs with respect to the LO signal, making the circuit operating nearly as an ideal frequency multiplier.

Future generations of mobile communications promise near unlimited access to information and data sharing with ubiquitous wireless connectivity for any kind of device and application that may benefit from being connected. This poses new design challenges and requires a renewal of the hardware platforms to meet the increased demand of capacity and reliability of the network. Next generation Radio Access Network (RAN) will evolve towards an increase of the radio bandwidth, with carrier frequency extended to the millimeter waves (mmW) range. Other technologies that will be extensively used to increase the RAN capacity will be beamforming and spatial multiplexing with advanced antenna systems (AAS) comprising hundreds of antenna elements. Optical technologies are the natural candidates to support the traffic increase, by providing high-capacity interconnection within the radio unit and towards the base band unit (BBU). However, the higher level of integration of AAS will cause an increase of the temperature to values >100C, due to power dissipation of RF power amplifiers (PA), digital front-end (DFE) and baseband processing Application Specific Circuits (ASIC). This makes the AAS an inhospitable environment for optical transceivers that include lasers, typically operating at temperatures <85C. A promising approach to solve this issue is to place the laser outside the transceiver as a remote laser source (RLS) in a less harsh environment or in a thermally controlled environment e.g. in the same cabinet of the BBU. In this article we demonstrate a fully integrated RLS enabling a reduction of power consumption and packaging cost.