## 1 Introduction
Each of the optical modules on the right-hand side is connected to an ASIC (green), which could, in an advanced antenna system, be a radio frequency processing ASIC connected to an RF antenna element. This architecture  
Figure 1: Hyperscale rack-scale demonstrator with H2020 COSMICC “Aurora” platform and test daughtercards with I/O core mezzanine cards connected [5; 6; 7]. can be easily scaled to higher capacities by adding further optical modules or to connect a higher number of antenna elements, thus accommodating various product variants with different bandwidths and antenna array sizes. Radio equipment generally requires a lower aggregate capacity than DCIs [13] but also the capability to scale up capacity and power consumption more effectively. Moreover, even if the transceiver volumes in radio are comparable or even higher than in DCI applications, the target cost is lower. This makes it more difficult for transceiver manufacturers to assume the risk of investing in new technologies while maintaining a sustainable business model. The adoption of standardized solutions and the definition of a limited number of implementation options can definitely mitigate this issue avoiding excessive market fragmentation. Power consumption constraints are also more demanding in radio access networks (RANs) than in DCIs. Data center equipment operates indoors in environments with controlled temperature and humidity. In addition, active cooling systems, e.g., line fans, are commonly implemented, with more advanced solutions such as liquid cooling gaining prominence. Constraints in weight, power supply and installation practices make these solutions impractical or not feasible for radio equipment deployed outdoors or at the top of an antenna pole. Therefore, current RANs require transceivers capable of operating in the so-called industrial temperature (I-Temp) range, from \(-40\) to \(85\,\mathrm{\SIUnitSymbolCelsius}\), but the increase in component density in radio boards, due to miniaturization and increase in processing capabilities, is pushing the upper bound of the temperature range above \(100\,\mathrm{\SIUnitSymbolCelsius}\). While silicon photonic devices, such as modulators and photodetectors, are tolerant to high temperatures, lasers are very sensitive to them. Transceiver operation beyond \(85\,\mathrm{\SIUnitSymbolCelsius}\) requires the use of either external laser sources, placed outside the transceiver [14], or quantum dot lasers, which are very promising to withstand high temperatures [15]. Variations in temperature are particularly problematic for wavelength division multiplexing (WDM) systems. In 5G (and beyond) radio systems operating in the millimeter range, radiating antenna elements and photonic interconnects will be co-integrated in a single hardware unit. This will not only increase the operational temperature but also calls for miniaturized devices--for example, co-packaged transceivers integrated into single multi-chip modules with digital integrated circuits (ICs) and analogue radio frequency (RF) front-ends. This scenario needs more accurate fiber attachment techniques for silicon photonic chips [16] as well as a radical decrease in the power consumption, from the current tens of picojoules per bit to a few picojoules per bit [17]. The IPSR-I integrated photonics roadmap for transceivers [18] makes clear that photonic packaging technology must improve substantially to achieve better energy efficiency. Integrated photonic solutions for radio applications can bene  
Figure 2: Possible interconnection scenario in radio systems. fit from the current DCI technology, such as single-mode silicon photonic transceivers; however, operating temperatures, power consumption and cost margins will be much more challenging in 5G+ environments going forward and will require new solutions.