Mitigating outage regions is of particular interest for emerging millimeter-wave (mmWave) and future sub-terahertz frequency cellular networks. Whereas smart radio environments based on metasurfaces constitute a key concept for 6G research, current networks cannot be served as new control procedures are required. Although small prototypes are already emerging, costs will be higher for the dense deployment of reconfigurable intelligent surfaces (RISs) than for passive reflectors. It is argued that passive reflectors can be used to effectively, cost-efficiently, and permanently boost connectivity in well-defined service areas. Leveraging the advantages of 3D printing and spray-painted conductive varnish, we introduce the Holistic Enlightening of bLackspots with passIve reflectOr moduleS (HELIOS) approach which is characterized by its scalability and parametrizability to meet the reflection requirements derived from sophisticated network planning. These slim reflectors meet the core criteria of ease of installation and minimal visual impact on the cityscape, which are imperative for market success. Our measurements-based comparison of prototypes against typical metal/aluminum reflectors shows at least equal reflectivity at a higher practicality of the proposed manufacturing process. The conducted simulation study validates the modular reflector pattern design process and finds a trade-off between the reflector efficiency and the minimum protrusion depth, which relates to the number of modules in the designated mounting area. An urban ray-tracing simulationbased case study further underlines the high applicability of the proposed approach, with the growth of the beyond line-of-sight (LOS) connectivity region being nearly twice as large for a site-tailored HELIOS configuration as for a simple reflector plate.INDEX TERMS Millimeter-wave, passive reflector, beyond line-of-sight connectivity, additive manufacturing.

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