The rapid evolution of fifth-generation (5G) mobile communications and the anticipated transition toward sixth-generation (6G) networks are fundamentally transforming the role of indoor wireless infrastructure. Although nearly eighty percent of mobile data traffic is generated indoors, in-building wireless systems are still frequently designed as isolated radio coverage projects centered on signal strength and capacity optimization. Such an approach is increasingly inadequate for modern buildings that function as digitally connected environments supporting enterprise operations, public safety, transportation, healthcare, industrial automation, and large-scale public venues. Indoor connectivity has consequently evolved into a multidisciplinary systems engineering problem involving radio architecture, building characteristics, fiber infrastructure, operational governance, lifecycle management, multi-operator coordination, and continuous technological evolution. 

This paper proposes a Systems Engineering Framework for Next-Generation In-Building Wireless Networks (SEF-NGIWN) that reconceptualizes indoor wireless infrastructure as an adaptive cyber-physical system rather than a static radio deployment. Instead of optimizing individual engineering components independently, the proposed framework integrates service requirements, radio architecture, physical infrastructure, operational governance, verification, lifecycle evolution, and continuous performance optimization within a unified architectural methodology. The framework further introduces a lifecycle-oriented engineering perspective in which design decisions are evaluated according to their long-term adaptability to emerging spectrum allocations, evolving operator requirements, digital infrastructure integration, and future wireless technologies. 

The conceptual framework is supported by professional engineering experience obtained through the design, implementation, and lifecycle management of more than 250 in-building wireless venues, including complex transportation hubs, stadiums, commercial facilities, and enterprise environments. These practical implementations demonstrate that successful indoor wireless systems depend less on maximizing initial radio performance than on engineering architectures capable of accommodating continuous operational evolution. The experiences further illustrate the value of integrating requirements engineering, radio planning, physical infrastructure, governance, verification, and operational learning into a unified systems engineering process. 

The study argues that future indoor wireless networks should no longer be evaluated solely through conventional coverage and throughput metrics. Instead, engineering success should be assessed according to the ability of the infrastructure to maintain performance, absorb technological evolution, support heterogeneous services, coordinate multiple operators, and continuously adapt throughout its operational lifecycle. The proposed Systems Engineering Framework extends current indoor wireless literature by positioning lifecycle engineering, governance, verification, and architectural adaptability as fundamental engineering principles for future indoor connectivity.