The Base Architecture
How to relate requirements and architecture on different abstraction levels
The SYSMOD zigzag pattern describes the different levels of abstractions and the relationship between requirements and architecture.
Your system requirements do not start at the very top of the levels, i.e. they already include some technical decisions. The base architecture is the architecture one level above your top level requirements. The following figure shows a simple base architecture of a forest fire detection system (FFDS).
The FFDS base architecture clearly specifies that the system will have local forest fire detection sensors and forest fire detection drones and it will use external satellites. The top level requirements will relate on the base architecture and directly mention satellites, sensors and drones.
In practice the base architecture description is a block diagram and some additional textual descriptions. You could also formalize the base architecture with constraint requirements.
Many projects I know do not explicitly describe the base architecture. It is implicitly described in the requirements and leads to lot of misunderstandings, i.e. errors, effort and costs. It is simple to avoid that obstacle in your project.