ADR 0003: Canonical FAPI IR

Status

Accepted

Context

The DU-high layer must support at least a local DU-low backend and a future Aerial backend without leaking backend-specific details into core orchestration logic.

Decision

Represent southbound intent as a versioned canonical IR centered on a slot_batch.

The IR includes:

• ir_version
• cell_group_id
• ue_ref
• frame
• slot
• profile
• ordered messages
• metadata

Backends negotiate capabilities against the IR rather than redefining the DU-high boundary.

Consequences

Positive:

• one contract for local, stub, and future Aerial profiles
• validation and artifact capture can be standardized
• scheduler outputs remain backend-agnostic

Negative:

• IR design must stay disciplined or it will drift into lowest-common-denominator ambiguity
• some backend-specific features may require extension fields or capability flags

Alternatives Considered

• Backend-specific request objects: simpler short term, but leaks backend details into DU-high.
• Message-by-message pass-through: weaker for batching, rollback planning, and verification.