Enabling and Validating F1 Interoperability Between Open-Source 5G RAN Stacks: OCUDU and OAI

Abstract

The telecommunications industry is shifting toward disaggregated network architectures that enable multi-vendor interoperability. The Third Generation Partnership Project (3GPP) defines standardized interfaces separating Radio Access Network (RAN) components, with the F1 interface connecting the Centralized Unit (CU) and Distributed Unit (DU) in disaggregated Base Station (BS) deployments. The Open Radio Access Network (O-RAN) Alliance builds upon the 3GPP Technical Specifications (TSs) by promoting open, interoperable RAN ecosystems through extensions to the specifications and badging programs in their certified O-RAN Testing and Integration Centers (OTICs). Despite the maturity of open-source 5G platforms such as Open Centralized Unit Distributed Unit (OCUDU) and OpenAirInterface (OAI), no published research has demonstrated F1 interface Interoperability (IOT) between these two software stacks, leaving a gap in validating whether multi-vendor CU/DU deployments can be realized using open-source components. In this thesis we present the first demonstrated F1 interface Interoperability (IOT) between OCUDU and OAI. We perform a code-level comparison of the two baseline codebases to identify the implementation differences that prevent interoperable multi-vendor operation, and we apply targeted software modifications to OAI and OCUDU's User-Plane (U-Plane) and Control-Plane (C-Plane) functions to close the gap required for functional IOT. We validate the multi-vendor system Over-the-Air (OTA) at the Commonwealth Cyber Initiative (CCI) xG Testbed in a 5G New Radio (NR) Standalone (SA) deployment, verify protocol conformance through Packet Capture (PCAP) analysis against two Test Cases (TCs) from the O-RAN Alliance Working Group (WG) 5 F1 IOT Technical Specification (TS), and characterize performance across ten deployment scenarios that compare single-vendor baselines with multi-vendor pairings. We find that the throughput of our IOT pairings matches the corresponding single-vendor baseline within measurement uncertainty when each side is deployed with parameters compatible with the other, and that idle latency remains within the envelope set by the two baselines. One interoperable configuration exposes a failure mode in which Downlink (DL) throughput collapses to roughly 30% of the air-interface ceiling while both stacks remain fully compliant with the 3GPP and O-RAN requirements, a degradation that neither the 3GPP specifications nor the O-RAN Working Group (WG) 5 IOT suite currently constrains. From this finding we derive five backward-compatible specification recommendations, two addressed to 3GPP and three to the O-RAN Alliance, designed to interlock so that 3GPP defines the underlying requirements and the O-RAN Alliance binds their use as a condition of OTIC badging. As the first demonstration of cross-stack F1 IOT between OCUDU and OAI, this work enables multi-vendor experimentation on fully open-source RAN stacks. We release our software modifications in a public GitHub repository.