Abstract
3GPP Release 17 integrates multicast-broadcast services (MBS) into the 5G system, yet open and instrumentable over-the-air implementations remain scarce. We present a 5G MBS testbed combining a 5G-MAG core, an MBS-enabled OCUDU gNB, and modified OpenAirInterface UEs operating over software-defined radios. The prototype implements broadcast-session signaling, multicast N3mb ingest, Group Radio Network Temporary Identifier (G-RNTI) addressed scheduling, blind UE decoding, and application delivery. In a 30-minute point-to-multipoint (PTM) trial, 1,801 sequence-numbered text messages were broadcast at one message per second. The first 29 were transmitted while the UEs were still completing registration and multicast group joining, and are therefore outside the observable interval rather than radio losses. Over the remaining interval, both UEs recovered all 1,772 messages with byte-exact payloads: 100\% delivery over the observable interval and 98.389\% of all messages transmitted. This open-source platform provides a reproducible foundation for over-the-air 5G MBS experimentation and future research.