Satellite IoT drops cover codes as GNSS-free access proves harder than expected

Satellite IoT capacity ran into a wall this quarter: RAN1 spent effort studying orthogonal cover codes (OCC), a technique to let several devices share one radio resource via distinct signal patterns, and concluded it could not agree whether OCC even helps for the random-access channel. The result is that Release 19 IoT-NTN devices keep the older, non-multiplexed access procedure — a capacity gain that simply won't ship this round.

The bigger fight is over what happens when GPS disappears. A dedicated study confirmed that in the worst case — a phone with no location fix anywhere in a satellite beam — no existing 5G preamble format, long or short, can handle the combined delay and Doppler shift across all elevation angles. In S-band, tolerance limits for long preambles range from just 0.85 kHz up to 24.6 kHz depending on configuration; in Ka-band, short preambles reach 54 kHz at 60 kHz spacing and 114 kHz at 120 kHz spacing. Only with a rough prior location fix does a single format — 'C2' — work across all satellite types, and only within a 5 km radius in S-band or 1 km in Ka-band. That gap is why RAN1 also prioritized a list of candidate fixes, from network-side pre-compensation to longer detection windows at the satellite.

A quieter but real dispute involves phase continuity for OCC-multiplexed uplink transmissions. Ericsson, Huawei, HiSilicon, NTT DOCOMO, Nokia, LGE and Panasonic wanted explicit rules forcing a phone not to adjust its timing advance mid-OCC-group; Samsung, Vivo and CATT argued this is a network scheduling responsibility, not a phone behavior rule. RAN1 could not reach consensus, so the specification stays silent — networks must now avoid the conflict purely through careful scheduling.

NB-IoT satellite voice picked up its first firm commitments: at least one uplink and one downlink semi-persistent scheduling (SPS) configuration, explicit activation/release commands rather than automatic RRC activation, and a rule that downlink SPS reception wins any collision with uplink SPS on the same device. But the signaling method for those activation commands — Downlink Control Information (DCI) versus a MAC Control Element (MAC CE) — remains open, and it is nearly Qualcomm alone favoring a closer look at MAC CE against a bloc including Samsung, Ericsson, Huawei, Apple and Google that leans DCI.

RAN1#124bis is where the SPS signaling choice is due — the group set its own deadline there, citing only two remaining meetings to decide between DCI and MAC CE for satellite voice. The outcome determines how many simultaneous voice calls a satellite operator can pack into its control channel, and whether chipset vendors design around a fast but capacity-limited DCI path or a heavier MAC CE alternative.