6G-era MIMO locks onto one-shot channel signals, but overhead cuts stay contested

Samsung, Huawei and Ericsson want the network to hand a phone its first uplink grant for channel reporting directly inside the MSG4 connection message; NTT DOCOMO, Qualcomm, vivo, Spreadtrum, NEC and Lenovo want it scheduled afterward with a separate command. Nineteen companies back the first approach, six the second, and after a full quarter of discussion RAN1 still could not close the gap — the proposal was noted, not adopted, leaving chipset vendors designing against an unresolved uplink-grant procedure.

Companies converged on aperiodic-only signaling for the moments that matter most: right after a phone connects, and right after a secondary cell wakes up. Tracking reference signals will arrive in up to N bursts triggered by a MAC-CE in MSG4, with the exact burst count still open; twenty-one companies backed this over periodic alternatives, with only Futurewei and Panasonic dissenting. For secondary cells, the activation command itself will now explicitly name which SRS and CSI resources to use — a scheme backed by 21 companies against a single holdout, Spreadtrum, which preferred a simpler pre-configured approach.

For massive MIMO arrays of 64 or 128 antenna ports, companies agreed to allow a channel-reference-signal density as low as one resource element per eight resource blocks per port — half the overhead of the previous 1/4 floor. Apple and Qualcomm pushed back on accuracy grounds, so the compromise restricts the thinnest setting to wideband reports using the Release 19 Type-I codebook and makes it a separate, optional phone capability. That means not every 6G-era phone will support the lowest-overhead mode, and networks planning to exploit it will need to check device capability first.

Two related overhead-saving ideas failed outright. Mixing different densities within one aggregated CSI-RS resource set was rejected 18-to-7, with Huawei and InterDigital on the losing side against Samsung, Apple, Nokia and 15 others; cycling the frequency position of reference signals across transmissions lost 13-to-7, with Huawei and Nokia again backing it against a majority led by Samsung, Apple and Ericsson. Both would have squeezed more accuracy from sparse signals but added receiver complexity that the larger camp wasn't willing to accept.

The uplink-scheduling split between the Samsung/Huawei and DOCOMO/Qualcomm camps is the item to watch — RAN1's next meeting needs to pick MSG4-embedded scheduling or a post-MSG4 grant, because chipset teams can't finalize the connection procedure until that's settled. Separately, whether the 1/8 density extends beyond wideband reporting to subband and Type-2 codebooks remains for further study, which will decide how much overhead savings networks actually get in practice.