3GPP Release 16: 5G Phase 2 — Industry 4.0 and Precision
If Rel-15 defined 5G, Rel-16 completed it. The first release targeted almost exclusively eMBB — enhanced mobile broadband. Rel-16 delivered the other two 5G use cases: URLLC for industrial automation and machine control, and positioning accuracy sufficient for indoor navigation. It also brought 5G to vehicles, unlicensed spectrum, and wireless backhaul — dramatically extending the reach and versatility of the 5G standard.
Overview
Rel-16 was the first 5G release completed entirely under pandemic conditions — the June 2020 freeze was postponed by three months because remote standardisation meetings could not maintain the pace of in-person sessions. Despite this, Rel-16 delivered more new work items than any previous 5G release. The central commercial rationale was simple: operators and their enterprise customers needed 5G to replace industrial Ethernet cables, not just upgrade consumer mobile broadband.
The air-interface enhancements in Rel-16 are tightly coupled: Configured Grant removes scheduling latency, PDCP duplication removes packet loss, and mini-slot aggregation manages HARQ timing — the three mechanisms work together to achieve the URLLC targets that factory automation requires.
URLLC and IIoT — The Industrial 5G Case
Ultra-Reliable Low-Latency Communications targets applications where packet loss or latency spikes cause physical consequences: factory robot arm collisions, conveyor belt mis-timing, and remote surgical tool overshoot. The requirements are strict — end-to-end air-interface latency below 1 ms and packet error rate below 10⁻⁶ (five-nines reliability per packet). Three Rel-16 mechanisms work in concert to meet these targets:
In normal NR scheduling, an uplink transmission requires the UE to send a Scheduling Request, wait for a UL grant from the gNB, and then transmit — a sequence that takes 4–8 ms. With a Configured Grant, the network pre-allocates periodic uplink resources to the UE at configuration time. When data arrives, the UE transmits immediately into the next CG occasion without any scheduling exchange. Effective uplink latency collapses to one slot — approximately 0.5 ms at 30 kHz numerology. Multiple simultaneous CG configurations can be active with different periodicities for different traffic types.
The same PDCP packet is sent simultaneously over two independent radio paths — for example, two component carriers or two cells in Dual Connectivity. If one path suffers an outage (interference, shadowing, handover gap), the other carries the packet successfully. The receiver discards the duplicate. The tradeoff is exactly double the resource consumption for duplicated bearers, but in URLLC scenarios this is acceptable because the payload size is small and the reliability gain is essential.
Mini-slot aggregation completes the URLLC toolkit. Mini-slots can be as short as 2 symbols (versus 14 for a full slot). Rel-16 allows consecutive mini-slots to be aggregated with independent HARQ processes per mini-slot — each attempt is acknowledged separately rather than waiting for a full-slot HARQ timer. The gNB can retransmit individual failed mini-slots immediately, reducing the retransmission latency from several milliseconds to a fraction of a millisecond.
TSN Integration — 5G Meets Factory Ethernet
Time-Sensitive Networking (IEEE 802.1 TSN) is the industrial Ethernet standard used by factory automation PLCs, Profinet, and EtherCAT controllers. Its defining requirement is deterministic timing: packets must arrive within microseconds of their scheduled window. A factory robot arm expects a position command at exactly the right moment — arriving 100 µs late is as bad as not arriving at all.
Rel-16 maps the 5G network into a TSN bridge. From the perspective of the factory PLC, the 5G network appears as a standard IEEE 802.1Q Ethernet switch with a known, bounded latency. The mechanism has two key components:
- PTP grandmaster synchronisation: the gNB synchronises to the factory's IEEE 1588 Precision Time Protocol (PTP) grandmaster clock, achieving sub-microsecond time alignment. All NR timing — slot boundaries, HARQ timers, and configured grant periodicities — is locked to factory time.
- UPF as TSN translator: the UPF translates between TSN traffic profiles and 5G QoS flows. TSN stream reservations are mapped to 5G QoS classes with matching latency and jitter bounds. The factory controller configures TSN; the 5G network honours those parameters transparently.
The result is that existing TSN-capable industrial equipment connects wirelessly over 5G without any modification. A factory floor can remove Ethernet cables entirely and gain mobile machine placement flexibility — a key enabler for reconfigurable production lines.
NR-V2X — 5G Vehicle Communications
NR sidelink replaces LTE-V2X PC5 with a significantly more capable variant. Where LTE-V2X (Rel-14) supported only broadcast — one vehicle transmits, all nearby vehicles receive — NR-V2X adds unicast and groupcast. Key improvements:
A direct bidirectional session between two vehicles with full HARQ acknowledgement — the receiving vehicle sends ACK/NACK back to the transmitter, enabling reliable retransmission. LTE-V2X broadcast had no feedback mechanism; if the packet was lost, neither side knew. Unicast HARQ brings reliability from roughly 90% to exceeding 99.9% in typical V2X environments.
NR-V2X can use FR1 bands with up to 100 MHz channel bandwidth, compared to LTE-V2X's 10 MHz at 5.9 GHz. This opens the door to sensor-data sharing applications that require continuous high-rate streams — a following vehicle receiving a leading vehicle's compressed radar point cloud at 5–10 Mbps to see around corners.
Two resource management modes are defined. Mode 1 is network-scheduled — the gNB assigns sidelink resources, giving the network full visibility and control.Mode 2 is autonomous — the UE senses which resources nearby UEs are using and selects unoccupied resources, enabling operation without network coverage. The Rel-16 Mode 2 algorithm is a significant improvement over the Rel-14 sensing baseline, reducing collision probability in dense vehicle environments.
IAB — Wireless Backhaul
Integrated Access and Backhaul (IAB) allows a gNB to relay its traffic wirelessly back to the network using the same NR air interface, rather than requiring a fibre connection at every site. An IAB node plays a dual role:
- As a UE (MT — Mobile Termination): the IAB node connects to a donor gNB or another IAB node on the backhaul link, appearing as a regular UE on the Uu interface.
- As a gNB (DU — Distributed Unit): the IAB node serves regular UEs on the access link using standard NR. From those UEs' perspective, it is a normal base station.
Multiple IAB hops can be chained, allowing coverage to extend several hops from a fibre-connected donor. The backhaul and access links share the same NR radio channels but are time-division multiplexed — the IAB node does not transmit on backhaul and access simultaneously on the same resource, avoiding self-interference. Timing and synchronisation propagate hop-by-hop from the donor.
IAB directly addresses the economics of dense mmWave deployment. Running fibre to every lamppost or building facade where a 28 GHz small cell would go is often physically impossible and commercially prohibitive. IAB removes that constraint, enabling dense FR2 small-cell grids in city centres without trenching.
NR-U and 5G Positioning
NR unlicensed operates in the 5 GHz and 6 GHz bands alongside Wi-Fi, using a Listen-Before-Talk (LBT) mechanism to share the channel fairly. NR-U can operate standalone — no licensed anchor required — or as supplementary downlink to a licensed NR carrier. A private enterprise 5G network using NR-U pays zero spectrum licence fees while still enjoying 5G QoS, network slicing, and 5GC integration. The LBT Category 4 procedure is equivalent to Wi-Fi's EDCA, ensuring coexistence.
Rel-16 achieved better than 1 m horizontal accuracy in controlled indoor conditions using a combination of: DL-TDOA (Time Difference of Arrival from multiple transmission/reception points), UL-TDOA (arrival time measured at multiple TRPs), AoD (Angle of Departure from gNB beams), and AoA (Angle of Arrival). At mmWave, beam widths are narrow enough to estimate angle to within a fraction of a degree — combining angle and time-of-arrival gives sub-metre resolution even without GPS.
Why Rel-16 Mattered
- Configured Grant URLLC made private 5G for factory automation commercially viable — the combination of pre-allocated resources and PDCP duplication delivers the reliability that replaces industrial Ethernet, something LTE could never offer.
- TSN integration enabled wireless replacement of factory Ethernet for the first time — existing industrial controllers connect over 5G without modification, removing the largest barrier to 5G adoption in manufacturing.
- NR-V2X unicast set the stage for cooperative automated driving — HARQ feedback and wider bandwidth enable sensor-data sharing between vehicles, the key capability for Level 4 convoy automation.
- IAB reduced the cost of dense 5G deployments dramatically — removing the fibre-to-every-site requirement unlocks mmWave at scale in dense urban areas where underground fibre is cost-prohibitive.
- Sub-metre 5G positioning opened new revenue streams in retail (indoor navigation, location-based offers), logistics (asset tracking in warehouses), and emergency services (caller location in multi-storey buildings).