By RAN1 A-IoT Rapporteurs; Xiaodong Shen (CMCC) and Matthew Webb (Huawei)

First published June 2025, in Highlights Issue 10

3GPP is developing new IoT technologies supporting passive devices - battery-less or with limited energy storage - powered by harvesting ambient energy from the environment, with ultra-low complexity and small form factor.

The new IoT technology will provide complexity and power consumption orders-of-magnitude lower than the existing 3GPP LPWA technologies (e.g. NB-IoT and eMTC) and will address use cases and scenarios that cannot otherwise be fulfilled based on existing 3GPP LPWA IoT technologies.

In Release 18 WG SA1 and TSG-RAN analyzed use cases, deployment scenarios, traffic, connectivity topologies, device constraints, service requirements, KPIs, design targets, and required functionalities for Ambient IoT solutions. A follow-up Study Item (SI) in Rel-19 explored more-detailed radio aspects. It is worth noting that WGs SA2 (Architecture) & SA3 (Security and Privacy) are also carrying out studies in their areas.

The WG RAN1 Rel-19 A-IoT Work Item (WI) focuses on two representative use cases, i.e. indoor inventory (rUC1) and indoor command (rUC4) defined in TR38.848. Deployment scenario 1 with Topology 1 (D1T1) is considered, where D1 refers to device and base station reader (BS) indoors, and T1 refers to device directly and bidirectionally communicates with the BS reader. The targeted spectrum is FR1 licensed FDD spectrum, with R2D in DL spectrum and D2R and carrier-wave in UL spectrum.

The overall Rel-19 objective is to standardize the so-called A-IoT ‘device 1’, which has ~1 µW peak power consumption, energy storage, RF envelope detector receiver, initial sampling frequency offset (SFO) up to 10⁵ ppm, neither R2D nor D2R amplification in the device. The device’s D2R transmission is backscattered on a carrier wave (CW) provided externally.

Rel-19 A-IoT supported traffic types include Device-terminated (DT) and Device-originated device-terminated triggered (DO-DTT). Device-originated autonomous (DO-A) is not supported due to active transmission in D2R not being supported.

The A-IoT devices envisioned for Rel-19 consume tiny amounts of power, at very low cost. This will be achieved using components with wide manufacturing tolerances and variability under environmental conditions e.g. by temperature profile. They will contain no active RF components with their device-to-reader (D2R) transmissions done via passively modulating an external RF carrier wave using simple circuitry - powered for a short time from stored energy.

The stored energy can be harvested from ambient RF, thermal or dedicated energy supply nodes. This aspect is unspecified in Rel-19. The device’s receiver, for R2D, is typically a very simple envelope detector, which can detect how the overall power of the signal varies, but cannot detect any detailed structure of it, unlike conventional UEs which can decode complicated OFDM signals.

The Study Item generated a reference architecture for device 1, shown in Figure 1. More advanced devices were also studied, which included adding a reflection amplifier to the passive device (2a), and also very low-cost active devices (2b). The standardization of some of those concepts is targeted for Rel-20.

Such a device architecture requires the simplest modulations. In R2D, square-wave on-off keying (OOK) is used, and in D2R the device chooses from OOK or BPSK, both based on square wave signals. Although the R2D design is based on OFDM from the reader transmitter’s point of view, this is to allow convenient co-generation with NR. Many steps of the conventional 3GPP L1 are removed to accommodate the device’s low power: in both R2D and D2R, there is no scrambling, interleaving, MIMO, separate control channels, L1 HARQ, nor use of complex numbers in designing signals. There is no channel coding in R2D, to avoid device processing load, but the LTE convolutional code is used in D2R, to benefit from the sophisticated reader’s receiver.

Manchester line-coding is used in R2D and D2R, which does not have a comparable step in NR. This takes the square-wave modulation and inserts additional frequent ON-OFF signal transitions. In R2D the device’s receiver can use these edges as a timing reference of the high-quality timing reference in the BS transmitter to adjust its internal receiver clock to maintain tracking. This removes any need for absolute synchronization between reader and device. In D2R transmissions, a binary sequence-based preamble and periodic midamble (e.g., m-sequence) are inserted to allow the reader to keep track of the device’s clock drift.

R2D multiplexing is only TDMA. The reader can vary the R2D data rate by modulating the OOK square wave at different rates – this does not permit FDMA, due to the wideband envelope detector in the device. For D2R, the line-code can be used to effectively repeat an information bit at a faster rate, and create a higher-frequency square wave, allowing devices to be frequency-separated according to their repetition rates i.e., FDMA. This provides higher capacity during an inventory procedure with hundreds of devices to read in a few seconds.

The overall access scheme of A-IoT is network-triggered, so a device only responds to a transmission from the reader. D2R access is based on slotted-ALOHA, a long-known protocol providing simple randomization and back-off among multiple addressed nodes contending for resources in the time domain. There is also a contention-free access when a particular device is targeted. The protocol layers for A-IoT remove the vast majority of L2 and L3: there is no RRC, PDCP, SDAP, ARQ, mobility, AS security, and other traditional higher-layer functions. The concept of paging is retained, by which the network indicates the beginning of e.g. an inventory process.

There is a new network architecture, shown in Figure 2, introducing the ‘A-IoT Function’ (AIOTF) which can communicate directly with the RAN, or indirectly via an AMF. The AIOTF is the termination of the NAS protocol with the device. It manages the services relating to triggering RAN operations, and other aspects relating to transfer of information between an application and the RAN or device.

A globally unique A-IoT permanent device identifier is allocated to each device. The identifier has several parts to it, which can correspond to e.g. a PLMN ID, information type ID, electronic product code (EPC) etc. The CN can indicate different matching ‘masks’ to address subsets of the devices based on those parts of the ID, and only matched devices respond with e.g. their EPC or other stored information. The structure of a permanent device ID is shown in Figure 3.

In Rel-19, WG RAN4 studied the RF requirements impact, CW interference cancellation feasibility, testability issues for an A-IoT device, and co-existence between A-IoT and NR. RAN4 first discussed the deployment scenario for co-existence and decided to focus on macro BS outdoor and A-IoT system indoor. Depending on different spectrum usage and interference conditions, RAN4 evaluated co-existence between A-IoT and NR for 36 interference cases. According to the evaluation results, co-existence between NR and A-IoT system is feasible. In some cases, the interference from another system (NR or A-IoT) is negligible, while in other cases, interference is observed and some potential mechanisms to mitigate the interference are required. Details can be found in TR38.769.

Finally, evolution targeting more advanced devices is expected to significantly enhance key performance metrics including coverage, capacity, and data rates. With higher permitted device power consumption and complexity, whilst remaining orders of magnitude below 3GPP’s LPWA technologies, active D2R transmission with internally generated signals can provide superior coverage performance, and Zero-IF/IF receivers exhibit better link performance compared to RF envelope detectors.

Although Topology 2 - which enables intermediate UEs to function as a reader by relaying to a BS - is not included in Release 19, its potential incorporation in Release 20 could expand the application scope of A-IoT. It is expected that the deployment of A-IoT solutions on licensed spectrum will introduce novel capabilities and create new market opportunities in the future.