What is reduced capability (RedCap) NR and what will it achieve?
The driving force behind 5G is New Radio (NR). Now, Reduced Capability (Red Cap) New Radio is helping expand the NR device ecosystem, enabling the growth of even more 5G use cases. Find out what exactly NR RedCap is, and what it intends to achieve.
So far, the rollout of 5G in terms of services, subscriptions and availability of 5G-capable devices is outpacing that of 4G Long-Term Evolution (LTE), a fact clearly shown in the latest M2M Systems Mobility Report. In addition, the momentum behind 5G is expected to continue to be strong in the coming years, with a forecast of 5G subscriptions reaching 3.5 billion in 2026.
The engine for driving 5G forward for fast growth and rapid adoption is its radio access technology, referred to as New Radio (NR). The flexibility and scalability of 5G NR makes it possible to introduce timely enhancements to address new use cases to help expand the 5G ecosystem and connect more and more devices to the network. One recent example is NR support for reduced capability (RedCap) devices. This work item has recently been approved in the 3GPP RAN plenary in December 2020 and the feature will be introduced in 3GPP Release 17. The introduction of reduced capability NR devices can facilitate the expansion of the NR device ecosystem to cater to the use cases that are not yet best served by current NR specifications.
Use cases
The use cases that motivate the specification work on NR RedCap include wearables (e.g. smart watches, wearable medical devices, AR/VR goggles, etc.), industrial wireless sensors, and video surveillance. The key requirements of these use cases, described in the 3GPP document RP-2023933, New WID on support of reduced capability NR devices are summarized in Table 1. To maximize the benefit of economies of scale, it is desirable that all these three use cases can be addressed by a common NR RedCap framework.
Referring to Table 1, these three use cases have less stringent data rate requirements than enhanced mobile broadband (eMBB) use cases, and do not require tight or deterministic latency requirement as time-critical communications use cases. Therefore, starting from the Release 15 NR devices as a baseline, there is room for trading off device capabilities for complexity or cost reduction.
On the other hand, these use cases have very different requirements than the low-power wide-area (LPWA) use cases currently addressed by the LTE-M and NB-IoT solutions. For example, the data rates need to be higher than for LPWA. Furthermore, there is a constraint on device form factor for certain wearable use cases. The consideration of use-case requirements drives the choices of key physical-layer parameters for RedCap. These choices have a direct impact on the complexity and cost of the device hardware platform. We foresee that RedCap devices will be positioned as a lower segment than eMBB, but higher than LPWA devices.
The technology positioning of RedCap is illustrated in Figure 1. Generally speaking, RedCap is positioned to address use cases that are today not best served using eMBB, ultra-reliable low-latency communications (URLLC) or LPWA solutions.
Reduced device capabilities
So how is cost reduction achieved? The capabilities of a RedCap device compared to those of Release 15 NR devices are summarized in Table 2 and illustrated in Figure 2. Bandwidth reduction, reducing the maximum number of MIMO layers, and the relaxation of the maximum downlink modulation order all help reduce baseband complexity. Reducing the minimum number of required receive branches and allowing half-duplex (HD) operations in all bands help reduce the bill of material costs in terms of antennas and RF components. Each of these reduced capability features are described in more details below.
Maximum device bandwidth: A baseline NR device is required to support 100 MHz in frequency range 1 (FR1), and 200 MHz in FR2, for transmission and reception. For RedCap, these requirements are reduced to 20 MHz and 100 MHz, respectively. Such bandwidth reductions however still allow all the physical channels and signals specified for initial acquisition to be readily reusable for RedCap devices, therefore minimizing the impact on network and device deployment when introducing RedCap to support the new use cases.
Minimum number of device receive branches: The number of receive branches is related to the number of receive antennas. Reducing the number of receive branches therefore results in a reduction in the number of receive antennas and cost saving. The requirements on the minimum number of receive branches depends on frequency bands. Some frequency bands (most of the FR1 frequency-division duplex (FDD) bands, a handful of FR1 time-division duplex (TDD) bands, and all FR2 bands) require a baseline NR device to be equipped with two receive branches, whereas some other frequency bands, mostly in the FR1 TDD bands, require the device to be equipped with four receive branches.
For the bands where a baseline NR device is required to be equipped with a minimum of two receive branches, a RedCap device is only required to have one receive branch. For the bands where a baseline NR device is required to be equipped with a minimum of four receive branches, it is yet to be decided whether a RedCap device is required to have one or two receive branches.
Maximum number of downlink MIMO layers: The maximum number of downlink MIMO layers for a RedCap device is the same as the number of receive branches it supports. This is a reduction compared to the requirements for a baseline device.
Maximum downlink modulation order: A baseline NR device is required to support 256QAM in the downlink in FR1. For a RedCap device, the support of downlink 256QAM is optional. For FR1 uplink and FR2, both downlink and uplink, a RedCap device is required to support 64QAM, same as the requirement for a baseline device.
Duplex operation: Regarding duplex operations, the only relaxation is for operations in FDD bands. A baseline NR device is required to support a full duplex (FD) operation in an FDD band, i.e., transmitting and receiving on different frequencies at the same time. A typical full-duplex device incorporates a duplex filter to isolate the interference between the device’s transmit and receive paths. In practice, the same device may need to support multiple FDD bands; therefore, multiple duplex filters may be needed to support the FD-FDD operation.
For a RedCap device, the support of FD-FDD is optional, i.e., it is not required to receive in the downlink frequency while transmitting in the uplink frequency, and vice versa. Such a duplex operation is referred to as half duplex FDD (HD-FDD). HD-FDD obviates the need for duplex filters. Instead, a switch can be used to select the transmitter or receive to connect to the antenna. As a switch is less expensive than multiple duplexers, cost savings are achieved.
Furthermore, a RedCap device is expected to operate in a single band at a time and will not support carrier aggregation and dual connectivity.
Based on the study in 3GPP, the total reduction in bill of material cost and complexity metric, relative to a baseline NR device is summarized in Table 3. It can be seen that substantial cost and complexity reduction can be achieved. This helps establish RedCap as a distinct device segment from the eMBB or time-critical communication device segment.
Given the reduced capabilities described above, a RedCap device supports a peak physical layer data rate of 85 Mbps in FR1 for devices with one receive branch, sufficient to fulfill all the data rate and latency requirements of the intended use cases. For RedCap devices supporting more receive branches, the peak physical layer data rates are much higher.
The LTE-to-NR migration path
The 3GPP Release 17 work on the support for reduced capability NR devices is an important step to further expand the addressable market of 5G NR. It enables a reduced capability device to operate in any of the NR frequency bands.
It’s worth mentioning that some of the wearable and video surveillance use cases are currently addressed by LTE-based solutions. NR RedCap offers a path for migrating from LTE to NR for these use cases. Such a migration path is important as it can accelerate the spectrum re-farming from LTE to NR a number of years down the road.
From the performance point of view, both network and device, there is also an incentive to take such an LTE-to-NR migration path, as RedCap is a native NR technology, which embraces all the key NR building blocks, including beamforming, scalable numerology, network energy efficiency, and so on. A RedCap device will support full coexistence on an NR carrier that is configured to be optimized for eMBB or time-critical communication performance.
This is particularly important for industrial wireless sensor use cases as the network for enabling fully automated factories in Industry 4.0 would need to support both time-critical communications with more capable devices and lower-end sensor devices. The configuration of such networks may be optimized for ensuring the performance of time-critical communications while requiring the lower-end sensor devices to still operate efficiently.
