RedCap: A New Cellular IoT Technology for the 5G Era

Author : Dylan McGrath, Keysight Technologies

15 June 2023

5G Network
5G Network

Over the years, the mobile communication industry has introduced several wireless technologies that relate to Internet of Things (IoT) connectivity. These have included LTE Cat M, EC-GSM-IoT and NB-IoT. All were designed with the objective of adhering to the acute constraints that IoT presents.

The latest and most powerful cellular IoT technology to emerge is the first to be capable of leveraging the power of 5G. The 3GPP’s Release 17 contains enhancements that enable reduced capability (RedCap) devices to operate on 5G networks.

As the name implies, RedCap devices connect to the Internet via 5G, but do not have the full array of capabilities (or performance) of conventional devices, like smartphones or other more advanced items of user equipment (UE). While RedCap devices, such as wearables or IoT hardware, lack the full throughput, bandwidth and latency capabilities of the 5G specification, they typically have much lower power budgets and unit costs.

The 3GPP outlines three main RedCap use cases - industrial sensors, surveillance devices and wearables. Its 38.875 technical report specifies the maximum data rate, end-to-end latency and service availability for each of these use cases.
Cost/performance trade-offs
To lower complexity, cost and power consumption, RedCap devices use fewer antennas than standard 5G hardware. This not only curbs cost, but also reduces the maximum number of multiple-input multiple-output (MIMO) layers. RedCap devices only support 2x2 MIMO for the downlink and a single-input single-output (SISO) uplink. RedCap also limits device bandwidth capability compared to standard 5G UEs, in order to keep power amplifier costs down. Only bandwidths of 20MHz for frequency range 1 (FR1) and 100MHz for frequency range 2 (FR2) are used.

Another cost-saving enhancement introduced in 3GPP Release 17 for RedCap devices is support for half-duplex frequency-division duplex (FDD) transmission. While half-duplex FDD significantly reduces cost by enabling RedCap devices to use switches rather than the duplexers required for full-duplex FDD, it has the drawback that transmission and reception cannot be performed simultaneously. Consequently, such devices are unable to:
• Detect scheduling information for downlink and uplink in the same set of symbols.
• Monitor downlink messages while configured in uplink mode.
• Send uplink control information while monitoring the downlink.
In case of conflict, RedCap devices can decide what to do based on their particular implementation.
Power-saving enhancements
RedCap also implements several simplifications of 5G specifications to increase power efficiency, minimise power consumption and extend devices’ battery lives. For example, RedCap employs a streamlined network monitoring scheme compared to full 5G. RedCap devices limit the number of blind decoding and control elements monitored in the physical downlink control channel (PDCCH), reducing the amount of power required for these tasks. 

Another power-saving RedCap feature is extended discontinuous reception (eDRX). RedCap increases eDRX cycles when the device disconnects from the network or becomes idle, thereby substantially prolonging battery life. Longer eDRX cycles are particularly valuable in use cases like stationary wireless sensors, because eDRX cycle increases come with the trade-off of decreasing devices’ mobility performance. 

RedCap also relaxes radio resource management (RRM) requirements for devices not located at the cell edge and introduces a radio resource control (RRC) inactive state, which allows the UE to make small data transmissions without transitioning the RRC-connected state. Both enhancements are designed to save power and extend device battery life.
RedCap device complexity reductions
The final aspect of RedCap is reduced complexity. The upshot is that RF components can fit in devices with very small or unconventional form factors. This is very appealing in a wearable context, for instance. RedCap simplifications include support for only a single carrier (with no support for carrier aggregation) and support for single connectivity, enabling RedCap devices to operate in 5G standalone mode only. RedCap also supports 5G power class 3, which limits devices’ effective isotropic radiated power (EIRP) to reduce the size of their accompanying batteries.
Implications for the 5G network
The Release 17 enhancements outlined above are focused mainly on RedCap devices. RedCap enhancements also have implications for network infrastructure too though. The biggest impact on 5G networks is that they must adapt to the specific features of RedCap during the random-access process and when the device stays connected. New information elements (IEs) introduced for RedCap enable the bandwidth to adapt dynamically based on the actions taken by the device. For example, some of these IEs pertain to the permission for a RedCap UE to camp in a cell or do so using half-duplex mode. Some IEs are required to enable RedCap devices to connect to a cell. When these IEs are not detected, the cell will bar devices from connecting and they will have to start the RRC procedure to connect to another cell where the required IEs are present.

Some of the IEs relate to bandwidth part (BWP) configuration, which allows flexible spectrum usage to achieve power savings by dynamically adapting the assigned bandwidth depending on what the UE is doing. Low bandwidths affect the random-access channel (RACH) procedure used by devices for network access. The network can specify a BWP for RedCap devices or reduce BWP size for devices to attach to the network. However, this enhancement has limited value for RedCap, since these devices operate at a 20MHz maximum bandwidth.

The new signalling parameters and procedures introduced in Release 17 to enable network support of RedCap devices require engineers to check the compatibility of their devices to ensure connectivity. 5G device development teams also need tools to check RedCap parameters for debugging purposes.
RedCap market outlook
There is the prospect of widespread RedCap device adoption in the near future. Analysts expect initial 5G RedCap chipsets to appear later this year, with the first commercial RedCap devices entering the market in 2025/2026. After 2026, RedCap device deployment is expected to rise steeply. The drivers here will be consumer uptake of 5G-connected wearables for health monitoring, along with other applications like low-cost wireless sensors for industrial data collection and asset-tracking, plus surveillance devices for use in smart cities.

As with all cellular devices, comprehensive test methodologies and equipment are critical to bringing 5G RedCap devices to market. The right testing approach is needed to validate the protocol compliance and performance of 5G RedCap devices.

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