5G RedCap (NR-Light): the natural successor to 4G LTE Cat 1 and Cat 4

This article originally appeared in the Nov'23 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

The following chapter in the cellular connectivity story is, of course, 5G. We’re already seeing widespread adoption of 5G, particularly in the consumer smartphone space. But in the IoT, take-up has been noticeably slower, and, in a couple of segments. This is because areas such as consumer wearables, heavy industrial equipment, and smart factory devices, are not especially well catered for by the original 5G specifications, which debuted in 3GPP Release 15 in 2018.

If you’re involved in the design of any of these classes of device, then there’s good news: 3GPP Release 17 introduces 5G New Radio (NR) Reduced Capability. This generally gets shortened to 5G RedCap or referred to as NR-Light. This provides the missing piece of the 5G jigsaw, making 5G a viable option for these classes of product.

In this article, we’ll identify the gap that RedCap fills, then explore its key characteristics. We’ll also touch on how it differs from 4G LTE Cat 1 and Cat 4, and some of the factors to consider when deciding whether 5G RedCap is the right cellular connectivity solution for your devices today and over the coming years.

A quick look at the original 5G specifications

To understand why RedCap is needed, we need to take a quick look at the original 5G specifications. 5G NR debuted in June 2018, as part of 3GPP Release 15. This defines three main use case pillars, with differing characteristics. First, there’s enhanced mobile broadband (eMBB). This has become the main deployment driver for 5G, primarily in the consumer smartphone space. eMBB provides higher data rates and lower latencies than 4G LTE. Second, is ultra-reliable low-latency communication (uRLLC). As the name suggests, this is aimed at mission-critical and latency-sensitive applications, by providing the lowest-possible latency and highest network reliability. It’s expected to see a boom in popularity with the expansion of autonomous vehicles, industrial automation, and robotics.

The third area is massive machine type communication (mMTC). Aimed at the IoT, this focuses on minimising power consumption and enhancing indoor coverage, compared to legacy cellular technologies. Interestingly, while LTE-M and NB-IoT technically sit under the 4G LTE umbrella, they met ITU IMT-2020 5G mMTC requirements, so are officially part of the 5G mMTC family.

At this point, we also need to note a couple of other points about 5G NR. Developed specifically for 5G, it uses two frequency ranges. FR1 is for bands up to 7.125GHz, and FR2 is for bands between 24.25 GHz and 71.0 GHz. The range above 24 GHz is also referred to as millimeter wave (mmWave).

There’s a gap…

For all that’s good about the original 5G specification, there was a piece missing: a spec tailored to the needs of mid-range IoT and consumer-grade applications, many of which will currently be using LTE Cat 1 or Cat 4 for their cellular communication.

In the consumer space, this will be things like smartwatches and other wearables. Elsewhere, examples include smart factory applications such as sensor networks, video surveillance, industrial wearables, or autonomous guided vehicles, all of which require medium data rates. The telematics, remote diagnostics, and fleet-management devices used in construction, agriculture, and mining equipment, are other examples. These too require medium data rates, as well as the ability to remain operational for many years to come – in some cases, potentially beyond the retirement of 4G networks.

In these use cases, both eMBB and uRLLC are over-specified, and therefore, among other considerations, not cost-effective. At the same time, mMTC solutions, such as LTE-M and NB-IoT, don’t meet the performance demands of these types of mid-range device.

Enter 5G RedCap: the successor to LTE Cat 1 or Cat 4

Happily, 3GPP Release 17 introduced the missing piece of the puzzle: 5G NR Reduced Capability – or RedCap for short. As we touched on above, it’s sometimes also referred to as 5G NR-Light. In terms of its capabilities, 5G RedCap sits below eMBB and uRLLC, but above LTE-M and NB-IoT, so is in the sweet spot for applications that currently use LTE Cat 1 or Cat 4.

In Release 17, RedCap devices must support bandwidths of up to 20MHz in FR1 and 100 MHz in FR2. Given that cost considerations mean we expect most RedCap devices to be deployed in the FR1 spectrum, the maximum bandwidth many devices will need to support is 20MHz.

RedCap supports data rates up to the LTE Cat 4 range when deployed at 20MHz bandwidth. Rates may differ, based on the network setup and type of duplex operation. For example, operation in full duplex frequency division duplexing (FD-FDD) at 20MHz bandwidth (considering receive diversity) with 256-QAM, could achieve up to 227Mb/s (downlink) and 91Mb/s (uplink).

FR1 and FR2 band range support, in theory, is the same as with 5G eMBB devices. This will help facilitate a simpler rollout for mobile network operators worldwide. Having said that, mobile operators may ultimately decide to deploy RedCap on a more limited set of bands than eMBB kit.

From a power consumption perspective, RedCap adds extended discontinuous reception (eDRX) cycles in inactive and idle modes and relaxes neighbor-cell measurements for stationary devices. This helps to bring down power consumption compared to eMBB.

RedCap also does other things to bring down device complexity, including fewer antennas and downlink multiple input multiple output (MIMO) layers, downlink modulation order, and duplex operation. To learn more about all these areas, our ‘Cellular technology evolution for IoT applications in the 5G era’ whitepaper investigates the technicalities of 5G RedCap in more detail.

Comparing 5G RedCap to 4G LTE Cat 1 and Cat 4

For most designers creating devices requiring the type of cellular connectivity that 5G RedCap will provide, it will come down to a straight choice between RedCap and 4G LTE Cat 1 or Cat 4. In certain configurations, you’ll be able to achieve higher peak data rates and lower latencies with RedCap than with LTE Cat 4 (which has a peak downlink rate of around 150Mbps). RedCap will also give you an intrinsic connection to the 5G Core in the private network, which will be of benefit if network longevity is a key concern.

Another factor will be your timelines. When will your device be launching, and what geographical regions will it need to operate in? We expect to see the first 5G RedCap devices coming to market during 2024 in some of the early cellular adopter countries, such as those in North America, as well as China and other Asia-Pacific nations.

How long will customers expect your equipment to remain operational for? If the kit has a very long expected lifespan, then adopting the newer technology is likely to make more sense, to ensure it can continue to connect to mobile networks well into the 2030s.

The evolution of RedCap: 3GPP Release 18

As ever, the 3GPP standards continue to evolve at pace, and Release 18, expected in early 2024, is set to expand RedCap’s support to cover additional use cases. These are mostly lower-tier IoT devices with capabilities that sit between existing low-power wide-area (LPWA) user equipment and Release 17 RedCap user equipment. Examples include industrial wireless sensor networks, and smart grids.

Release 18 RedCap will target a peak data rate of around 10Mb/s. This could be achieved by lowering the user equipment baseband bandwidth to 5MHz for data channels in FR1, while keeping the user equipment’s RF bandwidth at 20MHz. It would also help to minimize device ecosystem fragmentation. Importantly, Release 18 RedCap is not seeking to replace existing LPWA solutions, since these will remain better in terms of indoor coverage and power consumption.

Where to find out more

For those creating solutions that currently use LTE Cat 1 or Cat 4 to send data to and from remote devices, the launch of the 5G RedCap specification signals it’s time to start exploring how you’ll eventually replace 4G cellular in your designs and ecosystems. Because even though 4G networks are some years away from being retired, the long-lived nature of many devices using LTE Cat 1 and Cat 4, means it’s smart to begin mapping out your next steps today