The stripped-down version of 5G New Radio, called 5G Red Cap (Reduced Capability) came to prominence with the release of 3GPP Release 17, which supports it, in mid-2022, even though its full capabilities will not arrive until Release 18 enters products during 2024.
Originally dubbed NR-Light, Red Cap is pitched as the version of 5G fit for many IoT applications in both the enterprise and consumer spheres and work is already underway incorporating the technology in smartwatches, as well as industrial monitoring devices. However, its emergence has also drawn criticism and even cynicism from those who see it as a fix for 5G’s own problems and an attempt to smother alternative non-cellular low-power WAN technologies for IoT, notably LoRaWAN.
It is certainly an attempt to unify cellular’ s own 4G-based LPWAN variants, NB-IoT and LTE-M, with a single 5G system going forward from Release 18. It is seen as the glue that could connect enhanced mobile broadband (eMBB) to certain capabilities in the other two main 5G use cases, ultra-reliable low-latency communications (URLCC), and massive machine-type communication (mMTC). This is depicted in a famous Venn diagram, originally drawn by Ericsson and widely reproduced by analysts, bloggers and consultants, where Red Cap intersects the other three and acts as the join.
Some critics argue that the IoT applications for which Red Cap is positioned are already well served by the four main LPWAN variants that are optimized for low-power wireless communications – NB-IoT, LTE-M, LoRaWAN plus Sigfox. But that misses two key points.
The first is that Red Cap was designed in the first instance as an intermediate option or layer between LPWAN and full-blown 5G for an emerging range of IoT use cases. The smartwatch is one example where, to date, connection to the Internet has usually been enabled by tethering to smartphones via Bluetooth or possibly WiFi. Red Cap, with its lower power requirements, is a viable option for direct connectivity, for which the LPWAN options would be unsuitable for various reasons, such as lack of the required bandwidth.
The second is that Red Cap has been designated as the 5G successor to NB-IoT and LTE-M, which are both rooted in LTE. This prepares the ground for eventual retirement of LTE, while providing higher performance than LPWAN with similar lower cost and power consumption. It is true that LPWAN and Sigfox as the non-cellular options can respond but it is difficult seeing them being preferred for some of the new use cases, with enhancements likely at best likely just to shore up their existing base.
It is also worth considering Red Cap in the wider context of cellular evolution under 5G and beyond, and in this context Ericsson’s Venn diagram is already looking a bit dated. The 3GPP has already recognized four, rather than three prongs or ‘vectors’ in 5G evolution, adding local sidelink communication out-of-band as this becomes increasingly prominent as a separate grand use case. And the industry itself identifies many more broad use case categories, often separating AI and extended reality (XR) capabilities, for instance.
Under 3GPP’s revised schema, eMBB, as specified in Release 15 for consumer handsets and fixed wireless access (FWA), is the first vector, as before. The second vector is also the same, URLLC as specified in Release 16, addressing time-critical edge computing and future XR applications. The third vector is now recast for the IoT generally, extending beyond just mMTC to embrace Red Cap from Release 17 onwards.
The fourth vector is sidelink, which came in for cellular V2X in the automotive sector, for local communications between vehicles with each other and with roadside infrastructure. However, from Release 17 onwards, sidelink will almost certainly be supported for other use cases, including telemetry and Industrial IoT, where it will incorporate Red Cap. In fact, Red Cap is also likely to be embraced within automotive C-V2X where the extra capacity and performance will be welcome too.
So, although the original motivation for Red Cap was to support wireless devices with reduced capabilities compared with 5G, it has come to be viewed equally as a more capable successor to LPWAN. So, it supports devices that are less complex and costly, and require enhanced power efficiency for prolonged battery life in the field, compared with say smartphones, but at the same time still delivers more 5G-like performance.
This can be seen by considering the three main use cases set out for Red Cap devices by 3GPP. Even the lowest category, wireless industry sensors, carries a specification of 2Mbps data rate and under 100ms latency, which only LTE-M, of the four primary LPWAN variants, is capable of supporting.
Next up the ladder come wearables at data rates of up to 25Mbps, so even though this has greater latency tolerance than the first Red Cap use case, it is way beyond any LPWAN option.
Finally comes surveillance which, as it includes video, requires up to 150Mbps for the downlink and 50Mbps up, and latency below 500ms.
To meet these requirements while cutting cost and power consumption significantly in full 5G, a number of stratagems are being employed. One is to rein back on antenna proliferation under 5G, which immediately reduces complexity and cost while still leaving enough headroom to meet the more modest demands of IoT. Red Cap only supports 2×2 MIMO for the downlink, and single-input single-output (SISO) for the uplink. Maximum operating bandwidth is also curtailed, with a maximum of 20 MHz for frequency range 1 (FR1) in midrange spectrum and 100 MHz for frequency range 2 (FR2) in the millimeter wave, reducing amplifier costs.
We note then that Red Cap does support mmWave, which at first sight might seem surprising given the focus on reduced capabilities. But the mmWave has the advantage for IoT of low range, which means the same frequency band can be reused many times within an area of application, in agriculture for example. In these cases, there would be no need for costly beamforming or other methods to overcome signal degradation.
There are also techniques to reduce power consumption, both in the device and network. Among various stratagems, Red Cap implements the system frame number (SFN) technique to increase extended discontinuous reception (eDRX) cycles when the device disconnects from the network or becomes idle, as a result reducing power consumption significantly.
Such extended eDRX cycles are especially useful in industrial sensors that only require intermittent connectivity. SFN is a counting mechanism to help synchronize protocols at the physical layer to ensure smooth handover between downlink and uplink channels in half duplex operation where they must take turns.
Red Cap then can be seen as the unifier of LPWAN and 5G. But more than that, it is part of 5G’s evolution towards energy efficient and cost-effective operation.