What is RedCap technology in 5G R17?

A few days ago, 3GPP announced the freezing of the 5G R17 standard. In the R17 version, the "Little Red Riding Hood" RedCap is particularly prominent and is hailed as an indispensable piece of cake for the 5G Internet of Things.

So what exactly is RedCap? Let’s talk about it below.

Why define RedCap?

Let's first look at the network capability requirements of three types of terminals: wearable devices (including wearable watches, AR/VR glasses, etc.), industrial wireless sensors, and surveillance cameras:

As we all know, 5G eMBB supports carrier bandwidths of more than 100MHz and a peak rate of up to 10Gbps; uRLLC supports millisecond-level latency and ultra-high reliability; and mMTC evolved from NB-IoT and eMTC in the 4G era, mainly supporting low-speed, low-cost, and low-power IoT connections with a bandwidth of less than 1.4MHz and a rate of less than 1Mbps.

However, it is not difficult to find from the above table that the network capability requirements of use cases such as wearable devices, industrial wireless sensors, and video surveillance are between the eMBB, uRLLC, and mMTC capabilities.

Therefore, in order to match these use cases with requirements for rate, device cost and battery life between eMBB, uRLLC and mMTC, 3GPP defined support for RedCap NR devices in Release 17.

From the perspective of technological evolution, on the one hand, 4G has lower-cost LTE Cat-1, Cat-3, and Cat4 medium-rate IoT standards, but there is no smooth upgrade path for 5G NR; on the other hand, 5G does not yet have a medium-rate IoT standard corresponding to 4G, and the existing modules that support 5G eMBB are relatively expensive. Therefore, in order to expand the IoT market in the 5G era, the industry urgently needs to define a simpler and lower-cost 5G medium-rate IoT standard.

RedCap not only has simpler functions and lower costs, but more importantly, compared with the 4G LTE medium-rate IoT solution, it is native to 5G and naturally has the advantages of 5G NR, such as support for a very wide range of frequency bands including millimeter waves, higher network efficiency, support for beamforming, and connection to the 5G core network. It can also coexist with NR eMBB and uRLLC on the NR carrier. Therefore, RedCap is expected to replace 4G medium-rate IoT and support a larger IoT market.

In R17, RedCap mainly supports the following use cases:

• Industrial sensors: pressure sensors, humidity sensors, thermometers, motion sensors, accelerometers, actuators, etc.
• Surveillance cameras: used in smart cities, factories and other industrial sites
• Wearable devices: smart watches, electronic health-related equipment, medical monitoring equipment, AR/VR glasses, etc.

In the R18 version, RedCap will further reduce terminal complexity, support a wider range of rates, and may add capabilities such as support for positioning, sidelink protocols, and unlicensed spectrum to support more use cases.

What capabilities were cut?

RedCap, the full name of which is Reduced Capability NR, literally means reducing NR capabilities. During the standard research process, some manufacturers also called it NR-Light or NR-lite. Later, 3GPP officially named it RedCap.

What capabilities did RedCap cut?

As shown in the table above, in R17, RedCap mainly cut the following functions, thereby reducing terminal complexity and cost by 50% to 65%.

• The maximum bandwidth is reduced from NR's 100MHz to 20MHz
• The minimum antenna configuration is reduced from NR's 2T4R to 1T1R or 1T2R
• The minimum number of downlink MIMO layers is reduced to 1
• The maximum modulation order can support at least 64QAM
• Half-duplex mode introduced

After maximizing functional tailoring, RedCap can support theoretical peak rates of approximately 20Mbps-100Mbps for both upstream and downstream in the FR1 frequency band, which is sufficient to meet most medium-speed IoT needs.

Of course, in order to achieve higher peak rates, RedCap terminal devices do not need to be tailored so drastically, and more advanced features can be selected, such as support for 2 receive antennas, 2 downlink MIMO layers, 256QAM, full-duplex FDD, etc.

Through tailoring, RedCap is close to the data rate capabilities of LTE Cat-4 terminals.

In addition to trimming the physical layer functions listed above, RedCap also trims some higher-layer functions, such as reducing the maximum number of DRBs from 16 to 8, reducing the length of PDCP SN and RLC-AM SN from 18 bits to 12 bits, and optionally supporting ANR functions.

What are the key technologies?

In addition to function tailoring, it is important to mention that in order to reduce the power consumption of terminal devices, RedCap introduced two energy-saving technologies in R17 that can extend battery life: eDRX and RRM measurement relaxation.

(1) eDRX

On the one hand, the data flow between the terminal and the network is usually intermittent or bursty; on the other hand, if the terminal keeps listening to the network paging, it consumes a lot of power. Based on this, in order to make the terminal more power-saving, the 5G R15 version introduces the DRX (discontinuous reception) mechanism for NR. In the RRC_IDLE (idle state) and RRC_INACTIVE (inactive state) states, the terminal periodically monitors the PDCCH channel to receive paging messages, and enters the sleep state (turns off the receiver) for the rest of the time without listening to paging, which can greatly reduce power consumption.

In NR, the maximum DRX cycle in RRC_IDLE and RRC_INACTIVE states is 2.56 seconds.

RedCap introduced the eDRX mechanism, which is an extended DRX mechanism as the name suggests. It extends the DRX cycle in the RRC_IDLE state to 10485.76 seconds (about 3 hours) and the DRX cycle in the RRC_INACTIVE state to 10.24 seconds, so that the terminal can sleep longer, save more power and extend battery life.

It is worth mentioning that when using the DRX mechanism, a trade-off needs to be made between terminal power consumption and latency, that is, the longer the DRX cycle, the greater the downlink latency may be. Since RedCap does not have strict latency and reliability requirements like eMBB/uRLLC use cases, this makes it possible for RedCap to adopt the eDRX mechanism.

(2) RRM measurement relaxation

In the RRC-IDLE and RRC_INACTIVE states, the terminal frequently performs RRM (Radio Resource Management) measurements to ensure that the terminal resides in the best available cell. RRM measurements mainly measure the RSRP and RSRQ values ​​of the serving cell and neighboring cells. Although RRM measurements can ensure that the terminal can obtain the best connection quality, they will consume battery power even when there is no data transmission between the terminal and the network.

In response to this, in the R16 version, an RRM measurement relaxation mechanism is defined for low-speed mobility and non-cell edge scenarios. When the low-rate condition is met for a certain period of time or the low-rate and non-cell edge conditions are met at the same time, the device is allowed to relax the adjacent cell measurement, such as increasing the RRM measurement period to reduce the number of adjacent cell measurements and the number of cell measurements, thereby reducing terminal power consumption. In the R17 version, the relaxation time is further extended for RedCap terminals.

In addition, as mentioned above, since RedCap is native to 5G and naturally has the advantages of 5G NR, RedCap can also be combined with 5G slicing, UPF sinking, uRLLC capabilities, coverage enhancement, 5G LAN, 5G positioning, terminal energy saving and other technologies to flexibly match a variety of medium-speed IoT applications, thereby better empowering all walks of life. ​