What is reduced capability (RedCap) NR?

到目前为止，在支持5G的设备的服务，订购和可用性方面，5G的推出速度已经超过了4G长期演进（LTE），这在最新的《爱立信移动性报告》中已清楚地表明了这一事实。此外，在未来几年中，预计5G背后的势头将继续强劲，预计2026年5G用户将达到35亿。

推动5G快速增长和迅速采用的引擎是其无线接入技术。作为新电台（NR）。5G NR的灵活性和可扩展性 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-202933, “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.

Table 1: Requirements of wearables, industrial wireless sensors, and video surveillance use cases

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.

Figure 1: Illustration of technology positioning of RedCap in relation to eMBB, URLLC, and mMTC.

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.

Table 2: Device capabilities, Release 15 baseline NR devices versus Release 17 RedCap devices.

Figure 2: Illustration of the differences between RedCap device and baseline NR device capabilities.

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.

对于RedCap设备，对FD-FDD的支持是可选的，即，在上行链路频率中传输时不需要在下行链路频率中接收，反之亦然。这种双工操作称为半双工FDD（HD-FDD）。HD-FDD消除了对双工滤波器的需求。取而代之的是，可以使用开关选择发射器或接收器以连接到天线。由于交换机比多个双工器便宜，因此可以节省成本。

此外，预计RedCap设备一次只能在单个频带中运行，并且将不支持载波聚合和双重连接。

根据3GPP中的研究，表3总结了相对于基线NR设备的物料清单成本和复杂性指标的总体降低。可以看出，可以实现成本和复杂性的大幅降低。这有助于将RedCap建立为与eMBB或时间紧迫的通信设备段不同的设备段。

鉴于上述降低的功能，对于具有一个接收分支的设备，RedCap设备在FR1中支持85 Mbps的峰值物理层数据速率，足以满足预期用例的所有数据速率和等待时间要求。对于支持更多接收分支的RedCap设备，峰值物理层数据速率要高得多。

	FR1 FDD	FR1 TDD	FR2
总减少量	~65%	如果RedCap设备支持2个接收分支，则约为58％ 如果RedCap设备支持1个接收分支，则约为71％	~48%

表3：相对于基线NR设备，RedCap设备的成本/复杂性指标的总降低。

LTE到NR的迁移路径

3GPP Release 17支持低容量NR设备的工作是进一步扩展5G NR潜在市场的重要一步。它使能力降低的设备能够在任何NR频带中运行。

值得一提的是，基于LTE的解决方案目前已解决了一些可穿戴和视频监控用例。对于这些用例，NR RedCap提供了从LTE迁移到NR的途径。这样的迁移路径很重要，因为它可以在未来数年内加速从LTE到NR的频谱重新分配。

从性能的角度来看，无论是网络还是设备，都存在采取从LTE到NR迁移的动力，因为RedCap是本机NR技术，它涵盖了所有关键的NR构建块，包括波束成形，可扩展命理，网络能效等。RedCap设备将支持在NR载波上完全共存，该NR载波已配置为针对eMBB或时间紧迫的通信性能进行了优化。

这对于工业无线传感器用例尤为重要，因为在工业4.0中实现全自动工厂的网络需要同时支持与功能更强的设备和低端传感器设备的时间紧迫的通信。可以优化此类网络的配置，以确保时间紧迫的通信性能，同时要求低端传感器设备仍然有效运行。