Unlocking RedCap UE performance: Challenges and solutions in 5G technology

The world of 5G is evolving rapidly. A prime example of this progress is the introduction of New Radio (NR) support for reduced capability (RedCap) devices in Release 17, completed in Q3 of 2022.

Release 17 has ushered in a new category of user equipment (UE) that expands the NR device ecosystem, introducing fresh features and advancements to 5G mobile networks. These UEs are known for their reduced complexity and cost, making them well-suited for power-efficient 5G applications like wearables, video surveillance, and industrial sensors.

However, while this new UE innovation promises to reshape the 5G landscape, its development has presented unique challenges. In the following sections, we will delve into these challenges and explore how Ericsson helped address them.

Exploring the key hurdles

To support these emerging applications, new devices must strike a balance between reduced cost and complexity while delivering extended battery life – distinguishing them from existing 5G New Radio (NR) devices.

Achieving lower complexity and cost in the reduced capability UEs is realized by reducing the supported channel bandwidth and the number of receive antenna branches. Meanwhile, reduced energy consumption in the UE is made possible through the use of an extended discontinuous reception mechanism and more relaxed radio resource management (RRM) measurements compared to conventional 5G NR devices. As a result, the 3GPP UE performance requirements originally designed for more advanced devices needed to be relaxed to match the capabilities of RedCap UEs.

Figure 1: Ensuring that both legacy UE and RedCap UE can be operated in the same cell.

Development of requirements

In response to these challenges, it became crucial to develop specific requirements for RedCap UEs. Within the 3GPP standardization framework, Ericsson led the effort to define these requirements within 3GPP RAN Working Group 4. In this section, we will discuss the specific goals we aimed to achieve and the methods we employed to address these challenges.

Key challenges

1. Early and broad deployment
   Our first goal was to enable the early and widespread deployment of RedCap in commercial networks as shown in Figure 1. To achieve this, we focused on:
   • Ensuring that RedCap UEs could support all legacy NR bands.
   • Reusing requirements from legacy NR UE with 2 receiver (Rx) branches in Frequency Range 1 (FR1) and Frequency Range 2 (FR2) and from NR UE with limited transmission and reception channel bandwidth.
   • Minimizing any impact on base station RF characteristics, including avoiding the need for hardware changes. As a result, no new base station RF requirements were defined for RedCap meaning that the installed base of network equipment can support RedCap trough a software update.
2. Power efficiency
   Our second goal was to enable power-efficient operation (or lower battery consumption) by:
   • Introducing a new FR2 power class 7 (PC7) UE with a minimum effective isotropic radiated power (EIRP) of 16.4 dBm. This solution enables considerable UE power saving, compared to the legacy handheld-type FR2 Power Class 3 (PC3), which has a minimum EIRP of 22.4 dBm.
   • Supporting mobility in the network: To support UE mobility within the network, UEs periodically perform measurements on neighboring cells, which consume battery power. To address this, we introduced new relaxed UE measurement requirements during low-activity states (known as RRC IDLE and RRC INACTIVE). These new requirements allow RedCap UEs to measure reference signal quality less frequently, or in some cases, not at all, for up to 4 hours when specific mobility conditions were met. These conditions considered the mobility level of the UE, distinguishing between fully stationary devices and those with limited mobility, providing more power-saving options.
   • Additionally, thanks to Ericsson leading the RedCap work, we defined requirements for devices configured with extended discontinuous reception (eDRX) periodicity for up to three hours in one of the low-activity states (RRC IDLE). This further enhanced power efficiency while supporting robust mobility and paging reception for all RedCap UEs, including those supporting HD-FDD.
3. Complexity reduction
   The third and final goal was to reduce the complexity of RedCap UEs in various scenarios, including:
   • Supporting 1 Rx branch operation.
   • Removing the duplexer to enable HD-FDD operation.
   • Reducing the assumed total antenna elements, going from 8 in FR2 for legacy NR PC3 UEs to 4 antenna elements in FR2 for PC7 RedCap UEs.
   • Operating using non-cell-defining (NCD) synchronization signal block (SSB).

   Given that existing 3GPP UE radio resource management (RRM) and demodulation requirements were primarily based on 2 Rx branches, reducing the number of Rx branches to 1 called for new downlink performance requirements. New requirements specific to 1 Rx operation had to be defined for all Radio Resource Control (RRC) states and existing procedures. Furthermore, operating half-duplex mode UEs in NR FDD bands was a new challenge, requiring special handling to ensure the availability of all necessary downlink/uplink resources for HD-FDD UEs within a cell, needed for various operations such as paging reception, cell identification, and handover.

   To address these challenges, we conducted simulation studies to assess the impact of 1 Rx branch RedCap UE operation compared to legacy operation using 2 Rx branches, proving the need for new measurement accuracy requirements for the 1 Rx branch UE. We led the effort to identify solutions to address this impact and standardized those solutions.

   In the context of synchronization signals in NR, there are two types: cell-defining SSB (CD-SSB) and non-cell-defining SSB (NCD-SSB). Legacy NR measurement and mobility procedures are primarily based on measurements performed on CD-SSB, which is transmitted on the carrier frequency during initial cell access. However, RedCap UEs could operate in a narrower RedCap-specific bandwidth part (BWP), where NCD-SSB could be transmitted, as indicated by the network after connection establishment. We played a leading role in defining new requirements for mobility procedures based on NCD-SSB, identifying relevant and practical mobility scenarios using NCD-SSBs and ensuring timely definition of requirements. Additionally, we led efforts to introduce new definitions for intra- and inter-frequency measurements based on NCD-SSB.

   Finally, the limited bandwidth of RedCap compared to legacy NR devices had an impact on legacy UE demodulation and Channel State Information (CSI) reporting requirements. We initiated the work to define demodulation requirements for all downlink channels, including PDSCH, PDCCH, and PBCH. For the CSI reporting component, we introduced new requirements, considering the 1 Rx capability, half-duplex Frequency Division Duplex (FDD) operation, and the limited bandwidth.

Leading the RedCap work in 3GPP standardization

As the rapporteur of the RedCap work item in 3GPP, Ericsson is proud to have played a pioneering role in the standardization of RedCap technology within the 3GPP framework. Our efforts have revolved around defining essential UE requirements for reduced capability UEs, spanning three critical domains: RF characteristics, RRM requirements, and demodulation performance. These contributions have been instrumental in shaping the future of 5G technology, particularly for applications requiring cost-effectiveness, energy efficiency, and streamlined complexity.

As stated earlier, the low complexity and cost aspects had impact on legacy 3GPP technical specifications resulting in most of the legacy NR UE requirements could not be reused and therefore defining of new RedCap specific requirements required was necessary. In addition, Ericsson team also had the privilege to lead the work on defining the performance test cases to verify the correct UE behavior according to the core requirements and to ensure overall UE and also network performance to quick and easy commercial roll-out the RedCap by the operators.

The author thanks esteemed colleagues who are part of the work and contributed to this blog post: Kazuyoshi Uesaka, Chunhui Zhang, Muhammad Kazmi, Zhixun Tang, Zhanxian Griselda Wang

Learn more

Read the article: RedCap - expanding the 5G device ecosystem for consumers and industries

Read the blog post: What is reduced capability (RedCap) NR and what will it achieve?

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