# 5G New Radio

## Summary


5G defines a new radio interface called **5G New Radio (NR)**. Rather than being something new, it should be seen as an evolution of LTE technology. In fact, the term **Next Generation Radio Access Network (NG-RAN)** is commonly used and it covers both 5G NR and LTE/E-UTRA radio access. 

5G attempts to address many new use cases not possible in earlier generations. 5G NR plays an important role in fulfilling these new cases. All layers of the 5G NR protocol stack are enhanced but perhaps most changes are within PHY. 

While there are many 3GPP specifications that define 5G NR, beginners can start with TS 38.300 and TS 38.401 that give high-level descriptions.

## Discussion

### Which are the 5G NR technical enablers for achieving 5G use cases?

The three broad 5G use cases supported by 5G NR are Enhanced Mobile Broadband (eMBB), Massive Machine-Type Communications (mMTC) and Ultra-Reliable Low-Latency Communications (URLLC). 

5G NR spans spectra from sub-GHz to mmWave bands. This enables deployments of macro cells to picocells. Both licensed and unlicensed bands are allowed. Wide bandwidths at mmWave bands enable eMBB. Multiple bands can be combined to offer higher data rates and boost capacity. 

For URLLC, reliability is improved via multi-antenna transmission, multiple carriers and packet duplication. Latency is reduced by mini-slot transmission, grant-free uplink access, and eMBB resource pre-emption. Control and reference signals appear at the start of a slot. 

To cater to wide range of bands and deployments, OFDM Sub-Carrier Spacing (SCS) is flexible while maintaining time-domain alignment. As SCS goes up, the slot duration shrinks. For example, wider SCS might suit URLLC and a narrower SCS for MBB/mMTC. 

There are many ways to aggregate slots with guard periods between uplink and downlink. Thus, we can have short transmissions for URLLC and longer ones for eMBB. 


### What are the key design principles behind 5G NR?

We identify the following: 

    + **Flexibility**: OFDM sub-carrier spacing and symbol duration are flexible such that services with different requirements of bandwidth and latency can coexist. The design allows for wide range of deployments from outdoor macrocells to indoor picocells. Wide spectrum from sub-GHz to mmWave bands are supported. Symbol allocation within a slot is flexible.
    + **Forward Compatibility**: Design is such that it's easy to introduce new use cases in future. This is enabled by self-contained slots and beams, that is, they can be decoded without dependency on other slots or beams. This also relates to design flexibility: details are configured at runtime rather than fixed in specifications.
    + **Ultra-Lean Design**: LTE transmits regular reference signals, synchronization signals and system broadcasts. 5G is designed to minimize such "always on" transmissions. This improves network energy efficiency and reduces interference in high traffic load conditions. This also assists forward compatibility. Another related approach is device-centric mobility. A mobile sends out periodic reference signals that a base station measures, rather than monitor reference signals from many nearby cells.

### What's the architecture of 5G NR?

**Next Generation Radio Access Network (NG-RAN)** consists of gNB and ng-eNB. *gNB* serves a 5G UE over 5G New Radio (NR), a new air interface developed for 5G. gNB connects to 5G Core, though some can connect to 4G EPC as well. *ng-eNB* connects to 5G Core but serves a 5G UE over E-UTRA radio. 

gNB and ng-eNB are interconnected via **Xn interface**. In the user plane, Xn uses GTP-U over UDP/IP. In the control plane, Xn uses XnAP over SCTP. Thus, signalling packets have guaranteed delivery whereas user plane packets don't. Xn-U does data forwarding and flow control. Xn-C facilitates UE mobility management and dual connectivity. 

5G NR nodes connect to 5G Core via **NG interface**. Like Xn, NG-U uses GTP-U over UDP/IP, and NG-C uses NG-AP over SCTP. More specifically, NG-U connects to User Plane Function (UPF) and NG-C connects to Access and Mobility Management Function (AMF) in the core. NG-C and NG-U are also called N2 and N3 interfaces respectively. 


### What's the 5G NR protocol stack?

The air interface between the UE and gNB is usually described in two planes: 

    + **User Plane (UP)**: Carries user data. Consists of PHY, MAC, RLC, PDCP and SDAP. SDAP is new in 5G (doesn't exist in LTE).
    + **Control Plane (CP)**: Carries signalling such as sending system broadcasts, paging UEs, establishing connection, handover, measurement reporting, NAS messaging, etc. Consists of PHY, MAC, RLC, PDCP and RRC. In addition, the UE has Non-Access Stratum (NAS) layer that terminates on the network side in the 5G Core.PHY is commonly called Layer 1. MAC/RLC/PDCP/SDAP are called sub-layers of Layer 2. 

When transmitting, SDAP maps QoS flows to data radio bearers. PDCP maps (data or signalling) radio bearers to RLC channels. RLC maps RLC channels to logical channels. MAC does the scheduling and maps logical channels to transport channels. PHY maps transport channels to physical channels. When receiving, the layers map in the opposite direction. 


### What are the key principles or features of NG-RAN?

An essential principle of NG-RAN is the logical separation of signalling and data transport networks. NG-RAN and 5GC functions are separated from transport functions, even if they happen to reside in the same equipment. Mobility of an RRC connection is fully controlled by NG-RAN. 

NG-RAN nodes can be further disaggregated into Radio Unit (RU), Distributed Unit (DU) and Centralized Unit (CU). By locating these parts at cell site or at cloud edge different deployment options are possible. Operators can decide based on the use case, CAPEX and OPEX. 3GPP has standardized the **F1 interface** between gNB-DU and gNB-CU, the **W1 interface** between ng-eNB-DU and ng-eNB-CU, and the **E1 interface** between gNB-CU-CP and gNB-CU-UP. Other industry bodies might standardize the RU-DU interface. 

NG-RAN has many functions: Radio Resource Management (RRM); routing of packets to AMF or UPF; encryption and integrity protection of data; IP and Ethernet header compression; connection setup and release; scheduling and transmission of system broadcasts and paging; QoS flow management; radio access network sharing; dual connectivity; support of network slicing; and more. 


### What role does each protocol layer play in NG-RAN?

SDAP maps a QoS flow to a data radio bearer. It's not relevant to the control plane. 

PDCP does header compression/decompression using ROHC or EHC protocols. It encrypts/decrypts packets. It adds/checks integrity of packets. It supports both in-order and out-of-order delivery. It discards duplicates. 

RLC supports three transmission modes: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). An RLC entity's functions are based on its mode. In AM, there's error correction through ARQ, duplicate detection and error detection. In AM and UM, SDUs can be segmented and there's sequence numbering; plus, SDUs can be discarded. 

MAC maps SDUs from one or more logical channels to transport channels. It does padding. It does error correction through HARQ, one per Component Carrier (CC). It prioritizes transmission towards UEs (downlink only), across logical channels of a UE, and UE resources. 

Among the PHY procedures are link adaption, power control, cell search, random access, and HARQ. PHY processing includes code block segmentation and CRC attachment, LDPC coding, rate matching, scrambling, modulation, layer mapping, and mapping to assigned resources and antenna ports. 


### How do 5G NR L2 sub-layers differ from LTE's E-UTRA sub-layers?

**SDAP** is new in 5G NR. 5G Core can configure different QoS requirements for different IP flow of a PDU session. SDAP maps IP flows to data radio bearers that can provide the necessary QoS. 

**PDCP** does packet duplication when sending, and reordering and duplicate detection when receiving. This is done to satisfy URLLC use case. PDCP also includes integrity protection for user plane data. 

Unlike LTE, **RLC** doesn't concatenate RLC SDUs since something similar is done at MAC. Likewise, 5G RLC doesn't do reordering since the reordering done at PDCP is deemed adequate. 

**MAC** carries signalling for beam management done at PHY. 

Whereas LTE RRC has only two states (*Idle* and *Connected*), 5G **RRC** includes *Inactive* as an additional state. This enables devices to save power and quickly reconnect with minimal signalling. UE can also request specific system information instead of NG-RAN periodically sending the same. 


### What are the enhancements to 5G NR in Release 16?

We summarize the main enhancements:  

    + **MIMO**: MU-MIMO support of higher rank, multiple transmission and reception points (multi-TRP), better multi-beam management (useful in mmWave band), and extended uplink coverage (full power UL transmission).
    + **URLLC**: Using Coordinated Multi-Point (CoMP) reliability is improved. CoMP uses multi-TRP. Improved HARQ, flexible scheduling, inter-device service multiplexing and intra-device channel prioritization are other changes.
    + **Time-Sensitive Networking (TSN)**: For industrial automation, TSN provides time-deterministic delivery of data packets.
    + **Power Save**: Wakeup Signal (WUS) lets a device skip the next low-power DRX (discontinuous reception) monitoring period.
    + **Integrated Access and Backhaul (IAB)**: IAB allows a base station to provide wireless backhaul to neighbouring base stations.
    + **NR-U**: 5G can operate in unlicensed spectrum, either with a licensed/shared spectrum as anchor or in standalone mode.
    + **Non-Public Network (NPN)**: For industrial IoT, private networks with small cells, dedicated resources and low latency are possible.
    + **Cellular-Vehicle-to-Everything (C-V2X)**: NR-based sidelink is introduced. Multicast groups are defined based on distance and applications.
    + **Positioning**: Meets the accuracy requirements of 3 meters (indoor) and 10 meters (outdoor).


## Milestones

Mar  
2017

First drafts of two high-level documents *TS 38.300: NR and NG-RAN Overall description; Stage-2* and *TS 38.401: NR-RAN; Architecture description* are published.  

Dec  
2017

3GPP approves the first specifications for 5G, called "early drop" of Release 15. Specifically, it ratifies **Non-Standalone (NSA) 5G New Radio (NR)** specification. This enables vendors to start implementing the first 5G products. NSA 5G will allow operators to leverage existing 4G infrastructure. However, it can't support some use cases that require ultra-low latency and higher capacity. 

Jun  
2018

Also part of Release 15, and called "main drop", 3GPP approves many specifications for 5G including the **Standalone (SA)** option. This allows operators without 4G networks to offer 5G service. 

Mar  
2019

3GPP approves "late drop" of Release 15. This might aid in migrating from 4G to 5G, or NSA 5G to SA 5G. However, some vendors and operators don't see this as essential since Release 16 or 17 specifications could offer alternatives.  

Jul  
2020

3GPP finalizes **Release 16** specifications. This adds support for unlicensed spectrum. It improves on latency, power consumption, positioning and cellular-to-vehicle connectivity. Existing features enhanced by Release 16 include MIMO, beamforming, Dynamic Spectrum Sharing (DSS), Dual Connectivity (DC) and Carrier Aggregation (CA).