5G NR

Overview

5G NR (New Radio) is the radio interface of 3GPP’s fifth-generation system. Release 15 was the first full set of 5G specifications. According to 3GPP, the non-standalone NR specifications came first, in late 2017, with NR working inside an LTE network. Release 15 then grew to cover standalone operation with the new 5G core. Release 16 completed 3GPP’s IMT-2020 submission, and Release 17 added reduced-capability (RedCap) devices, NR up to 71 GHz and non-terrestrial networks, according to the 3GPP releases timeline. 3GPP applies the 5G-Advanced name and logo from Release 18 onwards.

NR keeps LTE’s foundations: OFDM with a cyclic prefix, HARQ and a split between control and shared channels. What changed is flexibility. One radio has to serve 410 MHz up to 71 GHz, wide and narrow carriers, and devices from phones to sensors. This hub is for engineers who know LTE or OFDM and need the NR concepts that make the specifications readable.

Key ideas

Scalable numerology

TS 38.211 (NR physical channels and modulation) defines the subcarrier spacing as 15 kHz × 2^μ, with μ from 0 to 6, so 15, 30, 60, 120, 240, 480 and 960 kHz. A slot always holds 14 OFDM symbols with the normal cyclic prefix. A wider spacing gives shorter symbols, so the slot shrinks from 1 ms at μ = 0 to 0.125 ms at μ = 3. The extended cyclic prefix exists only at 60 kHz. Wider spacing tolerates more phase noise and Doppler, which is why the millimetre-wave bands use it. Narrower spacing gives a longer cyclic prefix for large cells with long delay spread.

The frequency ranges are set in TS 38.101-1 and 38.101-2. FR1 runs from 410 MHz to 7125 MHz. FR2 has two parts, FR2-1 (24.25 to 52.6 GHz) and FR2-2 (52.6 to 71 GHz). Band n78, for example, is 3300 to 3800 MHz TDD. The OFDM and NR numerology reference tabulates the derived symbol and cyclic prefix lengths.

Bandwidth parts

An NR carrier in FR2 can be 400 MHz wide (TS 38.101-2), but a phone does not need to process all of it all the time. A bandwidth part (BWP) is a contiguous subset of the carrier with its own numerology. TS 38.213 allows a UE to be configured with up to four downlink and four uplink BWPs per serving cell. TS 38.300 states that one BWP is active per cell at a time, except in the supplementary uplink case. Switching to a narrow BWP when traffic is light saves power. BWPs also let a device with a smaller bandwidth capability use a wide carrier.

Beam management

In millimetre-wave bands, path loss forces high-gain antenna arrays, and high gain means narrow beams. The gNB sends synchronisation signal blocks (SS/PBCH blocks) in different directions at different times. TS 38.213 allows up to 64 candidate SS/PBCH blocks in a half frame in some configurations. The UE measures them and reports the best, and the network then refines the beam with channel state information reference signals. When the link fails, the UE starts beam failure recovery through a random access procedure (TS 38.300). Giordani and co-authors give a clear tutorial on NR beam management. The MIMO and beamforming hub covers the array side.

NG-RAN, the 5G core and Open RAN

TS 38.300 describes the NG-RAN. Its nodes are gNBs (NR) and ng-eNBs (LTE connected to the 5G core), linked to each other over Xn. They connect to the 5G core (5GC) over NG-C to the Access and Mobility Management Function (AMF) and over NG-U to the User Plane Function (UPF). The Session Management Function (SMF) controls the UPF. The 5GC architecture itself is specified in TS 23.501. A gNB can itself be split into a central unit and distributed units over the F1 interface (TS 38.401).

Open RAN goes further. The O-RAN Alliance was founded in February 2018 by AT&T, China Mobile, Deutsche Telekom, NTT DOCOMO and Orange, according to its about page. Its specifications define open interfaces between parts from different vendors. Polese and co-authors explain the architecture in Understanding O-RAN (IEEE Communications Surveys and Tutorials, 2023). The radio unit (O-RU) and distributed unit (O-DU) are joined by the Open Fronthaul interface using split 7.2x, in which the O-RU keeps the FFT and cyclic prefix handling. A near-real-time RAN Intelligent Controller (RIC) runs control loops of 10 ms to 1 s through applications called xApps over the E2 interface. A non-real-time RIC works above 1 s with rApps and guides the near-real-time RIC over A1.

Where to start

If OFDM is new to you, start with OFDM from scratch and the OFDM and multicarrier hub. Read the LTE hub for the system NR grew from. The 3GPP spec map shows which TS covers which layer, and the OFDM link with channel estimation project builds a pilot-based receiver of the kind NR uses. Research directions beyond Release 19 are in the 6G and research hub.

Tools and platforms

Vendor platform hubs: Ericsson, Nokia and Qualcomm for networks and chipsets, Keysight and Rohde & Schwarz for test equipment.

Common mistakes

• Treating “5G” as one thing. Say which release, which frequency range and whether the network is standalone or non-standalone.
• Assuming a network implements every feature of the release it supports.
• Mixing numerologies when computing slot timing. Always state μ.
• Taking a 15 kHz LTE intuition into FR2, where phase noise and beam alignment dominate.
• Forgetting that the active BWP, not the carrier, sets what the UE actually receives.
• Pointing at Open RAN as one product. It is a set of interfaces, and interoperability still has to be tested part by part.

Going deeper

• 3GPP TS 38.211, TS 38.213 and TS 38.300.
• Dahlman, Parkvall and Sköld, 5G NR: The Next Generation Wireless Access Technology, Elsevier, 2018.
• Lin and co-authors, 5G New Radio: Unveiling the Essentials of the Next Generation Wireless Access Technology, arXiv, 2018.
• Giordani and co-authors, A Tutorial on Beam Management for 3GPP NR at mmWave Frequencies, IEEE Communications Surveys and Tutorials, 2019.
• Polese and co-authors, Understanding O-RAN, IEEE Communications Surveys and Tutorials, 2023.