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5G New Radio: Unveiling the Essentials of the Next Generation Wireless Access Technology

Xingqin Lin, Jingya Li, Robert Baldemair, Thomas Cheng, Stefan Parkvall, Daniel Larsson, Havish Koorapaty, Mattias Frenne, Sorour Falahati, Asbjörn Grövlen, Karl Werner

arXiv:1806.06898v1cs.NI

TL;DR

5G NR must support diverse use cases and spectrum conditions. This article surveys 3GPP NR specifications, focusing on physical-layer channels, signals, and design rationales. It presents NR as a flexible air interface for varied requirements, deployments, and future wireless services.

  • Problem

    5G NR must accommodate use cases from eMBB through URLLC to mMTC, across varied spectrum and deployment conditions.

  • Method

    The article provides an accessible overview of 3GPP NR specifications, emphasizing physical-layer channels, reference signals, and their design rationales.

  • Results

    The article presents NR as a flexible air interface capable of meeting a wide range of requirements, use cases, and deployments.

  • Takeaways & Limitations

    NR provides a foundation for the future evolution of wireless communications services.

Abstract

from arXiv · show

The 5th generation (5G) wireless access technology, known as new radio (NR), will address a variety of usage scenarios from enhanced mobile broadband to ultra-reliable low-latency communications to massive machine type communications. Key technology features include ultra-lean transmission, support for low latency, advanced antenna technologies, and spectrum flexibility including operation in high frequency bands and inter-working between high and low frequency bands. This article provides an overview of the essentials of the state of the art in 5G wireless technology represented by the 3GPP NR technical specifications, with a focus on the physical layer. We describe the fundamental concepts of 5G NR, explain in detail the design of physical channels and reference signals, and share the various design rationales influencing standardization.

I. THE BIRTH OF 3GPP 5G NEW RADIO

5G NR was developed by 3GPP to support diverse use cases, while this article explains its technical specifications with an accessible focus on the physical layer.

  • 5G NR targets enhanced mobile broadband, ultra-reliable low-latency communications, and massive machine type communications.
  • 3GPP developed NR specifications through Release 15, completing non-standalone NR in December 2017 and standalone NR in June 2018.
  • The article examines detailed NR technical specifications and design rationales while remaining accessible to wireless communications and networking audiences.
  • Its treatment mainly focuses on Layer 1 physical-layer specifications, with higher layers such as MAC and RRC kept to a minimum.
  • The article covers NR design fundamentals, synchronization and access channels, shared and control channels, reference signals, and conclusions.

A. Waveform, Numerology, and Frame Structure

NR combines flexible waveform, numerology, frame, spectrum, and resource designs to support deployments ranging from sub-1 GHz to millimeter-wave bands while reducing latency, interference, and device power use.

  • A. Waveform, Numerology, and Frame Structure: NR adopts CP-OFDM for downlink and uplink, while supporting DFT-S-OFDM in the uplink to improve coverage.CP-OFDM supports low implementation complexity and low cost for wideband and MIMO operation.
  • A. Waveform, Numerology, and Frame Structure: NR supports subcarrier spacings of 2^μ·15 kHz for μ = 0, 1, …, 4 across FR1 and FR2 spectrum deployments.The cyclic prefix scales down by 2^-μ from LTE’s 4.7 μs, adapting numerology to carrier frequency and deployment conditions.
  • A. Waveform, Numerology, and Frame Structure: A 10 ms frame contains 10 subframes, while mini-slot transmission can begin at any OFDM symbol and use only the symbols required.This supports very low latency for critical data and can minimize interference to other links.
  • B. Resource, Carrier, and Bandwidth Part: NR supports up to 400 MHz carriers, substantially exceeding LTE’s 20 MHz maximum, while ultra-lean operation minimizes always-on transmissions.The design is associated in the passage with higher network energy efficiency and lower interference.
  • B. Resource, Carrier, and Bandwidth Part: Carrier aggregation, NR-LTE spectrum overlap, supplementary uplink, and supplementary downlink support high-frequency coverage and migration from LTE.NR can map PDSCH around LTE CRS and flexibly place channels to minimize collisions with LTE reference signals.
  • B. Resource, Carrier, and Bandwidth Part: NR can reserve configurable unused PDSCH resources to support forward compatibility with physical-layer solutions for currently unknown use cases.
  • B. Resource, Carrier, and Bandwidth Part: A bandwidth part is a contiguous subset of RBs, with up to four configurable per direction but only one active per direction at a time.Bandwidth adaptation lets the UE use narrow bandwidth during ordinary periods and switch wider for bursty traffic.

C. Modulation, Channel Coding, and Slot Configuration

NR uses LTE-like modulation options alongside new control and data-channel coding schemes, and flexible slot symbols enable configured or dynamically determined uplink and downlink allocations.

  • C. Modulation, Channel Coding, and Slot Configuration: NR modulation includes B/QPSK and 16-, 64-, and 256-QAM, while control channels use Reed-Muller and CRC-assisted polar codes.NR data channels use rate-compatible quasi-cyclic LDPC codes rather than LTE turbo codes.
  • C. Modulation, Channel Coding, and Slot Configuration: NR supports FDD, semi-static TDD, and dynamic TDD, with dynamic TDD suited to traffic variation in small or isolated cells.Semi-static TDD may better address interference in large over-the-rooftop cells.
  • C. Modulation, Channel Coding, and Slot Configuration: Each slot symbol can be configured as DL, UL, or flexible, with cell-specific and UE-specific RRC settings determining allocations.The framework can reproduce LTE TDD slot patterns.
  • C. Modulation, Channel Coding, and Slot Configuration: When no slot configuration is provided, resources default to flexible and DCI can dynamically determine whether symbols carry downlink or uplink transmissions.

A. Synchronization Signals and Physical Broadcast Channel

NR synchronization, broadcast, and random access designs provide cell identification, timing, system information, beam-swept initial access, and deployment-flexible uplink timing acquisition.

  • A. Synchronization Signals and Physical Broadcast Channel: An SSB combines synchronization signals and PBCH, allowing a UE to obtain cell identity, downlink synchronization, PBCH timing, and basic system information.SSB subcarrier spacing is 15 or 30 kHz in FR1 and 120 or 240 kHz in FR2.
  • A. Synchronization Signals and Physical Broadcast Channel: NR PSS uses a BPSK-modulated length-127 m-sequence to address time and frequency offset ambiguity associated with LTE’s Zadoff-Chu PSS.NR SSS is generated using a BPSK-modulated Gold sequence.
  • A. Synchronization Signals and Physical Broadcast Channel: An SSB occupies 4 OFDM symbols and 240 contiguous subcarriers, while an SS burst set can transmit up to 64 SSBs across beams within a 5 ms window.The default burst-set periodicity is 20 ms, supporting beam sweeping while minimizing always-on transmissions.
  • B. Physical Random Access Channel: PRACH transmits a UE random-access preamble to indicate an access attempt and help the gNB adjust uplink timing.NR uses Zadoff-Chu sequences for their constant amplitude and correlation properties.
  • B. Physical Random Access Channel: NR random-access preambles support long and short sequence lengths with multiple formats for a wide range of deployment scenarios.The length-839 formats target large FR1 cells, while length-139 formats target small, normal-cell, indoor, and FR2 deployments.

IV. PHYSICAL SHARED CHANNELS

NR shared channels support downlink user-data delivery through PDSCH, with flexible processing, layer mapping, and resource handling. PDSCH also supports configurable resource avoidance for coexistence and forward compatibility.

  • PDSCH carries downlink user data, UE-specific higher-layer information, system information, and paging.
  • Downlink transport blocks undergo CRC attachment, LDPC base-graph selection, code-block segmentation, LDPC encoding, and rate matching.NR defines one LDPC base graph for small transport blocks and another for larger ones.
  • Scrambled and modulated PDSCH symbols map onto up to 4 MIMO layers, while two codewords support up to 8-layer transmission.Beamforming and MIMO precoding remain transparent to the UE and are determined by network implementation.
  • PDSCH transmissions can avoid configurable resources at RB-, symbol-, or RE-level granularity, including LTE CRS resources on shared carriers.This enables resource blanking for future services without disrupting backward compatibility.
  • Physical-layer processing for NR PDSCH is summarized in the left part of Figure 4.

B. Physical Uplink Shared Channel

PUSCH carries uplink shared-channel data and Layer 1/2 control information using flexible layer mapping and optional transform precoding. Its frequency-before-time mapping supports early receiver decoding.

  • PUSCH carries uplink shared-channel data and Layer 1/2 control information.The uplink shared channel is the transport channel for an uplink transport block.
  • PUSCH supports one codeword mapped onto up to 4 layers, with optional DFT transform precoding for single-layer transmission.
  • Uplink layers can use either non-codebook-based or codebook-based transmission for mapping to antenna ports.
  • Unlike LTE, PUSCH mapping is performed in frequency before time to enable early decoding at the receiver.

V. PHYSICAL CONTROL CHANNELS

NR control channels provide configurable downlink scheduling and uplink feedback mechanisms. PDCCH uses configurable CORESET resources, while PUCCH flexibly carries uplink control information across time and frequency.

  • PDCCH: PDCCH carries downlink scheduling assignments and uplink scheduling grants through downlink control information.
  • PDCCH: NR PDCCH transmission is confined to configurable CORESETs whose RB and OFDM-symbol sets are defined with search spaces.This differs from LTE control channels distributed across the entire system bandwidth.
  • PDCCH: A CORESET spans 1-3 consecutive OFDM symbols, and PDCCH uses UE-specific DMRS, 1, 2, 4, 8, or 16 CCEs, and interleaved or non-interleaved CCE-to-REG mapping.
  • PUCCH: PUCCH carries uplink control information including HARQ feedback, CSI, and scheduling requests.
  • PUCCH: NR PUCCH flexibly allocates time and frequency resources and defines five formats for short and long transmissions.Formats 0 and 2 use 1 or 2 OFDM symbols; formats 1, 3, and 4 are long PUCCHs.
  • PUCCH: A UE selects among up to 4 PUCCH resource sets for HARQ-ACK transmission according to UCI size.The first set supports at most 2 HARQ-ACK bits, while other sets support larger UCI sizes.

VI. PHYSICAL REFERENCE SIGNALS

NR reference signals follow lean-carrier principles by being on-demand and configurable, reducing unnecessary overhead while adapting to channel and mobility requirements. DMRS supports channel estimation for each associated physical channel.

  • NR reference signals are transmitted on demand when possible, with configurable time-frequency distributions that minimize overhead.At low load, transmissions can be extremely sparse, reducing energy consumption and intercell interference.
  • NR replaces functions tied to LTE’s always-on CRS with multiple UE-specific reference signals.
  • DMRS enables receiver channel estimation for demodulating its associated physical channel and is designed separately for PBCH, PDCCH, PDSCH, PUSCH, and PUCCH.DMRS is UE-specific, on demand, and normally confined to the scheduled physical resources of its channel.
  • PDSCH/PUSCH DMRS symbol counts and frequency density are configurable across mobility scenarios and overhead requirements.The first DMRS instance occurs early in the transmission so channel estimation can begin early.

B. Downlink and Uplink Phase-Tracking Reference Signals (PTRS)

PTRS tracks oscillator phase to suppress phase noise and common phase error, especially at high carrier frequencies. Its density and association are adapted to transmission conditions, while CSI-RS supports several measurement, tracking, and precoding functions.

  • PTRS: PTRS tracks local-oscillator phase at the transmitter and receiver to suppress phase noise and common phase error, especially at millimeter-wave frequencies.Its frequency density can be low while its time density is high because of phase-noise properties.
  • PTRS: PTRS can accompany both downlink PDSCH and uplink PUSCH, but when transmitted it remains tied to one DMRS port and the scheduled bandwidth and duration.Time and frequency densities adapt to SNR and scheduling bandwidth.
  • CSI-RS: NR CSI-RS supports downlink CSI acquisition, mobility RSRP measurement, beam management, demodulation tracking, and reciprocity-based uplink precoding.The design also supports UE-specific transmission and retained backward compatibility when new features are introduced.
  • CSI-RS: CSI-RS configuration is highly flexible: resources support up to 32 ports, configurable density, variable symbol spans, and periodic, semi-persistent, or aperiodic operation.For time-frequency tracking, a single port is transmitted in bursts across one or two slots.

D. Sounding Reference Signals (SRS)

SRS provides uplink channel sounding for link adaptation and scheduling, and can support downlink precoder selection under reciprocity operation. UE-specific configuration enables flexible symbol placement, sounding capacity, coverage extension, and antenna switching.

  • SRS: SRS performs uplink channel sounding for uplink link adaptation and scheduling, and under reciprocity operation supports downlink precoder selection.The reciprocity-based uses include massive multi-user MIMO.
  • SRS: NR SRS is UE-specifically configured, enabling flexible resource arrangements and system operation.This differs from the LTE counterpart described in the passage.
  • SRS: An SRS resource spans 1, 2, or 4 consecutive symbols within the last 6 symbols of a slot.Multiple SRS symbols provide coverage extension and increased sounding capacity.
  • SRS: Multiple configured SRS resources support intraslot antenna switching when a UE has fewer transmit chains than receive chains.The passage identifies this capability as important for the system's flexibility.
  • Conclusion: 5G NR is presented as a flexible air interface intended to meet diverse requirements, use cases, deployments, and future service evolution.The article focuses on the physical layer of the 3GPP NR specifications.
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