# 5G NR PHY

## Summary


5G NR PHY is designed to meet the main 5G use cases, namely eMBB, mMTC and URLLC. While it's an evolution of LTE PHY, there are many aspects that are unique to 5G NR PHY.

PHY layer sits at the bottom of the 5G NR protocol stack, interfacing to MAC sublayer higher up via transport channels. It provides its services to MAC and is configured by RRC. PHY supports downlink (gNB-to-UE), uplink (UE-to-gNB) and sidelink (UE-to-UE) communications. 

Some of the main features include a wide spectrum from sub-GHz bands to mmWave bands, an OFDM-based air interface, scalable numerology, deployments from indoor picocells to outdoor macrocells, FDD and TDD support, flexible and self-contained slot structure, modulation up to 256QAM, polar and LDPC codes, Hybrid-ARQ (HARQ), bandwidth parts, CORESETs, beamforming, and massive MIMO.

## Discussion

### Could you share some technical details of 5G NR PHY?

**5G spectrum** spans a wide range: FR1 (410-7125 MHz) and FR2/mmWave (24250-52600 MHz). **UE bandwidth** per component carrier is of range 5-100MHz (FR1) and 50-400MHz (FR2). Higher bandwidth allocations can be achieved with carrier aggregation. 

**Waveform** used in 5G is OFDM with Cyclic Prefix (CP). In uplink, an optional transform precoding of DFT spreading is done before sub-carrier mapping. 

**Sub-Carrier Spacing (SCS)** is flexible from 15kHz to 120kHz, with higher values applicable in FR2. **Slot duration** is also flexible from 1ms at 15kHz SCS to 125µs at 120kHz SCS. SCS 240kHz is only for control. **Cyclic prefix** at 60kHz SCS can be normal or extended. 

For **duplexing**, both FDD and TDD are supported in FR1. Only TDD is applicable in FR2. In TDD, DL/UL split can be dynamically adjusted. 


### What modulation schemes and channel coding are supported in 5G NR PHY?

**Modulation schemes** supported are QPSK, 16QAM, 64QAM and 256QAM. For DFT-s-OFDM in uplink, 5G NR introduces π/2-BPSK for better power efficiency at lower data rates, necessary for mMTC services. DFT spreading in uplink helps coverage-limited scenarios. 

**Channel coding** is based on Low Density Parity Check (LDPC) code, applied on transport blocks. A large TB is segmented into multiple equal code blocks and LDPC coding is applied on the code blocks. Polar code is used for BCH, DCI and UCI. In addition, block code is used for UCI. 


### Which are the main physical radio resources in 5G NR?

Time and frequency are the two main resources. **Time** is organized into OFDM symbols, slots, subframes and frames. **Frequency** is organized into sub-carriers as needed for OFDM with SCS determined by numerology. Unlike LTE, both have flexible configurations due to 5G's scalable numerology.

5G NR defines the following: 

    + **Resource Element (RE)**: The smallest unit of resource. It's one sub-carrier for one OFDM symbol duration.
    + **Resource Block (RB)**: 12 consecutive sub-carriers in the frequency domain. It's not defined for the time domain. *Common Resource Blocks* are numbered from zero for each SCS. A UE is configured one or more bandwidth parts. *Bandwidth part* is a contiguous set of common RBs. *Physical Resource Blocks (PRBs)* are numbered from zero within the bandwidth part. Thus, a UE uses PRBs for actual communication.
    + **Resource Grid**: A combination of subcarriers and OFDM symbols. Defined for each numerology, carrier and antenna port. One set of resource grids is defined for downlink, uplink and sidelink each.
    + **Resource Element Group (REG)**: One PRB and one symbol.
    + **Control Resource Set (CORESET)**: Multiple PRBs with 1, 2 or 3 symbols.

### What's the frame and slot structure in 5G NR?

A frame is 10ms. A subframe is 1ms that's divided into slots. Slot duration depends on numerology. At 15kHz, a subframe has a single slot. At 30kHz, a subframe has two slots, each slot being 500µs. Likewise, we have slot durations 250µs@60kHz, 125µs@120kHz and 62.5µs@240kHz. 

A slot has 14 OFDM symbols but only 12 symbols at 60kHz when using extended cyclic prefix. At higher SCS, symbols and slots are shorter. 5G also permits mini-slot transmissions of 2, 4 and 7 symbols. 

Because the different numerologies are of the form \(2^µ\), they can coexist. Regardless of numerology, symbols and slots are time aligned. Services with different requirements of bandwidth and latency can be multiplexed on the same frequency. 

TDD slot structure is **self-contained**. It allows for fast and flexible TDD switching. DL control, DL data, guard period and UL control are in the same slot. Thus, DL data and its acknowledgment can happen in the same slot. This is also possible for UL data. Symbol allocation to DL or UL can be switched every slot. 


### What are the different physical channels used in 5G NR PHY?

We note the following physical channels (with transport channel in parenthesis): 

    + **Downlink**: PBCH (BCH), PDSCH (DL-SCH, PCH), PDCCH
    + **Uplink**: PRACH (RACH), PUSCH (UL-SCH), PUCCH
    + **Sidelink**: PSBCH (SL-BCH), PSSCH (SL-SCH), PSCCH, PSFCHPDCCH, PUCCH, PSCCH and PSFCH are standalone physical channels, that is, they're not mapped to transport channels. PDCCH has Slot Format Indicator (SFI) and Downlink Control Information (DCI) fields. The latter informs scheduling for PDSCH and PUSCH. PUCCH carries Uplink Control Information (UCI). UCI carries channel reports, HARQ-ACK and scheduling request. 


### What are the different signals used in 5G NR PHY?

PHY has a few signals for the following purposes:  

    + **Synchronization**: Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). These are transmitted along with PBCH.
    + **Acquisition and Channel Estimation**: Demodulation Reference Signal (DM-RS) in downlink and uplink. Sounding Reference Signal (SRS) in uplink when PUCCH and PUSCH are not scheduled.
    + **Positioning**: Positioning Reference Signal (PRS) in downlink. SRS in uplink.
    + **Phase Tracking**: Phase Tracking Reference Signal (PT-RS) for PDSCH and PUSCH. Helps combat path delay spread and Doppler spread.
    + **Beam Management**: Channel State Information Reference Signal (CSI-RS) in downlink towards a connected UE.

### What are the main functions of 5G NR PHY?

The main functions include error detection on the transport channel and indication to higher layers; FEC encoding/decoding of the transport channel; HARQ soft-combining; rate matching of the coded transport channel to physical channels; mapping of the coded transport channel onto physical channels; power weighting of physical channels; modulation and demodulation of physical channels; frequency and time synchronisation; radio characteristics measurements and indication to higher layers; MIMO antenna processing; RF processing. 

Consider the PHY model for DL-SCH. At gNB, Transport Blocks (TBs) arrive from MAC on transport channels. PHY adds CRC to each TB, perhaps involving code block segmentation. Channel coding and rate matching are performed, including for HARQ retransmissions. Data modulation is next, followed by mapping to physical resources. Finally, there's antenna mapping before transmission. At UE, the reverse process happens with CRC used for error detection and indication to MAC. 

In the figure, blue boxes are configurable by higher layers. For other channels, some steps may be hardcoded in the specification. For example, all steps are fixed for BCH; coding and rate matching are fixed for PCH. 


### What are the main procedures in 5G NR PHY?

The main procedures include Cell search; Power control; Uplink synchronisation and Uplink timing control; Random access related procedures; HARQ related procedures; Beam management and CSI related procedures; Sidelink related procedures; Channel access procedures. 

We describe a few:

    + **Cell Search**: UE acquires time and frequency synchronization for a cell and detects the Cell ID. UE receives PSS, SSS and PBCH.
    + **Uplink Power Control**: For PUSCH, PUCCH, SRS and PRACH transmissions. When Dual Connectivity (DC) is active, UE is configured with maximum power for both MCG and SCG.
    + **Random Access**: Of either Type-1 or Type-2. Involves preamble transmission on PRACH (and PUSCH MsgA in Type-2), random access response (RAR) reception on PDCCH/PDSCH, PUSCH transmission based on RAR UL grant (fallback in Type-2), and PDSCH contention resolution.
    + **PDSCH Reception**: Typically first requires decoding DCI from PDCCH.
    + **PUSCH Transmission**: Scheduled dynamically by UL grant in DCI or semi-statically by Type-1 or Type-2 grants configured by RRC.
    + **CSI Measurements & Reporting**: Periodic, semi-persistent or aperiodic. Reports are sent on PUCCH or PUSCH and triggered by DCI when applicable.

### Which are the main 5G NR PHY specifications?

For a high-level overview of 5G NR PHY, PHY section of **TS 38.300** is worth reading. This specification gives an overall description of NR and NG-RAN. 

A general description of 5G NR PHY layer is found in **TS 38.201**. Beginners can start with this document. For more details into PHY, the following are useful: 

    + **TS 38.202**: Services and functions provided by PHY, downlink/uplink/sidelink models.
    + **TS 38.211**: Physical channels and modulation, frame structure, PHY resources, modulation mapping, OFDM signal generation, scrambling, modulation, up-conversion, layer mapping, precoding.
    + **TS 38.212**: Multiplexing and channel coding, rate matching, transport channels, control information.
    + **TS 38.213**: Physical layer procedures for control, synchronization procedures, uplink power control, random access procedure, UE procedure for reporting and receiving control information.
    + **TS 38.214**: Physical layer procedures for data, power control, procedures related to physical shared channels.
    + **TS 38.215**: Physical layer measurements, UE and NG-RAN measurement capabilities.To learn about RF requirements including operating bands, channel bandwidth and transmitter/receiver characteristics, documents to read are **TS 38.101** in two parts (for FR1 and FR2) for the UE,  and **TS 38.104** for base station. 

## Milestones

Dec  
2017

3GPP approves the first specifications for 5G, called "early drop" of Release 15. 

Dec  
2019

First updates to specifications towards Release 16 are published. Some of the new features are Remote Interference Management (RIM), two-step RACH procedure, NR-based access to unlicensed spectrum, Integrated Access and Backhaul (IAB), V2X, eURLLC, NR positioning, MIMO enhancements, Dynamic Spectrum Sharing (DSS) enhancements, Multi-RAT DC/CA, NR-DC and cross-carrier scheduling with different numerologies. 

Jul  
2020

3GPP finalizes **Release 16** specifications.