# 5G NR Channels

## Summary


On the air interface between a UE and a 5G base station, 5G New Radio carries information on various **physical channels**. These channels carry both User Plane (UP) or Control Plane (CP) information.

However, the 5G NR protocol stack has many layers, each layer communicating with its peer at a different level of abstraction. Higher-layers PDUs are not mapped directly to physical layer for transmission. Instead, PDUs at a layer are mapped to channel types that suits that layer's functionality and abstraction. Thus, RLC delivers its PDUs to MAC over **logical channels**; MAC PDUs to PHY are on **transport channels**. 

Channels help us organize and simplify the design and development of the stack. Each channel can also be prioritized and optimized differently. Downlink channels are distinct from uplink channels, though some channel names may be the same.

## Discussion

### What are the channel types defined in 5G?

There are three types of channels:

    + **Logical Channels**: Offered by MAC to RLC. *Control channels* carry CP packets. *Traffic channels* carry UP packets. Each logical channel maps to an *RLC channel* coming from RLC layer.
    + **Transport Channels**: Offered by PHY to MAC. MAC layer multiplexes one or more logical channels to a transport channel. Whereas logical channels describe *what* is carried, transport channels describe *how* they're carried.
    + **Physical Channels**: Channels that carry information on the air interface. Transport channels map to physical channels. There are also a few standalone physical channels that don't carry higher-layer information.

### In 5G NR downlink, what's the channel mapping?

Downlink logical channels include Broadcast Control Channel (BCCH), Paging Control Channel (PCCH), Common Control Channel (CCCH), Dedicated Control Channel (DCCH) and Dedicated Traffic Channel (DTCH). Downlink transport channels include Broadcast Channel (BCH), Paging Channel (PCH) and Downlink Shared Channel (DL-SCH). Downlink physical channels include Physical Broadcast Channel (PBCH), Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH). 

In terms of mapping we note the following:  

    + BCCH for Master Information Blocks (MIBs) goes on BCH, which goes on PBCH. BCCH for System Information Blocks (SIBs) goes on DL-SCH, which goes on PDSCH. PBCH is used by UE for cell acquisition, selection and re-selection.
    + PCCH goes on PCH, which goes on PDSCH. This is used to page a UE whose cell location is unknown.
    + CCCH is used by a UE during initial access when there's no RRC connection yet. Once a connection is established, DCCH and DTCH become available to the UE.
    + PDSCH carries a variety of transport channels. It can be adapted to current link conditions.
    + PDCCH is used to schedule transmissions on PDSCH and PUSCH.

### In 5G NR uplink, what's the channel mapping?

Uplink logical channels include Common Control Channel (CCCH), Dedicated Control Channel (DCCH) and Dedicated Traffic Channel (DTCH). Uplink transport channels include Random Access Channel (RACH) and Uplink Shared Channel (UL-SCH). Uplink physical channels include Physical Random Access Channel (PRACH), Physical Uplink Control Channel (PUCCH), and Physical Uplink Shared Channel (PUSCH). 

In terms of mapping we note the following:  

    + PRACH carries RACH, which has no mapped logical channel. PRACH is used by UE for initial access to the network. Random access preambles carried on RACH help overcome message collisions due to multiple UEs transmitting at the same time.
    + PUSCH carries UL-SCH, which carries CCCH, DCCH and DTCH logical channels. Like, PDSCH in downlink, PUSCH has flexible configuration that can be adapted to link conditions.
    + PUCCH carries uplink control information although these could be sent on PUSCH as well.
    + CCCH, DCCH and DTCH are also present in the downlink. DTCH is the only one in the user plane. As an example, we note that RRC signalling messages and NAS messages (that also go through RRC) use DCCH. Application data use DTCH.

### Which are the standalone physical channels?

By standalone physical channels, we mean channels that don't map to any transport channel. Information on these channels are only for exchanging control information at the physical layer. These help in gathering feedback and quickly adjusting configuration or resource allocation to respond to varying conditions on the air interface.

In the downlink, **PDCCH** carries Slot Format Indicator (SFI) and Downlink Control Information (DCI). DCI does scheduling for PDSCH and PUSCH. It carries Transmit Power Control (TPC) commands. DCI comes in many formats and one of them carries SFI to inform the UE which slot format is to be used. 

In the uplink, **PUCCH** carries Uplink Control Information (UCI), although UCI can also go on PUSCH. UCI carries channel reports, HARQ-ACK and scheduling request.  


### Which are the 5G NR physical signals?

There are a few physical signals closely associated with the operation of physical channels. For downlink synchronization, there are **Primary Synchronization Signal (PSS)** and **Secondary Synchronization Signal (SSS)**. These are transmitted along with PBCH. 

For acquisition and channel estimation, **Demodulation Reference Signal (DM-RS)** is used in both downlink and uplink. It's applicable for PBCH, PDCCH, PDSCH, PUCCH and PUSCH. In the uplink, **Sounding Reference Signal (SRS)** is used for channel estimation when PUCCH and PUSCH are not scheduled. 

For downlink positioning, **Positioning Reference Signal (PRS)** is used. Uplink positioning is based on SRS. 

For phase tracking and phase noise compensation for PDSCH and PUSCH, **Phase Tracking Reference Signal (PT-RS)** is used. This combats path delay spread and Doppler spread via fine time and frequency tracking.

For beam management to a connected UE, **Channel State Information Reference Signal (CSI-RS)** is used in the downlink. 


### What are the sidelink channels?

We commonly talk about downlink (base station to UE) and uplink (UE to base station) channels. However, there are also sidelink channels that enable direct device-to-device communications. This is particular to Vehicle-to-Everything (V2X) communications. 

Sidelink physical channels include Physical Sidelink Broadcast Channel (PSBCH), Physical Sidelink Control Channel (PSCCH), Physical Sidelink Shared Channel (PSSCH) and Physical Sidelink Feedback Channel (PSFCH). PSCCH and PSFCH are standalone channels. PSCCH carries a part of Sidelink Channel Information (SCI), the other part going on PSSCH. Sidelink Feedback Control Information (SFCI) goes on PSFCH. PSFCH carries HARQ feedback for PSSCH reception. 

Transport channels include Sidelink Broadcast Channel (SL-BCH) and Sidelink Shared Channel (SL-SCH). Logical channels include Sidelink Broadcast Control Channel (SBCCH), Sidelink Control Channel (SCCH) and Sidelink Traffic Channel (STCH). Mapping of these are PBCCH/SL-BCH/PSBCH, SCCH/SL-SCH/PSSCH and STCH/SL-SCH/PSSCH. 

Sidelink physical signals include DM-RS, CSI-RS, PT-RS, Sidelink Primary Synchronization Signal (S-PSS) and Sidelink Secondary Synchronization Signal (S-SSS). PSCCH is associated with DM-RS. PSSCH is associated with DM-RS and PT-RS. 


### What are the differences between 4G/LTE and 5G NR channels?

PBCH-DMRS, PDCCH-DMRS, PUCCH-DMRS, PDSCH-PTRS and PUSCH-PTRS are new in 5G NR. CRS, EPDCCH and EPDCCH-DMRS in LTE don't exist in 5G. Some LTE transmission modes don't need PDSCH-DMRS since Cell-Specific Reference Signal (CRS) fulfils the role. 

While DCI on PDCCH is present in 5G, LTE's downlink controls CFI on PCFICH and HI on PHICH are absent in 5G. These channels carry HARQ ACK/NACK for PUSCH transmissions since LTE uplink HARQ is synchronous. These are not needed in 5G since HARQ is asynchronous in both uplink and downlink, that is, HARQ process number is carried in DCI. 

LTE's downlink logical channels Multicast Control Channel (MCCH) and Multicast Traffic Channel (MTCH) are absent in 5G. Their associated transport and physical channels MCH and PMCH are also absent. However, multicast is being standardized in Release 17 and some of these channels may be introduced into 5G. 

## Milestones

Dec  
2017

3GPP publishes Release 15 "early drop". 

Jun  
2018

3GPP publishes Release 15 "main drop". This includes LTE-based Cellular Vehicle-to-Everything (C-V2X) feature, first introduced in Release 14 (Q1 2017). 

Dec  
2018

3GPP publishes Release 15 "late drop". This includes 5G NR-based C-V2X using **sidelink channels**. 

Jul  
2020

3GPP publishes Release 16 specifications. This release introduces dedicated **5G positioning** signals, measurements and procedures. This includes PRS for downlink and SRS for uplink. 

2022

3GPP Release 17 specifications is expected to come out in 2022. This release might introduce new channels for **multicast**. It will enhance sidelink channels include sidelink relay capability. Positioning will also be better.