PUCCH in 5G NR

Introduction

What is PUCCH in 5G NR?

The Physical Uplink Control Channel (PUCCH) in 5G New Radio (NR) is the channel through which the UE (User Equipment) sends critical uplink control information (UCI) to the gNB (base station). UCI consists of:

• HARQ (Hybrid-ARQ) feedback: ACK/NACK responses for downlink data

• Scheduling Requests (SR): Requests for uplink resources

• Channel State Information (CSI): Measurements and feedback about the downlink channel condition

PUCCH enables efficient, reliable feedback without requiring extra uplink data channel resources, optimizing radio resource usage and improving latency and reliability.

Why is PUCCH Used?

• Reliable Uplink Control: Allows the gNB to adapt scheduling and resource allocation in real time.

• Resource Efficiency: Separates control from data (which uses PUSCH), keeping control lightweight and consistent.

• Support for Different Feedback Needs: From simple ACK/NACK to rich CSI reporting—using various PUCCH formats.

PUCCH Formats in 5G NR

PUCCH in 5G NR is categorized into five formats, distinguished by the payload size they carry and the time-frequency occupation:

PUCCH formats are grouped based on their duration (short/long) and the payload size they support:

• Short Duration: Formats 0 & 2 (1–2 OFDM symbols): For low-latency or small payloads.

• Long Duration: Formats 1, 3 & 4 (4–14 symbols): For robustness or large payloads.

Format 0: Short, Small UCI

• Duration: 1–2 symbols

• Payload: 1–2 UCI bits

• Multiplexing: Supports multi-user (via cyclic shift)

• Scenario: Quick ACK/NACK or SR when latency is critical (e.g., TDD systems, cell edge with good SNR)

• Example: Uplink response to DL data, SR with minimal coding.

Format 1: Long, Small UCI

• Duration: 4–14 symbols

• Payload: 1–2 UCI bits

• Advantages: Higher robustness due to more symbols and frequency hopping.

• Scenario: Control information from UEs at bad radio conditions or requiring reliability. Used where multi-path/interference is high.

Format 2: Short, Large UCI

• Duration: 1–2 symbols

• Payload: >2 UCI bits (typically up to several bits for CSI)

• Multiplexing: Not suitable for multi-user; uses more PRBs

• Scenario: Quick transmission of rich UCI (like CSI) requiring low latency (e.g., URLLC scenarios, fast channel feedback).

Format 3: Long, Large UCI

• Duration: 4–14 symbols

• Payload: >2 UCI bits (up to 170 bits for 5G NR)

• Multiplexing: Single user

• Scenario: Large UCI payload (e.g., CSI report, multiple HARQ-ACK bits). Used in eMBB where reliability for UCI is needed.

Format 4: Long, Large UCI, Multi-User

• Duration: 4–14 symbols

• Payload: >2 UCI bits (multi-user multiplexing)

• Multiplexing: Multiple users share same resources (OCC used)

• Scenario: Simultaneous large UCI from several UEs; typically when bandwidth is scarce or group feedback is needed.

The allocation of PUCCH (Physical Uplink Control Channel) resource elements in 5G NR is defined in both the frequency and time domains and is adaptive to the numerology (subcarrier spacing, slot duration) used for the carrier, as well as the specific requirements of the PUCCH payload (control information size, latency).

 Modulation:

QPSK or BPSK is used depending on cases as below.

• Long PUCCH with 2 or more bits of information : QPSK

• Short PUCCH with more than 2 bits of information : QPSK

• Long PUCCH with 1 bit information : BPSK

How PUCCH Multiplexes UEs: Cyclic Shifts and OCCs

The 5G NR Physical Uplink Control Channel (PUCCH) uses advanced signal processing techniques—cyclic shifts and Orthogonal Cover Codes (OCCs)—to allow multiple UEs (User Equipments) to transmit their control information on the same physical resources without mutual interference. Here’s a detailed look at how each mechanism works to achieve user multiplexing, why they’re used, and the underlying principles.

1. Multiplexing Using Cyclic Shifts

• Each PUCCH transmission is based on a base sequence (often a Zadoff–Chu sequence), which has the property that its cyclically shifted versions are orthogonal (non-interfering) if the shift values are chosen correctly.

• Each UE uses a different cyclic shift index, which rotates the base sequence by a specific phase. The receiver knows (or searches) the possible shifts to recover the intended signal per UE.

How It Works

• The network allocates the same PRB/time resource to several UEs, with each UE using a distinct cyclic shift of the base sequence.

• At the receiver, a correlation process identifies the peak corresponding to each unique cyclic shift, thus separating user signals efficiently.

Example

• Suppose up to 12 cyclic shifts are mathematically possible per Zadoff–Chu sequence; each can be assigned to a different UE. In practice, fewer shifts may be used for robustness (for example, 4 cyclic shifts mean up to 4 UEs can transmit simultaneously on the same resource).

• For PUCCH Format 1, 7 cyclic shifts are often used for multiplexing up to 7 UEs per PRB and symbol group.

2. Multiplexing Using Orthogonal Cover Codes (OCCs)

• OCCs are binary sequences designed so that any two sequences are mathematically orthogonal. Applying these codes spreads the modulated symbols across either the time or frequency domain, depending on format and configuration.

• This orthogonality prevents cross-interference even if two UEs use the same cyclic shift.

How It Works

• In practice, the base sequence is first modulated, then "multiplied" (spread) by the OCC.

• Each UE gets a unique OCC within the set defined for that PUCCH configuration.

• Depending on the format:

  • Short PUCCH: OCCs often span the frequency domain (across subcarriers in the PRB).

  • Long PUCCH: OCCs may span time (across OFDM symbols) or frequency.

• In the receiver, OCC processing "unscrambles" each UE’s information, exploiting orthogonality.

Example

• If there are 4 OCCs defined for a given resource, up to 4 UEs can transmit their uplink control information on the exact same PRB/symbols, each using a different OCC.

• For more demanding scenarios, both OCC and cyclic shift can be used together, further increasing the number of multiplexed UEs.

Combined Use: Cyclic Shifts + OCCs

• Often, both cyclic shift and OCC are jointly used to push the number of multiplexed UEs even higher—by selecting unique pairings of cyclic shift and OCC for every user signal mapped to the same PRB and OFDM symbol bundle.

• The total number of multiplexed UEs equals the product of available cyclic shifts and OCCs (e.g., 4 cyclic shifts × 4 OCCs = 16 UEs share a PRB).

Table: User Multiplexing in PUCCH with Cyclic Shifts and OCCs

Technique	Main Domain	Typical Multiplexing Depth	Application	Example Benefit
Cyclic Shift	Time/Freq	4–12 per PRB	All PUCCH formats	4 UEs per PRB
OCC	Time/Freq	2–12 per PRB	Mainly formats with large symbol or subcarrier allocations	4 UEs per PRB
Combined	Both	Up to 16+ per PRB	High UE-density scenarios	16 UEs per PRB

Key Parameters for PUCCH

Common parameters for all formats include:

• startingPRB: Starting Physical Resource Block (frequency position)

• nofSymbols: Number of OFDM symbols used

• startingSymbolIndex: Symbol index where PUCCH starts

• intraSlotFrequencyHopping: Enables frequency hopping within slot

• SecondHopPRB: PRB for frequency hopping

• Initial Cyclic Shift (CS): Used for separation between users

• Time/frequency domain OCC: Orthogonal Cover Codes for multiplexing

• Additional DMRS: Additional reference signals for channel estimation

• Max Code Rate: For determining channel coding strength

Parameters used depend on format type and scenario.

PUCCH Components and Key Parameters

• PUCCH Resource: Defines time-frequency allocation for a PUCCH transmission (configured via RRC: pucch-Resource).

• Format: There are 5 formats (0–4), differing in duration, payload size, and modulation.

• Cyclic Shift/Orthogonal Cover Code: Ensures orthogonality for multiplexing UEs.

• Demodulation Reference Signal (DMRS): Embedded for coherent demodulation at gNB.

• RRC Parameters: PUCCH configs are delivered over RRC signaling and linked to specific Serving Cells/BWPs.

Important RRC/PHY Parameters:

Parameter	Description	Formats
pucch-Fmt0-1-initial-cyclic-shift	Initial cyclic shift for orthogonality	0, 1
pucch-Fmt1-time-domain-OCC	Orthogonal cover code (OCC) index	1
dataScramblingIdentityPUSCH	Scrambling initialization	2, 3, 4
pucch-Fmt4-preDFT-OCC-index	Pre-DFT OCC for multi-user support	4
frequency hopping	Inter- and intra-slot hopping for robustness	1, 3, 4 (optionally)

PUCCH Formats, Their Use Cases, and Examples

Format	Symbols per Slot	Payload Size	Use Cases	Example UCI
0	1–2 (short)	1–2 bits	Simple ACK/NACK, SR	HARQ, Positive SR
1	4–14 (long)	1–2 bits	High-reliability ACK/SR	HARQ, Positive SR
2	1–2 (short)	>2 bits	Small to moderate CSI	CSI, HARQ+CSI
3	4–14 (long)	>2 bits	Moderate/large CSI/etc.	HARQ+CSI combo
4	4–14 (long)	Large (>2)	Multi-user, big CSI	Group CSI, Group HARQ

• Short Formats (0/2): Used for quick, small feedback, last 1–2 OFDM symbols.

• Long Formats (1/3/4): Span 4–14 symbols, support more payload & better coverage.

Example Scenarios:

1. HARQ feedback (2 bits): UE configures Format 0 (1-symbol) for immediate ACK/NACK, or Format 1 for robust transmission across more symbols.

2. SR (1 bit): UE encodes a scheduling request using Format 0 or 1, occupying a short (or long) window as per configuration.

3. CSI reporting (>2 bits): UE uses Format 2 (short) for quick CSI or Format 3/4 (long) for larger, more complex reports (e.g., MIMO feedback).

How the UE Generates PUCCH payload

• Step 1—Resource and Parameter Retrieval:

  UE gets its PUCCH resource configuration from gNB (via RRC, often during setup or reconfiguration).

• Step 2—Determine Payload:

  UE collects UCI it needs to send (HARQ, SR, CSI, or combinations).

• Step 3—Select Format:

  • 1–2 bits: Use Format 0 (short) if low latency suffices, Format 1 (long) for coverage/robustness needs.

  • 2 bits: Format 2 for quick small UCI, Format 3 or 4 for longer/robust transmissions or multiplexed users.

• Step 4—Mapping and Modulation:

  Map bits onto resource, apply cyclic shift/OCC for multiplexing, add DMRS as needed.

• Step 5—Transmission:

  Send the coded waveform over the configured PUCCH resource at the right instant.

Example

• UE gets a DCI requiring HARQ feedback (ACK + NACK), and also has an SR pending.

• The UE’s configuration indicates:

  • Format 0 for up to 2-bit feedback, 1 OFDM symbol (if low latency)

  • Format 1 over 7 symbols for better coverage (e.g., at cell edge)

• For larger CSI, Format 3 or 4 is used, with UE mapping the bits to the longer time/frequency resource.

  
  

PUCCH Format Selection by UE:

UCI Content	Example Format(s)	Typical Scenario
1–2 bit HARQ/SR	0, 1	ACK/NACK, basic SR
3–6 bit HARQ+CSI	2, 3	Small-to-moderate CSI
Large CSI/Group Feedback	3, 4	Massive MIMO, UE groups

Comparison between 4G LTE & 5G PUCCH

Feature	4G LTE PUCCH	5G NR PUCCH
Resource location	Fixed at carrier edge	Anywhere (PRB-scheduled) in BWP
Formats	4 main formats	5 formats, short (0,2) and long (1,3,4)
Payload and UCI	Small, simple, less combo	Small to large, low/high complexity, combo UCI
Duration	1 ms, fixed	1–14 symbols, highly flexible
Multiplexing	Limited	Multi-user, OCC/cyclic shift, scalable
Modulation	BPSK/QPSK	QPSK, higher orders, flexible
Bandwidth/Num. SCS	Up to 20MHz, 15kHz SCS	Up to 400MHz, multiple SCSs
Dynamic config.	Limited	Extensive, per-BWP, RRC

In essence, PUCCH enables UEs to efficiently inform gNBs about channel state, scheduling needs, and feedback with format and resource flexibility fitting a range of application scenarios, from fast ACK/NACK to rich channel state reports.

References:

• 5G NR Architecture, Technology, Implementation,   and Operation of 3GPP New Radio Standards - Sassan Ahmadi

• https://www.sharetechnote.com/html/5G/5G_PUCCH.html

• 3GPP TS 38.300 v15, 38.212 v15