PUCCH in 5G NR Phy

 

Short PUCCH can be small and large payload size

NR PUCCH Formats

 Short duration - PUCCH format 0 and 2.

Short duration PUCCH format 0 - with small payload size.
Short duration PUCCH format 2 - with large payload size.

Long duration - PUCCH format 1,3 and 4.

1. SHORT DURATION PUCCH
Short duration PUCCH can span 1~2 symbols within a slot and may carry either small (1~2 UCI bits) or large (more than 2 UCI bits) payload size.

1.1 PUCCH Format 0
1. 1~2 symbols in a slot and 1~2 UCI bits. HARQ-ACK or SR bit or both.
2. Two different approaches for design here
a) Demodulation Reference Signal based design.
b) Sequence based structure.

DMRS based design & Sequence based structure

For fading channels - DMRS and sequence based degrades.
Frequency selective based fading with large delay spread - DMRS based sequence
structure degrades more then sequence based structure design. This is due to channel estimation error.
For 2-bit HARQ-ACK - Sequence based outperforms DMRS.
Under High delay spread Sequence Based delivers better BER.
Increase in SCS => Higher delay spread => results in channel estimation error.
In sequence based approach, orthogonal sequences used are robust under high-frequency selectivity.

1.2 PUCCH Format 2

Used to carry more than 2 UCI bits.
a. CSI report + more than 2 HARQ-ACK feedback bits.
Multiple PRB's up to 16 are used.
what is coherent detection here ?
FDM of DMRS and UCI symbols indicates, PF2 is CP-OFDM waveform.
DMRS - comes from pseudo-random - comes from a seed => fun(symbol, slot, configurable ID).
This can facilitate to randomize the inter-cell interference on a symbol level basis for
short PUCCH transmission.

DMRS pattern for PUCCH Format 2

20 bits of payload in PF2 occupies 4 PRB's.


2. LONG DURATION PUCCH

Span for 4~14 symbols within a slot.
May carry 1~2 bits or large payload size,more than 2 bits.

Long PUCCH - small payload size - PF1.
Long PUCCH - large payload size - PF3 or PF4.
The above depends on UE multiplexing capacity.

PF3 - No multiplexing capacity.
PF4 can multiplex up to 4 UE's of the same time-frequency resource (same PRB)
using DFT-OCC (Orthogonal Cover Code).
In long PUCCH formats, DMRS and UCI ---> time division multiplexed
and based on DFT-s-OFDM.
Long duration PUCCH is to enhance coverage for cell edge UE's.


2.1 PUCCH Format 1
NR PF 1 - 4~14 symbols in a slot.
Carries 1~2 UCI bits [ HARQ-ACK bits or SR or both ].
Here in this format - DMRS symbols are formed with low PAPR Computer Generated Sequence
(CGS) with cylic shift in frequency domain and Orthogonal COver Codes (OCC) in time domain.
Modulation - BPSK or QPSK.
OCC is applied in Time Domain.

2.2 Design Approach
1) Extension Method.
2) Puncturing Method.

2.2.1 Extension Method.

Candidate structure for PF1
Length 7 using extension method - Fig 7a - Shortest length of PF1

In Extension method, any length n PF1 (4 ~= n ~= 14) consists of floor of n/2 UCI symbols and
n to floor n/2 DMRS symbols.
DMRS + UCI Symbols - Interleaved in TDM Manner.
DMRS Symbols occupy the even indexed symbols locations within length n.
Any length-n PF1 n greater than 4 can be conceived as an extension of length-4 PFI

2.2.2 Puncturing Method.

In this method, DMRS symbols are bundled together ad punctured into the middle of the length-PF1.
UCI symbols are placed at the beginning and at the end of PUCCH length. Like LTE...
DMRS overhead is 1/2.

2.2 PUCCH Format 3
More than 2 UCI bits.
CSI and more than 2 HARQ-ACK bits.
Large payload - up to 16 PRB's.
DMRS overhead is same as PF2, 1/2.
DMRS => low PAPR sequence - Computer generated sequence in frequency domain.
UCI bits - encoded + scrambled + QPSK modulated + DFT precoded = UCI Symbols.
UCI => pi/2 QPSK is also supported for UE Specific configurable modulation scheme.


PUCCH Format 3 Structure
Same as PF1 and PF3 - spans over 4 to 14 symbols in a slot.

Length 4 PF3 - 1. 
Here there are multiple design options based on the DMSRS symbols and their locations.

Length 5 DMRS - 1 or 2 DMRS symbols.

Length-10 PF3 - can have any number of DMRS symbols less than 5 (2 or 4) for less than 1/2 DMRS overhead.