Section 6 - NR Control Channels

Question 1

What are the functionalities of the NR Control Channel that are similar to LTE

Answer 1

PDDCH carries DCI which includes UL and DL grant allocation, and the Transmit Power Control (TPC) for closed loop power control. PUCCH carries the UL Control Information (UCI), with the Scheduling Request (SR), HARQ feedback, and CSI report

Question 2

What are neu functionalities of the NR Control channel

Answer 2

The PDCCH has a more flexible time-frequency structure where it can only occupy part of the carrier bandwidth (CORESET). Has support for beamforming, and introduces new control data such as Slot Format Indication (SFI) and Pre-emption Indication (PI). The PUCCH has 2 formats: long and short, which provide flexibility when transmitting different UCI payloads and transmission duration. New fields introduced to CSI report Strongest Layer Indicator (SLI), CSI-RS Resource Indicator (CRI), L1-RSRP

Question 3

What are the characteristics of NR CORESET?

Answer 3

The can be semi-statically configured by RRC. They occupy up to 3 symbols in the time domain and multiple of 6 PRBs in the frequency domain. It might not exist in all BWP as they are a cell-level configuration and UE specific. User data can be allocated in unused CORESET

Question 4

What is the structure of Resource Element Groups (REG) in 5G?

Answer 4

They have 12 subcarriers and 1 symbol. 6 REG form a CCE (Control Channel Element), REGs from the same CCE can be bundled and interleaved with REG bundles from another CCE. REG bundle size can be 2, 3, 6. The same precoding is applied to the REG bundle.

Question 5

What similarities has the NR PDCCH monitoring with LTE?

Answer 5

• There are common and UE-specific search space sets
• DCI is transmitted with different CCEs aggregation levels
• UE performs fixed number of blind decoding to detect the DCI transmitted

Question 6

What are the difference in NR PDDCH with respect to LTE?

Answer 6

• Each search space set has its own periodicities for a UE to monitor, ranging from 1 to 2560 slots. The search space set can last more than one slot within a period.
• Association between search spaces and CORESET is provided to the UE. Up to 10 search space sets can be configured per BWP mapped to one or more CORESETs
• The number of blind decoding candidates is configured for each aggregation level per Search Space
• Aggregation level can be up to 16 CCEs.

Question 7

What are group common PDCCH?

Answer 7

Was created to lower the overhead by conveying a common signaling to a group of UEs. Some of the common signaling includes:
- Slot configuration: SFI sets the slot format per slot over a number of slots. DCI 2-0 and SFI-RNTI is used when addressing the UE
- Interruption Transmission Indication: DL Pre-emption information indicates the PRBs and symbols with no transmission within the TB allocated to UE. DCI 2-1 and INT-RNTI are used when addressing the UE
- Group TPC commands: Power control during SPS scheduling on PUSCH and PUCCH. TPC-PUCCH-RNTI/TPC-PUSCH-RNTI and DCI 2-2 are used
- SRS switching: SRS power control (PC) is independent from PUSCH PC. SRS-TPC-RNTI and DCI 2-3 are used
- GC-PDCCH CORESET can be the same or different than CORESET for monitored PDCCH
- GC-PDCCH blind decoding is configured with up to 2 candidates for a configured aggregation level

Question 8

What are the PDCCH Transmission mode?

Answer 8

One-port transmit diversity scheme with REG bundling. MU-MIMO is supported using non-orthogonal DMRS. Through localized or distributed mapping of a REG to a CCE it is possible to do contiguous or non-contiguous mapping within the frequency domain. For each CORESET, precoder granularity in frequency domain is configurable between: REG bundle size, Number of contiguous RBs within the CORESET

Question 9

What is the Slot Format Indicator (SFI)?

Answer 9

It is a mechanism for supporting dynamic TDD scheduling. Each slot format represents a combination of OFDM symbols denoted DL/UL/Flexible. Patterns provided can be divided into 3 groups:
- 0 switching point
- 1 switching point (starts with DL and changes to UL)
- 2 switching points (UL and then alternates between UL and DL twice)
SFI introduces the usage of the flexible symbol to handle the necessary guard period during DL to UL transition, and to reserve resources when needed.

Question 10

What is included in the DCI?

Answer 10

The MCS, the redundancy version (RV) for HARQ, the CRC of the DCI is scrambled with the RNTI that is used to address the scheduled message to the UE.

Question 11

How are the DCI formats labeled?

Answer 11

The first number can be 0 (uplink), 1 (downlink), 2 (group). The second number for type 0 and 1 can determine if the grant is for fallback 0 or regular 1. There are a total of 8 DCI formats

Question 12

What are the functions and format of PUCCH?

Answer 12

PUCCH carries UCI, including CSI, ACK/NACK and scheduling request.
The formats are defined for different payload sizes:
- There are long and short durations available
- UE may transmit up to two PUCCHs within one slot duration
- Frequency hopping is supported
Formats 0 and 2 are short (up to two symbols), 1,3,4 are long which carry more than 2 bits UCI except 1.

Question 13

Describe characteristics of PUCCH format 0

Answer 13

Is a short format occupying 1 or 2 OFDM symbols and with UCI payload less than 2 bits.
Its length is 1 symbol for simultaneous transmission of 2-bit HARQ-ACK and SR or when only CGS sequences of length 12 with consecutive mapping within a PRB are supported in Rel15.
It has 2 symbols when we want to transmit two 1-symbol PUCCHs conveying the same UCI. Sequence hopping between the two symbols is supported and frequency hopping is supported at least for localized (contiguous) PRB allocation in each symbol

Question 14

Describe characteristics of PUCCH format 2

Answer 14

It is a short format but payload is larger than 2 bits. If payload is larger than expected, CSI is dropped and priority is given to HARQ feedback. The number of PRB is proportional to payload, which keeps effective code rate constant.

Question 15

Describe characteristics of PUCCH format 1

Answer 15

It is a long format with between 4 and 14 OFDM symbols but UCI payload is less or equal to 2 bits. It is similar to format 0 in sequence and cyclic shift. More symbols are used than format 0 to transmit the same data:
- More effective frequency diversity is achieved when hopping
- More energy per UCI bit
- Coverage and robustness is enhanced
HARQ ACK via BPSK or QPSK modulation is repeated in time domain and multiplied with sequences
Two state SR is based on ON-OFF keying
Includes DMRS transmission occurring in all even symbols to include CS accuracy. It is not possible to apply time domain OCC over multiple UCI/DMRS symbols per frequency hop

Question 16

What are the characteristics of PUCCH 3?

Answer 16

Is similar to format 2 but more symbols to use because larger payload. It uses multiple RBs in frequency. Frequency hopping is supported in time domain. In time domain, symbols are shared between control information and DMRS.

Question 17

Describe PUCCH format 4

Answer 17

Similar to format 3 but code multiplexing is supported for allocating the same resources to 2 or 4 users. Only 1 PRB is allocated in the frequency domain.

Question 18

How is the repetition of long PUCCH configured

Answer 18

The number of slots for long PUCCH is RRC configurable. The number of repetitions is UE-specific and can be 1, 2, 4 or 8 slots. For long PUCCH with more than 2 bits over multiple slots all UCI bits are encoded and transmitted in each slot.
Each slot in the multi-slot transmission is a repetition. Starting symbol in the first slot is indicated by PUCCH resource allocation. Starting symbol in subsequent slots is the same as the starting position in the first slot
It is possible to apply frequency hopping. Inter slot hopping is performed on every slot. For intra and inter-slot hopping, the 1st hop and 2nd hop are performed on the frequency resources indicated by PUCCH resource allocations by separate RRC configuration

Question 19

What is the high-level configuration for PUCCH?

Answer 19

Some PUCCH resources are semi-statically configured:
- PUCCH resource for SR only transmission
- PUCCH resources for P-CSI only transmission
A configurable max code-rate, per PUCCH format (2,3,4) used by the UE to determine the format of the PUCCH
Max number of CQIs in UCI is higher layer configurable. Generally the UE assumes a single CQI in UCI meaning up to four MIMO layers in RI report. The RI payload depends on the rank restriction signaling.