Power Control and DCI in 5G NR

Jyothi
6 min readFeb 16, 2022

For Power Control implementation in 5G NR, various signals and various physical channels are used. This story talks about DCI in the context power control. DCI is used to provide Uplink (UL) Power control (PC) commands.

  • 2 bits in DCI Format 0_0, DCI Format 0_1 , DCI Format 1_0, DCI Format 1_1 are used indicate Closed loop Power Control (CL PC) TPC cmds.
  • DCI Format 2_2, DCI Format 2_3 are used for closed loop PC cmds.
  • DCI Format 2_0, 2_1, 2_2 and 2_3 are used to provide UE group common signaling, designed to address group of UEs and can accommodate payloads for each UE within the group. Each UE able to extract its own information and ignore other information.

DCI Format 0_0 and DCI Format 0_1

2 bits in DCI Format 0_0 and DCI Format 0_1 indicate the CL PC TPC cmd for scheduled PUSCH. Interpretation of these 2 bits depends upon whether TPC command accumulation is configured.

Below tables shows the TPC cmd values and adjustment state values:

DCI Format 1_0 and DCI Format 1_1

2 bits in DCI format 1_0 (RNTI can be C_RNTI, TC_RNTI, CS_RNTI, MCS_C_RNTI) and in DCI format 1_1 indicate TPC command for scheduled PUCCH. These cmds are used for CL PC. Interpretation of these 2 bits is based upon these standardized lookup table which maps the 2 bits onto values of 1, 0, -1, and 3dB.

RNTI

Radio Network Temporary Identifier (RNTI) used to identify a connected UE or group of UEs or a specific radio channel. RNTI are used to scramble/ change the CRC bits attached to DCI payloads.

The TPC-PUSCH-RNTI, TPC-PUCCH-RNTI and TPC-SRS-RNTI arc used to provide Transmit Power Control (TPC) commands for the PUSCH, PUCCH and SRS respectively.

In PC , below 3 types of RNTI are used:

Below are the Information elements(IE) used for Power Control in the context of DCI:

CellGroupConfig is a radio resource control (RRC) control information element used to provide configuration parameters for a cell group. PhysicalCellGroupConfig structure in CellGroupConfig IE is used to configure cell group specific L1 parameters. This IE optionally consists of TPC-PUSCH-RNTI, TPC-PUCCH-RNTI and TPC-SRS-RNTI values.

PDCCH-Config IE is used to configure UE specific PDCCH parameters. This IE optionally consists of tpc-PUSCH, tpc-PUCCH and tpc-SRS fields of PUSCH-TPC-CommandConfig, PUCCH-TPC-CommandConfig and SRS-TPC-CommandConfig types. These fields indicate to enable and configure reception of group TPC commands for PUSCH,PUCCH and SRS.

SRS-TPC-CommandConfig IE used to configure UE for extracting TPC cmds for SRS from a group TPC messages on DCI Format 2_3. fieldTypeFormat2–3 specifies the number of bits in the field are for SRS request (0 or 2). startingBitOfFormat2–3 field specifies the starting bit position of the block within group DCI SRS request fields and TPC commands. Value 1 corresponds to first/left most bit of format2_3, 2 corresponds to 2nd bit of format 2_3 and so on. startingBitOfFormat2–3SUL field specifies the starting bit position of the block for SUL (Supplementary Uplink) carrier.

PUSCH-TPC-CommandConfig IE used to configure UE for extracting TPC cmds for PUSCH on DCI Format2_2. targetCell field specifies serving cell to which the acquired PC cmds are applicable. If absent , UE applies the TPC commands on which it received. tpc-Index and tpc-IndexSUL fields specify the position of the first bit of TPC cmd inside DCI format2_2.

PUCCH-TPC-CommandConfig IE used for extracting TPC commands for PUCCH from group TPC messages on DCI Format 2_2. tpc-Index and tpc-IndexSUL specify the position of the first bit of TPC cmd inside DCI format2_2. UE can be provided with TPC indices for the Primary cell and Primary SCG (Secondary cell group) cell (tpc-IndexPCell and tpc-IndexPUCCH-SCell). This TPC index value can be 1–15.

PUSCH-PowerControl used to configure UE specific PC parameters of PUSCH. tpc-Accumulation field if enabled or if absent UE applies TPC commands with accumulation, else UE applies TPC commands with absolute. twoPUSCH-PC-AdjustmentStates field specifies the number of PUSCH PC adjustment states maintained by UE. If the field is present UE maintains two power control states. If the field is absent UE maintains one power control state.

PUCCH-PowerControl IE is used to configure UE specific PC parameters of PUCCH. twoPUCCH-PC-AdjustmentStates field specifies the number of PUCCH PC adjustment states maintained by UE. If the field is present UE maintains two power control states. If the field is absent UE maintains one power control state.

Phy-Parameters IE is used to convey physical layer capabilities. Phy-ParametersXDD-Diff and Phy-ParametersFRX-Diff structures having twoDifferentTPC-Loop-PUSCH and twoDifferentTPC-Loop-PUCCH fields. UE signals its supports for power control adjustment states using these fields. gNB will subsequently configure these adjustment states in corresponding fields of PUSCH-PowerControl and PUCCH-PowerControl IEs.

tpc-Accumulation field in SRS-Config IE is used for TPC cmd accumulation for SRS. If field absent UE applies TPC cmds via accumulation. If disabled, UE applies TPC cmds without accumulation.

srs-PowerControlAdjustmentStates in SRS-ResourceSet structure inside SRS-Config IE used to specify whether to apply the same TPC cmds to both PUSCH and SRS.

srs-TPC-PDCCH-Group field in SRS-CarrierSwitching IE used to configure UE to receive type ‘A’ or type ‘B’ TPC commands. Type ‘A’ means that UE is configured to decode one block with DCI Format 2_3 which can contain multiple TPC cmds for multiple UL carriers. Type ‘B’ means that UE is configured to decode multiple blocks within DCI Format 2_3 where each block provides a single TPC cmd for a single uplink carrier.

Power Control Adjustment states

There can be 2 PC adjustment states or one PC adjustment state. Standard says having 2 PC adjustment states is beneficial when switching between uplink beam pairs. gNB can dynamically request UE to switch uplink beam pairs using SRS Resource Indicator in DCI Format 0_1. Maintaining 2 PC adjustment states allows UE to switch between PC calculations and at the same time switch between beams. This helps to ensure that UE always transmits with appropriate power.

DCI Format 2_2

DCI Format 2_2 is used to provide CL PC TPC group common signaling cmds for PUSCH and PUCCH. UE is addressed by TPC-PUSCH-RNTI or TPC-PUCCH-RNTI to scramble CRC bits which are added to DCI payload of DCI Format 2_2.

Below table shows the contents of DCI Format 2_2. It can accommodate upto 15 blocks. Each block is specific to each UE. A block can be of size 3 bits or 2 bits. First 2 bits of block indicate TPC cmd. When using two power control adjustment states block size is of 3 bits, 3rd bit specifies power control adjustment state. Using TPC-PUSCH-RNTI/TPC-PUCCH-RNTI and TPC indices, UE will decode and extract its specific block. Padding is added to make the size same as DCI Format 1_0.

DCI Format 2_3

DCI Format 2_3 is used to provide TPC commands for SRS and also to request the transmission of an aperiodic SRS. UE is addressed by TPC-SRS-RNTI to scramble CRC bits which are added to DCI payload of DCI Format 2_3.

Below table shows the contents of DCI Format 2_3. A block can be of size 2 bits, 4 bits, 6 bits and so on. Each block may include an SRS request(0 or 2 bits) and can include one or more TPC cmds (2 bits each). SRS request 00 means no SRS transmission is requested. Using TPC-SRS-RNTI, fieldTypeFormat2_3, startingBitOfFormat2_3, startingBitOfFormat2_3SUL, tpc indices, type A or B information UE will decode and extract specific block/blocks. Padding is added to make the size same as DCI Format 1_0.

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