3GPP TS 38.213 7.1 explains PUSCH Power Control.
The main aim of UL PC is to reduce interference and to reduce UE power consumption. Interference can be intracell interference or intercell interference.
There are 2 power control schemes : conventional PC schemes and fractional PC schemes.
Conventional PC schemes
These schemes attempt to maintain a constant Signal to Interference plus Noise Ratio (SINR) at the receiver. UE increases its transmit power to compensate an increase in path loss.
Fractional power control schemes
These schemes allow the received SINR to decrease as pathloss increases. When compared to conventional power schemes, in fractional power control schemes UE transmit power increases at reduced rate as pathloss increases. These PC schemes can improve air interference efficiency and increase average cell throughputs by reducing intercell interference.
3GPP TS 38.213 specifies fractional power control schemes but with an option to disable and fallback to conventional power control schemes.
Fractional PC schemes are part of OL PC computations.
Power control computations also depends on the transmission schemes. There are 2 types of UL transmission schemes : codebook based transmissions and non-codebook based transmissions.
Codebook based transmissions allow base stations to instruct UE to switch between its transmit beams. The UE can maintain separate sets of PC parameters to change the beams and corresponding PC parameters.
PUSCH Power control parameters IE:
The IE PUSCH-PowerControl is used to configure UE specific power control parameters for PUSCH.
PUSCH transmit power computation
· Max Power PCMAX ,f, c(i) is the configured UE transmit power defined in [8–1, 3GPP TS 38.101–1] and [8–2, 3 GPP TS38.101–2] for carrier f of serving cell c in PUSCH transmission occasion i .
· Nominal UE transmit power P O_PUSCH,b, f, c(j) is a parameter composed of the sum of a component PO_NOMINAL_ PUSCH,b,f,c(j) and a component PO_UE_PUSCH,b,f,c( j) .
PO_NOMINAL_ PUSCH,b,f,c(j) and PO_UE_PUSCH,b,f,c( j) parameters change based on type PUSCH transmission (MSG3 transmission, Configured grant PUSCH transmission, Codebook based dynamic grant PUSCH transmission, non-codebook based dynamic grant PUSCH transmission).
§ Codebook based Dynamic grant PUSCH transmission
· PO_NOMINAL_ PUSCH,b,f,c(j) represents a cell specific power level which is applicable to all UE within cell. PO_NOMINAL_ PUSCH,b,f,c(j) is given by p0-NominalWithGrant IE within PUSCH-ConfigCommon IE.
· PO_UE_PUSCH, b,f,c( j) represents a UE specific offset which can be used to adjust UE performance. Its value is specified by p0 IE within P0-PUSCH-AlphaSet. UE selects the first instance of P0-PUSCH-AlphaSet within PUSCH-PowerControl IE if SRI-PUSCH-PowerControl IE not included. Base station provides SRI value with DCI format 0_1. This is pointer to an instance of SRI-PUSCH-PowerControl IE which has a P0-PUSCH-AlphaSet.
· M PUSCH RB, b, f, c ( i) is the bandwidth of the PUSCH resource assignment, number of resource blocks for PUSCH transmission occasion i on active UL BWP b of carrier f of serving cell c. This value is multiplied with the subcarrier indexing to scale the transmit power in proportion with the bandwidth of resource allocations.
· α b,f,c (j) is fractional power control multiplier. If set to 1 , conventional power control scheme is applied. If set to value lesser than 0, fractional power control scheme is applied. Fractional power control reduces the gradient of the UE transmit power characteristic, which constantly causes the received power density to decrease.
o Codebook based Dynamic grant PUSCH transmission
§ Its value is specified by alpha IE within P0-PUSCH-AlphaSet. UE selects the first instance of P0-PUSCH-AlphaSet within PUSCH-PowerControl IE if SRI-PUSCH-PowerControl IE not included. Base station provides SRI value with DCI format 0_1. This is pointer to an instance of SRI-PUSCH-PowerControl IE which has a P0-PUSCH-AlphaSet.
· PL b,f,c (qd) is a downlink pathloss estimate in dB calculated by the UE using reference signal (RS) index qd for the active DL BWP, as described in Subclause 12, of serving cell c.
Pathloss = reference signal power — higher layer filtered RSRP value
Where referenceSignalPower is provided by higher layers and RSRP is defined in 3GPP TS 38.215 for the reference serving cell and the higher layer filter configuration provided by QuantityConfig is defined in [12, TS 38.331] for the reference serving cell.
o Codebook based Dynamic grant PUSCH transmission
§ The downlink transmission used for path loss measurements is given by an instance of PUSCH-PathlossReferenceRS within the PUSCH-PowerControl parameter structure.
§ In codebook based dynamic grant PUSCH transmission, base station provides the UE with an SRS Resource Indicator (SRI) within DCI Format 0_1. The SRI is used as a pointer to an instance of SRI-PUSCH- PowerControl which includes a pointer towards an instance of PUSCH-PathlossReferenceRS. If the Base Station has not provided the UE with an instance of SRI-PUSCHPowerControl, the UE selects the first instance of PUSCH- PathlossReferenceRS IE.
·Delta TF,b,f,c(i)is used to control UE transmit power based upon the allocated Modulation and Coding Scheme (MCS). The requirement for this mechanism depends upon the implementation of Link Adaptation at base station.
o If Base station uses Fast Adaptive Coding (AMC), then base station changes the allocated MCS according to channel conditions. If channel conditions are good, UE is allocated high MCS and receives high SINR. In this case UE need not increase its transmit power, Delta TF,b,f,c(i) can be set to 0 dB.
o If base station allocates MCS by not considering channel conditions, DeltaTF,b,f,c(i) is used at UE to increase its transmit power when using high MCS which ensures that base station receives high SINR.
o DeltaMCS flag IE in PUSCH-PowerControl IE is used to specify the inclusion of Delta TF,b,f,c(i) in power control computation.
o If PUSCH transmission is using more than single layer or if DeltaMCS is excluded Delta TF,b,f,c(i) is set to 0 dB.
· f b,f,c(i,l) specifies the closed loop power control calculation for active UL BWP b of carrier f of serving cell c in PUSCH transmission occasion i. l represents power control adjustment state.
UE can be configured with 1 or 2 power control adjustment states. UE maintains separate power control calculation for each adjustment state.
Base station sends TPC commands along with power control adjustment state information. TPC commands are used update specific adjustment states.
In case of Configured Grant resource allocation, PowerControlLooptoUse IE within ConfiguredGrantConfig IE is used to specify power control adjustment states.
In case of codebook based dynamic grant PUSCH transmissions, the SRI within DCI format 0_1 can be used to indicate an instance of SRI-PUSCH-PowerControl IE which specifies specific adjustment state using sri-PUSCH-ClosedLoopIndex. If UE does not receive an SRI then the first adjustment state is used.
In case of non-codebook based dynamic grant PUSCH transmissions, UE uses first adjustment state.
If UE receives the TPC cmds in DCI format 0_0 or 0_1 and TPC cmds in DCI format 2_2 specific to PDSCH, these commands are used for PUSCH CL PC computations.
CL PC can may use or may not use TPC accumulation.
A TPC command occupies 2 bits and maps onto one of the four values specified in above table. TPC command accumulation allows fine tuning of the UE transmit power, while potentially allowing fb,f,c(i,l) to have a large value. The value of 0 dB means that a TPC command can be sent without impacting the UE transmit power
Closed loop power control operates in ‘absolute’ mode when TPC command accumulation is disabled. In this case, TPC commands are mapped onto the {-4,-1, 1,4} dB values, and fb,f,c(i,l) = dPUSCH,b,f,c(i,l) ie. In this case, closed loop power control can generate a maximum change of 4 dB.
