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Below is updated equation for closed loop power control,
PUL = minimum( Pmax , P0 + f(PL) + f(μ) + f(nPRB) + f(ΔMCS) + δ )
where δ is determined by base station and informed to UE.
δ is known as "TPC command" and it is part of DCI.
As indicated in earlier section, all parameters of above equation are known to base station except path loss (as experienced by UE).
To determine the path loss at base station, MAC Power Headroom (PHR) reporting is used.
As the name suggests, PHR is difference between Pmax and estimated UL power.
UE can be configured (SRB1 MAC Configuration) to send PHR at certain intervals.
By solving below equation for PL, base station can find out the path loss [6] and come out with adjustment (δ value) to maintain optimal UL SINR.
PPHR = Pmax - ( P0 + f(PL) + f(μ) + f(nPRB) + f(ΔMCS) )
In a way, PHR acts like a feedback loop for uplink power control.
Refer 3GPP TS 38.321 [4] (sections 5.4.6 and 6.1.3.8) for more information on PHR.
With uplink power control equation, base station will have overall cell level, UE level, and slot level control over uplink power.
Scheduling logic at base station need to make use of these parameters to achieve desired uplink data rate while ensuring minimum uplink energy and interference.
CSI-RS
DCI
MCS
MIB
PHR
PL
PRB
RAN
SINR
SSB
TPC
UE
ULPC
| Channel State Information Reference Signal
Downlink Control Information
Modulation and Coding Scheme
Master Information Block
Power HeadRoom
Path Loss
Physical Resource Block
Radio Access Network
Signal to Interference and Noise Ratio
Synchronisation Signal Burst
Transmission Power Control
User Equipment
UpLink Power Control
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1. 3G Evolution: HSPA and LTE for Mobile Broadband
2. 5G NR: The Next Generation Wireless Access Technology
3. 3GPP TS 38.213 Physical layer procedures for control
4. 3GPP TS 38.321 MAC protocol
5. Scheduling or Radio Resource Selection by Samir Amberkar
6. 5G NR in Bullets
7. 3GPP TS 38.321 MAC protocol
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