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Target of power control is to maintain SINR at receiver,
so the first step in power control is to list down the parameters that affect uplink power.
1) Nominal power value
Simply speaking, this is the value of power when the effect of other parameters is zero.
Let us call it P0.
We will also consider limiting the power to certain value, let us call it Pmax.
2) Path loss (as covered earlier section)
Let us call it PL. Higher the path loss, more will be the transmit power required.
3) Bandwidth (of the allocated resources)
Wider the bandwidth, more will be the power required.
Higher numerology (bigger spacing between subcarriers) will take up more bandwidth.
So, UL power will depend on numerology (μ) and the number of physical resource blocks (nPRB).
nPRB is number of resource blocks assigned to UE in a particular slot.
4) Modulation and coding
Let us call it ΔMCS.
Higher the MCS, more are the chances of errors and/or decoding failures.
To improve the chances of successful decoding, power need to be increased.
If we collate all, the equation will look like,
PUL = minimum( Pmax , P0 + f(PL) + f(μ) + f(nPRB) + f(ΔMCS) )
Please refer 3GPP TS 38.213:7.1 [3] for more detailed equation.
Base station is aware of all parameters except PL.
If all parameters are kept same, uplink power depend on path loss.
Path loss (as experienced by UE) is not known to base station, so base station cannot find out actual power with which UE did the uplink transmission.
This type of power calculation or control is known as open loop power control.
In above equation,
μ is part of MIB (cell level),
Pmax is part of SIB1 (cell level),
P0 is part of RRC (DRB) Configuration (UE/RRC level), and
nPRB and MCS are part of DCI (slot/UE level).
That leaves out PL.
UE need to estimate the path loss.
A simple way of determining path loss is to calculate difference between received power and expected power of the downlink reference signal.
Reference signal could be SSB or CSI-RS.
Open loop power control ensures that UE adjusts the uplink power to suit the transmission and compensates the path loss.
This however may not guarantee that UL SINR at base station is at intended value at all times.
If there is a way for base station to adjust/tune uplink power, base station will have more control over UL SINR.
Closed loop power control allows base station to do the same.
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