# 5G RACH Power Control – Open Loop Power

**5G NR PRACH Power Control**

In any wireless System, when a device (UE) needs to get access it has to send a signal or preamble (MSG1) to base station (gNB), then energy is needed. One might have question, at what power it needs to send the preamble so that it is detected successfully ? The 3GPP specification, 38.213 gives the following formula about RACH power control calculation.

The formula says RACH power shall be minimum of two, Pcmax (i) or PRACH Target + PL (ii). Pcmax is depend on UE category and usually consider as **23dBm**. PRACH power control use Open Loop power control, which mean base station only indicate UE for target power and does not control further with TPC commands.

## RACH Power Control Procedure

**Step 1: ** Calculate PPRACH, target power using following formula.

- preambleReceivedTargetPower is available by RRC parameter
**“preambleReceivedTargetPower”** **DELTA_PREAMBLE**can be obtained using prach-configuration Index translation to preamble format and then lookup in following table

**PREAMBLE_POWER_RAMPING_COUNTER**is 1 for the initial transmission, and the retransmission is incremented by one each time**PREAMBLE_POWER_RAMPING_STEP**is derived from the RRC parameter “**powerRampingStep”**

**Step#2 **Then, calculate Path Loss **(PL) **using following formula

**referenceSignalPower**is determined by the parameters**ss-PBCH-BlockPower**and**powerControlOffsetSS**according to the SSB or CSI-RS associated with PRACH**HigherlayerfilteredRSRP**is the**RSRP**measured when sending**PRACH**

**Example #1:**

We have following RACH parameter received as part of RRC reconfiguration message within rach-ConfigGeneric.

Assuming following RSRP at the time of RACH procedure is -85dBm

**Step: 2** Calculating path loss

- PL= refence signal power – RSRP (from above log
**ss-PBCH-BlockPower**= -12) - PL= -12 – (-85) =
**73 dB**

**Step 1:** PRACH Target Power

**prach-Configuration Index**is associated with preamble format —-> 0 so*DELTA_PREAMBLE*—-> 0 dBm**preambleReceivedTargetPower**as per logs is -100dBm**powerRampingStep**as per logs is 4dB- PRACH target = -100 + 0 +(1-1)*4 =
**-100 dBm**

**Step 0: **Calculating PRACH Tx Power

- PRACH Tx Power = min {Pcmax , PRACH Target + PL } dBm
- PRACH Tx Power = min {23 , -100 + 73 } dBm
- PRACH Tx Power = min {23 , -27 }
**dBm** **-27 dBm**

So UE will transmit the RACH with **-27 dBm** power at first time. If RACH procedure fails at **Msg2** or **Msg4** fails, UE will trigger Msg1 retransmission. The maximum number of Msg1 retransmissions is defined by the parameter **preambleTransMax **. If a retransmission of Msg1 occurs, the power of each retransmission will increase by the step of **powerRampingStep **which is** 4dBm **as per above log snippet until UE reach the maximum power or the maximum number of retransmissions is reached. It should be noted here that the retransmission to increase the power is conditional, given that the SSB associated with PRACH has not changed.

**Example#2**

**prach-ConfigurationIndex**17 —-> preamble format 0 —–>*DELTA_PREAMBLE = 0dB***preambleReceivedTargetPower —>**-100

—-> 4*PREAMBLE_POWER_RAMPING_COUNTER**Pathloss —> 105*+**PRACH Tx Power**= preambleReceivedTargetPower*DELTA_PREAMBLE*+ (*PREAMBLE_POWER_RAMPING_COUNTER*– 1) ×*PREAMBLE_POWER_RAMPING_STEP*）*+ Pathloss*(matching with logs)**PRACH Tx Power =（-100 + 0 + （4-1）*2） + 105 = 11dbm**

**Related Post:**

**5G NR RACH Procedure Overview – CBRA and CFRA****5G NR RA-RNTI Calculation****5G NR Msg2 Random Access Response (RAR)****5G NR Timing Advance: RAR TA and MAC-CE T**