LTE RAP : How does an UE estimate initial power to transmit Random Access Preamble ?

Question

Actually i am trying to learn RACH procedures and getting lots of hurdles to understand. I am posting my queries whatever is coming to my mind so that i will get the descriptive answers. I hope someone will help me here to learn it. Thanks in advance.

Answer 1

After receiving SIB-2, UE comes to know about RACH related parameters.

UE applies "preambleInitialReceivedTargetPower" to send RACH request to eNB and if eNB doesnot respond back with RAR then UE will add "powerRampingStep" to "preambleInitialReceivedTargetPower" and again it wil transmit RACH. For example, In SIB-2, UE receives values like below.

preambleInitialReceivedTargetPower = -96 dBm
powerRampingStep = -4 dBm

Then First RACH request will be sent by applying -96 dBm and if eNB is not replying then UE will apply
( -96 - (-4)) dBm i.e., -92 dBm.

Answer 2

SIB2 carries "RadioResourceConfigCommon" IE which in turn has "RachConfigCommon" IE .
RachConfigCommon has following informations:

powerRampingParameters SEQUENCE {
powerRampingStep ENUMERATED {dB0, dB2,dB4, dB6},
preambleInitialReceivedTargetPower ENUMERATED {
dBm-120, dBm-118, dBm-116, dBm-114, dBm-112,
dBm-110, dBm-108, dBm-106, dBm-104, dBm-102,
dBm-100, dBm-98, dBm-96, dBm-94,
dBm-92, dBm-90}
},

I think when UE reads SIB2 it came to know how much initial power it has to use to send Random Access Preamble to eNodeB.
It is just my guess since I am not confirm.
Waiting for others to respond.