Observe the attached graphic that shows the familiar procedure for PRACH power control. Preambles of increasing power are sent by the UE until an ACK is received. Then the PRACH message itself can be sent.
Logic would dictate that the PRACH message would be sent with the same amount of power the last preamble used i.e the parameter powerOffsetPpm would be set to 0dB or even higher.
In E*******'s recommended setting however for powerOffsetPpm is actually -4dB which to me doesn't make sense. Can anyone explain the logic behind this or share the value they use in their network.
For reference Alcatel Lucent recommend a setting of 0dB.
Hello!
The first signaling message on the PRACH channel RRC_Connection_Request is under error protection which uses FEC type - convolution coding scheme.Applying this it's possible to win around 3-4 dB.In order to decrease interference in UL we can set -4 dB
I can say it is smart way to reduce UL interference ....
BUT .... that reduction of power is just for the control part @ Q-branch (pilot & TFCI) ... because when it get to the rach message on I-branch the power will be increased again by almost 3dB ..... (using the gain factor B/W contorl part & data part ).
So we can say that the power of RACH message is approximately equal to the power of last preamble ..
Though i cant see the attachment,hope below explaination might help!!
PowerOffsetLastPreamblePRACHmessagein PRACH(Pp-m)
•The power offset between the last transmitted preamble and the control part of the PRACH message (added to the preamble power to receive the power of the message control part)
•range: -5 ... 10 dB, step 1 dB default: 2 dB
•The power offset between last preamble and and message part should ensure decoding the RACH message at BS with high probability. Still, it should be mimised to reduce UL interference
Thanks: 
PRACH power control question
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