Why preamble request don’t require any UL grant because Rap response only carries up grant in these cases?
How does the UE initiate RACH preamble in UL direction without any grant?
Let’s go through this:
A) From SIB-2 UE will get some power information from the eNB that are RadioResourceConfigCommon message contains all the parameters required for to establish RACH (prach-config & RACH-ConfigCommon).
powerRampingParameters SEQUENCE
{
powerRampingStep ENUMERATED {dB0, dB2,dB4, dB6},
preambleInitialReceivedTargetPower,
},
So what are power ramping parameters,and how these actually will useful to synchronise with node?
Preamble Initial Received Target Power:
The power level the eNB would like to receive for a random access. The default value is -104 dBm and it will estimate the Pathloss.
Power Ramping Step:
The amount of additional power to be used every time the random access is attempted again. This can be 0, 2, 4 or 6 dB.
So based on Preamble Initial Received Target Power, UE will initiate preamble request.