5G/NR 学习笔记: 基本问答 RACH PRACH

What is the functionality or purpose of RACH process?

RACH (Random Access Channel) looks over-complicated, why do we still need it? The main purpose can be described as follows.

1. Achieve UP link synchronization between UE and eNB
2. Obtain the resource for Msg3

It is not efficient if UE is using broadcasting/always-on synchronization mechanism. The uplink synchronization process should meet following criteria:
1. The synchronization processs should happen only when there is immediate necessary
2. The synchronization should be dedicated to only a specific UE

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Fundamental difference from LTE RACH

The major difference between LTE RACH and NR RACH would lie just before RACH Preamble gets transmitted. It is due to Beamforming which would be supported by default in NR. When NR is operating in Beamforming mode, UE need to detect tha select a best beam for RACH process. The beam selection process would be trhe fundamental difference between LTE RACH and NR RACH process.

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How to determine which preamble format to use?

PRACH Configuration Index determines the Preamble Format to be used. Then you may ask who determines PRACH Configuration index? The answer is eNB determines it via prach-Configuration IE in SIB2.

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How does Network knows exactly when UE will transmit the RACH?

It is simple. Network knows when UE will send the RACH even before UE sends it because Network tells UE when the UE is supposed to transmit the RACH.

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How many RACH-Preamble can be used?

Theoretically 64 PRACH preambles are available in total, but the number of the preambles available in a specific condition are determined by a couple of SIB2 parameters.

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How to generate 64 PRACH preamble sequence?

i) generate a Zaddoff Chu sequence using rootSequenceIndex (we can call it as based sequence)
ii) generate 64 different sequence by doing cyclic shift of the base sequence. The cyclic shift interval is determined by Ncs and the Ncs is determined by zeroCorrelationZoneConfig and Highspeedflag.

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How to generate RACH signal

You do not have to know the details of this procedure unless you are the DSP or FPGA engineer implementing LTE PHY.

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How is the RACH Preamble power determined

The RACH Preamble (PRACH) power varies depending on cases as decribed below.
Basically PRACH power is determined by OpenLoopPower control algorithm.

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What is the RACH Process overview

It is presented in the below diagrams

step	Uplink or downlink	process
0	UE <— BS	SIB2
1	UE —> BS	PRACH Preamble (Msg1)
2	UE <— BS	RACH response - RAR (Msg2)+ UL Grant for Msg 3
3	UE —> BS	RRC Connection Request (Msg 3)
4	UE <— BS	HARQ ACK for Msg 3
5	UE <— BS	CR+RRC Connection Setup (Msg4)
6	UE —> BS	HARQ ACK for Msg4
7	UE <— BS	DCI 0 (UL Grant) for RRC Connection Setup Complete
7	UE —> BS	RRC Connection Setup Complete

RAR window: step 1 to step 2.
Contention Resolution Timer: step 3 to step 5
RRC delay: step 5 to step 7