RACH stands for Random Access Channel. This is the first message from UE to eNB when you power it on. Even though they use a little bit different name, in all cellular technology (GSM, WCDMA, LTE, NR) there is a specific signal that perform the same function. In CDMA, they call it ‘Access Probe’ , in GSM they call it ‘Channel Request’, and in WCDMA / LTE they call it ‘RACH’. In terms of eNB point of view, it would seem that it is getting this initial UE signal in almost random fashion (e.g, in Random timing , Random Frequency and in Random Identification) because it has no idea when a user turn on the UE (Actually it is not completely random, there are a certain range of agreement between UE and Network about the timing, frequency location and possible indentification, but in large scale it would look like working in random fashion). In terms of Radio Access Network implementation, handling RACH would be one of the most challenging job. Even in terms of protocol design, RACH design can be one of the most important / critical portions.
What is RACH for?
It is for establishing Uplink Synchronization
Two types of RACH Process: Contention based and Non Contention Based
Possible Subcarrier Spacing of RACH Preamble : 1.25 Khz only
Preamble format and Sequence Length : Format 0,1,2,3,4
What are main differences among the Preamble Formats ? : Length of Preamble
Why are there different Preamble types ? The biggest reasons are to cover various range of distances between UE and Cell and to enhance reliable reception of the preamble with the optimal use of network resources.
RRC Parameter to determine Preamble Type, Sequence Length, PRACH Transmission Time : prach-ConfigIndex