In previous 2 chapters, we had a a brief of RACH procedure. In the next chapters we shall have indepth understanding of all the messages.
In brief:
1. CBRA procedure is as below:
i) UE –> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)
ii) UE <– NW : Random Access Response (Timing Advance, T_C-RNTI, UL grant for L2/L3 message)
iii) UE –> NW : L2/L3 message
iv) Message for early contention resolution
2. CFRA procedure is as below:
i) UE <–NW : RACH Preamble (PRACH) Assignment
ii) UE –> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)
iii) UE <–NW : Random Access Response (Timing Advance, C-RNTI, UL grant for L2/L3 message)
Now coming to the actual topic, to send a Random Access Preamble through PRACH channel.
To send , we need to answer below 2 questions:
1. How to generate a Preamble ?
2. How to select the correct PRACH time-frequency resource to send the selected preamble ?
In this chapter we shall answer first question, in the next chapter we shall answer the second question.
What is PRACH Preamble?
Similar to LTE, NR preamble sequence is based on Zadoff Chu sequence.
To generate a sequence we follow below steps:
Now we shall understand in detail about the formulas mentioned above:
PRACH Preamble:
A PRACH Preamble is a ZaddOff Chu (ZC) sequence {x(0),x(1),…x(LRA-1)} of length L_RA (839 or 139).
Below is the definition of Xi
Here u is used to indicate that the ZC sequence is generated according to the root sequence number u.
UE needs to generate 64 preambles according to the configuration parameters.
There are 2 ways to generate the sequences. Either use different “u” in above formula or use the same root sequence number and generate based on cyclic shift by using below formula.
According to TS 38 211, we need to perform cyclic shift on a root sequence. If that is not possible to generate 64 preambles and change the root sequence till all 64 preambles are generated.
We use below formula to get the spread over frequency domain.
Different types of preamble format:
Similar to LTE, in NR there are 2 types of preamble depending upon the subcarrier spacing:
1. Long Preamble: The subcarrier spacing of PRACH preamble is 1.25 or 5 Khz, long sequence (L_RA = 839) is used.
They are divided into 4 formats as 0/1/2/3 as shown in below table.
TS 38 211
2. Short Preamble: The subcarrier spacing of PRACH preamble is 15,30,60 or 120 Khz, short sequence (L_RA = 139) is used
They are divided into 9 formats as A1/A2/A3/B1/B2/B3/B4/C0/C2 as shown in below table.
TS 38 211
A preamble consists of 3 parts: Cyclic Prefix(CP) + Preamble Sequence (repeat) + Guard Period (GP).
Below Image shows some of the example of example of different preamble formats.
Now let us take the format 2 from the table 1 as an example and see how much it will occupy the time domain:
Below table shows the number of RBs in the frequency domain.
Generation of PRACH Preamble
One PRACH preamble corresponds to 64 preamble.
The preambles are generated in the order of the cyclic shift of the first logical root sequence and then the increasing order of the logical root sequence until there are 64 preambles
prach-RootSequenceIndex indicates the starting logical root sequence.
