LTE Interview Question-Answer Part 1 | SIB | MIB |Cell Search Procedure | RACH

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LTE Interview Question Part 1 | SIB | MIB |Cell Search Procedure | RACH
What is Functionality & RACH process?.
RAPID in RACH response and Back of indicator.
There is another case where UE needs to retry PRACH. It is the case where UE received RAR from the network, but the RAPID is not for it (It means that RAR is not for some other UE). In this case, it is highly probable that a Backoff Indicator value is transmitted with RAR to control the PRACH retransmission timing.
Contention Resolution
Same PRACH preamble from multiple UE reaches the NW at the same time. This kind of PRACH collision is called "Contention" and the RACH process that allows this type of "Contention" is called "Contention based" RACH Process.
i) RACH Preamble (RA-RNTI, indication for L2/L3 message size)
ii) Random Access Response (Timing Advance, T_C-RNTI, UL grant for L2/L3 message)
iii) L2/L3 message
iv) Message for early contention resolution

Exactly when & where a UE transmit RACH?
Through PRACH config index, it is determined by SIB2.
Answer:
The set of 64 preamble sequences within a cell are divided into two groups:
This group of sequences can be further divided into 2 sub-groups:
· 'Group A' sequences selected by the UE when the uplink data quantity to be sent is relatively small, or when the UE is in poor coverage.
· 'Group B' sequences selected by the UE when the uplink data quantity to be sent is relatively large, and the UE is in good coverage.
The division of PRACH sequences is illustrated in Figure 167. The number of 'Group B' preamble sequences can be set to 0 by configuring the value of 'Size of RA Preambles in Group A' to equal the value of 'Number of RA Preambles'.

The maximum allowed value for 'Number of RA Preambles' is 64 so it is possible to use all of the sequences for contention based random access, and not reserve any sequences for the non-contention based procedure.
Group A sequences are indexed from 0 to 'Size of RA Preambles in Group A'- 1, where the 'Size of RA Preambles in Group A' parameter ranges from 4 to 60 and is broadcast in SIB2.

Group B sequences are indexed from 'Size of RA Preambles in Group A' to 'Number of RA Preambles'- 1, where the 'Number of RA Preambles' parameter ranges from 4 to 64 and is broadcast in SIB2.
The UE selects a group B sequence if both of the following conditions are satisfied:
Preamble format?
How to determine which preamble format to use PRACH configindex determine which preamble format will use.
Why multiple preamble format? Especially why we need various PRACH format with different length of time?
How does Network knows exactly when UE will transmit the RACH
When do I have to retry? What should be the time delay between the previous transmission and the next transmission?
Do I have to retransmit the PRACH with the same power as previous one? Or try with a little bit higher power? If I have to try with a little bit higher power, how much power do I have to increase?
Back of Indicator:
If ARQ is implemented on RLC Layer the why HARQ is implemented at MAC Layer.
Difference between TX diversity and MIMO??
Difference between Close loop spatial multiplexing and Open loop spatial multiplexing?
HOW and WHEN UE start acquiring SIBs
Minimum of how many SIBs are required for the UE to initiate Attach procedure?
On which channels SIBs are transmitted?
Why SIBs are needed?
How many types of SIBs are there?
I am working in wireless technology where what I learn the same knowledge, technology and technical things shared with you in video format.
How to determined which Preamble format to use ?
Apart from Technology my involvement in creating online money and also learn related to some technical and social things which belong to my Learning Habit, I do not want to keep these things confined to myself.

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