What is the purpose of Random Access Preambles group A and B in LTE?
And if we increase the Group A share > 50%, what is the impact?
To avoid collision?
It’s to avoid RACH congestion. Packet fundamentals of RACH not to deal with any collision.
Small packets and large packets topics.
There are two groups of RA-Preambles, Group A and Group B. Group A always exists and Group B exists only with the specific configuration in SIB 2 parameter. The determination of Group A and Group B is described in 36.321 5.1.1 Random Access Procedure initialization as follows.
If sizeOfRA-PreamblesGroupA is equal to numberOfRA-Preambles then there is no Random Access Preambles group B. The preambles in Random Access Preamble group A are the preambles (0 to sizeOfRAPreamblesGroupA – 1) and, if it exists, the preambles in Random Access Preamble group B are (the preambles sizeOfRA-PreamblesGroupA to numberOfRA-Preambles – 1) from the set of 64 preambles.
The set of 64 preamble sequences within a cell are divided into two groups:
– group for the contention based random access procedure. UE are responsible for randomly selecting a sequence from the group of sequences advertised by SIB2. These sequences are shared by all UE and there is a danger that multiple UE select the same sequence. 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.
This is the exact reason why we need two groups, its also based on coverage.
Type A (small data)- for requesting a normal UL resource (RACH message 3).
Type B Large volume- for requesting a larger resource (RACH message 3) due to Message Size AND Pathloss (PL) < threshold.
Small volume means up to 256 bits.
How does we differentiate that its small volume or large volume?
I mean any specific indication in parameter or in message?
pathloss is PCMAX – preambleInitialReceivedTargetPower - deltaPreambleMsg3 where PCMAX Ue o/p power, initial Rx target power is typically -104 dBm ( Range -90 to -120) & deltaPreambleMsg3 is 2 dB
Yes in Nokia parameter is LNCEL: raSmallVolUl.
In general, in any network we keep the Group A share 80% only…
For example, out of 64 preambles, 40 are used for non-dedicated (contention based) & out of these 40, 32 for small volume only.
GroupB uses higher number of bits over more UL resource.
This is done to help UEs in bad coverage areas to get their Preamble received at eNB.
Higher % of Group B means coverage issues and potentially more contention at PRACH stage.
messageSizeGroupA in SIB2
Typically 56 bits which is the size of RRC Connection Request