Explain LTE RACH Configuration and Capacity. My Question: - How many UE preambles can be handled in 10 ms frame by eNodeB? - How many UE preambles can be handled in T300 by eNodeB?

1. Why RACH ?
http://www.sharetechnote.com/html/RACH_LTE.html
• Main purpose
– Achieve UP link synchronization between UE and eNB
– Obtain the resource for Message 3 (e.g, RRC Connection Request)
• UL sync meet following criteria
– should happen only when there is immediate necessity
– should be dedicated to only a specific UE

2. When RACH Process occurs ?
• In LTE, RACH process happens in following situation (3GPP specification, 10.1.5 Random Access
Procedure of 36.300 )
– Initial access from RRC_IDLE
– RRC Connection Re-establishment procedure
– Handover (Contention Based or Non Contetion Based)
– DL data arrival during RRC_CONNECTED requiring random access procedure
E.g. when UL synchronization status is non-synchronized�
– UL data arrival during RRC_CONNECTED requiring random access procedure
E.g. when UL synchronisation status is "non-synchronised" or there are no PUCCH
resources for SR available.
– For positioning purpose during RRC_CONNECTED requiring random access procedure;
E.g. when timing advance is needed for UE positioning

3. Two types of RACH process :
• Contention Based RACH procedure
step1. UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)
step2. UE <-- NW : Random Access Response (Timing Advance, T_C-RNTI, UL grant for L2/L3 message)
step3. UE --> NW : L2/L3 message
step4. Message for early contention resolution
In this case, the UE with the successful L2/L3 decoding on NW side will get the HARQ ACK from Network. This
HARQ ACK process for step 3 message is called "contention resolution" process.
• Contention Free RACH procedure
step1. UE <-- NW : RACH Preamble (PRACH) Assignment
step2. UE --> NW : RACH Preamble (RA-RNTI, indication for L2/L3 message size)
step3. UE <-- NW : Random Access Response (Timing Advance, C-RNTI, UL grant for L2/L3 message)
• 64 PRACH Preamble Sequences are divided into three group
1. Number of Contention Based Preambles, Group A = size-OfRA-PreamblesGroupA
2. Number of Contention Based Preambles, Group B = (64 – numberOfRA-Preambles) - sizeOfRA-PreamblesGroupA
Preamble Group B is used when msg3 size is large with low propagation loss.
3. Number of Contention Free Preambles = 64 – numberOfRA-Preambles

4. How the information is encoded into PRACH (RACH Preamble) ?
• Information in PRACH Preamble is represented by purely physical properties.
1. PRACH Preamble transmission Timing (t_id)
2. Location of PRACH transmission in frequency domain (f_id)
3. Sequence of the whole I/Q data of PRACH signal (one example shown below)

5. Exactly when and Where a UE transmit RACH ?
3GPP specification TS36.211 - Table 5.7.1-2. prach-ConfigIndex
F_offset is specified by prach-FreqOffset in SIB2
F_width is fixed to 6 RBs (1.08MHz)
R_slot is specified by prach-ConfigIndex in SIB2
R_length is specified by preamble format
One System Frame = 10 ms
One Slot

6. Preamble structure

7. How to Generate 64 PRACH Preamble Sequences ?
Index of
PRACH Sequence
PRACH Sequence
0 22
1 48
30 802
31 828
32 22
33 48
62 802
63 828
rootSequenceindex 22
Highspeedflag FALSE
zeroCorrelationZoneConfig 5
base of Zadoff-Chu Sequence
Decided by rootSequenceIndex
1
cyclic shift of sequence
Decided by zeroCorrelationZoneConfig
and HighSpeedFlag
26
Higher value, More repetition in 64 sequences
- zeroCorrelationZoneConfig
- base of Zadoff-Chu Sequence
- cyclic shift of sequence
Higher speed flag, More repetition in 64 sequences

8. typical RACH configuration

9. max RACH Capacity
http://lteuniversity.com/get_trained/expert_opinion1/b/lauroortigoza/archive/2012/03/21/rach-capacity-part-1-of-2.aspx
Max RACH capacity is dominated by number of UEs per frame.
For example, if eNodeB can handle 4 UE per subframe, max RACH capacity is 40 UE/second.
Max RACH capacity is limited by numberOfRA-Preambles)
numberOfRA-Preambles, number of contention based RA preambles.

10. min RACH Capacity
http://lteuniversity.com/get_trained/expert_opinion1/b/lauroortigoza/archive/2012/03/21/rach-capacity-part-1-of-2.aspx
min RACH capacity is dominated by T300 timer.