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
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• Contention Based RACH procedurestep1. 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 procedurestep1. 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 group1. Number of Contention Based Preambles, Group A = size-OfRA-PreamblesGroupA2. Number of Contention Based Preambles, Group B = (64 – numberOfRA-Preambles) - sizeOfRA-Pre-
amblesGroupA Preamble Group B is used when msg3 size is large with low propagation loss.3. Number of Contention Free Preambles = 64 – numberOfRA-Preambles
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)
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 SIB2F_width is fixed to 6 RBs (1.08MHz)R_slot is specified by prach-ConfigIndex in SIB2R_length is specified by preamble format
base of Zadoff-Chu SequenceDecided by rootSequenceIndex 1cyclic shift of sequenceDecided by zeroCorrelationZoneCon-fig and HighSpeedFlag
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Higher value, More repetition in 64 se-quences - zeroCorrelationZoneConfig - base of Zadoff-Chu Sequence - cyclic shift of sequence
Higher speed flag, More repetition in 64 se-quences
typical RACH configuration
max RACH Capacityhttp://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.