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 Re-

quest)

• UL sync meet following criteria– should happen only when there is immediate necessity– should be dedicated to only a specific UE

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 proce-dure;

    E.g. when timing advance is needed for UE positioning

Two types of RACH process :

• 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

One System Frame = 10 ms

One Slot

Preamble structure

How to Generate 64 PRACH Preamble Se-quences ?

Index of PRACH Se-

quencePRACH Se-

quence0 221 48

30 80231 82832 2233 4862 80263 828

rootSequenceindex 22Highspeedflag FALSEzeroCorrelationZoneConfig 5

base of Zadoff-Chu SequenceDecided by rootSequenceIndex 1cyclic shift of sequenceDecided by zeroCorrelationZoneCon-fig and HighSpeedFlag

26

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.

min RACH Capacityhttp://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.