RACH LTE NB-IOT

Time and frequency structure:

Following are main points for NB-IOT RACH sequence.

1. Each Preamble consists of 4 symbol groups transmitted without gaps.
2. Each symbol group consists of cyclic prefix(CP) and 5 identical symbols(total length T_SEQ) as shown in figure 10.1.6.1–1.
3. There are 2 types of RACH preamble format 0 and 1. refer table 10.6.1.1–1 for their CP length and sequence length.
4. Each preamble will be transmitted N_Rep^NPRACH times.

to understand the location of preamble we need to understand higher layer parameters of RACH (SIB2) , below is ASN for NPRACH parameters.

NPRACH-Parameters-NB-r13::= SEQUENCE {

nprach-Periodicity-r13 ENUMERATED {ms40, ms80, ms160, ms240, ms320, ms640, ms1280, ms2560}, : NPRACH resource periodicity

nprach-StartTime-r13 ENUMERATED {ms8, ms16, ms32, ms64, ms128, ms256, ms512, ms1024}, :NPRACH start time in a radio frame where NPRACH can be transmitted.

nprach-SubcarrierOffset-r13 ENUMERATED {n0, n12, n24, n36, n2, n18, n34, spare1}, :frequency location of the first subcarrier allocated to NPRACH , In number of subcarriers, offset from subcarrier 0

nprach-NumSubcarriers-r13 ENUMERATED {n12, n24, n36, n48}, : Number of sub-carriers in a NPRACH resource

nprach-SubcarrierMSG3-RangeStart-r13 ENUMERATED {zero, oneThird, twoThird, one}, :Fraction for calculating starting subcarrier index for the range of NPRACH subcarriers reserved for indication of UE support for multi-tone msg3 transmission

maxNumPreambleAttemptCE-r13 ENUMERATED {n3, n4, n5, n6, n7, n8, n10, spare1}, : Maximum number of preamble transmission attempts per NPRACH resource

numRepetitionsPerPreambleAttempt-r13 ENUMERATED {n1, n2, n4, n8, n16, n32, n64, n128}, :number of NPRACH repetitions per attempt

npdcch-NumRepetitions-RA-r13 ENUMERATED {r1, r2, r4, r8, r16, r32, r64, r128, r256, r512, r1024, r2048, spare4, spare3, spare2, spare1}, : Maximum number of repetitions for NPDCCH common search space (CSS) for RAR, Msg3 retransmission and Msg4

npdcch-StartSF-CSS-RA-r13 ENUMERATED {v1dot5, v2, v4, v8, v16, v32, v48, v64}, : Starting subframe configuration for NPDCCH common search space (CSS), including RAR, Msg3 retransmission, and Msg4

npdcch-Offset-RA-r13 ENUMERATED {zero, oneEighth, oneFourth, threeEighth} : Fractional period offset of starting subframe for NPDCCH common search space

}

NPRACH Transmission Timing

from the above diagram its clear that NPRACH will be transmitted in a frame where nfmod(nprach-Periodicity/10)=0. and in that radio frame it will be transmitted after nprach-StartTime30720Ts (shown in yellow).

Frequency location of preamble is calculated based on following equation

lets understand above equation in more detail by breaking it down.

• Frequency location of each random access symbol group will be calculated as n_start+N_SC^RA(i). value of n_start is dependent on N_SCOFFSET^NPRACH which is read from SIB2 (nprach-SubcarrierOffset). and is fixed for all symbols.
• now for each symbol group frequency location depends on other factor i.e. N_SC^RA. and N_SC^RA varies for each symbol group.
• N_SC^RA value is selected based on 5 equations, for each symbols group one equation will be satisfied and value will be calculated.From the equation it seems this value will be different for each symbol group.
• Because of different N_SC^RA for different symbols , frequency location of each symbols will be different (hopping).

RACH Procedure

RACH Process

• UE firsts read SIB-2 information to get PRACH related parameters and RACH resources (time and frequency).
• UE transmits RACH(msg-1) in Repetition unlike in legacy systems.
• eNB sends RAR(msg-2 ) back to UE by sending first NPDCCH in repetition and then NPDSCH.
• UE reads RAR and gets to know the scheduling details of MSG 3 , and it then sends MSG-3 in repetition .
• eNB receives MSG-3 and sends MSG-4 (contention resolution ) to UE by sending first NPDCCH in repetition and then NPDSCH.
• In case UE receives RAR(DL SCH subframe ending in subframe n )but does not contain response to transmitted preamble , it should transmit a new preamble 12 ms after the subframe n.
• When no RAR is received in subframe n (last subframe for RAR window), UE should send new preamble 12 ms after subframe n.