Terminology » LTE » LTE PRACH Channel: Physical Random Access Channel Explained

LTE PRACH Channel: Physical Random Access Channel Explained

lte prach random access channel preamble

This page describes the LTE Physical Random Access Channel (PRACH). It also provides links to WCDMA PRACH and GSM RACH channel basics. This channel is used to carry random access preambles, which are used for initiating the random access procedure.

The basic structure is shown in the figure below. A random access preamble includes a CP (Cyclic Prefix), a sequence, and a guard time. This carries the random access preamble. The RACH transport channel is mapped to this.

Carries the random access preamble that a UE sends to access the network.
Consists of 72 sub-carriers in the frequency domain.
There are 4 different RA (random access) preamble formats defined in LTE FDD specifications.

The same have been mentioned in Table 1 below. It consists of different preamble and CP durations to accommodate different cell sizes.

LTE PRACH channel

Table 1: LTE PRACH Preamble Formats

Preamble Format	CP Length (ms)	Sequence Length (ms)	Guard Time (ms)	Total Length (ms)	Guard time equiv. dist. (Km)	Typical Max. cell range (Km)
0	0.10	0.8	0.10	1	30	15
1	0.68	0.8	0.52	2	156	78
2	0.2	1.6	0.2	2	60	30
3	0.68	1.6	0.72	3	216	108

The preamble format to be used in a specific cell is signaled to the UE using the PRACH configuration index. This information is broadcast in SIB-2 (System Information Block Type 2). The PRACH configuration index also indicates the SFN (System Frame Number) and subframes, providing the exact position of the random access preamble.

Table 2: LTE PRACH Channel Configuration Index

PRACH Configuration Index	Preamble Format	SFN	Subframe number
0	0	Even	1
1	0	Even	4
2	0	Even	7
3	0	Any	1
4	0	Any	4
5	0	Any	7
6	0	Any	1, 6
7	0	Any	2, 7
8	0	Any	3, 8
9	0	Any	1, 4, 7
10	0	Any	2, 5, 8
11	0	Any	3, 6, 9
12	0	Any	0, 2, 4, 6, 8
13	0	Any	1, 3, 5, 7, 9
14	0	Any	0 to 9
15	0	Even	9
16	1	Even	1
17	1	Even	4
18	1	Even	7
19	1	Any	1
20	1	Any	4
21	1	Any	7
22	1	Any	1, 6
23	1	Any	2, 7
24	1	Any	3, 8
25	1	Any	1, 4, 7
26	1	Any	2, 5, 8
27	1	Any	3, 6, 9
28	1	Any	0, 2, 4, 6, 8
29	1	Any	1, 3, 5, 7, 9
30	-	-	-
31	1	Even	0
32	2	Even	1
33	2	Even	4
34	2	Even	7
35	2	Any	1
36	2	Any	4
37	2	Any	7
38	2	Any	1, 6
39	2	Any	2, 7
40	2	Any	3, 8
41	2	Any	1, 4, 7
42	2	Any	2, 5, 8
43	2	Any	3, 6, 9
44	2	Any	0, 2, 4, 6, 8
45	2	Any	1, 3, 5, 7, 9
46	-	-	-
47	2	Even	9
48	3	Even	1
49	3	Even	4
50	3	Even	7
51	3	Any	1
52	3	Any	4
53	3	Any	7
54	3	Any	1, 6
55	3	Any	2, 7
56	3	Any	3, 8
57	3	Any	1, 4, 7
58	3	Any	2, 5, 8
59	3	Any	3, 6, 9
60	-	-	-
61	-	-	-
62	-	-	-
63	3	Even	9

The preamble uses a subcarrier spacing of 1.25 kHz instead of the usual 15 kHz. The random access preamble occupies 1, 2, or 3 subframes in the time domain (1, 2, or 3 ms) and 839 subcarriers in the frequency domain (1.05 MHz). There’s a 15 kHz guard band on both sides, resulting in a total bandwidth of 1.08 MHz (equal to 6 RBs - Resource Blocks).

The position of the LTE random access preamble is defined by the PRACH frequency offset parameter carried in SIB-2.

There can be a maximum of 1 random access preamble in a subframe, but multiple UEs can use it. Multiple UEs using the same preamble resource allocations are differentiated by their unique preamble sequences. As mentioned in Table 2, a maximum of 64 preamble sequences are divided into Group A and Group B.

The LTE UE selects a sequence from these two groups based on the size of the uplink packet and radio conditions. This helps the eNodeB to estimate the PUSCH (Physical Uplink Shared Channel) resources needed for UE uplink transfer.

Sequences in Group A are used for smaller-sized packets or larger-sized packets under poor radio conditions. Sequences in Group B are used for larger-sized packets under good radio conditions.

LTE PRACH channel reference: 3GPP TS 36.211