LTE PRACH Channel-Physical Random Access Channel

LTE PRACH describes LTE Physical Random Access Channel(PRACH).It mentions links for WCDMA PRACH and GSM RACH channel basics.

This channel is used to carry random access preambles used for initiation of random access procedure. The basic structure is mentioned in the figure. As shown a random access preamble includes a CP, 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 a UE sends to access the network

•  It 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 the table-1 below. It consists of different preamble and CP duration to accommodate different cell sizes.

LTE PRACH Preamble Format	CP Length	Sequence Length	Guard Time	Total Length	Guard Time Equiv.Dist.	Typical Max. Cell Range
0	0.10ms	0.8ms	0.10ms	1ms	30Km	15Km
1	0.68ms	0.8ms	0.52ms	2ms	156km	78km
2	0.2ms	1.6ms	0.2ms	2ms	60Km	30Km
3	0.68ms	1.6ms	0.72ms	3ms	216Km	108Km

The preamble format to be used in a specific cell is informed to the UE using PRACH configuration index. This is broadcasted in SIB-2. PRACH configuration index also indicates SFN and subframes. This gives the exact position of random access preamble. Table-2 beloe mentions LTE PRACH channel configuration index, preamble format, allowed SFN and allowed subframes.

PRACHConfiguration 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 subcarrier spacing of 1.25KHz instead of 15KHz. The random access preamble occupies 1,2 or 3 subframes in the time domain(1,2,3ms) and 839 subcarriers in frequency domain(1.05MHz) . There will be 15KHz guard band on both the sides and hence it uses total of 1.08MHz (equal to 6 RBs). The position of LTE random access preamble is defined by PRACH frequency offset parameter carried in SIB-2.

There is a max. of 1 random access preamble in a subframe but more than one UEs can use it. Multiple UEs using same preamble resource allocations are differentiated by their unique preamble sequences.

As mentioned in table-2 max. of 64 preamble sequences are divided into group-A and group-B. LTE UE selects the sequence from these two groups based on size of uplink packet and radio conditions. This helps eNodeB to calculate PUSCH resources needed for UE uplink transfer. Sequences in Group-A are used for smaller size packets or larger size packets in poor radio conditions. Sequences in Group-B are used for larger size packets in good radio conditions.

LTE PRACH channel reference: 3GPP TS 36.211