RachRootSequence Planning
Prepared ByRAJESH KUMAR
Random access preamble transmission
•
First step in the random access procedure is the transmission
of a random access preamble
•
Indicates to the eNodeB the presence of a random access
attempt and to allows the eNodeB to estimate the delay
between the eNodeB and the UE
•
Random access preamble is transmitted on Physical Random
Access Channel (PRACH)
•
In a first step of the random access procedure, the UE selects
one preamble to transmit on the PRACH
Preamble sequence generation
•
The preamble consists of two parts:
– Preamble sequence
– Cyclic prefix
•
•
In each cell, there are 64 preamble sequences available
Preamble sequences are constructed from one or several root Zadoff-Chu
(ZC) sequences, by combining different cyclic shifts of each root sequence
– Logical root ZC sequence index is broadcasted as part of the System Information
– There are 838 different root ZC sequences used for random access and each ZC
sequence consists of NZC = 839 samples
– A new parameter is introduced in the current release for the Random access process:
rachRootSequence Planning
It can be beneficial to plan rachRootSequence such that the same root
sequences are not used in neighboring cells
– If the same root sequences are used in neighboring cells, and the PRACH
allocation overlaps, this will lead to an increased false alarm ratio, where a
random access preamble transmitted by a UE in the neighboring cell could be
detected by the eNodeB
– This is mainly a problem when the random access load is high
– In cells, where UEs are traversing with high speed, the restricted set of root ZC
sequences should be used
• It could be applicable for cells that cover railways and highways
• It is not applicable in USA because speeds on railways and highways do not
exceed 105 mph (limit for 1900 MHz)
500
450
400
350
UE speed [km/h]
•
300
250
200
150
100
50
0
700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600
Carrier frequency [MHz]
rachRootSequence Planning
RECOMENDATION
›
It is recommended to set the parameter
rachRootSequence to different values in neighboring
cells to reduce the probability for false RACH
detections
›
The values must differ by at least 10 between any
two neighbors (and at most 827 since there is a
wrap-around between the first and last value in the
value range)
rachRootSequence GROUPING
rachRootSequence
Group
rachRootSequence
Group
1
5
15
25
15
425
435
445
2
35
45
55
16
455
465
475
3
65
75
85
17
485
495
505
4
95
105
115
18
515
525
535
5
125
135
145
19
545
555
565
6
155
165
175
20
575
585
595
7
185
195
205
21
605
615
625
8
215
225
235
22
635
645
655
9
245
255
265
23
665
675
685
10
275
285
295
24
695
705
715
11
305
315
325
25
725
735
745
12
335
345
355
26
755
765
775
13
365
375
385
27
785
795
805
14
395
405
415
28
815
825
835
rachRootSequence PLANNING Process
›
PN planning tool in Planet will be used to generate a PN plan and then
convert the PN plan to the rachRootSequence plan
1.
2.
3.
–
–
4.
–
Best server layer will be generated
Neighbor list will be created by utilizing best server layer in Planet
A PN plan with an increment of 7 will be generated
Gives 24 groups, no reserves
rachRootSequence grouping includes 28 groups (see slide 6)
Take the first 24 groups and replace the PN Plan created in Planet with values listed on
slide 6
4 spare groups left
THANKS