Tuesday, 5 January 2016

Cell Radius Limited by Prach Parameters Issue Analysis


1 Description

During the drive test in a project, find some place with not too bad coverage (RSRP=-106 dBm) but the UE cannot random access successfully.



2 Analysis

2.1 Random Access Failure

This snapshot show that the UE RSRP is about -105 dBm, and UE send Random Access Request(MSG1) for many times but do not receive the response from the network. This cause the random access failure. In this case we notice that the UE location is about 10.2 km from the site Sparta (and other surrounding sites)
 
. 
 

2.1 Prach Parameters Analysis

The cell access radius is determined by the Prach Parameter (Ncs). This parameter name in OMM is “Ncs Used to Generate Prach Preamble(Ncs)”and its short name is ncs. Here is the relationship between ncs and the cell radius. Currently the the whole network ncs configuration is 10, that is 9.8 km for the access radius. So this random access failure case is caused by the Prach configuration. 
 

Ncs configuration
High Speed Ncs (120km/h)
Cell Radius (km)
# of root sequence needed per cell
Num_u
Not High Speed Ncs
Cell Radius (km)(<120km/h)
# of root sequence needed per cell
Num_u
0
15
1.08
2
0
118.93
64
1
18
1.51
2
13
0.79
1
2
22
2.08
2
15
1.08
2
3
26
2.65
2
18
1.51
2
4
32
3.51
3
22
2.08
2
5
38
4.37
3
26
2.65
2
6
46
5.51
4
32
3.51
3
7
55
6.8
5
38
4.37
3
8
68
8.66
6
46
5.51
4
9
82
10.66
7
59
7.37
5
10
100
13.23
8
76
9.8
6
11
128
17.23
11
93
12.23
8
12
158
21.52
13
119
15.95
10
13
202
27.81
16
167
22.82
13
14
237
32.82
22
279
38.84
22
15
-
-
-
419
58.86
32
 
 
 
 
 
  
 
 Solution
 

3 Optimization Suggestion

 

3.1 Ncs optimization

 
.According to the DTC network, ncs=12 (corresponding cell radius is 15.95 km) is more reasonable. So we suggest optimizing the ncs from 10 to 12.

3.2 Other Prach parameters re-planning

According to table Table 1  ncs configuration, when ncs is changed, the number of root sequence needed per cell will change. That means the root sequence parameters need optimization too. The corresponding parameters in OMM are “Logical Root Sequence Start Number Used to Generate Prach Preamble” with short name rootSequenceIndex and “Transmission time configuration of random access preamble” with short name Prach Configuration Index. It is necessary to re-plan the rootSequenceIndex for each cell.


Budi Prasetyo

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