1 Random Access Success Rate:-
1.1 Definition:-
It is
the ratio between the Total no of failed random access (CS & PS) & the total
no of RACH requests (CS & PS).
1.2 Formula:-
Random Access Success Rate: = (RAACCFA/
(CNROCNT+PDRAC+RAACCFA))*100
Counter Details:-
RAACCFA: =Total No of failed random access both CS &
PS
CNROCNT: =Total No of accepted Random Access
PDRAC: = Number of
accepted PS random accesses
MS requests for the allocation
of SDCCH through RACH. This request from the MS on the uplink could either be
as a page response (MS being paged by the BSS in response to an incoming call)
or due to user trying to access the network to establish a call. Availability
of SDCCH at the RBS will not have any impact on the Random Access Success.
1.3 Reason for degradation & Solution:-
* Poor BCCH &
BSIC Plan (Especially CO BCCH-BSIC).
* Poor
Coverage/Spillage
*Cells with High
TA (Open Area Cells) & overshooting cells.
* Incorrect
parameter settings (ACCMIN, MAXRET, CRO)
* Hardware Issue
(Faulty Antenna & Cable, Faulty TRX)
* High Path
Imbalance
* Phantom RACH.
* Incorrect Timer
settings
1.3.1 Poor BCCH & BSIC Plan (Especially CO BCCH-BSIC):--
If in a BSC the no of CO BCCH & BSIC usages are very
high then it leads to high RACH Failures in that BSC & also if the cells in
neighboring BSCs are also having CO BCCH BSIC then that will also impact on
RACH Success. Change the Frequency & remove CO BCCH BSIC.
1.1.1 Poor Coverage/Spillage:--
Cells with Poor Signal Strength can cause High RACH
failures. This happens for the cells located at cell boundary with Weak
Coverage. Percentage of Call samples can be checked from MRR Recordings and can
plan for Tilts, Azimuth Antenna Height Changes, and Power Changes.
1.1.2 Cells with High TA (Open Area Cells) & overshooting cells: ---
Cells which are facing towards Open area and can cause
high RACH Failure due to Poor Coverage.
From MRR samples Tilt can Planned for the Cells & Azimuth
change also if possible can be recommended.
1.1.3 Incorrect parameter settings: ---
A.ACCMIN: -
It is the cell parameter that indicates the minimum
received signal level at the MS required for accessing the system.
Range: 47 to
110
Default: 110
Incorrect
ACCMIN values may lead to attempts from poor coverage regions. Optimized values
for ACCMIN would ensure that MS camps on a cell with good downlink signal
strength reducing the chances of failures. Normally it is defined as -110 due
to which MS will attempt from poor coverage regions. Increase the Value to
-102,-100 as MS will camp on a better cell as compared to ACCMIN -110.
B.CRO (Cell Reselection Offset (CRO) :-
Defines an
offset to encourage or discourage MSs to select the cell while it is camping on
another cell, i.e. perform a cell reselection.
Range: 0 to 63
Unit: dB
Default: 0
CRO is a
value from 0 to 63. Each step represents a 2 dB step (0 to 126 dBm). This value
is added to C1. A higher CRO value will make the cell tower more attractive to
the MS. The higher the CRO, the more attractive the cell will be. The network
might assign a CRO value to a cell if the network wanted to encourage mobile
stations to utilize that cell. The network might want to do this in order to
reduce the load on other cells during times of high traffic volume or to force
MS's to a certain band.
If the neighbouring cells to a cell which is suspected
to have poor RA Success Rate due to excess traffic from poor signal strength cell
boundary , is kept higher than the cell in question , it will make the
neighbouring cells more appealing for idle mode MS reselection across the cell
boundaries. This approach can improve the RA Success Rate for cells with excess
traffic on weak cell boundary (with good neighbours).
C.MAXRET:-
Defines maximum number of
retransmissions an MS may do when accessing the system on RACH.
Type: Numeral
Range: 1, 2, 4, 7
Default: 4
Recommendation:--
·
In the areas where the radius of the cell is smaller than 3 km and the
traffic volume is normal, such as urban non-busy areas, the MS MAX
Retransmission should be set to 4.
·
For micro cells, the MS MAX Retransmission should be set to 2.
·
For micro cells where the traffic volume is heavy and cells where
congestion occurs, the MS MAX Retransmission should be set to 1.
1.1.1 Hardware Issue: ---
- Check Hardware Faults (TRX, Cell, Neighbour Cell blocked or down) and Follow the Hardware Failure Identification Procedure and Rectification.
·
Check for faulty antenna, loose /damaged
jumper/feeder cables, VSWR alarms etc.
1.1.2 High Path Imbalance:--
From MMR Recording we can
find out the RX level average in DL & UL then we can calculate the Path
loss simply by subtraction & if the output is high then we can say the path
loss is high.
1.1.3 Phantom RACH:--
·
The phantom RACH phenomenon is a general
phenomenon in digital radio technology. It is therefore common to all GSM
systems, not only Ericsson.
·
In the transceiver, the timeslot handler in
charge of the RACH channel is listening for access bursts from the mobiles.
These bursts contain a check sequence (8 bits) that is used to determine if the
message is valid.
·
When the traffic is low in the cell, there is
not many access bursts coming to the BTS. Most of the received signal is noise.
If the receiver is sensitive, some of this noise might be interpreted as an
access burst by the BTS: a certain bit pattern in the noise might match the
valid check sequence.
·
When the traffic is high, the problem is not so
disturbing as most of the noise will be covered by genuine access bursts. So
there will not be so many phantom RACHs.
·
The more traffic you have, the less noise will
be received in the RACH channel handler, and thus, the less phantom RACHs will
be reported by the BTS.
·
This RACH phenomenon is unavoidable unless you
diminish the receiver sensitivity or introduce a powerful filtering function in
the signaling processor. But then you might lose some genuine access bursts.
·
When there is low traffic in a cell, it is
possible to receive up to about 120 phantom RACHs per hour (one every 30
sec).
1.1.1 Incorrect Timer Settings (T3122 Timer change):--
·
The
timer is used to control how long the MS should wait with sending a new channel
request after having received immediate assign reject. The timer value
corresponds to the waiting indication value sent in the Immediate Assignment
Reject message
·
Start
Event:-When the MS receive Immediate Assignment Reject Message
·
Stop
Event:-When the MS Sends RACH Request for the second time as in the 1st attempt
the request gets rejected.
·
Default
Value=10 Sec
·
Increase
the value to 15 sec so that the RACH request attempt will decrease & will
improve the RACH Success Rate
