Presented by: Ryma KISMA

This presentation describes the random access and discusses the main steps of the investigation and analysis in the case of a low random access success rate and the actions to take in order to improve it.

The Random Access Channel RACH:

Random Access Channel (RACH) is used on the “uplink” to request for the allocation of an SDCCH. This request from the MS could either be as a paging answer (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.

Common Control Channel (CCCH): PCH, RACH and AGCH.

MS BTS BSCPaging

command(1)

Paging request (PCH)(2)

Channel request (RACH)(3)Channel required(4)

Channel activation(5)

Channel activation Ack(6)

Immediate assignement cmd(7)

Immediate assignement(AGCH)(8)

Random Access formula:

Failed Random Accesses of Total Number of Random Access Attempts

Random Access Success Rate = (CNROCNT)/ (CNROCNT+RAACCFA) * 100

Counters:

 RAACCFA: Failed Random Access CNROCNT: All accepted Random Access

 

Used tools:Business objects: This tool provides statistics, used to create reports, it gives

an access to the data base and presents it for analysis in tables or charts.

TEMS investigation:

Working with TEMS is very interesting, it is useful for deep analysis of drive tests to identify cases.

MCOM:

Its main advantage is to show the sites positions on a map, it gives a complete description of the site, shows co-channel and adjacent interference and neighboring relations.

X-Manager:

It allows to visualize alarms at real time, and it can also give the history of all detected faults. Perform PMR which is an estimation of TA, RX-level, RX-QUAL, in addition to NCS and FAS…

Technical procedure: A low Random Access success rate may be due to :

1/ Too high timing advance:

* In this case the parameter MAXTA should be checked and set to a highest value.

*Tilting is also possible.

2/ bad coverage:

low signal strength coverage within a cell can cause poor RA success Rate.

3/ Interference and Phantom RACH:

When the traffic within a cell is too low, there will not be many access burst coming to the RBS. In such cases most of the received signal will be noise and if the receiver at the RBS is sensitive, some of this received noise will be interpreted as an access burst. It is called as Phantom RACH.

To minimize this effect, it is possible to optimize the type of filtering that is used in the RBS and/or optimize CRO and ACCMIN for the cell.

4/ Bad BSIC plan:

the excess use of co-BCCH/BSIC across the same BSC and also for close in cells from the surrounding BSCs is one of the random access failure causes.

5/ Faulty Antenna / Cable:

If the other root causes are not fitting the profile check for faulty antenna, loose /damaged jumper/feeder cables, VSWR alarms etc.

6/ MAXRET and TX:

A good setting of MAXRET and TX is recomended for a satisfactory random access success rate.

TX: the time between two RA is definded by TX( TX chosen randomly), it is recommended to tune it between 32 and 50.

MAXRET: Number of random access retries. It is recommended to set between 2 and 4.

7/ Software congestion:

If there is no interference and the parameter tuning didn’t improve the random access performance, then the SAE(software file congestion) has to be verified on the BSC level, it may be the reason of low random access success rate.

The action to take to face this problem is to Increase the software data record.

Troubleshooting procedure:

1/ check Rxlevel, TA, site location ( Enable PMR). 2/ check the hardware, alarms and if there is no transmission

problem on the Abis.3/ check if the surrounding cells are not down or congested.4/ Perform a drive test.5/ Overcome interference and BCCH/BSIC plan optimization.6/ Parameter tuning and check MAXTA, MAXRET, TX,

ACCMIN,CRO.7/ Check SAE if the degradation occured to many cells.

Is there transmission pb on the

Abis?

Open TT to BSS team

Check Hardware

alarms

Low Random Access success

rate

Start

yes

yes

No

Reset cell

Is there a problem?

Open TT to BSS team

yes

yes

No

No

* **

Whether there is a

bad frequency

plan

MAXTA too low?

Check if there is

bad BSIC planning

*

yes

No

Change BSIC or frequency

plan

yes

No

yes

No

Increase MAXTA

**

***

Check if site is too

hightilting

yes

No

Check if parameter setting is

wrong

Check if software

file is congested

ACCMIN, TX, MAXRET

tuning

yes

No

Increase SAEyes

No

***

If Random Access is still low

restart the investigation

yes No

END

Reference list:

ALEX.STS analysis guide document.Best knowledge competence.