Phantom - Downtilt

[justdream]

Experts,

one of the consultant that I've worked with, has used to downtilt some antennas to reduce RACH Phantom

My question:

what's the critrion of selecting cells that need downtilting due to RACH Phantom? how to know these cells?

[RF engineer]

Experts,

one of the consultant that I've worked with, has used to downtilt some antennas to reduce RACH Phantom

My question:

what's the critrion of selecting cells that need downtilting due to RACH Phantom? how to know these cells?
Check cells with low immediate assignment or you have indicator related to RACH .
BR

[justdream]

Check cells with low immediate assignment or you have indicator related to RACH .
BR

I don;t have direct inducatir for phantom RACH, and I've small inquiry regarding cells with low immediate assignment:
may be this cells has aproblem on Abis link, so this may misguide me, am I right ???

[RF engineer]

I don;t have direct inducatir for phantom RACH, and I've small inquiry regarding cells with low immediate assignment:
may be this cells has aproblem on Abis link, so this may misguide me, am I right ???
May be so you can check Alarm related to this ,
BR
RF

[justdream]

Experts, no more advice regarding this case :hug:

[danikd]

Experts, no more advice regarding this case :hug:

Phantom RACH is a process when specific mobile is sending Random Access burst to a serving cell (cell which specific mobile is camping on). The Random Access burst contains TSC (Training Sequence) the TSC is derived from BCC number which is part of BSIC transmitted on Synchronization Channel (SCH) on BCCH TRX i.e. TSC=BCC for BCCH TRX.

Now when mobile transmits Random Access burst, this transmission is OMNI directional i.e. transmitted not only toward serving cell but to all surrounding cells as well.
When network utilizes a very tight BCCH reuse (less than 25 BCCH in use) in urban area, then the BCCH reused with in the limited geographical area many times and BCCH allocation requires proper allocation of BCC in order to prevent BCCH/BCC (not BSIC) reuse.

More over the Phantom RACH is related to UL performance of the cell. The UL performance of the cell is 100% depends on antenna configuration i.e. antenna type and tilt. Correctly designed antenna configuration will prevent appearance of Phantom RACH as well.

If you would like to know more...just ask!!!!

[plannerguy]

Hi

which vendor your working with...

[justdream]

Thanks a lot danikd,

I'm working with Siemens BR.9
and I want to know how could I use statistics to know worst cells causing RACH Phantom?

[kevin_h811]

hi JUSTDREAM,

Probably u could use SD Failure Rate as indicator for this issue.

Regards//

[plannerguy]

Hi justdream

expalntion on RACH low sucess rate in Siemens


Even without any real MS RACH access there are always at least some ‘noise’ signals on the RACH, which are evaluated and tried to decode by the Layer 1 of the BTS.( This is only captured by the Siemens BTS). It checks the received signal with respect to specific criteria, e.g. it measures the receive level and checks if it exceeds a hardcoded minimum threshold (RSSI level, not equal to RACHBT), the signal-to-noise ratio (SNIR), evaluates a soft decision criterion (SOVA), tries to decode the training sequence, checks the Convolution Code, checks for block CRC errors and measures the delay of the RACH burst to determine the MS-BTS distance.
The NINVRACH subcounter 3 counts all RACHs which are detected with a sufficient quality in terms of signal processing (the above mentioned criteria) but show a CRC error.

Reasons for High Values / Allowed Range
It is mainly triggered by the following signals:
• RACHs detected out of noise / interference signals,
• RACHs from GSM signals on other cells on same frequency (these RACHs show typically CRC errors since the BSIC is wrong).

In GSM05.05 the following RACH rates are allowed with random input: Less than 0.02 % of frames shall be detected as error free. That allows approximately 2.6 valid RACHs / minute or 156 RACHs / hour for one BCCH. For our NINVRACH subcounter 3 this would allow 64 * 156 / hour = 9984 / hour (6 bit CRC causes factor 64).

Some Examples with Different Scenarios
The following diagrams were delivered from one real-life network and show an evaluation of the following counters for 1 week:
CCCH.IMMASSAtts = ATIMASCA (all subcounters for SDCCH)
CCCH.RAFail3 = NINVRACH (subcounter 3 'other causes')


Regards
Plannerguy

[codebox]

Hi,

In Siemens BR9, if changing the parameter RACHBT has an effect on SDCCH drop rate, this means this cell can possibly be redesigned for optimum coverage.

Regards,

[riyantoyadi]

1. first cek with subcounter that you haved then please specific what value is bigger. If Phanthom RACH is most contributor please follow next step.

2. Adjusting environment : antenna height (tilting n azimuth) more higher TA makes some high RACH rejected, then Load of Traffic SDCCH, last but not least is interference (external or internal)