Immediate Assignment failure

[danikd]

Friends, can you give me some information about Phantom RACH
and how to optimize it?

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!!!!

[Nasrin]

hi all
phantom RACH can degrade imm ass fail on sdcch ,what about sd drop?can affect it ? other qusetion when HLR check the channel requset is from MS or not?

[RF Engr]

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!!!!

Its a great reply.But dont you think interference on uplink can also cause Phantom rach e.g external interference in network causing phantom rach

[danikd]

Its a great reply.But dont you think interference on uplink can also cause Phantom rach e.g external interference in network causing phantom rach

As far as I understand, you are referring to non-GSM signal source that transmits with in the UL GSM band on BCCH frequency?

If I understand you right, then the answer is following:

The probability that Non-GSM signal will introduce Phantom RACH effect is very low, since in order to be decoded on on BTS side it should be GMSK modulated and should contain Random Access message with Access Cause inside.

The external (non-GSM) interference you can recognize by analyzing the ICM (Idle Channel Measurements) statistics.

Some GSM vendors like E*******, Siemens, Huawei have an efficient features to monitor UL interference for entire band (similar to UMTS UL RTWP measurements).
Those features (their results) can be very helpful for external UL interference recognition and identification.

Best regards,

[danikd]

hi all
phantom RACH can degrade imm ass fail on sdcch ,what about sd drop?can affect it ? other qusetion when HLR check the channel requset is from MS or not?

SDCCH drop could not be affected by Phantom RACH effect. Since SDCCH drop counted only after SDCCH assignment by specific mobile (immediate assignment complete and RRM - radio resource management engaged for specific connection). Just to remind Phantom RACH just initiates Immediate Assignment procedure but never competes it.

To be more precise - HLR never check channel request. All channel requests are managed on BSS level. HLR is involved only into Authentication procedure. This procedure is a DTAP procedure (Direct Transfer), where BSS side (air-interface, BTS, BSC) are transparent and not involved into transaction (they just used as transport pipe for message).

Best regards,

[shup7]

Phantom RACH is kind of non real channel request some time BTS decode noise as channel request and you can reduce it by change Parameter related to RACH and RX leve access min or RACH access min
BR
RF

thank you for your detailed explain..