When you say :
“BTS fails to decode is a Phantom RACH”
That’s the problem… and I’m not sure you are meaning it ! 🙂
The phantom “Channel Request” on the RACH is **succesfully** decoded and leads to a SDCCH Allocation. All other noises which are not “looking like” a Channel Request are rejected by the BTS and do not lead to any QoS degradation. And they are not called “phantom”. Just noise.
Whereas the Phantom RACH does lead to SDCCH Assignment Failure (the phantom does know seizes its SDCCH subchannel) and sometimes to SDCCH Congestion.
Now, where does the Phantom comes from… it could be from random external noise, self-generated noise or army radar signals… Or by MS which are actually sending the channel requests (or ho access) to another cell with same frequency and same BSIC.