[Rnp_]

Hi,
What might be the odd case if
1)I have a neighbor relation with a cell which have same BCCH of the serving Cell?
I wanted to know what might be the reason that HO will not happen with that neighbor?

2)2 Neighbor have same BCCH.In that case what are the difficulties to HO happen to any of the cells?

[Pix]

RNP,
Hi

1) Impossible scenario, if cell 1 and cell B are neighbors, they can’t have the same BCCH. It’s forbidden by the system. If it isn’t, then huge interference is bound to happen.

2) Cell B and cell C are neighbors of the cell A, and they both have the same BCCH. Just ensure B and C don’t have overlapping coverage, and they have different BSIC.
No problem at all.

If the coverage overlaps a bit, you’re going to see that HO from cell A to cell B might also be understood as a channel request (RACH) in cell C. And of course, a bit of interference on top of all that.

[RNP__]

Hi Pix,
1st of all,Thanks a lot.
(For the 2nd case,)as I know,MS will try to decode the BSIC whose signal its getting.So if it gets signal for both Cell B(lets say -75 dBm) and Cell C (lets say -99 dBm) and lets say the serving cell A have level (-82 dBm) and if the MS try to HO from A to B is there any chance to wrong BSIC decode for B (instead of B its decoding the BSIC of C) so HO will fail/not happened?

[Pix]

RNP,

YEs, it could happen. But there is a strong difference of level between B and C (C/I = 24dB), so the mistake might not happen that much.

But it will eventually, once in a while, so your QoS won’t be perfect: some HO will fail. Some call setup on cell B and cell C are going to fail. And also a lot of ghost RACH on cell B and cell C, which might lead to congestion.

Altogether, identical BCCH frequency on two cells that share coverage (even if it’s just a little bit) will degrade the QoS.

[RNP__]

Hi Pix,
Thanks again.
But why lots of ghost RACH on cell B and C?

[pix]

because channel requests and ho access on cell B are heard by cell C… And therefore processed as channel requests (by cell C).

And vice versa 🙂

[RNP__]

Sorry Pix,
I cannot understand why the channel request and HO access on cell B will be heard by cell C also?
Please explain a little brief…I am not clear about the cause of ‘Ghost RACH.'(why it happens?).

Pardon me to ask u question again and again 🙂

[Pix]

a MS sends a CHANNEL REQUEST to cell B, on its BCCH frequency.

This channel request is sent to cell B, but cell C can also catch it.

Cell C will start processing this channel request, because it is located on its BCCH frequency. Of course, the MS will not pursue this channel request with Cell C, but rather with cell B.
So that’s 1 ghost rach for cell C.

Same with HO ACCESS : the MS sends a HO access on the BCCH frequency of cell B (because the previous serving cell has allocated a target TCH on the BCCH TRX of cell B). This HO ACCESS is decoded by cell C, because it’s on its BCCH freq and it looks like a channel request (same message and burst format).
That’s 1 ghost rach for cell C.

Regards,
Pix

[RNP__]

thanks again Pix.
So its clear that if we have co-BCCH problem with overlapping coverage then it will increase the no. of GHOST RACH.

[GHOST RACH]

Hi
Please I need to know if there is difference between phantom RACH and Ghost RACH?????

Regards

[Pix]

RNP: yes 🙂 clear as a ghost.

GHOST_RACH: it is exactly the same.
ghost rach, phantom rach, dummy rach… it all means the same thing.

I can see that ghost RACH really are getting a lot of attention nowadays !

[ABE]

Hello,

Has anyone seen a GSM cell suddenly experiencing an abnormal increase of CHANNEL REQUEST with cause LOCATION UPDATE that lasts just a couple of hours and disappears?

They are not phantom RACH since the cause is clearly identified.

[Pix]

I’ve seen that once, when BTS from other operator goes down : all their subscribers move to our BTS, trying to LU into our PLMN.

[Ahmad]

Hi
Please I need to know if there is difference between phantom RACH and Ghost RACH?????

[pix]

it is the same thing

[Author]

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