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Same BCCH Neighbor

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  • #55813 Reply
    Rnp_
    Guest

    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?

    #55814 Reply
    Pix
    Guest

    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.

    #55815 Reply
    RNP__
    Guest

    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?

    #55816 Reply
    Pix
    Guest

    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.

    #55817 Reply
    RNP__
    Guest

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

    #55818 Reply
    pix
    Guest

    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 ๐Ÿ™‚

    #55819 Reply
    RNP__
    Guest

    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 ๐Ÿ™‚

    #55820 Reply
    Pix
    Guest

    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

    #55821 Reply
    RNP__
    Guest

    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.

    #55822 Reply
    GHOST RACH
    Guest

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

    Regards

    #55823 Reply
    Pix
    Guest

    RNP: yes ๐Ÿ™‚ 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 !

    #55824 Reply
    ABE
    Guest

    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.

    #55825 Reply
    Pix
    Guest

    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.

    #55826 Reply
    Ahmad
    Guest

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

    #55827 Reply
    pix
    Guest

    it is the same thing

Viewing 15 posts - 1 through 15 (of 15 total)
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