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phantom emergency Rach

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  • #52553 Reply
    peyo
    Guest

    Hi,

    Customer point to me a cell with a strange behavior. Having a closer look i’ve found a lot of Rach with cause “emergency call” all these rach are showing a TA value at 16 when real cell coverage cannot exceed 1. These kind of issues can be seen when a another cell (far from the one we are looking at) is same BSIC, same BCCH , in that kind of cases TA is the same for all rach but cause value is different.

    Any one allready faced that kind of issue?

    BR

    Peyo

    #52554 Reply
    andri
    Guest

    Hi peyo,
    I’m quitely don’t understand your question. Do you mean cell coverage doesn’t exceed 1 TA?? and there is neighbor cell with same bsic and bcch facing direct to its sector?

    #52555 Reply
    peyo
    Guest

    Hi Andri,

    it’a urban cell so trafic is concentrated in a small area which doesn’t not exceed 700 meters radius. These Rachs with TA at 16 are unexpected rach. we can discard those rach using rach filtering on TA value but the aim here is to understand where this trouble is coming from.

    BR

    Pierre

    #52556 Reply
    pix
    Guest

    hello peyo,

    You could perform some Abis interface traces, to isolate those “funky” Channel Requests. My guess is that an external interferer is generating a certain sequence of bits. These bits are decoded by the BTS as being “emergency call”, at a TA=16.

    Other possibility : the BTS itself generates this noise.

    Could you check when are those RACH transmitted, and how often ?

    Try changing the BCCH frequency to a frequency far away (2 or 3 MHz away) (for few hours, or one day, only), and see if the problem is still there. If yes, then the problem might come from BTS, because if it was from an external interference, it would not be heard on other frequencies (otherwise, other BTS’s would hear it as well)

    Regards,
    Pix

    #52557 Reply
    andri
    Guest

    yeah, it must be some noise decoded as RACH. If it is possible for you to set RACH Threshold, then you can adjust the value.
    Or, maybe you can downtilt teh antenna, since you said that it was concentrated in small area. Should be no problem..
    i’m waiting if the problem solved.

    Rgrds,

    Andri

    #52558 Reply
    Peyo
    Guest

    Pix:

    I’ve performed radio subsystem trace (TRX internal decoder ) to see those rach. Of course they could be generated by the bts itself but , from our last findings on phatom rach, there is an erratic behavior for these rach in our system . both TA and Cause are random. We also find a strange behavior of the system but only for LU and at TA=0 but nothing has been ever found on Rcah with emergency cause.

    I’ve allready asked for frequency changed. waiting for an answer from customer. (and according to the crazy amount 1/2h should be enough 🙂 )

    I’ve also checked rach time reception and they seems to be random. you can have 2 minutes without any rach then 2 in 15ms (which is not a racch repetition due to ccch_conf and tx_interger settings of the network)

    Andri:

    I’m afraid antenna downtilt is not the solution because it should not really affect reception . i was thinking in intermodulation issue.

    BR

    Pierre

    #52559 Reply
    Pix
    Guest

    Peyo,

    thx for the feedback. for info, which system is it ?

    the weird thing is the static value of the timing advance : it points to a problem in the BTS itself. If it was an external interferer, the funky bursts should not be “synchronized”.

    (the TA is a delay, based on synchronization between MS and BTS. without synchronization, the timing advance should be random)

    #52560 Reply
    peyo
    Guest

    the thing that generate this external interference can be synchronised in another place (on a GSM network i mean) in that case TA=16 doesn’t mean that the issue is at 16*0,55 km it means that it seems to come from a fix point. If you’have real interference with another cell with same BSIC,BCCH you can see the same behavior on TA but all cause are seen (LU, Page response, LMU, …).

    I agree that it could come from BTS itself that’s why BCCH change should tell us more

    #52561 Reply
    pix
    Guest

    it’s quite impossible that another BTS or another system is so sharply synchronized with the BTS… it could from another of your BTS’s, but only if they share a common clock.

    #52562 Reply
    peyo
    Guest

    Hi Pix: i’m afraid you’re 100% right. I will keep you posted on our findings on that issue

    #52563 Reply
    peyo
    Guest

    crazy stuff!! the BSIC has been changed and it solved the problem ….

    #52564 Reply
    pix
    Guest

    is there any cell about 8km away with same BSIC/BCCH
    or same BSIC/TCH
    or same TSC/BCCH
    or same TSC/TCH ???

    I think your BTS are synchronized, and there are MS’s 8km away that perform CHANNEL REQUEST or HANDOVER ACCESS towards a cell with same bsic or tsc, and same tch or bcch than the degraded bts.

    crazy stuff indeed ! if you manage to find anything let me know, i’ll certainly use that to illustrate possible problems generated by duplicated “radio ID”

    regards
    pix

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