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Inter BSC HO failure

Viewing 11 posts - 1 through 11 (of 11 total)
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  • #60053 Reply
    Optimizer
    Guest

    Hi all,

    An HO failure occured during drive test although a good neighbor is available.
    The pb is : the serving cell(couple BCCH-BSIC) is soon as one of the 6 strongest neighbors of him self.

    There is no other cell using this couple.

    can this pb cause a ho failure ?

    Please help !

    #60054 Reply
    Pix
    Guest

    Optimizer,

    Are you using TEMS ?

    Check the HO COMMAND message (layer 3) received by the MS. In this message you can seee to which frequency (TCH) the MS is going to handover to.

    BTW, in tems, the serving cell often appears as neighbour, but that’s not a bug. It shows the BCCH RXLEV. While the serving cell “active” measurements (in the main window – not in the neighbour window) show the TCH RXLEV.

    Cheers,
    pix

    #60055 Reply
    Optimizer
    Guest

    Pix,
    Thanx for response.
    We are using agilent.
    When I check layer 3 msg, a good neighbor is available (good BCCH RXLEV):

    “Measurements are valid
    N1: Freq 6[ 4] Level -70 dBm BSIC19(dec) 23(oct)
    N2: Freq 14[10] Level -78 dBm BSIC18(dec) 22(oct)
    N3: Freq 8[ 5] Level -84 dBm BSIC15(dec) 17(oct)
    N4: Freq 13[ 9] Level -84 dBm BSIC04(dec) 04(oct)”

    however, in RR HANDOVER COMMAND, the best neighbor is not chozen :
    “BCCH: 8 BSIC: 15(dec) 17(oct)”

    the result is a handover fail.

    Please note that the best neighbor is a cell of an other BSC and an other OMC and all declarations are right(ALCATEL BSS).

    Please help !

    #60056 Reply
    Pix
    Guest

    So the chosen neighbour is:
    N3: Freq 8[ 5] Level -84 dBm BSIC15(dec) 17(oct)

    Are the two other cells in different frequency bands, or in different layers, or congested ?

    If yes,
    Then you must tell me what are the bands/layers of serving cell and each neighbour.
    RxLev(serv) = ? (-70dBm ??)
    What is BCCH/BSIC of serving ?

    Also,
    In the HO CMD message : what is the target ARFCN (just after the BCCH/BSIC info). Is this ARFCN indeed a TCH frequency in the target cell ?

    Pix

    #60057 Reply
    optimizer
    Guest

    all cells (serving and neighbors) are in the same band, the same layer and not congested.

    the serving cell :
    BCCH: 5, BSIC: 22
    RxLev Sub: -89 dBm, RxQual Sub: 7

    the HO command msg:

    “003163 13:36:50.641 L3 DL RR Handover Command
    Cell ID: 23392, BCCH: 5, BSIC: 22, LAC: 34, Mode: Dedicated, TimeSlot: 6, MAIO: 3, Hopping Flag: 1, HSN: 15, Link: Down, Type: RR, Logical Channel: FACCH, TI/SI: 0, Primitive: 6, IEI Message: 65535, IEI: 1579, TDMA Frame Number: 325576
    BCCH: 8 BSIC: 15(dec) 17(oct)
    Channel Type: TCH/H + ACCHs, Subchannel 0
    TimeSlot: 5 TrainingSeq: 7
    Hopping: Yes, MAIO: 3 HoppingSeq (HSN): 14
    HandoverRef: 2 OrderedPowerLevel: 5
    Target CA List: 8 34 49 59 (Range 128)
    Channel Mode: Speech full rate or half rate version 1 (GSM FR/HR)”

    the HO fail msg:

    “003173 13:36:51.234 L3 UL RR Handover Failure
    Cell ID: 23392, BCCH: 5, BSIC: 22, LAC: 34, Mode: Dedicated, TimeSlot: 6, MAIO: 3, Hopping Flag: 1, HSN: 15, Link: Up, Type: RR, Logical Channel: FACCH, TI/SI: 0, Primitive: 6, IEI Message: 65535, IEI: 1576, TDMA Frame Number: 325684
    Abnormal release, unspecified (Hex 01)”

    TCH frequencies of chozen neighbor :
    34 ; 49 ; 59

    #60058 Reply
    Pix
    Guest

    Optimizer,

    Weird !

    Does the MS sends the HO ACCESS, right after the reception of the HO COMMAND ?

    The HO FAIL message is in UL, right ? (I forgot)

    How many times did this situation occur ? Only once ?

    Now, in RNO/NPO : can you check the indicators in “Adjacency” / MATRIX / BASIC (it is the type 180, I’m sure you know)

    How many HO Requests & Allocated & Success from this serving cell to each of the top 3 neighbours (in the list you gave)

    In the serving cell (Cell Indicator), what is the rate of Outgoing HO Failure Rate ?

    I assume you are using ALU ? (And are you one of my trainees I saw few weeks ago ?)

    Cheers 🙂
    Pix

    #60059 Reply
    Rex
    Guest

    Hi,
    the first strongest neig. BCCH is 6 (-70 dBm) and the serving is 5 (-89 dBm). So, adjacent channels, you were already interfered. Can’t HO to second neig. with BCCH 14 (-78 dBm) wich is again interfered by forth neig. adj channel with BCCH 13 (-84 dBm). I think you should do frequency re planning in the area. I agree with Pix, first check the indicators Pix mentioned adding the DL_Quality_HO, UL_Quality_HO, RMS Voice Quality due to Interference.
    Regards,
    Rex

    #60060 Reply
    Pix
    Guest

    Hi Rex,

    Nice analysis, I totally missed the obvious interference.

    However, interference is not checked before the HO is done, so the BSC shall still propose the first neighbour as the candidate.

    Unfortunately, it is really difficult to know why the BSC didn’t choose it here, because there are so many variables to take into account (previous HO ? load ? LINK FACTOR ? PRIORITY, cause of HO ? etc…)

    Pix

    #60061 Reply
    Amit dalal
    Guest

    One problem arise in metro city of gujarat that the call drop due to handover failure.

    call drag from the serving cell and no handover to the target cell.

    Rx level(Serving):- -95 dBm
    Rx Quality:- continuous 7

    Rx level(Target):- -70 dBm

    Mobile decode the target cell but no handover to the target cell..and then call drop occur..

    Neighbour already define .

    #60062 Reply
    pix
    Guest

    hi amit,

    the first step is to check the QOS stats:
    1/ are there HO REQ and HO SUCC between those 2 cells, over the last days ? Check hourly. Check both ways.

    YES -> go to 2/
    NO -> go to 3/

    2/ Is the target cell congested ?

    YES -> in such case, the incoming HO are rejected. Tune the target cell in order to reserve one or two timeslots for incoming HO.
    NO -> got to 4/

    3/ Are both cells in different BSC, or different LAC ?

    YES -> Check the core network configuration
    NO -> delete & recreate the relation

    4/ You need to perform Ater traces and A traces in order to understand at which stage the HO doesn’t work.

    regards
    pix

    #60063 Reply
    Brother
    Guest

    dear sir Amit dalal,

    these situations are common if cell layers are defined, i faced several times, please check the layers

    Rx level(Serving):- -95 dBm which cell layer

    Rx level(Target):- -70 dBm which cell layer

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