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INTERFERNCE in GSM

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  • #50197 Reply
    MKT
    Guest

    Hi all…

    Please read the lines below slowly and correct me wherever I am wrong.

    1) C/I and C/A has no effect on Mobile Stations in IDLE mode.
    2) C/I and C/A is something which comes into picture the moment MS goes into ACTIVE mode.
    3) If a cell is having 4 TRX and it has BCCH and two SDCCH/8(due to HR implemented).
    4) If I make BCCH+SDCCH/8+SDCCH/8 on TRX 1 and block the remaining 5 TS and compensate this by adding 5 TCH/HR on TRX2…Then….

    Will this arrangement eliminate the problem of C/I and C/A forever?

    #50198 Reply
    pix
    Guest

    i don’t understand how it would eliminate the interference, can you explain further ?

    anyway, the TRX that carries the BCCH timeslot will always transmit a dummy signal at full power on all the timeslots, even if there is no data to transmit. So blocking 5 TS just means that the uplink TS will not be used. The 5 TS in downlink will be used.

    In IDLE mode, the C/I will have an impact : the MS cannot decode the downlink system information messages and the downlink paging messages, or the BTS cannot decode the uplink channel request (RACH) or uplink HO ACCESS.

    Regards,
    Pix

    #50199 Reply
    MKT
    Guest

    Pix..you have confused me….just tell me one thing whether power control feature is applicable to BCCH TRX or not?

    I think it is a feature of cell/BTS rather then a TRX.
    Power control is enabled or disabled on cell basis and not on TRX basis.

    Second…blocking a TS will make the gate closed….BOTH uplink as well as downlink.

    RF hopping is the case iam considering. I know other TRX are hopping.

    By blocking the remaining time slots i just want to ensure that no MS will get a TCH on BCCH frequency which is non hopping one. In this way iam ensuring that
    when MS is active he will be hopping on TCH frequencies …and even if same BCCH is coming to this cell under question the C/I will have no effect and so do the C/A.

    #50200 Reply
    klumzy
    Guest

    Hello MKT,

    Firstly PC is indeed a cell level parameter. but PC algo stats thatBeacon frequency should always be transmitted with max power and no PC should be applied on it for reasons of selections and reselections for idle mode MS.

    Secondly as Pix advised, Idle mode will be affected by the BCCH TRX and its level of interference, so even if all active TS are on hopping TRXs (not affected by BCCH TRX`s level of interfernece), all idle MSs under this cells’s coverage will be affected. loss of paging, RACH, AGCH is possible.
    You are right when you get all active MSs on hoping TRXs you will avoid bad QoS, but generaly BCCH TRX interference degrades the cell’s performance significantly.

    #50201 Reply
    Pix
    Guest

    100% OK with Klumzy 🙂

    MKT, to be clear : yes, the PC is enable at cell level, for DL, UL or both directions.

    However, DL PC will NEVER work on the timeslots that are sent on-air onto the BCCH frequency (TS 0…7).

    Even if the TCH is not allocated on the BCCH TRX, here are the impacts of bad C/I on the BCCH & SDCCH :

    – poor paging success
    – poor immediate assignment success
    – poor channel request success
    – poor SDCCH assignment success
    – poor SDCCH drop
    – poor TCH assignment success

    you will also suffer from MS that cannot decode the SI messages in Idle mode, and which will consequently reselect another cell (even if RXLEV is lower..)

    It will also induce problems in GPRS (TBF establishment failure, and re-establishment failure)

    For all those reasons, the BCCH C/I should always be better than TCH C/I.

    Regards,
    Pix

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