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CELL SELECTION

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  • #50018 Reply
    MKT
    Guest

    GSM 900 is the network. A location which is hardly 400m from the BTS site.The location is higly dense urban. There are three BCCH ARFCN’s on which the MS is latching. First one is 113 having Rxlevel of 80dBm, second one is 74 having Rxlevel of 92dBm, third one is 80 with Rxlevel=95dbm.
    Since a MS has to select the stongest BCCH then what is making the MS to latch at weak BCCH signals also. is there any way by which we can make sure that MS are latched only to a specific BCCH which is having good Rx level at a particular location.

    #50019 Reply
    paraHO
    Guest

    RxLv is one component, another is RxQual.

    #50020 Reply
    pix
    Guest

    paraHO, there is no RXQUAL in idle mode. Cell SELECTION works only in IDLE MODE.

    MKT, CELL SELECTION is a process performed only at MS “Switch On”. All other cell selections are called “CELL RESELECTION”.

    For cell selection : RXLEV_ACCESS_MIN and MS_TXPWR_MAX_CCH are the parameters that define which CELL will be selected. The RxLev is the ONLY measurement which is performed.

    For cell reselection, it is the same, with on top of that the CELL RESELECT OFFSET and CELL RESELECT HYSTERESIS (if different LAC).

    In your situation, I assume you’re talking about the cell REselection. Then check that all rxlevaccessmin and ms txpwr max cch are the same on all cells, and put cell reselect offset to zero on all cells.

    in this situation, the cell with highest RXLEV will be reselected.
    if you want to force reselection on cell with lower RXLEV, increase the cell reselect offset on the cell you want to “favor”.

    #50021 Reply
    paraHO
    Guest

    MKT should have been clearer in my comments, sorry.

    MS may use different BCCH ARFCN in dedicated mode as oppose to idle mode. Did test calls show the BCCH ARFCN with lower RxLv being used and if used what was the RXQual for these?

    I ask as I have a similar issue.

    Pix agreed GSM 03.22 and 05.08 show C1 and C2 criteria for idle mode.

    #50022 Reply
    Pix
    Guest

    ParaHO,

    You mean in dedicated mode ? The cell on which is the call can have a lower RXLEV than the neighbors, because of the capture HO’s mechanism.

    For instance, HO will push a call from a 900 cell with good level to a 1800 cell with lower level.

    If no capture HO, it can be traffic HO : the cell A with good level is high loaded (or even congested), so the cell B will carry the call, even though its level for this MS is lower than cell A.

    #50023 Reply
    paraHO
    Guest

    Thank you Pix that answers it.

    #50024 Reply
    MKT
    Guest

    Thanks Pix for reply. One more question…Handover i think was initially meant or designed for Mobile stations on move. But i know there are traffic reasons hand overs also. Any way the doubt is if iam standing at a place(iam not moving) and this situation ie of CELL RE SELECTION is happening there every next second. Assuming that the CELL RE SELECTION process is initiated by the RX LEVEL criteria then does this mean the same will be applicable to the dedicated mode also. Means in dedicated mode also the HANDOVER will be at the same frequency as the CELL RE SELECTION . If so, then what is the solution to prevent this. Also is there any document on net which can provide a good understanding on BSS Parameters(other then GSM specifications,need patience to read ). I mean CELL RE SELECTION , cell re-selection offset, hysteresis etc.

    #50025 Reply
    MKT
    Guest

    Dear Pix…if you don’t mind will you
    introduce yourself. Your experience,age country etc. Iam deeply impressed by your knowledge about GSM. What you have done to attain such an expertise. Please guide me also.

    #50026 Reply
    pix
    Guest

    when you say “frequency”, you mean the occurence of the cell reselection ?
    the cell reselection can happen every 5 seconds : a stronger neighbor is measured for 5 seconds –> the MS will reselect another cell

    in dedicated mode, the HO can occur every second at max. but there are averaging window. Normally, HO can take place every 2 or 3 seconds at max. (it is variable).

    But keep in mind that the HO will be triggered ONLY if there is a better cell around. So normally, after one HO, the Ms is in the best cell already. There is no reason to move to another cell in the next 10 seconds 🙂

    I’m not sure i understood your question though…

    #50027 Reply
    Naren
    Guest

    Adding something to Prix here are my summarized comments..

    MS operates in two modes: idle mode and dedicated mode. In the idle mode, MS monitors the broadcast channels in order to “hear” if it is being paged. It also measures other BTSs’ BCCH carrier and decides whether it should camp on another cell. This is called cell reselection and the reselection algorithm used in GSM is detailed in [Reference: 3GPP TS 05.08: 6 Idle mode tasks]. In dedicated mode (i.e. during a call), changing cell is called a handover (HO).

    Cell selection
    Cell selection is performed immediately after MS is switched on. If MS is located in the same cell it in which it was previously was switched off, the SIM card should have the local BCCH frequency stored in memory and MS should find network quite expeditiously. If MS has moved to another cell since it was turned off, it enters a cell selection procedure

    #50028 Reply
    kali prasad
    Guest

    In which wabesite i can find the total call flow and knowledge about BTS .various systems of BTS
    please tell me

    #50029 Reply
    MKT
    Guest

    If i make the DTX (discontinuous transmission) off in a particular cell.Then,
    1) Will this will provide a better Rx level in that cell? If yes then,
    a)will it will have the impact on C1 & C2.

    2) If POWER CONTROL is enable in a cell then will the making DTX off in a cell will increase the interference
    in other cells.

    #50030 Reply
    pix
    Guest

    mkt,

    good questions 🙂

    1. downlink DTX off means that all TS that are not sending data/voice/signalling will not transmit anything (0 power).
    however there is an exception : the BCCH TRX (all the 8 TS) will always transmit a signal, even if there is nothing to transmit.
    Since the C1/C2 are calculated on the BCCH frequency, then having or not DTX will not change a single thing.

    2. DL power control and DL DTX are a way to decrease downlink interference in a network. Combining them is the most effective, but just using one of them is possible.

    #50031 Reply
    MKT
    Guest

    Rxlev-Sub and Rxlev-full two things…then…c1 and c2 equation depends on Rxlevel(don’t know whether it is full or sub value)…and DTX and Rxlev-sub/full also has some relation. that is why i asked whether c1 or c2 has something to do with DTX.

    #50032 Reply
    pix
    Guest

    DTX is used during dedicated mode, on non-BCCH TRX.

    C1 and C2 are used by MS’s to (re)select a cell : it is performed in idle mode, on the BCCH TRX.

    There is no interaction between the two : the MS are using the RXLEV ‘FULL’ because they don’t wait to measure just some special timeslots (however it shouldn’t be called ‘full’ : it is just a level measured on a signal, not on specific timeslots – as far as i think)

    Don’t forget the MS’ are measuring the BCCH of their serving cell + all the other frequencies in their bands…)

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