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Relation in GSM Spectrum & TRX _ urgent

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  • #56966 Reply

    Dear All,

    i need your help.. Pls clear what is the relation in GSM Spectrum & TRX? Like… we got 4.4 MHz GSM spectrum i.e 22 ARFCN. Now how many TRX in a BTS can be occupied?… i am very confused…

    #56967 Reply

    TRX in a BTS depends on BTS capcity and TS assigned for the BTS!
    Example: For SIEMENS 240XL you can have 12 TRX and can extend by adding a COAMCO(4TRX).but depends on E1.

    #56968 Reply

    I think what Bhupendra asked was how many TRX’s can be deployed per sector depending on the 22 ARFCN NOT the BTS Capacity. He’s worried of TIGHT frequency. In my case i work for an operator with 24 GSM 900 ARFCN and we are using maximum of 2 TRX’s per sector. We have many DCS Frequency where we use 6 TRX’s.

    #56969 Reply

    Yea TIMZ, u r right..
    i got the answer also…
    in 22 ARFCN, Say 10 ARFCN will be used for BCCH, 1 as Guard Band and remaining 11 for TCH.
    If we take hopping in 3/3/2 configuration BTS then 3 ARFCN will be used for BCCH TRXs (1 per sector) are non hopping & remaining 5 TRX will be in hopping mode. For these 5 TRX there will be 5 frequencies at a time, these 5 frequencies should not be adjacent so keeping gap of atleast 1 ARFCN the total requirement is 10 ARFCN.
    That is the reason why we can not go beyond 3/3/2 configuration with 4.4 MHz spectrum (in hopping case).

    Pls revert if i m not correct…


    #56970 Reply


    The computation is actually more complex than this, but the results are the same. For the sake of sharing, here is the full explanation 🙂

    If you want to use SFH 1×1, then you must apply a threshold of 12% :
    22 ARFCN – 10 BCCH ARFCN – 1 Guard
    = 11 ARFCN available

    11 ARFCN * 12% = 1.3 (approx)
    This magic number “1.3” means you should have an average of 1.3 hopping TRX per sector, therefore your max BTS config should be 2/2/3.

    #56971 Reply
    pramod kumar

    hi…. i want to know that if gos is 2% than how many traffic putup a site in 2,2,2 trx configuration
    and one trx will take how many erlangs if gos is 2%

    #56972 Reply

    To remain below 2% GoS, a cell with 2TRX shall not exceed 8.2 erlangs.

    Capacity of a BTS 2/2/2 = 3*8.2 erlangs

    If cell with 2 TRXs exceed 8.2 erlangs, then the GoS will get greater than 2%

    One TRX can accept up to 8 timeslots TCH, so that is 8 FR calls or 16 HR calls (and any possible mix in between)

    When you talk about capacity of a cell with 2 TRXs, you cannot talk about the individual Erl. capacity of 1 particular TRX.
    Erlang B Law doesn’t work this way. You must think at “Cell” level.

    #56973 Reply

    Hi PIX,
    This magic number “1.3” means you should have an average of 1.3 hopping TRX per sector, therefore your max BTS config should be 2/2/3.pls elaborate.

    #56974 Reply

    each sector can get 1 TRX “non hopping” (that carries the BCCH ts) + 1.3 TRX “hopping”.

    In average, that’s 2.3 TRX / sector

    a BTS with 3 sectors could have a config of 2/2/3, as long as the average number of TRX per BTS = 2.3 * 3 = 6.9, then you can do whatever mix you want.
    1/1/5, 2/2/3, 1/2/4, etc.
    of course it is much more manageable to have a regular BTS configuration.

    #56975 Reply

    Hi Pix,Can u pls explain the same for SFH 1*3 whose threshold value for RF load is much higher,pls provide max confg.of trx in such case,also try to elaborate it with an example.

    #56976 Reply

    hi guys,i am working in ericsson as a o&m engineer can anyone suggest me how to clear 2to 3 site downs i checked everything bts earthing,e1 path,power.i am not able to find how to clear this issue please help me.

    #56977 Reply


    you cans earch the forum for 1×3, you’ll find plenty of answers.


    #56978 Reply
    Monica voda..

    The threshold value for rf load for 1*3 is 30% with 11 ARFCN available (as explained earlier)–11*30%=3.3.this means you can have 3.3 hopping trx per sector,ie total of 10 trx.consider it as 9,+ 1 TRX per sector”non hopping” (that carries the BCCH ts)so max conf for SFH 1*3 will be 4*4*4, Pix correct it if i am wrong.

    #56979 Reply


    That’s it ! Just one clarification (but you didn’t say anything wrong : i just want to make it clear for readers):

    the total bandwidth needed is 33 frequencies for hopping TRXs (that’s 11 ARFCN per sector, as you said).

    #56980 Reply

    Hi PIX,
    Can u pls explain the relation of ARFCN and physica channel,is it that 1 AFRCN is a carrier that has 8 time slots (ie TDMA frames) so with 22 ARFCN ie on 4.4 mhz spectrum,there are 22 ARFCN so the pysical channel with be 22*8=176 avaliable for a cluster.Now we have to map the logical channels on this physical channel,using reuse pattern,am i right?if i am wrong pls give correct explaination..

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