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Higher BER in SDCCH TS

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  • #54510 Reply
    NYK
    Guest

    Normally SDCCH TS have higher BER then the TCH TS.
    What is the reason actually for this?

    #54511 Reply
    pix
    Guest

    SDCCH ts and TCH ts are using the same frequencies, the same frequency hopping mode, the same power control settings, etc..

    so no, there is no reason that they would have different BER.

    #54512 Reply
    NYK
    Guest

    I found that in a document.And I have also seen that from stat that SD TS have higher BER(both in uplink and downlink) than other TS.Does BER in SD TS increased with increase in SD traffic???
    I have just copied the lines from that document below:
    ‘SDCCH timeslots generally have higher BER than TCH, so if a carrier has high BER and the cause cannot be found, check to see if it has a very high amount of SDCCH traffic. If so, there may be nothing wrong with the carrier.’

    #54513 Reply
    pix
    Guest

    the only reason for writing this would be that the encoding and protection of the SDCCH bursts are different than the TCH burst. Meaning that the SDCCH have less protection, therefore a higher BER.

    But with HR and AMR (TCH with different codecs and protection), I doubt it makes any sense at all nowadays.

    Maybe I’m missing something here, but I’ve NEVER seen anything like this before. Of course, it doesn’t mean it’s wrong. It would prove that i don’t know all about radio, which is certainly a correct assumption :))

    Do they give any additional info about this? What’s the chapter about ?

    #54514 Reply
    NYK
    Guest

    Below is an example.This cell has AMR FR enabled.TS 1 and 2 are configured as SDCCH/8.And you will see that there DL BER is higher then the other TS.

    TS 0 1 2 3 4 5 6 7
    BER DL% 0 2.8 2.7 0.2 0 0 0 0
    BER UL% 0 0.4 0.4 0.2 0 0 0 0

    #54515 Reply
    Pix
    Guest

    That’s surprising. How did you get those measures?

    IMO, the difference can only be explained by different channel coding.

    #54516 Reply
    Raeef Saleh
    Guest

    I know this is old but anyway, it has nothing to do with coding or anything like this, it’s related to how BER is calculated. The system counts the number of bits that has an error, now to give the BER it divides the number of bits on a different number of bits depending on what kind of channel is used. For TCH, it will depend on the SACCH multiframe
    so BER= ( Sum of bits with errors)/

    sum of all bits)
    Now for SACCH multiframe
    you will have 4 multiframes ( 480 ms)
    each includes 6 blocks of voice ( 120 ms)each with 378 bits
    and one sacch block (456 bits)

    so total=
    4X6X378+456 = 9528

    now for SDCCH, you will have the 102 multiframe
    where each user will have 2 SDCCH
    blocks and one SACCH block
    =3X456= 1368

    so the denominator will be always much less in SDCCH BER than TCH BER. Hence BER for SDCCH is always higher.

    #54517 Reply
    pix
    Guest

    hi raeef,

    thanks a lot for bringing this old thread back to life !

    I understand the total of bits if different in SDCCH and in TCH timeslots. But the numerator (number of bits with error) is also different.

    The ratio should remain similar between TCH and SDCCH.
    for example, under the same radio conditions:
    on a TCH timeslot : 100 bits with error over 10,000 bits -> BER = 1%
    on a sdcch timeslot : 14 bits error over 1350 bits –> BER = 1%

    waiting for your answer, because i think you have more to say 🙂

    pix

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