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Link Budget Calculation (HOT TOPIC)

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  • #66872 Reply

    Hi, everybody. I want to know about Link Budget Calculation (In GSM900). I donnot how many parameters is considered in calculation. Assume that we have some parameters:

    MS output power = 33 dBm
    MS anten Gain = 0 dBi
    BTS anten Gain = 18 dBi
    Anten diversity = 2 dB
    Cable Loss = 3 dB
    Body Loss = 3 dB
    BTS sensitivity = -107 dBm
    Fading margin = 3 dB
    Penetration Loss = 20 dB

    => FSL max = 33 + 18 +2 – 3 – 3 – 3 – 20 + 107 = 131 dB

    Are those parameters enough for calculation? If it isnot enough, what else parameters?

    And if i want to calculate the maximum cell radius? How can i do?

    I have two ideas. The first, if we have FSL max, we will base on the formula:
    FSL max = 20*log(4*3.14*R*f/c)
    where R is maximum cell radius (m)
    f = 900 000 000 (Hz)
    c = 300 000 000 (m/s)
    => R = 94.1 (Km) (I think it is not reasonable).

    The second, we can apply Okumura-Hata model.
    Path Loss = 69.55 + 26.16*logf(Mhz) – 13.82*logH(m) – a(h) + [44.9 – 6.55*logH(m)]*logd(Km)

    where Path Loss = FSL max = 131 dB
    f = 900 (Mhz)
    H: BTS antenna height (assume H = 30m)
    d: cell radius (Km)
    => d = 1.3 Km
    I donot know which ideas is correct and when do we use FSLmax formula, when do we use Okumura-Hata model? I think two ideas isnot correct because Path Loss includes FSL max, Body Loss, Fading margin, Penetration Loss. I donnot what i think is correct or not.

    Please help me.

    #66873 Reply

    Somebody help me, please.

    #66874 Reply

    hi tt,

    your link budget parameters are fine

    and the idea to use okumura hata is good too, that’s what you should do.

    now the only problem is that you may want to use more margins:
    1/ slow fading margin (the one you used is the “fast” fading margin), which is about 10dB. It takes into account a lognormal distribution of obstacles in the area, blocking the signal. It’s difficult to explain 🙂
    2/ design margin, or safety margin, so that nobody will jump at your throat because your results were way too much optimistic, and there are coverage holes everywhere…
    3/ body loss, which is in reality closer to 6 or 9dB than 3dB.

    basically, you could remove 10+6+3
    = 19dB to your original FSL result.
    By the way, I didn’t check your computation, but you did both downlink and uplink ? then you took the worst FSL ?

    also, don’t forget that if yo intend to use more than 2 TRX per cell, you should take into account combining losses. (about 4dB loss)


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