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DAS eirp

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  • #69490 Reply

    Hello experts,

    i just want to know what should be the design crietrea with respect to eirp of indoor DAS, if we have gsm1800(33.5dbm bts tx pwr), umts2100(40dbm nodeb pwr,2 carriers) single sector site, how much eirp we should get from each indoor antenna. Whay should be our approach.


    #69491 Reply

    Hi Aly,

    Design criteria should be based on the RSSI and not necessarily on the EIRP. For instance, you can have a criteria like this:
    90% of the coverage area should be covered by -85dBm RSSI.
    This means that each indoor antenna can have different EIRP, depending how big the area to be covered. One antenna can have 2dBm because it only covers a small room. while others especially if located at the hall way, can have 8dBm because it has to cover more areas with concrete or wood partition.
    Using the Free Space Loss formula (FSL) together with some losses like the Body Loss and Partition losses you can have a reference value of RSSI on each antenna. FSL formula = 32.44+20*LOG(Distance to cover in km)+20*LOG(Frequency in MHz.
    You can have the same value of EIRP for each Antenna if you have the same target coverage area per antenna. There are components used in DAS for you to balance the power, you can put more power to one antenna and less on the other. These components are Power splitters/Power divider and directional couplers.

    #69492 Reply

    Thanks aye,

    i understand, also let me know, lets say if we have an omni antena which have eirp of 8dbm for 1800gsm, is there any fixed defference in umts eirp by which we can predict eirp value for umts for same antenna, as inone interview, i was asked if you have 10dbm eirp for gsm, what eirp you would get for umts.

    #69493 Reply

    Hi Aly,

    EIRP is a power computation, it does not really matter which Technology (GSM, CDMA, UMTS, LTE). I’m saying this because now we have different frequency bands that can be used for UMTS and LTE.

    If you are using purely active DAS there should be very slight difference between the EIRP of any Frequency Band (800,850,900, 1800, 1700, 1750, 2100, 2600), because the Technology will have the same Power at the Remote units, the slight difference will be due to a short jumper cable from the Remote unit to the Antenna.

    EIRP = Source Power – System Losses + Atennna gain

    Assuming you are using a Hybrid or a Purely Passive DAS, let us consider the equation above to answer your question.

    Difference between GSM900 and I assume you mean UMTS2100, there will be no fix difference at all because of two Variables:

    1. The system loss will be different depending on your product specification used in the DAS (Co-Axial Cable, jumper cables and the components Splitters and Couplers.
    2. The Source Power, if you have different Power source value before feeding to the DAS then that should directly impact the EIRP of the two Technology on each Atennna.

    In case you have the same power source then, the difference will be from the Product Specification, the loss in dB per Frequency Band of the Cable, Splitters and couplers used in the DAS.

    Now, computation for RSSI can be considered fixed in an ideal environment between Frequency bands. For instance the difference between GSM900 and GSM1800 is equal to 6dB.
    You can try different Frequency Band by using the Free Space Loss formula I have given on my first reply.
    Do not forget to convert meters to Km before using the formula.

    All the best,

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