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Need formula to selecting coaxial cable type and GSM antenna

I'm using a GSM modem for my application which is often installed at a basement where there is no GSM signal so I will need to attach it to an external antenna that is placed outside of the building.

Since low loss coaxial cables are very expensive, I'm trying to find a rough formula that will allow me to calculate the maximum loss/attenuation in dB per 100 feet of the coaxial cable that I can use, given my Antenna gain (DBi), cabling distance (in feet) required and GSM signal strength (RSSI ASU) obtained by the modem placed at the antenna location, and the minimum GSM signal strength which I can tolerate after cable loss.

For example, I'm using a 5DBi Antenna which has to be connected to the GSM modem 100 feet away in the building. If the GSM modem were to be placed at at the antenna, it could achieve a RSSI of 23 ASU which translates to -67dBm. I can tolerate a minimum GSM signal strength of 15 ASU (-83dBm). The formula should allow me to calculate the maximum loss/attenuation in dB per 100 feet of the coaxial cable that I can use.

Grateful for any thoughs. Thanks.

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    \$\begingroup\$ Are you sure you cannot move the modem near the antenna and extend the data cable? \$\endgroup\$ – jippie Oct 13 '12 at 19:58
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It is very simple. You should convert any amplifier and attenuation in dB format. and formula is like this Receiver_signal(dB)=Signal_at_antenna(dB)+Antenna_gain(dB)-Cable_Loss(dB)-other_loss(dB) As your Receiver_signal should be above minimum signal strength with a margin for example(15 dB). Place this amount for Receiver_signal and compute Maximum cable loss. Cables are standards. Please find cable loss at this table. So you can find which cable is suitable for your work. enter image description here

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