Link Budget

A link budget is the accountant's view of a radio link: tally every gain and every loss from the transmitter's amplifier to the receiver's input, and see whether what's left clears the minimum the receiver needs to decode. If it does, with margin to spare, the link works; the size of the margin tells you how far apart the two ends can be, which is what sets your cell range and ultimately how many sites you need.

The ledger runs roughly: transmit power, plus antenna gains at both ends, minus cable and connector losses, minus the big one — path loss over distance — minus an assortment of margins (fade margin for fast fading, building penetration loss for indoor coverage, body loss, interference margin). What survives must beat the receiver sensitivity. A couple of practical truths fall out of this. The uplink is usually the limiting direction, because a handset transmits at a fraction of a base station's power, so coverage is typically uplink-bound. And because the budget is denominated in dB, a few decibels of extra margin or loss moves your achievable range a surprising amount — which is exactly why people argue over every term in it.