Intro to Networking - Free Space Path Loss (FSPL)


As a radio signal leaves the transmitter antenna, it undergoes a phenomenon known as Free Space Path Loss (FSPL) or Path Loss. Path loss explains that as a signal propagates through space, it expands outward, resulting in a reduction in power levels. While true of all radio signals, higher frequency signals (e.g., 5 GHz) undergo greater path loss compared to lower frequency signals (e.g., 2.4 GHz). For this reason, outdoor wireless networks favor 5 GHz PtP backhaul, since path loss and EIRP are favorably-balanced for ultra-long distance links. Conversely, short distance links with ultra high-throughput favor higher frequency links like airFiber 24 GHz. Link Power Budgets consider TX/RX antenna gain and TX power as ways to compensate for path loss.


For this reason, lower frequency signals are particularly susceptible to in-band interference from neighbor networks, as signals may 'over-propagate' beyond the desired coverage area. To limit the impact of potential in-band interference by unwanted signals, keep transmitter power levels low and use the most directive antenna for the target application (i.e., semi-directional sectors instead of omni-directional antennas).

Path Loss Calculation

The amount of loss a signal can incur is twofold dependent on the 1- the frequency/wavelength of the signal, and, 2- the distance from the point reference traveled by the signal (e.g., distance from the transmitter).

The actual calculation for free space path loss is modeled by the following formula: FSPL (dB) = [(4πd)/λ]^2