Wireless signal strength decreases during transmission because of free-space loss, penetration loss, and device and connection loss. You need to consider these link budgets when calculating the signal strength.
Free-space loss model
The free-space loss model is used to calculate the link budget of indoor DAS APs and indoor APs. The following formulas are used:
20logf + 20logd - 28 (f: MHz; d: m)
20logf + 20logd + 32.4 (f: MHz; d: km)
20logf + 20logd + 92.4 (f: GHz; d: km)
The COST231-Hata model is used to calculate the link budget of outdoor APs and applies to 2000 MHz or lower frequency bands. To calculate the link budget on the 2.4 GHz frequency band, a correction parameter Cm is used: PL = 46.3 + 33.9lg(f) - 13.82lg(hb) - a(hm) + (44.9 - 6.55lg(hb))lg(d) + Cm
The Cm value depends on the environment:
Dense Urban: -3
In the formula, hb indicates the height of base station antenna (in meters), and hm indicates the height of mobile station antenna (in meters).
f indicates the antenna working frequency (in MHz), and d indicates the transmission distance (in km).
a is a function, which also depends on the environment:
Dense urban and urban: a(Hr) = 3.2log2(11.75 Hr) - 4.97
Suburban and rural: a(Hr) = (1.1log(f) - 0.7) Hr - (1.56log(f) - 0.8)
APs' coverage area is restricted by the multipath effect. Penetration and diffraction capabilities of wireless signals are weak; therefore, wireless signals attenuate greatly when blocked by obstacles. The following are penetration loss values of 2.4 GHz radios when penetrating various materials:
8 mm board: 1-1.8 dB
38 mm board: 1.5-3 dB
40 mm wooden door: 2-3 dB
12 mm glass: 2-3 dB
250 mm concrete wall: 20-30 dB
Brick wall: 15 dB
Inter-floor penetration: 30 dB
Elevator: 20-40 dB
Device and connection losses
Radio frequency (RF) devices, such as cable connectors, splitters, couplers, combiners, and AC filters, have insertion losses.
The insertion loss of a cable connector ranges from 0.1 dB to 0.2 dB.
The insertion loss of a combiner is 0.5 dB.
For the insertion losses of passive devices, see corresponding product manuals. The following table lists transmission losses of various cables.
Link budget calculation method
Transmit power + Tx gain - path loss + Rx gain = Signal strength
AP transmit power
An AP's transmit power depends on its specifications.
AP Tx antenna gain and STA Rx antenna gain
The antenna gain is determined by antenna specifications. Generally, the value is 2 dBi.
Path losses include free-space loss, penetration loss, and loss on cables.
The penetration loss cannot be calculated accurately because it depends on wall materials and signal transmission angle. Generally, the penetration loss count as 25 dB.