Wi-Fi Basics & RF Calculation
When dealing with RF and Wi-Fi, we will see a lot of different terms like dB, dBi, dBm, RSSI and SNR. It would be super confusing if we do not understand the meaning and difference among these units, not to mention if we need to do the RF mathematics to calculate the Wi-Fi signal strength and power.
So I thought it could be useful for us to do a quick explanation of what each of these measurements means when we are studying WLAN technology.
Watt (W) & Milliwatt (mW)
The common way to express power is by unit of Watt (W); while Milliwatt (mW) is equal to one thousandth (1/1,000) of a watt.
Decibel (dB)
Unlike W and mW, Decibel (dB) is not a unit of power or absolute measurement. It is a unit of comparison which is the relative measurement of two different power levels. It has to be referenced to something else to provide meaningful measurement.
Decibels relative to an isotropic radiator (dBi)
dBi stands for decibels measured against an isotropic reference antennas (isotropic radiators). An isotropic radiator radiate an equal signal in all directions. dBi is usually used for measurement of the amount of gain that an antenna puts on a signal.
Decibel relative to 1 mW (dBm)
dBm stands for decibel measured against 1 milliwatt (mW). Instead of comparing a signal to another signal. dBm is an expression for the amount of relative power transmitted per milliwatt.
It is important to know that dBm does not scale in linear like what we expect in Watt. But instead it is being logarithmic (with base 10).
Rule of 10s and 3s
The rule below does not give the exact values from logarithmic formula calculation. But it did highlight the logarithmic nature of dBm which is good for reference when considering the RF gain and attenuation.
3 dB of loss = -3 dB = halves the absolute power
e.g. (100mW - 3dB ≈ 20mW)
3 dB of gain = +3 dB = doubles the absolute power
e.g. (100mW + 3dB ≈ 200mW)
10 dB of loss = -10 dB = decreases the absolute power to one tenth
e.g. (100mW - 10dB ≈ 10mW)
10 dB of gain = +10 dB = increase the absolute power by ten-fold
e.g. (100mW + 10dB ≈ 1000mW)
dBm and mW Conversions Table
Received Signal Strength Indicator (RSSI)
dBm and RSSI both represent the signal strength. However, they are different because RSSI is not an absolute number. RSSI is a measure of the RF energy received. It is a metric to be used by the WLAN equipment manufacturer as a relative measurement of quality of a RF received signal to a client device. Different Wi-Fi chipset manufacturer might have RSSI varies.
Noise Floor
The noise floor indicates the amount of radio energy of background noise in the environment on a specific channel. If the noise level is too high, it can result in degraded strength and performance for the wireless signal strength.
Signal-to-Noise Ratio (SNR)
The SNR is the difference between the received wireless signal and the noise floor.
SNR = Signal Strength – Noise Floor
A higher SNR value means that the signal strength is stronger in relation to the noise levels, which allows higher data rates and fewer re-transmissions.
For below example, if the received signal for a client device is -70 dBm. And then the noise floor is measured at -95 dBm. The difference between the received signal and the noise floor is 25 dB. Thus the SNR is 25 dB.
