Choosing the Best Transmission Power (Tx)
All beacons allow you to adjust the transmitted power from, for example, -30dBm to +4dBm. dBm stands for Decibel-milliwatt, a unit used to measure radio frequency (RF) power level. dB (without the ‘m’) measures the power of a signal as a function of its ratio to another standardized value and the m in ‘dBm’ indicates we are comparing relative to 1 mW of power.
Transmission signals from a beacon expand outwards in a cone shape seen below. The relationship between distance and Tx power follows an inverse square law so that +3dBm is a doubling of power and -3dBm is a halving of power. +10dBm is a tenfold increase in transmission power.
The relationship actually follows an inverse square law and the logarithmic dB scale is used to represent this such that each +3dB change represents a doubling of power. -3bBm is a halving of power. +10 dB is ten times the transmit power. +4dBm is just over a doubling of power. -3dBm will be half power, -6dBm will be a 1/4 power and -9dBm 1/8 power.
Transmission power does not just affect the battery life of a beacon; it affects, of course, the range within which another smart device can pick up transmission. The lower the dBm power of a beacon, the more targeted of an area you can create. However, increasing the dBm to increase range is not as efficient due to the inverse square law.
There are many reasons to want to decrease the range of a beacon. You typically want to prevent regions of beacon coverage from overlapping. You may also want to deploy beacons which broadcast different types of packet data based on proximity. If you own a store and want beacons to target customers with specific ads within one meter of a product display, you will need to find the perfect Tx factor. Broadcasting a number of ads throughout a huge range defeats the purpose of contextual, targeted advertising.
There are also reasons to increase the Tx factor of your beacon. Stronger signal strengths have less margin for error when estimated the distances between two or more Bluetooth devices. If you are using beacons for proximity and location determination, higher Tx signals are necessary and overlap between regions is less problematic.
There is one more factor to consider when configuring the power level of your beacons. Many beacons have a value called ‘Measured Power’ which indicates what the expected RSSI (Received Signal Strength) is for a Bluetooth device at a distance of 1 meter from the beacon. This value is intended for calibration. Many factors affect the signal strength of beacons. Humid environments are less conducive for signal propagation. As a result, a facility by the ocean will need to adjust the ‘Measured Power’ of their beacons based on the real-life signal strength of beacons at one meter. The movement of human bodies, composed of ~70% water, affects signal strength, as well as the building materials and layout of facilities.
When you first receive your beacons, you will need to experiment with these two values to see what works best. Once you have a ‘Measured Power’ value which is unique to your climate and environment, you can better estimate how many beacons you will need to cover your square footage. In most cases, you should not need to change the ‘Measured Power’ value.