The Basics of Signal Attenuation
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Wireless signal attenuation is the reduction of energy or a radio frequency (RF) transmission, such as when sending data via WiFi, Zigbee, or some communication protocol for applications such as automated temperature monitoring. Attenuation is represented in decibels (dB), which is ten times the logarithm of the signal power at a particular input or source location in watts divided by the output or receiving end of a specified medium. For example, an office wall (the specific medium) that changes the strength of an RF signal from a power level of 10 milliwatts (the input) to 5 milliwatts (the output) represents 3 dB of attenuation. Consequently, positive attenuation causes signals to become weaker when traveling through the medium.

When the attenuation is high, signal power decreases to relatively low values, and the receiving device can encounter errors when trying to decode the radio signal. This problem gets worse when there is significant RF interference from other equipment in the environment. For example, IEEE 802.11 or WiFi, Zigbee, and other devices often operate on the same 2.5 GHz industrial, scientific, and medical (ISM) band that is also shared with microwave ovens. The occurrence of bit errors may prevent the receiving station from properly decoding wireless packets and sending a receipt acknowledgment to the source station. After a short period of time, the sending station will retransmit the frame; to the user, this will appear as slow communications.  In the worst case, signal power loss due to attenuation becomes so low that the system loses connectivity and all transmission stops.

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A common signal strength indicator is the Link Quality Indication (LQI) measurement based on the bit error rate [BER] of the current packet being received. The BER is expressed as a percentage calculated from the number of corrupt bits over the total number of bits in an individual wireless data packet. In a mesh network like Zigbee, it is calculated based on the previous hop of the inbound route, so that it provides information specific to the link-layer connection to the neighboring device relaying the current packet to the local device.

Any medium including air, water, wood, drywall, or concrete between the transmitter and receiver will cause attenuation of the signal strength and a reduction in the useable range between the endpoints. As the distance increases, attenuation also increases. Attenuation in outdoor applications is based on straightforward and basic free space calculations, but in contrast, indoor applications can be very complex to calculate.

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For more information on wireless monitoring, or to find the ideal solution for your application-specific needs, contact a CAS DataLogger Application Specialist at (800) 956-4437 or visit us at https://www.dataloggerinc.com.