Saturday, January 23, 2016

Understanding Milliwatt to dB conversion in the Wireless Survey

     Decibel to milliwatt conversion is important in most every aspect of 802.11 wireless. We use it for antennas, cable loss, signal amplifiers, radio transmitters, etc..

     One place that we do not always apply this knowledge and understanding is in the design stages of the wireless network. Particularly in the survey.

     Depending on the data rate you are wanting to provide to all of your devices in the network, you will survey accordingly. We have to understand that every three (3) dB gained or lost in our rf signal is a direct result of half of our power being gained or lost.

     For example: A transmitter set to 20dB power output is transmitting 100mW of power. If we transmit at 17dB, then our power output is cut in half to 50mW. That is a drastic difference to several applications and devices in 802.11 wireless based on the circumstance.

     If you are using the Air Magnet or Ekahau survey tool to design your wireless network, you have to pay close attention to your measurement tools provided:

RSSI - Received Signal Strength Indicator - The signal from your transmitter

     Noise Floor - Other devices in the area causing a signal to be detected on the frequency you are surveying with.

     SNR - Signal - to - Noise - Ratio - The difference between the signal level your transmitter is transmitting and the signal received from the noise floor created by other devices.

     The SNR is your usable signal and it is read in a positive decibel number compared to milliwatts. The RSSI and the noise floor are both read in a negative decibel number compared to milliwatts.

     If you have a RSSI level of -70 and a noise floor of -92 then your SNR is the difference between the two numbers 22 and it is read in decibels compared to milliwatts (22dBm).

     Where the understanding of the conversions comes into play is when your RSSI or the Noise Floor changes: -67 RSSI and a Noise Floor of -89. you still have 22dBm as a SNR. If the measurement of the RSSI is -64 on the meter, then you naturally think you have a better signal strength, but if the noise floor changes, then you have to factor in the change in the Noise Floor before believing you have a good signal for your design.

     Remember, the SNR is the signal that really matters, and a 3dB difference in that effects your power by half. Sure the industry standard for Voice over wireless is a -67dB RSSI at the edge, but that is assuming a noise floor of -92dB or better. That's a minimum SNR of 25dBm.

     If the Noise Floor goes from a -92dB to a -89dB then you have lost half of your power and that can have serious effects on certain applications in your network.

     If your Noise Floor gains 1dB then you should increase your RSSI gain by 1dB to compensate for it. This is why most all survey engineers go ahead and survey for a RSSI of -65dB in the event that the Noise Floor fluctuates. It gives you a safety zone for your SNR because that is the number that really matters.

    This is why a Spectrum Analysis is Imperative before starting your survey. You have to identify the noise floor you will be dealing with before you can provide a good design.

     3dB changes your transmit and/or receive power by half. This can lead to one way audio at times and it can lead to a downshift in data rates on either transmit, receive, or both. Keep your mind on the math as you survey.

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