Decibel and dBm in our GSM world

If you are familiar with gain issues, link budget, antenna installation and even signal-to-noise ratio calculations etc., sometimes abbreviations regarding decibel might cause little troubles and we might be confused to understand differences between these. That’s why I decided to prepare quick tips to help people.

Decibel of voice demostration

 

First, what is decibel? It is just simple logarithmic unit of input and output power ratio. A decibel is one-tenth of a Bel - named after Alexander Graham Bell, the inventor of telephone. As a simple demonstration;

Decibel = 10 log (P / Pref)
*P= signal power and Pref = reference power

Ok, why it is logarithmic? What is the benefit of it?

One of the answers is it is magic of logarithm so that we do not lost in quantity. Ratio is more meaningful and easy to make calculations in real world. Otherwise you might need to use very big amount of numbers.

dB is not just dB? It has some friends like dBm, dbi?

Let’s talk about dBm then. dBm is power in dB relative to 1 mW.

Thanks to Wikipedia, following formula is very useful to help you:

 

0 dBm equals 1 milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making −3 dBm equal to about 0.5 milliwatt or 500 microwatts.

Note: dB(isotropic) – the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.

Ok, lets imagine, you are talking with a friend via mobile phone. There should be value range of signal strength level in order to keep your call. Our GSM authorities say that; we can hear our friend’s voice if this level stays between from -110 dBm to -48 dBm.

Decibel Imagination :)

But do not forget, -110dBm is final edge for your call. If you get close to this value you would feel the decreasing voice quality.

As practically;

-110 to -100 = Call Drop and/or Bad Quality
-99   to -90   = Getting bad/signal may break up
-89   to -80   = it is ok  but maybe
-79 to -65 = Good
Over -65 = Excellent

Why these values are negative?

Since our reference is 1milliwatt, this (-) value indicates that signal power is less than 1mw.
As an example from Wikipedia again:

-80 dBm = 10pW  

Typical range (−70 to −90 dBm) of wireless received signal power over a network (802.11 variants)

80 dBm = 100kW 

Typical transmission power of FM radio station with 50-kilometre (31 mi) range

I hope this short quick tip type blog is useful in your life.

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