What is meaning of negative dbm in signal strength?

Question

When we try to get nearby cells and their LAC, MNC, signal (and while using other android apps) we are getting signal as negative value (like -85dbm). How should I take this? Should I ignore -ve sign and take absolute value or -85 is smaller strength than -60?

How this will affect my location finding?

Answer 1

The power in dBm is the 10 times the logarithm of the ratio of actual Power/1 milliWatt.

dBm stands for "decibel milliwatts". It is a convenient way to measure power. The exact formula is

 P(dBm) = 10 · log10( P(W) / 1mW )  

where

 P(dBm) = Power expressed in dBm    P(W) = the absolute power measured in Watts    mW = milliWatts    log10 = log to base 10 

From this formula, the power in dBm of 1 Watt is 30 dBm. Because the calculation is logarithmic, every increase of 3dBm is approximately equivalent to doubling the actual power of a signal.

There is a conversion calculator and a comparison table here. There is also a comparison table on the Wikipedia english page, but the value it gives for mobile networks is a bit off.

Your actual question was "does the - sign count?"

The answer is yes, it does.

-85 dBm is less powerful (smaller) than -60 dBm. To understand this, you need to look at negative numbers. Alternatively, think about your bank account. If you owe the bank 85 dollars/rands/euros/rupees (-85), you're poorer than if you only owe them 65 (-65), i.e. -85 is smaller than -65. Also, in temperature measurements, -85 is colder than -65 degrees.

Signal strengths for mobile networks are always negative dBm values, because the transmitted network is not strong enough to give positive dBm values.

How will this affect your location finding? I have no idea, because I don't know what technology you are using to estimate the location. The values you quoted correspond roughly to a 5 bar network in GSM, UMTS or LTE, so you shouldn't have be having any problems due to network strength.

Answer 2

I think it is confusing to think of it in terms of negative numbers. Since it is a logarithm think of the negative values the same way you think of powers of ten. 10^3 = 1000 while 10^-3 = 0.001.

With this in mind and using the formulas from S Lists's answer (and assuming our base power is 1mW in all these cases) we can build a little table:

|--------|-------------------| | P(dBm) |        P(mW)      | |--------|-------------------| |    50  |  100000           |     |    40  |   10000           |    strong transmitter |    30  |    1000           |             ^   |    20  |     100           |             | |    10  |      10           |             | |     0  |       1           | |   -10  |       0.1         | |   -20  |       0.01        | |   -30  |       0.001       | |   -40  |       0.0001      | |   -50  |       0.00001     |             | |   -60  |       0.000001    |             | |   -70  |       0.0000001   |             v |   -80  |       0.00000001  |    sensitive receiver |   -90  |       0.000000001 | |--------|-------------------| 

When I think of it like this I find that it's easier to see that the more negative the dBm value then the farther to the right of the decimal the actual power value is.

When it comes to mobile networks, it not so much that they aren't powerful enough, rather it is that they are more sensitive. When you see receivers specs with dBm far into the negative values, then what you are seeing is more sensitive equipment.

Normally you would want your transmitter to be powerful (further in to the positives) and your receiver to be sensitive (further in to the negatives).