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?

Solution 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.

Solution 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).

Comments

[Shishir Shetty]

Can you please share the code to get the dbm value?

[Octopus]

In terms of receivers -85dBm is more sensitive than -60dBm by a factor of 316. See my answer for an explanation.

[user1725145]

The question is about the transmitted network.

[Michel Feinstein]

Are you sure Signal strengths for mobile networks are always negative dBm values, because the transmitted network is not strong enough to give positive dBm values is true? As far as I know, the values are negative because 0 is the maximum power the piece of electronics doing the communication can read/transmit, so any readings will be less or equal to 0.

[user1725145]

If you transmit at a higher power, the formula gives a positive dBm value. Perhaps your understanding is correct for a particular transmitter?

[Michel Feinstein]

Yes, I think it is, the dbm is only going to be positive if the output power goes beyond 1mW and 1mW in radiated energy is A LOT, so your initial answer is right, on audio equipment that uses dB, which is not absolute I think 0 does represent the maximum the equipment can deal with, but correct me if I'm wrong.

[Jeff]

The statement regarding signal strengths always being negative is true with regards to received power. However, handsets will often times transmit at positive levels of dBm (typically max transmit power is +23dBm). The transmit power at the cell is almost always positive dBm.