Signal strength

Signal strength, in telecommunications, is the magnitude of an electric field at a reference point, which is located at a significant distance from the transmitting antenna. This is expressed in terms of the signal power of the receiver or the voltage per length recieved by the reference antenna.

High-power transmissions used in broadcasting are expressed in dB-millivolts per meter (dBmV/m). In low-power systems, including mobile phones, the signal strength is expressed in dB-microvolts per meter (dBμV/m).

Why SignalStrength is expressed in negetive values??

Because, the transmitted network is not strong enough to give positive dBm values.

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 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).