Why RX level value always negative ?

"Rx Level"

Why is it negative ??

Before i tell you why Rx level is negative, you must know about "dbm"

dbm => decibel-milliwatt
It is measured power referenced to one milliwatt (mW).
Its value is 10 * log[ P(W) / 1mW ]

Rx level is the strength of the signal that our mobile receives from a BTS (mobile tower). Here Rx level stands for Receiver Level

Mathematical formula to calculate the Rx level is
RxLev (dBm) = EIRP (dBm) - Path Loss (dB)

EIRP - Equivalent/Effective Isotropically Radiated Power is the amount of power that an antenna which evenly distributes power in all directions would emit to produce the peak power density observed in the direction of maximum antenna gain.
EIRP (dBm) = Pt (dBm) + Ga (dBi)

i - antenna gain in reference to isotropic antenna
t - output power transmitted

Path Loss - For a given antenna, the received power is inversely proportional to distance between the transmit and receive antennas & also to the square of frequency.

Ls ~ d^2 f^2
Path loss - Ls(dB) = 32.4(dB) + 20*log [ f (MHz) ] + 20*log [ d (km)]
where 32.4 is a constant of proportionality

The value of Path loss is always greater than EIRP, hence Rx level is negative

Example:

If we take power as 2 W i.e., maximum output from a UMTS/3G mobile phone of Power class 1
Pt will be 33 dbm (calculated using dbm formula).
Let gain Ga be 9 dBi => EIRP = 33 + 9 = 42 dBm

Frequency be 1800 MHz; Distance be 2 Km
Path loss = 32.4 + 65.1 + 6 = 103.5 (calculated using path loss formula)

Rx level = 42 - 103.5 = - 62 dBm ** negative value **

RX level Range
In few websites, i found Rx level mentioned in a range of 0 to 63 (positive value). Just subtract it from 110 dBm to get the value of Rx level. 0 to 63 equals -110 dBm to -47 dBm.
Upto - 65 dbm : Excellent
-65 to -75 : Good
-75 to -95 : Normal
> -95 : Weak

Rx Lev Sub and Rx Lev Full ??

Rx_Level_Full is measured when DTX is off.
RX Lev Full Its is nothing but the Mobile transmit the measurment report(SACCH multiframe) for every 480ms. This
multiframe containes 104 TDMA frames, in 104 TDMA frames 4 TDMA frames for Decode the BSIC and remaining 100 TDMA
frames for Average measurment of serving cell and neighbouring cell.This average measurment of 100 TDMA frames
are RX Lev Full

RX Lev Sub is when DTX is on.
DTX is a discontinouse trasmission, When the mobile conversation 40% of the time either Trasmitter or Receive is idle. When DTX is ON, DTX will switch off the Trasmitter or Receiver when they is no speech Pulses. only
few TDMA frames will trasmit, the average of this TDMA frames is called RX Lev Sub, give you proper measurment of RX level.