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 

Addon -  Check the table in the Unit conversions of this link

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Satya Sravan