1) Unit of power 
dBm (sometimes dBmW or Decibel-milliwatts) is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. Comp...
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This is decibels with respect to 1 mW of power. +30 dBm = 0 dBW.
Decibel level referenced to 1mW at 1004 Hz into 600 ohms impedance.
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Decibels rated above or below 1 milliwattDecibels to a Milliwatt
Decibels, dB, referenced to one milliwatt used in communications engineering as a measure of absolute power values. Zero dBm equals one milliwatt. S(dBm) = log10 P (milliwatts). So P = 10**(S/10). For example, the quoted mobile power levels for regulated UTRA - given in dBm translate as follows - 33 dBm = 1.9 watts 27 dBm = 0.5 watts (500 milliwatts) 24 dBm = 0.25 watts (250 milliwatts) 21 dBm = 0.125 watts (125 milliwatts) . And typical received signal strengths at a mobile phone are -90 dBm = 0.001 nanowatts (10-9 watts), -80 dBm = 0.01 nanowatts (10-9 watts) and -70 dBm = 0.1 nanowatts (10-9 watts).
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dBm -Decibels relative to one milli-Watt - is a measure of power output.
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dBM is an identifier meaning 'decibels referred to one milliwatt,' the common reference point for power levels in telecommunications circuits.
Variation on dB referenced to 0dB = 1mW into 600Ohms.
Decibels compared to one milliwatt. The higher the dBm, the higher the devices transmit or receive power.
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