Hi,
why in TLC having negative TX values is valid? How can we transmit something if the transmission power is in the negative?
Why for at least in fiber optics the “no signal received” was established to be -40 dbm rather than 0?
In: 0
From Wikipedia, dbm is a dimensionless unit. It’s used to compare values, so a negative value is perfectly reasonable and represents one value being lower than the other.
Dimensionless units are essentially numbers, framed using context for a particular use case. Another common example is a radian, used to represent an angle
It’s because you’re using decibels. Decibels measure how strong or weak something is relative to something else. Because it’s a relative measurement, negative values are allowed. Negatives mean less than (in this case weaker than) not non existent. So -40 db is a measure how how much weaker the signal can be (probably compared to noise), it’s putting a limit on how weak the signal can be relative to another thing (not 100% sure but probably noise).
RSSI is just a scale, like Celsius or Fahrenheit. 32°F is the same temperature as 0°C. It’s easier to measure water in Celsius and there are benchmarks to that. 0°C is ice, 100°C is gas.
However, for signal strength, there’s only a maximum amount I can have, but with sensitive enough equipment, there’s not really a lower limit. That makes it hard to put a positive maximum value on it, but easier to measure how much less there is. If I’m directly at the source, I have all the signal, so we mark that as the maximum, 0dbm. If I move away a bit, I now have 40dbm less of a signal, so the value is -40dbm. If I move farther, I have 80dbm less of a signal, so -80dbm, -120dbm, etc.
It is a relative measure. Logarithm of power ratios is the most common for RF. So a specific level is used as a reference point. Call it power level A. This gives a “bel”. The more common decibel = 10*bel
The signal to be measured is B. So if B = A, then B/A = 1 and log(1) = 0. Therefore a power level identical to the reference power level will get a dB measure of 0.
If B>A, then B/A > 1 and log (B/A) > 0.
If B<A, then B/A < 1 and log (B/A) < 0
dB makes sense if you understand the reference level. In RF communications, that reference level is usually 1mW in which case a customary notation of dBm is used (to indicate that the reference power level is 1mW)
In this case any measure of dBm > 0 means the signal is > 1mW and dBm < 0 means less than 1mW. Now for big transmission systems etc the signals can go into the kilowatts but for most hand held RF devices powered by batteries the signals will be below 1mW which gives negative dBm measures.
To add numbers to what others have said and make it more obvious that it isn’t negative power, -40 dBm is 0.1 microwatt. 0 dBm would be 1 milliwatt, 30 dBm is 1000 mW = 1 W.
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