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My background is computer science. Recently, I had to come across some concepts in wireless communication/cellular mobile communication. Despite having gone through many materials, I am still having a hard time understanding some basic concepts, especially the concept of "transmit power".

So, what exactly is transmit power? Is it:

  • the electrical power needed by a transmitter to transmit a signal to a receiver?
  • or the amount of electrical power transmitted from a transmitter to a receiver via a channel?
  • or is it a different kind of power?
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The transmit power for a single output transmitter typically refers to the output of the transmitter power amplifier that is connected to the transmit antenna, and that power is almost entirely radiated over the air. It is power in Watts.

The power needed by the transmitter includes this and everything else to run the transmitter. The power amplifier (PA) often dominates this power requirement which is why PA efficiency is such a big deal (For example, if a PA has a 50% efficiency and is transmitting 2W, it will require 4W of DC power to run). PA's are more efficient the closer we run them into saturation, hence constant envelope modulations such as FM (which can be saturated with no further distortion) are attractive when power efficiency is of prime importance.

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  • $\begingroup$ Hey Dan: I like your description, which matches the way I tend to think about it. The only thing that I'd add, from the perspective of the signal in the air, is that the antenna can also play a role: it may be more or less effective at radiating the power put out by the PA. And then there are directional antennas... (en.wikipedia.org/wiki/Effective_radiated_power). $\endgroup$
    – MBaz
    Feb 19, 2020 at 2:08
  • $\begingroup$ @MBaz Yes but directional antennas don't change the transmit power -- and this is why I said "almost entirely radiated" to not confuse it with what is coming out of the antenna. For example when we do a link budget, we start with Tx power, then we add antenna losses and gain, and then path loss etc $\endgroup$ Feb 19, 2020 at 2:14
  • $\begingroup$ This is actually in your link half way down "Relation to transmitter output power" is what I was trying to explain in my last comment. So we have to be clear if we are interested in Tx Power or EIRP. Tx Power in my experience has been specific to what is coming out of the connector that I would be connecting to the antenna. $\endgroup$ Feb 19, 2020 at 2:23
  • $\begingroup$ Thanks Dan, that's a very clear description. Also, is there any major difference in transmit power between uplink and downlink? Or they follow the same principles in general? $\endgroup$
    – foo
    Feb 19, 2020 at 2:23
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    $\begingroup$ @Foo Assuming you are not referring to actual power levels the principle (definition of what it is) is the same. $\endgroup$ Feb 19, 2020 at 2:25

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