Radio transmitter with (SW or manually) variable signal strength

I wonder if anyone here has heard of any ZigBee, Bluethooth, Wifi or RFID or any other kind of radio transmitting equipment, for which it is possible to alternate the transmission strength and hence its range distance. Any kind of example would do. Any suggestion of a ~2.4 GHz module which offeres the feature of alternating signal strength, would be very very welcome. Myself I've never heard of that kind of phenomena.

- Is it impossible to alter the transmission signal strength in radio wavelengths? I mean, by means of software or manually tuning a varistor or something like that.

- If it is not impossible, then how could it done? Hardware antenna redesign? Attenuation between antenna and MPU? How? And why do no product exist which offer this feature, or have my searches failed so totally that I've missed all of them?

Thank you.

Well, I am not a radio guru, but I know it is surely possible. In a very famous RF chip 'CC1100' and its other versions, there is a register memory called 'PA' which can be configured by software to increase or reduce the output power. Many such chips have this ability. A similar Zigbee chip by Texas instrument is CC2530 with programmable Output Power Up to 4.5 dBm.

well yeah, of course. Back in the 80's bell labs (lucent) developed adaptive power control for transmitters, to make it easy to coordinate frequencies. All cell phones do that now too. Generally, the way it works is you need just a little above the minimum received power for very low bit errors. So you watch received data, report back on the bit error rate, and the transmitter adjusts accordingly.

What screws it all up is if there is changing multipath (mobile, fixed but trucks or waves on a lake moving), and bursty interferers.

I was looking for something accessable to the amateur, and TI CC2530 seems to be perfect, thanks Genovator! -28 to 4.5 dBm is about a factor 2000 in watts, I think. Pretty good range.

The simplest power control that was used for many years including in cell phones was using a coupler at the PA output and a simple Schottky diode followed by an op amp that control the bias of the PA driver.
This gives about 20dB dynamic range, and even better with some compensations.

http://www.rfcafe.com/references/articles/DPAC.htm

My choice is to use a "Digitally Programmable RF Attenuator" at the output.Using this, you will..
-Linear attenuation in dB
-You don't have to play the bias of the PA will very likely create IM Distortions
-There won't be any mismatch loss between PA and Antenna (50 Ohm input/ 50 Ohm Output)
-If you find 5-6-7-8 bits attenuator, small incremental attenuation will give you a wide playing pool
Think about it..