You have to be careful with this kind of 'I'm on' signal.
A lot of regulations for un-licensed radio frequencies (such as the ISM band, etc) stipulate a maximum duty cycle on the transmission, meaning that you are only allowed to actively transmit for a certain % of the time. This prevents blocking of a particular frequency and allows better TDM of signals.
For example, for the ISM (Industrial, Scientific, Medical) range:
Dependent on the sub-band the transmission power is limited to 10 dBm … 27 dBm. The permitted time allocation (duty cycle) also varies with the sub-band. So interferences with other 868 MHz equipment are reduced and thus a better transmission quality can be achieved.
- ISM 433/868
So whatever solution you go for you will have to create some kind of periodic pulsed beacon that sends a short burst signal at pre-defined periods rather than just a constant 'on' signal.
This should also mean that you can save power in between the pulses as you can turn the transmitter off (a lot of TX chips have a 'sleep' or 'shutdown' mode) when it's not actively being used.
As has been mentioned in some of the comments, there is most likely a set of reinforcement bars in the concrete forming a 'Faraday cage'. This blocks a large number of signals from penetrating as they hit the bars and then get grounded by them. So, you need to pick a frequency that has a wavelength that is smaller than the space between the bars. Also, concrete can absorb the signal at certain frequencies.
From what I understand this is mostly due to the water content in the concrete. Water contains hydrogen. Hydrogen resonates at 2.4GHz. Many transmissions (WiFi for example) occur at 2.4GHz, so the hydrogen in the water in the concrete absorbs the transmission.*
So, the ISM 868MHz band has a wavelength of about 0.35m - this is probably going to be way too big to fit between the bars (I'm not sure what the regulations state about bar spacing). ISM 915 takes it down to 0.33m - still too big. The 2.4GHz is 0.125m - much more realistic but may not get through the concrete. So you'd be looking somewhere in the mid-to-high 1GHz range. Personally I'm not aware of a license free frequency range in that area. You'd need to check with the RF licensing people in your area (FCC, OfCom, etc).
*(This is purely my own conjecture - please correct me as I'd like to know the truth myself)
I'll assume you want to do this wireless, otherwise the solution is obvious.
I would go for infrared. The transmitter can be a small microcontroller sending a pair of bytes to an IR receiver module.
These receiver modules are tuned to a particular protocol. You can use an RC-5 module; RC-5 transmits 14-bit at a time using Manchester encoding. You can easily add a pair of bits to it, and define your own codes.
A 3 m range should not be a problem. I've used Vishay RC-5 receivers at 15 m range. At shorter distances you probably won't even have to direct the LED to the receiver. In a normal size room the signal will reflect off the walls.
A cheap RF solution is the RFM70:
Less than 5 dollar at Digikey. That's far less than an Xbee.
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This question assumes that connecting an RF signal to a power line does not convert the power line into an antenna. The power line will become an antenna and you will find that neighbors will in fact interfere with each other.
It's also quite probable (in some cases) that powerline comms will extend their range to neighbouring houses.
So, what you need to do is think about using some kind of protocol that can allow neighbours to interfere with each other without this necessarily causing an operational problem.