All I can say is that it did - I can only assume by very careful RF design.
Dunno.
I vaguely recall discussing things with one of the suppliers - I can't remember why now, but I imagine one of our clients was looking for something. It didn't go ahead.
Why not ? What happens in the network ? The base sends out a radio signal that is picked up buy the phone. The phone sends out a radio signal that's picked up by the base station. All this device is doing is picking up the weak signal from one side, and repeating it to the other side - basically acting as a transmission path with negative loss. As long as it doesn't distort the signal from one side to the other then neither end is going to see any difference.
BTW - this would have been when GSM (a.k.a. 2G) was "the" standard. And the discussion about multiple frequencies didn't apply as, IIRC, the GSM standard was a "time slot, end end talking at a time" system. Therein might be a clue - with careful design I imagine you could gate the repeater transmitters according to the received signal of the receiver on the other side of the unit. But I never looked into the details beyond talking with one of the suppliers and thinking it was "a bit iffy".
They actually rely on that ?
The way the system works is the base and mobile communicate on a control channel, when a call is set up the working channel is negotiated and a route through the network is established. Sure the systen is evolving but this basic format has always been there,
as I understand it. It is not possible to build a repeater using the same transmit and receive frequency without very significant physical isolation, even using impossibly directional aerials, so additional channels will have to be employed. By neccessity the additional channels will have to be identical to the range used by the base and mobile.
This is the simplistic layout of a full duplex repeater, be it analogue, digital, air tubes, string and tin cans etc
Base transmits on a 'GO' channel, which is rebroadcast on another 'GO' channel.
Mobile transmits on a 'RETURN' channel which is rebraodcast on another 'RETURN' channel.
Without worrying about how or why, base says to mobile listen on G1 and I'll listen on R1.
The repeater has to be able to comply with that instruction in some way. That instruction has to be translated by the repeater to G2 & R2 and instructions sent forward to the mobile which has to comply.
That has to happen, with the way the phone service works it is not possible to not work this way.
In the very unlikely scenario the repeater is able to repeat on the same channel [such as one set of aerials in a remote location like the roof and the other set in the room] then yes it would be possible, however the chances of being able to guarantee that sort of isolation are way beyond restrictive. But in any case the repeater would still have to channel hop to follow the base.
The next problem is the base and mobile establish contact [for now we'll just accept that is possible on the same channel] and the next step is they tell each other how strong the signals are and adjust their power, the phone says I have this amazingly strong signal [because I'm right beside the repeater] and the base calculates it can turn the power right down, way way down but the mobile reports the signal is still strong... get the picture. In the mean time the base is reporting a very weak signal from the mobile so the mobile cranks the power up to max, in fact all 32 mobiles in use are all cranking the power up to maximum to a level which is capable to reach 10 miles but the repeater is only 10 feet away so 32 people are now cooking their brains with a transmitter running at 5000 times the power needed.
Because the base has turned power down too low to reach the repeater properly the user now starts trying to get a signal at the window and all of a sudden base receives both signals [from mobile and repeater] which is of course completely out of phase and time so the base is unable to decode it.
So we're starting to see single channel working doesn't work and we accept the repeater has to run 4 channels. When base says listen on G1 the repeater has to do that and at the same time it has to find a clear channel and tell the the phone to go there. Additionally the repeater needs to communicate with the network to advise the presence of the additional channels being used. Power levels are then negotiated and adjusted correctly.
So far only three of the functions required have been described and you'll notice I've referred to channels, as mentioned before the channels are a combination of frequency and TDM.
Adding repeaters into the network is actually a complex thing to do as it doubles the number of channels used and requires negotion with the service provider, thinking about it I wonder if it may even need to be enabled by them.
Yes they actually rely on that. [additional capacity from the service providers]