[syhprum (OP)]

Full duplex operation on mobile phones is a source of wonder to me especially when I see two phones operating close together

[lyner]

You have to remember that each phone is not 'talking' directly to the other one. It is talking to its local base station. It is no important that they are using the same base station. The system would be the same if they were 100miles apart and using different bases. The data still goes back to an exchange somewhere.
What impresses me is the small data rate that is required for each voice channel and that one phone can find another one wherever it is.
It's still obscenely over priced, though!

[Pumblechook]

In GSM the phone and base are not transmitting data bits at the same time. 

[LeeE]

I think you'll find that CSMA is the answer.

Have a read of:

http://en.wikipedia.org/wiki/Carrier_sense_multiple_access

..and especially note the collision detection and avoidance sections.

Basically, the number of packets that can be sent across a link depends upon the frequency of the link, as the frequency of the link dictates how many times the state can change in a given period of time.

Although mobile phones use digital communication techniques it's perhaps easier to understand using an analogue illustration: if you recorded 1 second of speech with a 0-20kHz bandwidth you could then play it one hundred thousand times faster and broadcast it at 2GHz without any loss of resolution, which would only take 1/100000th of a second.  In theory then, a real-time 2GHz transmission link could cope with 100000 people talking simultaneously with a 0-20kHz bandwidth.

In practice, the data is digitised and the transmission link also has to carry control and multiplexing data as well as the user data, so whilst on the one hand the effective bandwidth per user might be less than 20kHz in practice, a significant proportion of the 2GHz bandwidth is used for protocol control i.e. identifying which packet of data has got to go where and coping with dropped packets and collisions.  In the end though, there's generally enough bandwidth to cope with very many conversations at the same time.

The capacity of this type of system can be further increased by splitting the system down into many discrete limited range 'cells' so that each 'cell' only has to cope with 'local' traffic and not the total traffic.

[Geezer]

It's still obscenely over priced, though!

How much do you pay in the UK? In the US I buy 1000 prepaid minutes for $100 (T-Mobile). The minutes last for one year, and if I buy another 1000 minutes before the first lot expires, they roll the unused minutes forward into the next year. So it's costing me around 5 pounds a month.

[JimBob]

I use my phone for business. I have an unlimited plan with voice, texting and web data costing about $150 a month or about 92. This may seem expensive yet compared to the cost, and inconvenience, of a land line connected to just one point about $250 a month, it is a bargain.   

[lyner]

One of my sons in in telecoms and says that people winge about spending 1p per minute on international land line tariffs but quite happily spend  £30+ per month on their mobiles.
How can £1.30 a minute be justified by ANYBODY for roaming charges? At least that milchcow will soon be extinct.

[techmind]

Full duplex operation on mobile phones is a source of wonder to me especially when I see two phones operating close together

With GSM, the "2G" mobile standard, there are several factors at play which help:
1) Primarily, the GSM signal works on a "time division multiplexing" arrangement with each carrier frequency being divided into 8 timeslots.
The basic handset transmission consists of carrier bursts of 0.577ms duration, repeating every 4.615ms (this give a repetition rate of 216.7Hz, the interference "buzz" with which we are familiar). The handset interleaves its transmission and reception so it doesn't have to do both at once. (In fact the handset cleverly advances the timing of its transmission so that it falls neatly into the correct timeslot when it arrives at the basestation, taking into account the time-of-flight!)

2) If you have two phones in close proximity then because each is only transmitting 1/8th of the time, there's a reduced likelihood that one is transmitting when another is receiving. Note also that the basestation can initiate a change of freqency and/or timeslot if it's suffering interference problems. The basestation also receives notification from the handset about how its doing, and so the basestation can increase its transmission power if the handset is struggling to receive it (the whole system continuously strives to use the minimum RF power necessary for reliable communication, given the instantaneous conditions of the radio-link).

3) On the GSM frequency allocations (both in the 900MHz and 1800MHz bands) there is a 20MHz "guard band" between the uplink band and downlink band. This permits some degree of analog filtering to be implemented at the antenna or receiver front-end.



I understand that the new 3G phones transmit and receive continuously (and therefore simultaneously). They do use digital coding systems to make signals on the same frequency non-interfering... but how they manage to transmit and receive simultaneously really must be magic!