Hi i have been doing some research into mobile phones and i came across a few terms that really confused me, i understand that when you talk on the phone the data is converted from your voice into digital signals, the part i dont understand is where modulation such as PSK occurs and how TDMA and CDMA fits into all of this can someone tell me what the relation is between these things.thank you
There are a few parts to a mobile phone system: most obviously you have the radio interface, some kind of two way radio link between the handset and the 'base station'. This transmitts and recieves digital data using quadrature phase shift keying (PSK). This is a kind of modulation very similar to FM, but is used with digital signals rather than analogue signals. So the digital data is used to phase shift modulate an RF carrier which is then transmitted, and picked up by the base station, which then demodulates the PSK signal to recover the digital data. This is all very well and good, but what do we do when we have many users? How can the base station distinguish between the signal from one hand set and another? This is where Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) come into play. TDMA (used in GSM systems) is a kind of time division multiplexing. Each user gets a certain 'time slot' in which they are allowed to transmit their PSK signal in. So each user takes a turn and sends a few bytes at a time. In GSM there are 8 time slots (I think, i cannot recall exactly) in each channel (frequency). The bit rate is low and special vocoders are used to get voice audio into a low bit rate. CDMA works differently, on principles originated by an Austrian actor (and electrical engineer and mathematician!) Hedy Lamarr
I don't know how that got started, but it is a possibility. Any transmitter/receiver combination has the possibility to interfere with some other combination. To my knowledge, it has never happened with aircraft. The law against cell phone use is not an FAA rule, but an FCC rule. Reason:You will tie up every tower in range of your cell phone while in an aircraft. As any radio operator should know, the higher your antenna the better reception you will get.
