Mobile Phones: How They Work Part 4 — the Mobile Phone Itself
Introduction
Mobile Phones: How They Work Part 3 — the Traffic Channels
Introduction
Mobile Phones: How They Work Part 4 — the Mobile Phone Itself
Introduction
In the previous sections of this introduction to mobile phnees (Parts 1, 2 and 3) we learnt about mobile telephone networks work and what happens when your phone connects to the network. To complete the picture of the mobile phone the final thing we need to consider is how the physical handset itself functions.
To do this, we shall return to the schematic of the mobile phone that was first used in part 1 of this discussion of mobile phones.
Mobile Phones — Overview of Previous Parts
Mobile telephones communicate with the mobile phone network using digital technology, yet the voice communication that still provides for the majority of mobile network traffic is purely analogue. A mobile phone must therefore convert voice communications into digital signals that can be transmitted over the network and it must convert digital signals received over the network into analogue voice data that can be heard by the mobile phone's user.
Now, in the first part of this introduction to mobile phones we've already seen how the microprocessor at the phone's heart is the mobile phone's true workhorse. We've seen how the telephone loads its operating system and then connects to the network. In part 2 we saw how the telephone communicates with the network over the control channel and part 3 described how data is transferred to and from the mobile phone using the mobile network's traffic channels.
The final part of the equation is to describe how the telephone encodes and decodes the traffic sent to and received from the network via the traffic channels.
Mobile Phones — How the Handset Works
From the previous parts of this introduction to mobile phones we know how the phone communicates with the network, how the network communicates with the phone and how the call is maintained as you move from cell to cell across the network. But how does your voice get transmitted across the cellular network? This is where we find out what happens within the telephone itself. First we'll look at the outgoing part of the phone call:
When you speak into your mobile phone's mouthpiece the analogue signal of your voice is converted into a series of elecric impulses (which are still analogue). So that it can be transmitted over the network this analogue electrical signal has to be converted into a digital electronic signal. To do this the phone's CPU passes the signal through a special chip called an analogue to digital converter which performs the actual task of converting the analogue voice data into a digital signal (ie a signal composed only of digital '1's and '0's) which is passed back to the microprocessor. This signal is passed through the telephone's dual band RF module where the signal is converted to a radio frequency pulse at the transmission frequency band of the network's traffic channel. This signal is then broadcast to the network via the mobile phone's antenna (which on most modern mobile phones is internal within the phone itself). The micro-processor also chops the voice data into the millisecond pulses that the network uses to transmit data so that the outgoing voice information is synchronized with the network.
For incoming voice data the situation is effectively reversed. The data is received in millisecond pulses on the reception band of the network's traffic channel. These are interpreted by the RF receiver in the telephone and are converted to incoming digital signals that the microprocessor can stitch together before passing to the analogue to digital converter so that the incoming digital data can be converted to analogue electrical impulses which are transmitted to the mobile phone's speaker and which emerge in the user's ear as voice data.
The situation is a little more complicated in that in transmitting its packets of digital information to the network the telephone sends two consecutive strings of bits each 57 bits long. But in betweeen these strings the telephone's CPU includes a special string of bits known as the 'training sequence'. You might think that this wastes bandwidth, but the training sequnce is crucial in overcoming a problem that plagued all analogue phones — the problem of interference. Because of buildings and local landmarks (from which radio signals can bounce) signals transmitted from the base station can reach the phone at different times and at different orders. However, because the telephone is listening for its own training sequence on all incoming data packets and distortion in this data packet as compared with the version stored in the mobile phone's main memory. The CPU then works out the differences and uses this information to clean-up any distortion within the incoming signal. This, along with the use of digital technology makes reception much clearer than for an equivalent analogue phone. This becomes all the more important as digital phones are used more and more to transmit purely digital information such as messages, photographs and digital clips where any distortion in received data would render the transmitted data completely meaningless.