Understanding Cell Phone Technology: The Model
Finally, we are ready to talk about the components of the modern communications system. The diagram attached to the end of this blog is the model we will have in our heads as we investigate the main components of our digital cell phones.
I’ve drawn two cell phones operating across the radio frequency (RF) spectrum. I’ve also drawn in an external antenna on each phone to remind you that every cell phone must have an antenna to transmit and receive communications across the RF channel. Of course, these days, the cell phone antenna is not externally visible because it’s hidden inside the cell phone case. Nonetheless, it remains an essential component. The cell phone would simply not work without its antenna.
Suppose I want to talk to a friend using my cell phone. It has a receiver, a device near the bottom of the phone (near my mouth when I hold it up to my ear) that is able to collect my voice for transmission. It not only “hears” my voice, but translates it from an analog (continuous) signal into a digital (discrete) signal so the other components of the phone can do their jobs.
The next component, labelled “source coding” compresses the signal down to its essential information content. We already talked about the earliest example of electrical compression, the Morse code developed for telegraphy. Last week, we talked about Claude Shannon’s development of information content, and related it to entropy.
“Channel coding” is the addition of redundancy back into the signal to provide error correction. Wait a minute, you say, I thought we just squeezed out all the redundancy from the signal so we only send the essential information needed to get the message through?! Yes, we did, but there’s another problem with which we have to contend: noise.
The channel, that is, the RF spectrum over which the communications are going to travel, is going to be noisy. There will be all sorts of other sources of energy running around in the spectrum, like other cell phones and electrical devices. The weather – humid, raining, and so on – can also affect how well the RF channel is performing. So, in order to mitigate for all the interferers, we’re going to add in some redundancy in clever ways so that we can detect and correct errors in message transmission. Don’t worry, there are a lot of complicated ideas here, and we will spend a couple weeks talking about them in the near future.
Finally, we come to modulation. Modulation combines the refined digital signal coming out of the channel coder with a carrier wave which will carry the message over the RF. Again, there are all sorts of ways of cleverly getting the information onto the carrier, and we will talk about some of them.
At this point, the signal goes out over the RF and the communications network and is routed to its destination. The receiver’s cell phone plucks the signal out of the RF, using its antenna. It then undoes all the things my phone did in reverse order: it “hears” the analog wave, taking discrete samples of it and removing the carrier. It uses the channel correction to try to correct errors that occurred due to transmission over a noisy channel. It decompresses the data to recover the original redundant message. Finally, it converts the digital signal back into analog voice for the receiver’s ear heard through the earpiece/speaker.
This is all just a quick once over. Beginning with this general model, we will investigate its individual components in more depth in the coming weeks.
