A Simple Model
What does a digital communications link actually look like?
This is a useful question to answer as it gives us a model we can continuously refer back to as we learn more about communications. Having a model means that when things get confusing later on, we can go back and see which technique, technology or innovation fits where. A simplistic model of a communications link is shown below, consisting of a source, transmitter, channel, receiver and destination.
The source is basically the signal we want to send. It could be your voice or a TV image or some music, or it could even be some digital data in the form of a frame. It could be a great many things, but for now let’s just accept that the source is some kind of generic information. We will get into the details later! Next is the transmitter responsible for (you guessed it) transmitting the signal to the other side.
The “channel” comes next. In this context we are using the word “channel” to collectively refers to the time and space that the information we are sending must travel through. The channel could be the glass tube of a piece of fiber, the twisted pairs of copper wires of an Ethernet cable or even the room you are standing in that separates you from the Wi-Fi router your phone is communicating with. It is important to realize, right now, that the channel in this context does not refer to the choice of radio frequency like when you switch the channel on the radio or TV, which is also a valid use of the word, but with a different meaning. To re-iterate, in the current context, the channel means the entire physical medium through which the transmitted signal must travel. A channel is characterized by the effects it has on the transmitted signal, which become apparent at the receiver. The receiver is what hears the transmitted signal as it has been affected by the channel and converts that signal back into an approximation of the original message to be processed by the destination. If the approximation is good enough, the destination will be able to recover the original message. If the approximation is not good enough, then the message is lost and the transmitter will have to try again.
Why Digital Communications?
I was asked this question in a job interview many years ago. My future boss stared at me as I fumbled with the answer. I blanked. I had never actually thought of it. Why the hell do we communicate using digital communications, instead of analogue? I could think of a hundred reasons, but couldn’t put my finger on one that summed the answer up in a sentence. But thankfully i have learned the answer: Noise immunity.
Digital signals lend themselves easily to being stored, and the information can be easily copied, true. But most significantly they are also significantly more immune to noise in a channel, making them easier to replicate at the receiver. Here is a picture (acquired from two separate introductory courses to digital communications, here and here) that illustrates the point extraordinarily well,
The Digital, Wireless Communications Link
Let’s go a little deeper and get to the key parts of this, the digital bit and the wireless bit! A more detailed block diagram of a digital, wireless communications link is shown below.
As we progress through this series of blog posts we will look at each of these blocks in more detail, but for now here is a brief summary of what each block does.
The Input Signal is very simply the information we want to transmit across the wireless link. This information is typically already in a digital format, although it could also be an analogue, continuous time domain signal (your voice entering the telephone perhaps?).
Source Coding is the process through which the original input signal is stored in some digital format and is compressed to reduce the storage and transmission requirements of the original information. You can think of source coding as removing redundant bits to lower (improve) the required storage space or data rate of some information. The simplest form of source coding could be analogue to digital conversion. An example of source coding of a digital signal would be the audio codecs used for a digital voice calls such as G.711 or G.729. For data, source coding would be built into the file or data you were sending, e.g. an MP3 music file that compresses digital information from raw, pulse code modulated audio of a .wav file.
Encryption exists to secure the message against interception (confidentiality), spoofing (authenticity) or from being tampered with (integrity).
Channel coding is the process of adding redundant information to the message to allow limited forward error correction and to minimize the need to resend messages that have been affected by channel induced errors. Effectively, channel coding adds a dimension of reliability to communications even in the presence of interference and noise. It would be accurate to state here that modern digital wireless communications technologies rely very heavily on channel coding techniques.
Modulation is the process of mapping the information in the coded information stream onto a carrier signal to create a digitally modulated waveform. This can be done in many ways, but typically involves manipulating the amplitude, frequency or phase of the carrier wave in a predetermined, finite number of ways. Each possible manipulation of the carrier wave is referred to as a symbol and carries a specific sequence of binary information.
The transmitter’s role is to further process and amplify the digitally modulated signal before it is fed into the antenna. The antenna then radiates the signal into the wireless channel which as we have mentioned is actually the physical space and time through which the signal must travel.
The antenna on the receiver is responsible for “hearing” and passing the electromagnetic signal into the receiver. The receiver amplifies the (typically very small) received signal and passes it to the demodulator so that it can be converted from the detected complex waveform back into a series of 1’s and 0’s.
The channel decoder then takes chunks of the received information and uses the redundant information (from the channel coder) to perform forward error correction on the received digital information, recovering the original encrypted message. The encrypted message is de-crypted before being passed to the source decoder which recovers the original information!
One of the things that I have not shown in the diagram above is the synchronization necessary between the transmitter and receiver. It is imperative that the receiver is synchronized to the same frequency and phase as the transmitter. There must also be synchronization of the symbols and of the frames sent so that data can be reliably reproduced on the other side!
That is all for now!