The Fundamentals Of Communication Systems
Communication is simply, the process of transferring information from one entity(the source) to another(the destination). This is the process involved when you talk to someone face to face or on the phone, when you listen to music on your iPod, when you use the internet, when you listen to your favorite show on TV or the radio.
Consider two people standing and talking on the street. Suppose that they keep moving away from each other. After a certain time, they are not able to hear each other. In technical terms, the amplitude of their speech signals die out-they get mixed with noise.. noise from the environment.
Here, we are just dealing with the speech signal which is an em wave, but not really what you would call an electrical signal. Now consider the two using a wire for communication. Now, they might be able to communicate for a longer distance with better clarity. But the signal(now an electric signal) eventually dies out.
Now there are two things to consider here-
1)The signal being “transmitted” now is electric in nature. So we must use a device that converts from speech to electric signal at the senders end and something that does the inverse operation at the receivers end.
These are called the input and output transducers respectively. The best transducers you can think of are the microphone(input transducer) and the speaker/headphones(output transducer).
2)The signal dies out after traversing a certain distance. Consider the old times when the kings used to send their messengers to other kingdoms to deliver messages. Walking those distances would tire the men-so, they started using horses. Similarly, we sent our signals “on” a high frequency carrier wave.(not exactly “superimposed on” but “modulated on”). We would then need to demodulate the signal(get the guy down from the horse to deliver the message).
So, we can now start forming our communication system-The modulation process takes place in the transmitter and the demodulation is performed in the receiver. We shall be only dealing with black boxes and not going into the circuits and glass boxes.
Now, between the transmitter and the receiver lies the channel which is the most susceptible to any noise signals or interferences. You will find that all communication system block diagrams are a mirror image about the channel. The channel may be wired or wireless/broadcast. Every channel has a limited capacity called the information capacity. The noise is unwanted energy that interferes with the message signal in the form of amplitude variations. Noise may be external(created outside the receiver) or internal(created inside the receiver).
The Process Of Modulation-Modulation is the process by which we vary a property of a high frequency carrier wave with respect to the modulating signal(message signal).
Hence, we are transmitting the high frequency signal giving it some property of out message signal in such a way that the message signal can be retrieved from the transmitted carrier using this transmitted property.
This property may be the amplitude, frequency or phase of the signal leading to the three types of modulation-Amplitude Modulation(AM), Frequency Modulation(FM) & Phase Modulation(PM). FM & PM are sometimes colletively referred to as forms of angle modulation.
The three forms are illustrated below(images by Ivan Akira)
It can be seen in the above illustrations that the type of modulation depends on the property of the carrier wave that varies. For instance, in case of amplitude modulation, the amplitude of the carrier varies with the amplitude of the modulation signal. Now there is one thing to understand carefully-
Even in case of angle modulation the frequency or phase of the carrier varies with the AMPLITUDE of the message signal and not with its freq/phase.
In other words, in case of FM, the frequency(carrier) varies with the amplitude(message) and in case of PM, the phase(carrier) varies with the amplitude(message). Hence, we always consider the amplitude of the modulation/msg signal.
We must then, also demodulate the signal at the receiver to generate the information signal back to the destination. This is the basic principle all communication systems follow.
Lets go back to the concept of noise being introduced in the channel as amplitude variations. For all analysis, study and design, we consider noise to be additive in nature. Which is to say, that the noise signal just adds to the transmitted signal like usual superposition.
Such a communication channel is called an additive noise channel. We also classified noise as external and internal noise. Further, External noise includes noise from the atmosphere, from industries, noise from space and solar radiations while internal noise includes shot noise, transit time noise and thermal agitation/white/Johnson noise.
Our basic aim as engineers is to design communication systems with very low noise or if possible, no noise at all. The basic idea is to get an exact replica of the information at the source to the destination.
Now, for all the right brained people like mi-self, lets take a look at a particular communication system. Consider two people talking over a certain distance using a walkie-talkie. Now, the microphone on the walkie-talkie of the person speaking is the input transducer.
It converts his speech signal into an electric signal which gets modulated by the modulator in the walkie-talkie circuit. This signal is sent by the antenna through the wireless channel to the antenna at the other person’s walkie-talkie whose receiver circuit comes into the action.
The signal, once demodulated by the receiver is then converted back to speech signal by the output transducer which is the speaker.
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