Introductory Communication Concepts
I. The Basic Communication System
In the Figure below a simplified version of a Communication System is
depicted. The message needs to be transmitted from point A to point B. The
arrows are shown as communication links from point A to point B and back from
point B to A if necessary. To send the message from point A a "Transmitter" is
used and a "Receiver" is used at point B. This process uses the top
communication link (arrow). The bottom communication link (arrow)
signifies a reversal of the above process, starting from point B and going back
to point A.
The communication links can be a number of different
physical conductors such as wires, coaxial cables, transmission lines,
waveguides, or fiber optic cables. They can also represent free space
which is the link used when the message is transmitted through antennas
traveling as a combination of electromagnetic signals.
II. Decibels as a Measurement
Measuring the difference between signal levels at different points of the Communication System can mean at times that very large changes in the magnitudes or levels of the signals have to be recorded. Hence, it is not enough to compare two signals using a ratio such as output_power/input_power, but additionally the logarithm of such a ratio is taken to make the number more appropriate. The definition of the figure called "Decibel" is to be: 10 times the logarithm (base 10) of such a ratio.
Thus in the case of output_power/input_power (output power divided by input power), we have that:
Ap(dB) = 10· log(Pout/Pin)
"dB" representing the "Decibel", and Ap(dB) meaning the power gain in decibels.Example: Let's say the output power level of a signal is 100 times its input power level, then the measurement in dBs is : Ap(dB) = 10·log(100) = 20 dB.
Decibels are commonly associated with sounds, which is the case if the signal is in the audible range. A decibel meter such as the one shown below can be used to measure the level of noise in decibels. Some common sound levels are indicated next to it, they are referenced to a Threshold of Hearing being 0 dB.
Normal conversation: 20 dB
Busy street traffic: 70 dB
Large Orchestra: 98 dB
Decibels can also be used to measure the difference of Voltage Levels of a signal. However, for this case one decibel is 20 times the logarithm of the ratio of output voltage to input voltage, instead of the factor of 10 in front of the logarithm term. This result comes from the fact that P = V2/R and the quadratic term inside the logarithm becomes 2 * 10 = 20, in front of the logarithm term. In short, for voltage levels:
Av(dB) = 20· log(Vout/Vin)
where Av stands simply for the voltage ratio of output voltage to input voltage of a Communication System.
III. Noise in Communication Systems
Noise is one of the factors that affects the information being delivered. It can create interference, alter the integrity of the signal and sometimes even degrade the signal to an unrecognizable pattern. Noise can come from a variety of sources, and can be external or internal. Sources of external noise can be man-made such as motors or ignition systems, can come from the atmosphere or even sometimes from outer space. Internal noise, on the other hand, can be classified as Thermal or Johnson noise due to the thermal interaction of particles in a conductor, Transistor noise caused by the random paths of motion in semiconductors, or Low Frequency noise called also "flicker" which occurs due to changes in dc current levels at low frequencies.
It is very difficult to measure Noise, and there exist only a few empirical formulas which can be applied only to specific instances of Noise, one of the examples being Thermal Noise.
The two figures below show how some small amount of Noise can affect a signal's shape. On the left figure the Noise "rides" on the original signal affecting its smoothness, the figure on the right displays the signal without any Noise.
Ultimately, to be able to indicate some measurement of Noise levels, a figure called the Signal-to-Noise Ratio is used. The S/N ratio is a ratio of the signal power to the Noise power in a Communication System, or in other words:
S/N = signal power/noise power = Ps/Pn
The S/N ratio is usually expressed in decibel form as:
S/N(dB) = 10·log10(Ps/Pn)
Although the S/N ratio measures the noise content at a specific point in the system, it is not that useful in indicating how much additional noise has been generated from input to output. For such purpose, there is another term called the noise figure or NF relating the S/N at the output to the S/N at the input like this:
NF = 10·log((Si/Ni)/(So/No))
where Si/Ni and So/No are the Signal-to-Noise ratios at the input and output respectively. NF is a quantity specified in decibels.
Example: An amplifier has a measured S/N power of 10 at its input and 5 at its output.
The noise figure is: NF = 10·log(10/5) = 10·log2 = 3dB
The goal is always to try to minimize the noise figure, to obtain good performance on a circuit or system.
Aside from Noise, the other main limitation to a Communication System is called Bandwidth. Bandwidth is the range of frequencies available to transmit a message, the larger it is the more information can be conveyed to a higher degree of accuracy. However, the FCC and the requirements of different Communication Systems limit the amount of "space" or Bandwidth available for different applications.