The noise can be introduced in any part of the system but it is most noticeable in the channel where the signal is weakest.
What are the fundamental limitations of the communications system?There are two fundamental limitations of any communications system which are
noise and
bandwidth.What is noise?In common use the word
noise means unwanted
sound or
noise pollution. In
electronics noise can refer to the electronic signal corresponding to acoustic noise (in an audio system) or the electronic signal corresponding to the (visual) noise commonly seen as '
snow' on a degraded
television or
video image. In signal processing or computing it can be considered
data without
meaning; that is, data that is not being used to transmit a
signal, but is simply produced as an unwanted by-product of other activities. In
Information Theory, however, noise is still considered to be
information. In a broader sense, film grain or even advertisements in web pages can be considered noise.
Noise can block, distort, or change the meaning of a message in both human and electronic communication.
In many of these areas, the special case of thermal noise arises, which sets a fundamental lower limit to what can be measured or signaled and is related to basic physical processes at the molecular level described by well known simple formulae.
What are the type of noise?
The following are the basic typs of noise in communications system:
1. External Noise - This noise is introduced in the transmitting medium or the channel. Examples of this noise type are:
a.) Industrial or Man-made Noise
This noise occurs randomly at frequencies up to around 500 MHz. The common sources are flourescent light, engine iginition systems, switching equipment, commutators in electric motors, leakage from high voltage lines, etc.
b.) Atmospheric or Static Noise
The intensity of this noise is inversely proportional with frequency and therefore it becomes less severe at frequencies above 30 MHz. The main source is lightning discharges during
thunderstorms.
c.) Extraterrestrial or Space Noise
This noise is observable at frequencies in the range from about 8 MHz to somewhat above 1.43 GHz. Some sources are the sun (solar noise) in the solar system, the stars (cosmic noise) in the universe.
2. Internal Noise - This type of noise is introduced at the receiver. The following are examples of internal noise of receivers :
a.) Thermal or White or Johnson Noise (Brownian or Gaussian Noise)
This noise was discovered by Robert Brown in 1827 and was first thoroughly studied by J.B. Johnson in 1928. It is dependent on temperature and its frequency content is spread equally throughout the usable spectrum. The primary source is the rapid and random motion of charge carriers inside a resistive component when heated.
Noise power is based on the thermal noise power at the input of the system, along with system gain and noise figure:
PThermalNoise = k * T * B (Watts), where 
Multiply by 1000 to obtain milliwatts and then convert to dBm units or convert to dBW units and add 30 dB:
PThermalNoise (dBm) = 10 * log10 (1000 * k * T * B)
or
PThermalNoise (dBm) = 10 * log10 (k * T * B) + 30
Now that we have the thermal noise at the input, add the system gain and the additional noise added by the system (the NF) to get the noise power at the output:
PNoise@Output (dBm) = PNoise@Input + GainSystem + NFSystem
Noise Voltage and Power
Thermal noise is to be distinguished from shot noise, which consists of additional current fluctuations that occur when a voltage is applied and a macroscopic current starts to flow. For the general case, the above definition applies to charge carriers in any type of conducting medium (e.g. ions in an electrolyte), not just resistors. It can be modeled by a voltage source representing the noise of the non-ideal resistor in series with an ideal noise free resistor.
The power spectral density, or voltage variance (mean square) per hertz of bandwidth, is given by
where kB is Boltzmann's constant in joules per kelvin, T is the resistor's absolute temperature in kelvins, and R is the resistor value in ohms.
For example, a resistor of 1kΩ at an average temperature (300 K) has
.
For a given bandwidth, the root mean square (rms) of the voltage, vn, is given by
where Δf is the bandwidth in hertz over which the noise is measured. For a resistor of 1kΩ at room temperature and a 10 kHz bandwidth, the RMS noise voltage is 400 nV or 0.4 microvolts.[1]
The noise generated at the resistor can transfer to the remaining circuit; the maximum noise power transfer happens with impedance matching when the Thévenin equivalent resistance of the remaining circuit is equal to the noise generating resistance. In this case the noise power transfer to the circuit is given by
where P is the thermal noise power in watts. Notice that this is independent of the noise generating resistance
Noise in Decibel
In communications, power is often measured in decibels relative to 1 milliwatt (dBm), assuming a 50 ohm resistance. With these conventions, thermal noise at room temperature can be estimated as:
where P is measured in dBm. For example:
-
-
| Bandwidth | Power | Notes |
| 1 Hz | -174 dBm |
|
| 10 Hz | -164 dBm |
| 1000 Hz | -144 dBm |
| 10 kHz | -134 dBm | FM channel of 2-way radio |
| 1 MHz | -114 dBm |
| 2 MHz | -111 dBm | Commercial GPS channel |
| 6 MHz | -106 dBm | Analog television channel |
| 2.4 GHz | -80 dBm |
For example a 6 MHz wide channel such as a television channel received signal would compete with the tiny amount of power generated by room temperature in the load of receiver, which would be -106 dBm, or one fortieth of a picowatt. The 6 MHz could be the 6 MHz between spectrum at 54 and 60 MHz (corresponding to TV channel 2) or the 6 MHz between 470 MHz and 476 MHz (corresponding to TV channel UHF 14) or any other 6 MHz in the spectrum for that matter. The 2.4 GHz in the chart should not be confused with the Johnson-Nyquist noise generated in a 6 MHz wide channel at that starting frequency, which would be -106 dBm.
Note, that it is quite possible to detect a signal whose amplitude is less than the noise contained within its bandwidth. The Global Positioning System (GPS) and Glonass system both have signal amplitudes that are less than the received noise at ground level. In the case of GPS, the received signal has a power of -133 dBm. The newer batch of satellites have a more powerful transmitter.
Noise CurrentThe noise source can also be modeled by a current source in parallel with the resistor by taking the Norton equivalent that corresponds simply to divide by R. This gives the root mean square value of the current source as:
Thermal noise is intrinsic to all resistors and is not a sign of poor design or manufacture, although resistors may also have excess noise.
Thermal Noise on Capacitors
Johnson noise in an RC circuit can be expressed more simply by using the capacitance value, rather than the resistance and bandwidth values. The rms voltage noise on a capacitance C is
independent of the resistor value, since bandwidth varies reciprocally with resistance in an RC circuit.[4] In the case of the reset noise left on a capacitor by opening an ideal switch, the resistance is infinite, and the formula still applies; however, now the rms must be interpreted not as a time average, but as an average over many such reset events, since the voltage is constant when the bandwidth is zero. In this sense, the Johnson noise of an RC circuit can be seen to be inherent, an effect of the thermodynamic distribution of the number of electrons on the capacitor, even without the involvement of a resistor.
The noise is not caused by the capacitor itself, but by the thermodynamic equilibrium of the amount of charge on the capacitor. Once the capacitor is disconnected from a conducting circuit, the thermodynamic fluctuation is frozen at a random value with standard deviation as given above.
The reset noise of capacitive sensors is often a limiting noise source, for example in image sensors. As an alternative to the voltage noise, the reset noise on the capacitor can also be quantified as the charge standard deviation, as
Since the charge variance is kBTC, this noise is often called kTC noise.
Any system in thermal equilibrium has particles with an energy of half of kT per degree of freedom. Using the formula for energy on a capacitor (E=1/2*C*V^2), the energy on a capacitor can be seen to also be 1/2*C*(k*T/C), or also kT/2.
b.) Shot Noise
This noise is due to short effect caused by random variations in the arrival of electrons or holes at the output electrode of an amplifying device and thus appears as randomly varying noise current superimposed on the output. When amplified, it is supposed to sound as though a shower of lead shots were falling on the metal sheet hence the name shot noise.
For Diode, the rms noise current is given by
In- 2 = 2 * e * Id * BW
where : e = charge of an electron = 1.6 x 10 – 19 C ; Id = Direct Diode Current ;
BW = Bandwidth of the system
c.) Transit-time Noise
This noise is due to the transit-time effect that is when the time taken by an electron to travel from the emitter to the collector of a transistor becomes comparable to the period of the signal being amplified. Its greatest effect is at higher frequencies especially in the microwave region. This is otherwise known as the "high-frequency noise".
Miscellaneous Noise
Flicker noise or Modulation noise or Excess Noise or 1/f or Pink Noise
This is a poorly understood form of noise found at low audio frequencies in transistors. It is proportional to emitter current and junction temperature, but since it is also inversely proportional to frequency, it may be completely ignored about above 500 Hz.
Resistance noise or Thermal noise in Transistors
It is due to the base, emitter and collector internal resistances, and in most circumstances the base resistance makes the largest contribution.
Noise in mixers
Mixers are much noisier than amplifiers using identical devices, except at microwave frequencies. This high value of noise in mixers is caused by two separate effects. First, conversion transconductance of mixers is much lower than transductance of amplifiers. Second, if the image frequency rejection is inadequate, as often happens at shortwave frequencies, noise associated with the image frequency will also be accepted.
