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2.3 Transmission Impairments

Attenuation
Signals loose power in time.

 -- --    --  --    ||  |
------------|------------|-----distance
   -  |-

Issues:

  1. Signals must be sufficiently strong so that the receiver will be able to detect and interpret them
  2. they should maintain a sufficient high level to make them distinguishable from noise
  3. Too strong signals can overload the circuitry of the transmitter and result in distortion.
  4. they should take into account that attenuation increases 1 , 2 , 3 with the frequency.
     -power-in-
log10 power out Bel
       -power-in-
10log10power  out deciBel
Pin = 100mW, Pout = 10mW, attenuation = 10 log 10100-= 10 deciBel
Decibels are commonly used because

  1. Signal strengths often falls off logarithmically
  2. Cascade losses and gains can be calculated with simple additions and subtractions.
Dispersion
Signals tend to spread as they travel, with the amount of spreading dependent on the frequency.

 ---|-     ---|--    ||-||-
-- ----   -- -- -   ---------|
-------------------------------distance

Delay distortion
Due to velocity of propagation that varies with frequency. Thus, various frequency components of a signal arrive at the receiver at different times.

 -|--      -|-      -|--
 |- -     -----     -----    |
---|---------------------------distance

Critical in particular for digital data, because signal components of bit positions spill into other bit positions, and so limiting the allowed rate of transmission.

Noise
Noise                         ||
                        ||
  ||   || |        |   |||||  | |||||||
--|||||||||-|----|||||||||-|-|-||||||||
   | |  | |        |  ||  |  |  |||
Signal   -| --| --| --| --| --|--| --| --| --|
  || | | | | | | | | | || | | | | | | |
  || | | | | | | | | | || | | | | | | |
--  -- --- --- --  --  -- --- --- ---
signal + noise                         ||
                        ||
 ||| |||  || --| |-|-|| ||| ||| |||  -|
 ||| ||| ||| |-| |||||| ||| -|| ||| --|
---------------------------------------
-- ||| ||| -|- --| || ||  -|- ||| ||  |
     | ||                         |
Signals are reconstructed by sampling.
Increased data rate implies "shorter" bits with higher sensitivity to noise.
Sources:
Thermal Agitates the electrons in conductors, and is a function of the temperature. It is often referred to as white noise, because it affects uniformly the different frequencies.
  • The thermal noise in a bandwidth W is

    N = kTW

    where T=temperature, and k= Boltzmann's constant = 1.38 . 10-23 Joules/degrees Kelvin.

  • Signal to noise ratio: (S/N)    = 10 log signal-power--
     dB         noise power Typically measured at the receiver, because it is the point where the noise is to be removed from the signal.
Intermodulation Resulting from interference of different frequencies sharing the same medium. It is caused by a component malfunction or a signal with excessive strength is used.
For example, the mixing of signals at frequencies f1 and f2 might produce energy at the frequency f1 + f2 . This derived signal could interfere with an intended signal at frequency f1 + f2 .
Crosstalk Foreign signal enters the path of the transmitted signal.
Impulse Irregular disturbances, such as lightning, and flawed communication elements. It is a primary source of error in digital data.

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