JPL's Wireless Communication Reference Website

Chapter: Analog and Digital Transmission

Noise

The performance of wireless communication systems is highly determined by noise. Particularly if signals are in a fade, the signal-to-noise ratio can be low and bursts of error can occur.

This is other argued that wireless systems, in particular cellular systems with dense frequency reuse, are interference-limited rather than noise-limited. However, any (digital) signal processing algorithm that attempts to remove, cancel or attenuate such interference increases the noise. The ability to separate multiple interfering signals is critically determined by the signal-to-noise ratio.

Additive White Gaussian Noise (AWGN)

A basic and generally accepted model for thermal noise in communication channels, is the set of assumptions that

Mostly it is also assumed that the channel is Linear and Time Invariant (LTI), sometimes also frequency non-selective.

Exercise

Which of the above assumptions are realistic in a wireless channel?


Thermal noise poses an insurmountable limit to the performance of communication systems. Even with perfectly designed equipment, a thermal noise floor with a spectral power density of k T0 = 4 10-21 watts/Hz is experienced. Here k is the Boltzmann's constant (k is about 1.38 10-23) and T0 is the temperature of the system. In practical systems, the noise power is n k T0 B, where B is the bandwidth and n is a factor by which the noise spectral density exceeds that of thermal noise. Most communication text books, express the performance of generic communication systems in terms of Eb/N0, where Eb is the received energy per bit and N0 is spectral noise density. Typically, n k T0.

Typical causes why the noise factor n is larger than unity are

At 900 MHz, the man made noise power level can easily be 20 dB (100 times) above the thermal level.

Exercise

Show that the ratio Eb/N0 is a dimension-free unit.


Further reading

     
  The matched filter principle for signals transmitted over LTI AWGN channels
     



JPL's Wireless Communication Reference Website 1999.