Communications, Radar & Electronic Warfare (201.. - Index of

66 Communications, Radar and Electronic Warfare4.2 Effects of NoiseIn the absence of intentional or unintentional energy, the fundamental limit on radiocommunications is caused in the receiver itself by the thermal noise floor. This iscaused by random fluctuations of electrons in the radio system, and is described bythe expression:N=kTBwhereN is the noise floor.k is the Boltzmann constant = 1.38 x 10 - 23 (J /K).T is the noise temperature of the receiver in Kelvin.B is the bandwidth in Hz.The noise floor is thus dependent on the receiver temperature and the bandwidth ofthe signal received. Note that there is no dependence on the frequency of the signal.Often, the concept of reference noise power is used, with T = 290 K for terrestrialreceivers. For a 1 Hz bandwidth, this is equal to 4 x 10 - 21 W, which is - 204 dBW (ina 1 Hz bandwidth). It should be noted that technically, the bandwidth is not the same asthe 3 dB limit normally quoted on equipment data sheets, but rather is the area underthe power transfer curve. However, in practice, the two values are normally typicallyfairly similar, so it is normally reasonable to use the 3 dB bandwidth figure.From the noise floor equation, it is clear that the thermal noise floor is predicated onthe temperature of the receiving system and also on the bandwidth of the system. Thismeans that highly sensitive systems should be cooled, and that the system should belimited in bandwidth if possible. Space borne receivers will normally be very cold,unless they are heated by sunlight.In practice, physically realisable systems suffer from additional noise above thisfundamental limit. To account for this, the concept of noise figure (Fr) is used. This isdescribed as an additional noise component that adds to the thermal noise floor toproduce the same re