Downlink CNR

Downlink CNR is an important figure in system TVRO design. Below are certain parameters used in CNR computation.

Figure of merit

Figure of merit is given as
Where t is the temperature and g is the gain of the receiver antenna.
For lossless case
and
where n is the noise factor, t_a is the noise temperature of the antenna and t₀ is the temperature of the environment.
F in db is simply

Path loss is defined as
Where is the wavelength of the carrier and the d is the distance in meters between the satellite and the receiver. For Geosynchronous satellites this distance is at the projection on the earth. In actual cases the distance is slightly more than this figure depending on the geographic location.. The Path loss in dB is
The same relation can be given in terms of frequency.
Where c is the velocity of light.
With metric units
Using km for d and GHz for f
Using miles for d and GHz for f

P_e is the Equivalent isotropically radiated power in dBW. It depends on the output of the transponders of the satellite and the antenna gain of the transmitting antenna. This figure is given by the service provider.
where p is the output power of the transponder and g is the antenna gain.

B is the baseband of the channel given in dB
Where b is the base band given in metric units.
When b is given in MHz, than

K is the Boltzmann constant given in dB units.

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