10.Calculation of link margin

  1.Down link rain attenuation    Rain fall introduces  attenuation by absorption  and scattering  of signal energy, and the absorptive attenuation introduces noise.
                 Effective noise temp of rain as
                                          T_rain =T_A(1-(1/A))
Where T_A is known as apparent absorber temperature. It is a measured parameter, which is a function of many factors, including physical temperature of rain, and scattering effect of the rain, and the scattering effect of the cell on the thermal noise incident up on it. The value of the apparent absorber temperature lies between 270 and 290 K .
Total sky noise temperature is the clear sky temperature plus the rain temperature
                                          T_Sky= T_cs+T_RAIN
Where T_Cs is the clear-sky noise temperature

2.Receiver antenna pointing loss  When a satellite link is established, the ideal situation is to have the earth station and satellite antennas aligned for maximum gain. There are two possible sources of off axis loss , one at the satellite and one at the earth station . The off axis loss at the satellite is taken in to account by designing the link for operation on actual satellite antenna contour. The off axis losses at the earth station is referred to as the antenna pointing loss.

Antenna pointing losses are usually only a few tenths of a decibel.

3.Eb/No required for BER of 1/ 10⁷  It is Signal to noise ratio required to achieve Bit Error Ratio of  1 error bit per 10⁷ message bits. 

4.Down link C/No (dB Hz)    It is the down link carrier to noise ratio. It can be calculated as follows.

Down link C/N_o(dB Hz) = Satellite operating EIRP - Downlink path loss - Down link rain attenuation - Receiving antenna pointing loss + Receiver G/T +228.6.

                                                                        ……………………………….[15] 

5.Over all Down link C/No (dB Hz)       It is the some of carrier to noise ratio at the down link and carrier to noise ratio at the up link. It can be calculated as follows

Over all Downlink(C/N₀) (dB Hz)= -10*LOG (10^-(C/No)U+10^-(C/No)D)…………..[16]
Here
                 (C/No)_U = Up link C/N ratio.
                 (C/No)_D = Downlink C/N ratio.
It  is the effective C/N ratio of the total system (Transmitter & receiver ) 

  Available Eb/No    It is the ratio of signal to noise ratio of the system. It can be calculated as follows.
           Available Eb/N_O = over all downlink (C/N_o) / Input data rate
           Available Eb/N_O (dB)= Over all downlink (C/N_o)(dB) - Input data rate (dB)
                                            = Over all downlink(C/N_o)(dB) – 10log (input data rate in KB*10³).
                                                                        ……………..………………………[17] 

Available link margin   It is the Difference between ‘Available Eb/N_o’  and ‘Eb/No required for BER of 1/10^7’    in dB.
                Available link margin = Available Eb/N_o(dB) – Eb/N_o required for BER of  1/ 10⁷ (dB)
                                                                        ………………………………………..[18]
                                              

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