Signal Fading Destructive interference due to multiple reflective paths of varying distances. This is a rare and transient phenomena, but it happens. Frequency Diversity - Use two signals at slightly different wavelengths. The probability that destructive interference will occur simultaneously at both wavelengths is extremely low. Tx Rx Space Diversity: Use multiple antennas separated by at least six wavelengths. The probability that destructive interference will occur simultaneously for both paths is extremely low.

Additional Losses in Multi-Link Systems Tx 1 Rx Tx N... P1P1 PNPN GTGT GRGR LSLS LFLF EIRP = P 1 + G T - L bT - L fT L fT (Feeder) L bT (Branching) Rx 1 Rx N L fR (Feeder) L bR (Branching) P r = EIRP – L S + G R - L F - L bR - L fR Since Fading is a transient phenomena, we usually provide a Fade Margin to account for L F in a statistical sense, e.g., 95%.

System Gain G SYS,i (dB) = P T,i (dBw) – P R,i (dBw) = P T,i - ( P T,i – L bT – L fT + G T – L S – FM + G R – L fR – L bR ) = L bT + L fT - G T + L S + FM - G R + L fR + L bR System Threshold System threshold is the input power required to achieve a specified S/N or C/N or E B /N 0 Effective noise power at the Receiver input is N 0 B or kT R B, where T R is the Receiver Noise Temperature = 290(NR R -1). For a specified S/N REQ, P R > S/N REQ (kT R B) (not dB) Using the convention we have adopted for use of the terms “Gain” and “Loss”, it would be more appropriate to call the above “Transmission Loss”.

G/T For Digital systems, we are interested in achieving a specified E B /N 0, therefore P R > S/N REQ (kT R B) = {(E B /N 0 )(f B /B)}(kT R B) = (E B /N 0 )f B kT R Neglecting Branching, feeder, and fading losses, the received power is P R = (EIRP/L S )(G R ) > (E B /N 0 )f B kT R Rewriting, (EIRP/L S )(G R /T R )/(f B k) > (E B /N 0 ) REQ Note that the term (G R /T R ) captures the two important parameters of the receiving system in one number. This number, G/T, is usually published to characterize receiving systems rather than giving the two values separately. The above relation, expressed in dB is therefore: (E B /N 0 ) REQ < E B /N 0 = EIRP(dBw) - L S (dB) + G/T(dB) – k(dB) - f B (dB) Note that eq in the text is totally wrong.