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+// Example 6.9
+// Calculation of the variance of the signal–ASE beat noise current.
+// Page no 290
+
+clc;
+clear;
+close;
+
+//Given data
+
+si=30;                // Electrical SNRs at the amplifier input
+so=25;               // Electrical SNRs at the amplifier output
+p=0;                 // Signal power at output 
+r=-126;             // Signal power at input 
+R=0.9;              // Planck constant
+f=195*10^12;       // Frequency
+b=20*10^9;         // Bandwidth
+
+// The variance of the signal–ASE beat noise current
+p1=10^(p/10)*10^-3;
+rn=10^(r/10)*10^-3;
+r1=rn*b;
+r0=2*R^2*p1*r1;
+
+
+//Displaying results in the command window            
+printf("\n The variance of the signal–ASE beat noise current = %0.2f x 10^-9 A^2  W/Hz",r0*10^9);
+
+
+// The value of noise power given in example as -126 but for calculation it is taken as -128 in book. Therefore answer is varying.