Transmit - Receive (with AWGN noise)

import java.util.ArrayList;
import java.util.Random;

public class App {

    // Variables
    static double stdDevOfNoise = 2 * Math.pow(10, -1);

    // Methods
    // 1. Random Integer
    public static int generateNumber(int n) {
        Random random = new Random();
        int Tx = random.nextInt(n);
        return Tx;
    }

    // 2. QPSK Modulation ---> Matches a number(0, 1, 2, 3) with a spot/vector in a 2-D field (s0, s1, s2, s3)
    // s0 = {sqrt(2)/2, sqrt(2)/2},   s1 = {sqrt(2)/2, -sqrt(2)/2},   s2 = {-sqrt(2)/2, -sqrt(2)/2},   s3 = {-sqrt(2)/2, sqrt(2)/2}
    public static ArrayList <Double> QPSKModulation(int Tx) {
        ArrayList <Double> s_t = new ArrayList <Double> ();
        // This vector has 2 elements ---> 1st = x, 2nd = y
        switch(Tx) {
        case 0:
            s_t.add(Math.sqrt(2) / 2);
            s_t.add(Math.sqrt(2) / 2);
            break;
        case 1:
            s_t.add(Math.sqrt(2) / 2);
            s_t.add(-Math.sqrt(2) / 2);
            break;
        case 2:
            s_t.add(-Math.sqrt(2) / 2);
            s_t.add(-Math.sqrt(2) / 2);
            break;
        case 3:
            s_t.add(-Math.sqrt(2) / 2);
            s_t.add(Math.sqrt(2) / 2);
            break;
        default:
            System.out.print("Not a valid input for function 'QPSKModulation'");
        }
        return s_t;
    }

    // 3. AWGN Box ---> This function inserts white Gaussian Noise and changes our signal coordinates
    public static ArrayList <Double> signalAfterAWGNoise(ArrayList <Double> s_t){
        ArrayList <Double> r_t = new ArrayList <Double> ();
        // Somehow, I have to insert noise. I will create new random coordinates, that will be added to our s_t
        // White Gaussian Noise has a mean value of 0 and standard deviation equal to No/2
        Random random = new Random();
        double noiseX = random.nextGaussian() * stdDevOfNoise;
        double noiseY = random.nextGaussian() * stdDevOfNoise;
        // System.out.print(noiseX + " " + noiseY);
        // Now, I add the noise coordinates in the vector I have sent
        r_t.add(s_t.get(0) + noiseX);
        r_t.add(s_t.get(1) + noiseY);
        return r_t;
    }

    // 4. QPSK Demodulation ---> It is based on the decision areas
    public static int QPSKDemodulation(ArrayList <Double> r_t) {
        // The demodulation function makes the reverse process
        if(r_t.get(0) >= 0 && r_t.get(1) >= 0) {
            return 0;
        }
        else if(r_t.get(0) > 0 && r_t.get(1) < 0) {
            return 1;
        }
        else if(r_t.get(0) < 0 && r_t.get(1) < 0) {
            return 2;
        }
        else if(r_t.get(0) < 0 && r_t.get(1) > 0) {
            return 3;
        }
        else {
            System.out.println("Error in function 'QPSKDemodulation'");
            return -1000;
        }
    }


    // MAIN FUNCTION
    public static void main(String[] args) {
        int successes = 0;
        int failures = 0;
        Random random = new Random();
        int iterations = random.nextInt(5001) + 5000;
        for(int i=0; i<iterations; i++) {
            int Tx = generateNumber(4);
            ArrayList <Double> s_t = QPSKModulation(Tx);
            ArrayList <Double> r_t = signalAfterAWGNoise(s_t);
            int Rx = QPSKDemodulation(r_t);
            if(Tx == Rx) {
                // System.out.println("Success");
                successes++;
            }
            else {
                // System.out.println("Failure");
                failures++;
            }
        }
        // Print the statistics
        System.out.println("Successes: " + successes);
        System.out.println("Failures: " + failures);
        System.out.println("Error rate: " + (double)failures/(double)(failures + successes));
    }   // END OF MAIN FUNCTION

}    // END OF CLASS