import numpy as np
import random

def channel(x):
    G = 10
    sigma2 = 10
    s = random.choice([1, 2])

    n = x.size
    Y = np.random.normal(0, np.sqrt(sigma2), n)
    if s == 1:
        x_even = np.array(x[::2]) * np.sqrt(G)
        x_odd = x[1::2]
    else:
        x_even = np.array(x[::2])
        x_odd = x[1::2] * np.sqrt(G)

    Y[::2] += x_even
    Y[1::2] += x_odd
    return Y