1. Consider the 4-PAM modulation system. The source generates IID symbols from S = {-3a, -a, a, 3a}, according to the following distribution: %3D 1 P({-3a}) 8 (la) P({-a}) (lb) P({a}) (lc) %3D P({3a}) (1d) Consider a block of N = 10 symbols, and assume that a > 0. What's the probability that the 10 symbols contain both positive symbol(s) and negative symbol(s)? 114112