1. A method of calculating an error probability of a communication system, comprising: receiving encoded communication information from another communication system; restoring information desired to be transmitted by another user by decoding the encoded communication information; and calculating the error probability by analyzing the restored information using a bivariate Gaussian Q function, the error probability indicating a probability that the restored information is different from the information desired to be transmitted by the other user, wherein the calculating comprises calculating the bivariate Gaussian Q function using a univariate Gaussian Q function.