In this paper, the achievable rate of a K-user interference channel with single antenna at individual node is studied. Interference alignment and zero forcing are applied at the transmitter and receiver, respectively. It is well known from the paper [1] of Cadambe and Jafar that a multiplexing gain (degrees of freedom) of K/2 is achievable. We propose two new pre-coding methods and study how they enhance the achievable rate in terms of the high SNR offset, while maintain the optimality of degree of freedom the Cadambe-Jafar scheme achieved.