A method for Symbol Error Rate (SER) approximation of an SER-based transmission power allocation operation for an Orthogonal Space Time Block Code in a DWCS equipped with multiple transmission Distributed Antennas (DA) geographically dispersed at random. The method for SER approximation includes the steps of: setting multiple combinable antenna subsets from the multiple DAs; selecting a quasi-optimal antenna subset Ag (1≦g≦2n−1) having a quasi-optimal power allocation weight wg based on predetermined power allocation, for each of the set multiple antenna subsets; and calculating an SER approximation value of the selected quasi-optimal antenna subset by applying a Probability Density Function (PDF) of a Signal-to-Noise Ratio (SNR) to the OSTBC SER having symbol constellation of a predetermined modulation scheme. The output of the SER approximation value can be output to a transmitter, or to a space-time encoder of a central processor for optimal power transmission.