  In this paper, we study single cell multi-user downlink distributed antenna
systems (DAS) where the antenna ports are geographically separated in a cell.
First, we derive an expression of the ergodic sum rate for DAS in the presence
of pathloss. Then, we propose a transmission selection scheme based on the
derived expressions to maximize the overall ergodic sum rate. Utilizing the
knowledge of distance information from a user to each distributed antenna (DA)
port, we consider the pairings of each DA port and its supporting user to
optimize the system performance. Then, we compute the ergodic sum rate for
various transmission mode candidates and adopt a transmission selection scheme
which chooses the best mode maximizing the ergodic sum rate among the mode
candidates. In our proposed scheme, the number of mode candidates are greatly
reduced compared to that of the ideal mode selection. Through Monte Carlo
simulations, we will show the accuracy of our derivation for the ergodic sum
rate expression. Moreover, simulation results with the pathloss modeling
confirm that the proposed transmission selection scheme produces the average
sum rate identical to the ideal mode selection with significantly reduced
selection candidates.
