  This paper investigates the performance of the downlink channel in
distributed antenna systems. We first establish the ergodic capacity of
distributed antennas, under different channel side information (CSI)
assumptions. We consider a generalized distributed antenna system with $N$
distributed ports, each of which is equipped with an array of $L$ transmit
antennas and constrained by a fixed transmit power. For this system we
calculate the downlink capacity to a single antenna receiver, under different
assumptions about the availability of the channel states at the transmitter.
Having established this information theoretic analysis of the ergodic capacity
of distributed antenna systems, this paper also investigates the effect of
antenna placement on the performance of such systems. In particular, we
investigate the optimal placement of the transmit antennas in distributed
antenna systems. We present a fairly general framework for this optimization
with no constraint on the location of the antennas. Based on stochastic
approximation theory, we adopt a formulation that is suitable for node
placement optimization in various wireless network scenarios. We show that
optimal placement of antennas inside the coverage region can significantly
improve the power efficiency of wireless networks.
