  In this paper, we propose a transmit covariance optimization method to
maximize the energy efficiency (EE) for a single-user distributed antenna
system, where both the remote access units (RAUs) and the user are equipped
with multiple antennas. Unlike previous related works, both the rate
requirement and RAU selection are taken into consideration. Here, the total
circuit power consumption is related to the number of active RAUs. Given this
setup, we first propose an optimal transmit covariance optimization method to
solve the EE optimization problem under a fixed set of active RAUs. More
specifically, we split this problem into three subproblems, i.e., the rate
maximization problem, the EE maximization problem without rate constraint, and
the power minimization problem, and each subproblem can be efficiently solved.
Then, a novel distance-based RAU selection method is proposed to determine the
optimal set of active RAUs. Simulation results show that the performance of the
proposed RAU selection is almost identical to the optimal exhaustive search
method with significantly reduced computational complexity, and the performance
of the proposed algorithm significantly outperforms the existing EE
optimization methods.
