  This paper investigates the performance of a massive multiple-input
multiple-output (MIMO) system that uses a large transmit antenna array with
antenna elements spaced densely. Under the assumption of idealized uniform
linear antenna arrays without mutual coupling, precoded quadrature phase-shift
keying (QPSK) transmission is proved to achieve the channel capacity of the
massive MIMO system when the transmit antenna separation tends to zero. This
asymptotic optimality is analogous to that of QPSK faster-than-Nyquist
signaling.
