  We consider uplink transmission of a massive multi-user multiple-input
multiple-output (MU-MIMO) system in the presence of a smart jammer. The jammer
aims to degrade the sum spectral efficiency of the legitimate system by
attacking both the training and data transmission phases. First, we derive a
closed-form expression for the sum spectral efficiency by taking into account
the presence of a smart jammer. Then, we determine how a jammer with a given
energy budget should attack the training and data transmission phases to induce
the maximum loss to the sum spectral efficiency. Numerical results illustrate
the impact of optimal jamming specifically in the large limit of the number of
base station (BS) antennas.
