  This paper proposes a flexible pilot assignment method to jointly optimize
the uplink and downlink data transmission in multi-cell Massive multiple input
multiple output (MIMO) systems with correlated Rayleigh fading channels. By
utilizing a closed-form expression of the ergodic spectral efficiency (SE)
achieved with maximum ratio processing, we formulate an optimization problem
for maximizing the minimum weighted sum of the uplink and downlink SEs subject
to the transmit powers and pilot assignment sets. This combinatiorial
optimization problem is solved by two sequential algorithms: a heuristic pilot
assignment is first proposed to obtain a good pilot reuse set and the data
power control is then implemented. Numerical results manifest that the proposed
algorithm converges fast to a better minimum sum SE per user than the
algorithms in previous works.
