The paper addresses the doubly iterative decoding of space-time turbo codes with a large number of transmit and receive antennas. In the doubly iterative scheme, the decoder uses preliminary soft values of the coded symbols and the soft estimates only for Quadratic-Phase-Shift-Keying (QPSK) modulation have been determined. In this paper we propose and explain the soft estimates related to the signal constellations for 8-PSK and 16-PSK modulations.