Transmit power reduction in MIMO-THP using tilted constellation

Tomlinson-Harashima Precoding (THP) is a good candidate in MIMO systems due to its simple implementation. Although THP method has a number of advantages in MIMO systems, however, it staffers from some drawbacks including modulo loss, shaping loss and power loss. In the THP technique, due to modulo operation, the transmitted symbols are distributed over a bounded region which is larger than the original constellation. Hence, the transmitted power will be increased. It is shown that the transmitted power can be reduced significantly by tilting or rotating the original constellation and hence, some of the power loss are recovered. In this paper, a tilted constellation method is developed for MIMO-ZF-THP and MIMO-MMSE-THP systems and in both centralized and decentralized (multiuser) scenarios. In the proposed methods, each transmit antenna uses a different rotating angle individually to tilt the constellation. The simulation results show that the proposed method outperforms the conventional MIMO-THP technique. In addition it is shown that the performance gain in the decentralized MIMO-THP scenario is more than the centralized MIMO-THP.