An Enhanced Tomlinson-Harashima Precoding Scheme for High-Order MU-MIMO Transmissions in 5G Networks

We propose a novel transmission scheme based on Tomlinson-Harashima precoding (THP) that serves as a practical downlink multi-user (MU) MIMO transmission scheme for future radio access networks. The transmission of reference signals (RSs) is an issue of THP because the modulo operation during the interference cancellation introduces ambiguities to the channel estimation if it is applied on RSs. Without the modulo operation, the power of RS symbols becomes uncontrollable which degrades the performance due to the transmission power constraints in practical systems. An existing solution is to use orthogonal resources to transmit RSs without interference cancellation, which increases the RS overhead especially for high-order MU-MIMO transmissions. We propose an enhanced precoding scheme that uses phase rotation to control the power of the RSs during the THP process and thus control the power of the RS symbols. Therefore, the loss in transmission power due to practical power constraints is less than that for legacy approaches, and the use of orthogonal resources is not needed to transmit the RS symbols. In practical application, a fast phase search method is introduced to find the feasible angles for the phase rotation with low complexity. Simulations show that the proposed scheme is superior to legacy processing schemes with respect to the bit error rate.