Performance Analysis of Tomlinson-harashima Multiuser Precoding in Multipath Cdma Channels

Tomlinson-Harashima multiuser precoding (THP) is a transmitter (Tx)-based multiple access interference (MAI) cancellation technique for Direct Sequence (DS) CodeDivision Multiple Access (CDMA) systems. This zero forcing (ZF)-based transmitter precoding method minimizes the mean square error at the receiver. It is suitable for the downlink since the receiver is as simple as the single user matched filter receiver. For frequencyselective fading channels, by combining THP with diversity techniques, we develop two specific designs, PreRAKETHP and Multipath Decorrelating THP (MDTHP), which provide desired performance-complexity trade-off. It is shown that the nonlinear THP designs outperform linear Tx-based methods and both linear and nonlinear receiver (Rx)-based decorrelating approaches. When the received users’ powers are unequal due to channel fading, the nonlinear precoders have better performance for weaker users. Therefore, they reduce total transmit power relative to linear precoders. In addition, in rapidly varying multipath fading channels, THP aided by the long-range channel prediction (LRP) method achieves almost the same performance as when the Channel State Information (CSI) is perfectly known at the transmitter.