SNR maximization and modulo loss reduction for Tomlinson-Harashima precoding

Compared to linear precoding, Tomlinson-Harashima precoding (THP) requires less transmit power to eliminate the spatial interference in a multi-user downlink scenario involving a multi-antenna transmitter and geographically separated receivers. However, THP gives rise to certain performance losses, referred to as modulo loss and power loss. Based on the observation that part of the users can omit the modulo operation at the receiver during an entire frame, we present an alternative detector, which reduces the modulo loss compared to the conventional detector. In addition, this contribution compares several existing and novel algorithms for selecting the user ordering and the rotation of the constellations at the transmitter, to increase the SNR at the detector and decrease the modulo loss for the alternative detector. Compared to the better of linear precoding and THP with conventional detector, the optimized alternative detector achieves significant gains (up to about 4 dB) for terrestrial wireless communication, whereas smaller gains (up to about 1 dB) are obtained for multi-beam satellite communication

Modulo loss reduction in spatial multiplexing systems with Tomlinson-Harashima precoding

When a multi-user communication system over a block-fading MIMO channel utilizes Tomlinson-Harashima precoding in the downstream direction, to eliminate the interference between the spatially multiplexed data streams, the conventional detection, involving a modulo operation at the receiving terminals, is known to yield a performance degradation that becomes considerable at low SNR. In this contribution, we propose a novel detection method that exploits sending one bit of extra information per user and per frame to the receiver, which indicates whether or not the considered user can detect all its data within the frame without performing a modulo operation. Moreover, in the case of M-PAM transmission it is possible to optimize the rotation of the constellations at the transmitter, maximizing for each frame the number of users for which no modulo operation is required. Numerical results show that in the case of 2-PAM the novel detection algorithm is able to completely recover the modulo loss experienced by the conventional detection method without an increase in transmit power, and to outperform 4-QAM (with conventional or novel detection) in terms of mutual information at low SNR