Abstract
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.
Highlights
When using spatial multiplexing in a communication system, consisting of a multi-antenna transmitter (TX), a frequency-flat channel, and a number of user terminals, each user receives a linear combination of the signals sent by the different TX antennas
The curves for the optimized Conventional detector (CD) and the non-optimized Alternative detector (AD) intersect, with the latter outperforming the former in the lower range of mutual information (MI), where the Modulo loss (MoL) of the non-optimized CD is large. For both types of channel, the best performance is achieved by the optimized AD; the resulting gain compared to the non-optimized CD is largest for 2-Phase amplitude modulation (PAM), because the fraction of the users that can remove the modulo operation is larger for 2-PAM than for 4-PAM and 4-Quadrature amplitude modulation (QAM)
We investigate the effect of reduced-complexity algorithms that select the symbol rotations at the TX and the ordering of the users, to reduce the MoL and to increase the Signal-to-noise ratio (SNR) at the detector; several of these algorithms are novel
Summary
When using spatial multiplexing in a communication system, consisting of a multi-antenna transmitter (TX), a frequency-flat channel, and a number of user terminals, each user receives a linear combination of the signals sent by the different TX antennas. In [17, 18], the constellations are rotated to maximize the number of users which can discard the modulo operation In this contribution, we consider MU-MISO THP and we assume that perfect channel information is available at the TX and the RX. Besides applying some existing and some new algorithms which optimize over either the user ordering or the constellation rotations, this contribution jointly maximizes the SNR at the detector and reduces the MoL, by optimizing over both the ordering and the rotations.
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