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
The signal-to-noise ratio (SNR) gap approximation provides a closed-form expression for the SNR required for a coded modulation system to achieve a given target error performance for a given constellation size. This approximation has been widely used for resource allocation in the context of trellis-coded multicarrier systems (e.g., for digital subscriber line communication). In this contribution, we show that the SNR gap approximation does not accurately model the relation between constellation size and required SNR in low-density parity-check (LDPC) coded multicarrier systems. We solve this problem by using a simple modification of the SNR gap approximation instead, which fully retains the analytical convenience of the former approximation. The performance advantage resulting from the proposed modification is illustrated for single-user digital subscriber line transmission.
Abstract-The Monte Carlo (MC) simulation of the error performance of a concatenated coding system with finite interleaving depth between the outer and inner codes is time-consuming, especially when targeting low error rates and examining several interleaver settings. In this contribution we present a semianalytical evaluation of the word error rate (WER) performance of a system with Reed-Solomon (RS) outer coding, finite block interleaving and systematic low-density parity check (LDPC) inner coding. The proposed evaluation method relies on a simple semi-analytical statistical model for the number of bit errors in a segment of the information word after LDPC decoding on the AWGN channel. Only the WER and bit error rate (BER) of the inner subsystem (LDPC code and the considered constellation) are required to compute the WER of the concatenated code, corresponding to different parameters of the RS code and the interleaver. We show that the semi-analytical WER of the concatenated system closely matches the WER resulting from MC simulations, for both the AWGN channel and the Rayleigh block-fading channel.
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