arXiv subjects

David Loeschenbrand

Publications and source records attributed to David Loeschenbrand.

3 recordsLinked to original sources

arXiv2019

Combating Massive MIMO Channel Aging by Orthogonal Precoding

In this work we investigate ultra-reliable low-latency massive multiple-input multiple-output (MIMO) communication links in vehicular scenarios, where coherence between uplink and downlink cannot be assumed. In such scenarios the channel state information obtained in the uplink will be outdated for the following downlink phase. To compensate for this channel aging we will utilize orthogonal precoding with two-dimensional precoding sequences in the time-frequency domain within an orthogonal frequency division multiplexing system. The channel hardening effect of massive MIMO transmission decreases, due to channel aging, with increasing frame duration and increasing velocity, while the channel hardening effect of orthogonal precoding (OP) increases with increasing time- and frequency-selectivity of the wireless communication channel. By combining massive MIMO and OP we can show by numeric link level simulation that the performance with outdated channel state information in terms of bit-error rate versus signal-to-noise ratio can be improved by two orders of magnitude.

eess.SP↗

arXiv2018

Iterative Detection for Orthogonal Precoding in Doubly Selective Channels

Ultra-reliable wireless communication links require the utilization of all diversity sources of a wireless communication channel. Hadani et al. propose a two dimensional discrete symplectic Fourier transform (DSFT) as orthogonal pre-coder for a time-frequency modulation scheme. In this paper we explore \emph{general} orthogonal precoding (OP) and its performance in time- and frequency-selective channels. We show that iterative parallel interference cancellation (PIC) and iterative channel estimation methods can be used for the detection of OP. A scalar signal model for OP transmission is obtained by PIC. Based on this signal model, we can prove that all constant modulus sequences, e.g. the DSFT basis functions or Walsh-Hadamard sequences, lead to the same performance for OP. We validate our receiver structure by numerical link level simulations of a vehicle-to-vehicle communication link with a relative velocity of $0\ldots200\,\text{km/h}$. We demonstrate that OP achieves a gain of about $4.8\,\text{dB}$ if compared to orthogonal frequency division multiplexing at a bit error rate of $10^{-4}$. Our performance results for coded OP are the best results for a fully documented receiver architecture, published so far.

cs.IT↗

arXiv2017

Low-Complexity Equalization for Orthogonal Time and Frequency Signaling (OTFS)

Recently, a new precoding technique called orthogonal time-frequency signaling (OTFS) has been proposed for time- and frequency-selective communication channels. OTFS precodes a data frame with a complete set of spreading sequences and transmits the results via orthogonal frequency division multiplexing (OFDM). OTFS uses two dimensional (2D) linear spreading sequences in time and frequency which are the basis functions of a symplectic Fourier transform. OTFS allows the utilization of time- and frequency-diversity but requires maximum likelihood decoding to achieve full diversity. In this paper we show performance results of a low-complexity equalizer using soft-symbol feedback for interference cancellation after an initial minimum-mean square error equalization step. Performance results for an implementation in the delay-Doppler domain and in the time-frequency domain are compared. With our equalizer, OTFS achieves a gain of 5dB compared to OFDM for a bit error rate of $10^{-4}$ and a velocity of $200\,\text{km/h}$.

cs.IT↗