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 general orthogonal precoding (OP) and its performance in timeand 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 vehicleto-vehicle communication link with a relative velocity of 0 . . . 200 km/h. We demonstrate that OP achieves a gain of about 4.8 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.

We investigate a parametric extension of the classical s-dimensional Halton sequence, where the bases are special Pisot numbers. In a onedimensional setting the properties of such sequences have already been investigated by several authors [5,8,23,28]. We use methods from ergodic theory to in order to investigate the distribution behavior of multidimensional versions of such sequences. As a consequence it is shown that the Kakutani-Fibonacci transformation is uniquely ergodic.

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The time-variant vehicle-to-vehicle radio propagation channel in the frequency band from 59.75 to 60.25 GHz has been measured in an urban street in the city center of Vienna, Austria. We have measured a set of 30 vehicle-to-vehicle channel realizations to capture the effect of an overtaking vehicle. Our experiment was designed for characterizing the large-scale fading and the small-scale fading depending on the overtaking vehicle's position. We demonstrate that large overtaking vehicles boost the mean receive power by up to 10 dB. The analysis of the small-scale fading reveals that the two-wave with diffuse power (TWDP) fading model is adequate. By means of the model selection, we demonstrate the regions where the TWDP model is more favorable than the customarily used the Rician fading model. Furthermore, we analyze the time selectivity of our vehicular channel. To precisely define the Doppler and delay resolutions, a multitaper spectral estimator with discrete prolate spheroidal windows is used. The delay and Doppler profiles are inferred from the estimated local scattering function. Spatial filtering by the transmitting horn antenna decreases the delay and Doppler spread values. We observe that the RMS Doppler spread is below one-tenth of the maximum Doppler shift 2f v/c. For example, at 60 GHz, a relative speed of 30 km/h yields a maximum Doppler shift of approximately 3300 Hz. The maximum RMS Doppler spread of all observed vehicles is 450 Hz; the largest observed RMS delay spread is 4 ns.INDEX TERMS 5G mobile communication, automotive engineering, communication channels, fading channels, intelligent vehicles, millimeter wave propagation, millimeter wave measurement, multipath channels, RMS delay spread, RMS Doppler spread, parameter extraction, time-varying channels, two-wave with diffuse power fading, wireless communication.

Massive multiple-input multiple-output (MIMO) systems are of high interest for ultrareliable low-latency communication (URLLC) links. They provide channel hardening, i.e. reduced channel variations, due to the large number of transmit antennas which exploit spatial diversity by beam-forming. Massive MIMO requires channel state information (CSI) on the base station side. For time-varying vehicular communication channels the CSI acquired during the uplink phase will be outdated for the following downlink phase, leading to reduced spatial channel hardening. We investigate a combination of massive MIMO with general orthogonal precoding (OP) to compensate this effect. OP uses two-dimensional precoding sequences in the time-frequency domain and provides channel hardening by exploiting time-and frequency diversity. We show that the combination of massive MIMO and OP is beneficial for time-varying communication channels. While the spatial channel hardening of massive MIMO decreases, the timefrequency channel hardening of OP increases with larger time-variance of the communication channel. An iterative receiver algorithm for massive MIMO with OP as well as a detailed analysis of the channel hardening effect is presented. We demonstrate a BER reduction by more than one order of magnitude for a velocity of 50 km/h = 16.6 m/s using the orthogonal frequency division multiplexing (OFDM) based 5G new radio (NR) physical layer.INDEX TERMS 5G, massive MIMO, orthogonal precoding, ultra-reliable low-latency communication (URLLC) links. I. INTRODUCTIONUltra-reliable low-latency wireless communication (URLLC) links are an important component for connected autonomous vehicles, industrial wireless control loops, and many other machine-to-machine communication applications [1]. The random fading process in wireless communication channels leads to signal strength fluctuations at the receive antenna and random unpredictable frame errors.Massive multiple-input multiple-output (MIMO) systems reach the capacity of multi-user MIMO systems by linear beam-forming over a large number of transmit antennaThe associate editor coordinating the review of this manuscript and approving it for publication was Jiankang Zhang.