Abstract
Massive multiple-input multiple-output (MIMO) systems are of high interest for ultra-reliable 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 For time-varying vehicular communication channels the CSI acquired during the uplink phase will be outdated for the following downlink phase, leading to reduced spatial channel hardening.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 time-frequency 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.
Highlights
Ultra-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]
We present a receiver structure for massive multiple-input multiple-output (MIMO) with general orthogonal precoding (OP) that uses parallel interference cancellation (PIC) [12] and iterative channel estimation [13], applying a well-known low-complexity multi-user detection framework [14]
A quadrature phase shift keying (QPSK) symbol alphabet A is used with symbol rate rs = 2 and an rc = 1/2 convolutional code for channel coding followed by a random interleaver
Summary
Ultra-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]. For mobile users the channel impulse response is timevarying, the CSI becomes outdated (channel aging) due to the time delay between uplink and downlink transmission This causes the channel hardening effect of massive MIMO to decrease with longer frame duration and increasing. Previous work either considers a quasi static scenario where the uplink and downlink phase take part within a so called coherence interval [2] or performs channel prediction between the uplink and downlink transmission [5], [6] using long-term statistical information Another method to improve the communication link reliability is orthogonal precoding (OP) [7]–[10].
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