In the 6th generation (6G), demand for high-capacity wireless communication systems is growing due to the emergence of a wide variety of applications. Small-cell use with millimeter waves and higher frequency bands is key to increasing system capacity. To achieve a large number of small-cell installations, a flexible backhaul is required to support the foundation of high-speed 6G accesses. The orbital angular momentum (OAM) multiplexing technique is effective for achieving high-capacity wireless backhauls. This paper presents the field experimental results of real-time wireless transmission using a combination of OAM and polarization multiplexing in the E-band, from 71 to 86 GHz, using an OAM-multiplexing prototype. Experimental results indicate successful real-time transmission of 139.2 and 104.0 Gb/s at distances of 22.5 and 45 m, respectively. We also demonstrated real-time OAM multiplexing transmission in a reflection scenario using a reflector placed in the middle of the propagation path and successfully achieved 139.2 Gb/s at a distance of 22.5 m.

This paper presents the design of two-stage beam alignment methods employing a hybrid analog-digital antenna array and exploiting the beam pattern in a point-to-point millimeter-wave (mmWave) radio for mmWave massive multiple-input multiple-output systems. We investigate an antenna deactivating approach that generates wider beams at the coarse alignment stage and exploit the theoretical beam pattern at the fine alignment stage. Our numerical results show that the proposed two-stage methods can achieve a better beam alignment than existing exhaustive methods and avail measurements/complexity reductions by tuning key parameters governing the alignment performance.

This work proposes a system that optimises multidimensional signal transmission, utilising signals with probabilistic shaping designed with the aid of end-to-end learning of an autoencoder-based architecture. For the first time, this work reports bit mapping optimisation for multidimensional signals and applied the newly derived optimised signals to the probabilistic shaping system. The autoencoder employs two neural networks for the transceiver, separated by the embedded channel. The optimisation of the autoencoder configuration is implemented for probabilistic shaping for n-dimensional signals. Specifically, We investigate a 4-dimensional (4D) signal employing 2 successive time slots that has better noise immunity relative to regular 2-dimensional quadrature amplitude modulation (QAM) signals. We propose a new application of autoencoders in communication systems based on 4D signals and apply machine learning to optimise the 4D probabilistic shaping on the basis of receiver signal-to-noise-ratio (SNR). The performance of the optimised probabilistically shaped 4D signals is evaluated in terms of the bit error rate (BER) and mutual information. Simulation results show that the proposed probabilistically shaped 4D signal achieves better BER performance relative to the unshaped 4D and regular 2D QAM. We demonstrate the mutual information of the proposed signal with varying SNR, showing its improved capacity in comparison with other constellations.

This work proposes and explores the use of probabilistic shaping for the non-orthogonal multicarrier spectrally efficient frequency division multiplexing (SEFDM) system. The system design considers the reverse concatenation architecture which cascades the constant composition distribution matching (CCDM) algorithm together with soft-decision forward error correction (SD-FEC)-LDPC code for the probabilistic shaping scheme. The non-orthogonal signalling is implemented by discrete Fourier transform (DFT)-based SEFDM modulation with matched filtering demodulation and advanced interference cancellation detection. The high achievable spectral efficiency, low computation complexity and reliability make SEFDM a good candidate for multicarrier signalling for beyond 5G communications. By adding extra shaping gain and flexibility of rate adaptation, the combination of two capacity-achieving techniques provides significant insight of further performance improvement. In this paper, we investigate the performance of the proposed probabilistically shaped-SEFDM (PS-SEFDM) system with regular QAM constellations. The presented results of the proposed system show less required power and bandwidth saving compared to OFDM when achieving the same error performance and same spectral efficiency.

The authors analyse and investigate the effectiveness of frequency domain equalisation (FDE) for single‐carrier (SC) transmission over a line‐of‐sight multiple‐input multiple‐output (LOS‐MIMO) channel with frequency selective fading. For this purpose, they propose a LOS‐MIMO system model employing SC‐FDE techniques together with a data frame structure having a cyclic prefix and postfix (CPP) overhead. The channel capacity of the proposed system is derived taking into account system design parameters, such as symbol rate and roll‐off factor, as well as multipath fading channel parameters. An efficient channel estimation method is also proposed for implementing and simulating the proposed system. Signature curves estimated in simulations are consistent with those expected from the theoretical capacity analysis. The simulation results show that the SC‐FDE approach to the LOS‐MIMO system can, with small CPP overhead, effectively reduce intersymbol and interstream interference.

In this paper, we present a novel phase error estimation scheme for low-density parity-check (LDPC) coded high-order quadrature amplitude modulation (QAM) transmission systems, and we provide a theoretical analysis of its performance. The scheme uses known pilot symbols which are assumed to be placed at equally spaced positions, and phase errors occurred at these pilot symbol positions are first estimated. They are then interpolated over the other positions by using a Wiener filter (WF), and estimation accuracy is next refined by using an LDPC code-aided phase-locked loop (PLL). We present an efficient implementation of this two-stage, Pilot-Symbol-Assisted and Code-Aided (PSA/CA) scheme and evaluate its performance by means of power spectrum analysis. We determine the mean square error at each step of the PSA/CA scheme, and we find an optimal value of the noise equivalent loop bandwidth of the PLL under the assumption that local oscillator phase noise can be modeled as a single-pole single-zero model. We show through simulations that the PSA/CA scheme has a very high phase noise tolerance comparable to that of state-of-the-art phase estimation schemes and provides a computationally efficient alternative.

In this paper, we present an efficient systematic encoding algorithm for quasi-cyclic (QC) low-density parity check (LDPC) codes that are related to cyclic maximum-distance separable (MDS) codes. The algorithm offers linear time complexity, and it can be easily implemented by using polynomial multiplication and division circuits. We show that the division polynomials can be completely characterized by their zeros and that the sum of the respective numbers of their zeros is equal to the parity-length of the codes.

Despite the lack of rich scattering which is necessary for MIMO systems in mobile environments, this paper proposes line-of-sight MIMO systems with the highest MIMO capacity in some tens of GHz microwave radios. The proposed scheme constructs spatially orthogonal MIMO channels by using geometric arrangement of antennas and can use individual local oscillator signals without feedback signals for a transmit matrix. It can also compensate for the fluctuation of the carrier phase caused by the geometric small shift of the antenna position.

A novel decoder architecture is presented for quasi- cyclic low-density parity-check (QC-LDPC) codes. The architecture implements a min-sum algorithm and uses feedback shift-registers to store all the messages exchanged within the algorithm. This shift-register-based min-sum (SRMS) decoder does not require complex interconnections between processors and registers and is amenable to a simple, high throughput hardware implementation. A codec with the SRMS decoder for high-rate codes of length 4095 bits has been fabricated with a 0.15 mum, 1.5 V CMOS technology. The decoder has a gate count of 693 K gates and achieves a throughput of 155.52 Mbps (STM-1) using high-order QAM schemes. Measurement results show that a microwave radio system employing this LDPC codec exhibits good error performance.