Cellular vehicle-to-everything (C-V2X) is an emerging technology to realize autonomous driving and Mobility as a Service (MaaS). In C-V2X communications, direct device-to-device communication without passing through the mobile core network, called Sidelink, is employed in vehicle-to-vehicle or vehicle-to-infrastructure communication scenarios. To extend the network coverage and improve the system performance, multi-hop Sidelink communication has attracted a lot of attention recently. In this paper, the performance of multi-hop communication using V2X Sidelink is evaluated by over-the-air experimental trials conducted in an anechoic chamber. The packet forwarding and routing functions are implemented by software in off-the-shelf V2X modules, and the end-to-end latency and the packet reception interval jitter are evaluated. The trial results show that the 99 percentile of the end-to-end latency is less than 100 milliseconds, even with 3 hops.

The field trial results for massive multi-user multiple-input multiple-output (MU-MIMO) transmission at 28 GHz with 500 MHz bandwidth are reported in this paper. In this experiment, hybrid beamforming with block diagonal precoding is employed, in which channel state information is acquired by uplink sounding assuming channel reciprocity in a time division duplex system. 8-user MU-MIMO scenarios with various user distributions are considered, and the downlink transmission performance and propagation channel characteristics are evaluated. The trial results show that digital precoding improves the system performance and that over 20 Gbps total throughput can be achieved for a variety of user distributions. The performance of the precoding is evaluated in terms of eliminating inter-user interference.

For 5G massive MU-MIMO (Multi-User Multiple-Input and Multiple-Output) systems in high frequency bands, a favorable scenario is a line-of-sight (LOS) environment where the direct paths dominate the spatial channels. In the LOS environment, spatial correlation between user equipments (UEs) tend to be high and its capacity decreases, especially in the case where we have densely distributed UEs. The authors have already proposed a nonlinear precoding scheme incorporating block multi- diagonalization (NL-BMD) for highly correlated channel and demonstrated its effectiveness by computer simulation. In this paper, the authors present indoor propagation measurement results using a channel sounding system with 44 GHz active phased array antenna in the condition that UEs are sparsely and closely distributed. In addition, this paper shows the spectral efficiency of NL-BMD based on the measurement results compared with the conventional precoding schemes.

In the paper, we propose a novel transmit precoding named “nonlinear block multi-diagonalization (NL-BMD) precoding,” composed of block multi-diagonalization and adjacent inter-user interference pre-cancellation, for multiuser MIMO downlink toward 5G. It is revealed through computer simulations that, the proposed NL-BMD precoding yields up to 77% performance improvement in average sum-rate throughput and enables large-capacity transmission regardless of the user configuration, compared with the conventional block diagonalization precoding.

For multiuser-MIMO (MU-MIMO) downlink, the authors propose a novel precoding concept of block lower multi-diagonalization. Extending the conventional block diagonalization method, the proposed precoder for the desired user is computed to incorporate a predetermined number of interfering users. Consequently, the concept enables us to ensure extra degrees of freedom at the transmit array even after null steering in a linear MU-MIMO strategy, and also to reduce computational load at transmit precanceling in a nonlinear MU-MIMO strategy. From an implementational viewpoint, we here examine two approaches: block bi-diagonalization, allowing one interfering user, and block lower tri-diagonalization, allowing up to two interfering users. In the paper, prior to a study on practical management schemes for residual interference, we reveal the significant potential of the proposed concept through the discussion and numerical evaluation using sum-rate capacity.

To meet the growing demand for wireless broadband communications in high-mobility cases, we study cell structures using millimeter-wave. Single antenna Cell (SC) and Linear Cell (LC) structures are here considered as typical approach. The cells in LC structures consist of strips while those in SC structures are circular. Distributed Antenna Cell (DAC) and Narrow Beamwidth Distributed Antenna Cell (NBDAC) have already been proposed for LC structures, but rigorous performance evaluations in high mobility case have not been conducted. Thus, in this paper, we compare the performances of SC, LC and NBDAC in a high-mobility environment. The comparison shows that NBDAC is the most suitable structure for high-mobility scenarios. In addition, we propose interference reduction methods for NBDAC to reduce adjacent cell interference and improve throughput at a cell edge. To evaluate effectiveness of the proposed methods, theoretical evaluations and computer simulations are conducted, and it is shown that NBDAC with the proposed methods can achieve 1Gbps throughput even in high-mobility environments.

In a multiple-input multiple-output (MIMO) system, cross-polarized antenna selection yields significant reduction in cost and hardware size. However, actual benefits of the technique are dependent on the propagation characteristics including channel polarization. To accurately characterize the target 2 GHz-band MIMO channels, the authors conduct 2 GHz cross-polarized channel measurement campaigns. Based on the measured data, novel channel models specifically for the 2 GHz bands are established. In addition, we evaluate the performance improvement obtained with cross-polarized antenna selection using the channel models. Simulation results reveal that antenna selection is particularly useful in the low SNR regime, and that the system capacity at cell edges can be increased up to 13%.

Recently, a multiuser MIMO system has attracted much attention. In the downlink of the multiuser MIMO system, since the base station simultaneously transmits signals to terminals, there is inter-user interference (IUI) at each terminal. Block diagonalization, which can achieve perfect IUI suppression, has been extensively studied to solve the issue. However, with the scheme we cannot obtain extra diversity gain due to the null steering at the base station. In this paper, we propose imperfect block diagonalization based on maximum eigenvectors of users and Gram-Schmidt orthonormalization, and its error rate performance is evaluated using computer simulations. The result shows that, despite its low complexity, the proposed scheme provides excellent performance, especially in many-user environments.

Frequency-domain equalization (FDE) has been studied for suppressing inter-symbol interference (ISI) due to frequency selective fading in single carrier systems. When a high- mobility terminal is assumed in the system, channel transition within an FFT block cannot be ignored. Then, the ISI reduction performance of FDE degrades since cyclicity of the channel matrix is lost. To solve this problem, a method of dividing the received data block into multiple subblocks has been proposed. In the paper, the subblock processing is applied to frequency-domain turbo equalization (FDTE). The results of numerical analysis show a remarkable improvement by the proposed method. Specifically, the FDTE with two-subblock processing can decrease the block error rate floor to less than 0.001 when the maximum Doppler frequency normalized by the block length is 0.4.

Frequency domain equalization (FDE) has been studied for reducing inter-symbol interference (ISI) caused by frequency selective fading in single carrier systems. When a high-mobility terminal exists in the system, the channel state may change within a DFT block. Then, the ISI reduction performance of FDE degrades since cyclicity of the channel matrix is lost. We propose to divide a received data block into multiple subblocks to decrease the channel transition within the DFT block in fast fading environments. Also, to satisfy periodicity of the received signal in each subblock, we introduce a pseudo cyclic prefix technique. The results of numerical analysis show that the proposed method can effectively decrease the error floor in fast fading environments.

In a frequency-selective MIMO channel, the optimum beam- forming is achieved by using eigenvectors, i.e., eigenbeam- space division multiplexing (E-SDM), in the frequency domain. However, the calculation load of eigenvalue decomposition increases in proportion to the number of FFT points. In addition, eigenvectors without consideration of frequency continuity increase the delay spread of the effective channel observed at the receiver. Addressing these issues, we have proposed a pseudo eigenvector scheme for the purpose of mitigating the calculation load and maintaining frequency continuity (or decreasing the delay spread). In this paper, we present the PE-SDM performance in a MIMO single-carrier system with MMSE-FDE. The simulation results show that PE-SDM single-carrier transmission provides almost the same or better throughput performance with less complexity in comparison to E-SDM.

The performance of MIMO-OFDM E-SDM systems may be seriously degraded in time-varying fading environments due to a channel transition. In the paper, we propose two methods of channel extrapolation to compensate for the channel change. Futhermore, we also consider a method to reduce the amount of header signal and the computational complexity for the systems. We conducted MIMO measurement campaigns at a 5.2 GHz frequency band to evaluate the system performance. Simulation results based on the measured channel data show that robust bit-error rate performance can be obtained by using our channel prediction methods in both line-of-sight (LOS) and non- LOS environments. It is also shown that the method of reducing the amount of header signal and the computational complexity may be used in the practical implementation point of view.

Performance of multiple-input multiple-output (MIMO) systems applying the eigenbeam-space division multiplexing (E-SDM) technique may be degraded in time-varying fading environments due to a channel change during the time interval between the transmit weight matrix determination and the actual data transmission. We have proposed some channel prediction methods to compensate for the channel change. Simulation results based on computer generated channels showed better performance when using the prediction methods in rich scatterer environments assuming the Jakes model. However, actual MIMO systems may be used in line-of-sight (LOS) environments, and even in a non-LOS (NLOS) case scatterers may not be uniformly distributed around a receiver and/or a transmitter. In addition, mutual coupling between antenna elements should be considered in actual implementation since it affects the system performance. We conducted MIMO measurement campaigns at a 5.2 GHz frequency band to evaluate the channel prediction techniques. In this paper, we present the experiment and simulation results using the measured channel data. It is shown that robust BER performance is obtained when using the channel prediction methods, and that the performance in LOS environments is better than that in NLOS ones.

Frequency domain equalization (FDE) has been studied for suppressing inter-symbol interference (ISI) caused by frequency selective fading in single carrier systems. When a high-mobility terminal exists in the system, channel state changes within a DFT block. Then, the ISI reduction performance of FDE degrades since cyclicity of the channel matrix is lost. Here, we propose to divide received data block into multiple subblocks to decrease the channel transition within the DFT block in fast fading environments. Also, to satisfy periodicity of the received signal in each subblock, we introduce a pseudo cyclic prefix technique. The results of numerical analysis show that the proposed method provides 1.3-fold increase in tolerable Doppler frequency for FER of 0.01.

In a frequency-selective MIMO channel, the optimum beamforming is achieved by using eigenvectors, i.e., eigenbeam-space division multiplexing (E-SDM), in the frequency domain. However, the calculation load of eigenvalue decomposition increases in proportion to the number of FFT points. In addition, eigenvectors without consideration of frequency continuity increase the delay spread of the effective channel observed at the receiver. To solve these issues, a pseudo eigenvector technique has been proposed. It has also been reported that a pseudo E-SDM (PE-SDM) technique can provide almost the same or better performance compared with the E-SDM when the receiver employs a simple channel estimator with time windowing. In this paper, focusing on the calculating process of pseudo eigenvectors, we propose a spatial windowing scheme improving the accuracy of the effective channel estimates. The simulation results show that the accuracy in the PE-SDM can be improved by combining the proposed scheme with the conventional time windowing scheme.

In a frequency-selective MIMO channel, the optimum transmission is beamforming based on eigenvectors, i.e., eigenbeam-space division multiplexing (E-SDM), at each frequency point. However, the calculation load of eigenvalue decomposition at the transmitter increases in proportion to the number of frequency points. In addition, frequency-independent eigenvectors increase the delay spread of the effective channel observed at the receiver. In this paper, we propose pseudo eigenvectors for the purpose of reducing the calculation load at the transmitter and maintaining frequency continuity of the effective channel. First, we demonstrate that pseudo eigenvectors decrease the delay spread of the effective channels. Next, the practical performance of the pseudo E-SDM (PE-SDM) transmission in MIMO-OFDM systems is evaluated. The simulation results show that PE-SDM with an interference canceller provides better performance than E-SDM when an estimated effective channel is used to detect the substreams at the receiver.