One of the possible methods for obtaining reduced latency is the use of belief propagation (BP) decoding on polar codes. We proposed an iterative approach to get the information set optimal for BP decoding in order to mitigate performance degradation caused by BP decoding. In order to estimate the block error rate (BLER) of the BP decoding, we employ the union bound. Specifically, an all-zero codeword is transmitted through a low-noise channel, and it is decoded by successive cancellation list decoder with a large list size to capture low-weight codewords. By employing the proposed iterative algorithm, the performance were improved compared to the performance employing the information set optimized for the successive cancellation decoding.

We propose two deep neural network (DNN)-based demodulators for log likelihood ratio (LLR) estimation to mitigate hardware impairments. At high-frequency bands, such as the sub-terahertz (sub-THz) wave bands expected to be used in the sixth-generation mobile communication system (6G), hardware impairments such as nonlinear distortion of power amplifiers (PA) and in-phase and quadrature (IQ) imbalance of direct IQ modulators are key challenges to be addressed. We describe the signal distortion effect of nonlinear distortion and IQ imbalance in the time-domain (TD) and frequency-domain (FD) signals. We compare the two DNN-based demodulators: 1) TD-DNN, which requires TD signals as the input data, and 2) Hybrid-DNN, which requires TD and FD signals with a conventional demodulator. The numerical results indicate that the proposed DNN-based demodulators improve the coded bit error rate (BER) performance and that the Hybrid-DNN demodulator enables more efficient learning and demodulation processing by taking advantage of the characteristics of each.

On sub-THz transmission, the coverage enhancement, especially in NLOS environment, is a technical challenge due to the significant distance attenuation and linearity. In addition, from the perspective of RF devices, hardware impairment suppression of sub-THz transceivers is also a technical challenge, which may further degrade the coverage performance. As a compensation scheme for hardware impairments, we proposed Hybrid-DNN demodulator which uses frequency-domain and time-domain signals to suppress hardware impairments, and evaluated it by simulation. However, the sub-THz transmission suffer from the combined effects of the channel environment and hardware impairments. In this paper, we conducted three indoor coverage experiments that demonstrate coverage enhancement using reflected waves and suppression of hardware impairments by employing Hybrid-DNN demodulator. We utilized 140 GHz-band Tx-RFIC and phased array antenna developed by Nokia Bell Labs as the sub-THz transceiver. The measurement results indicate that it is possible to obtain coverage performance comparable to that obtained in a line-of-sight environment, if a ray connecting the Tx and Rx antennas with one reflection by utilizing reflected waves from metal walls is available. In addition, the results also confirmed the coverage enhancement even under hardware impairment by using beam steering and Hybrid-DNN demodulation.

In this paper, we explore advancements in the 6th-generation mobile communication system (6G) to achieve extremely high-speed communication with peak data rates surpassing 100 Gbps. Our research focuses on enhancing MIMO spatial multiplexing capabilities by developing a New Radio Network Topology (NRNT) utilizing devices like Reconfigurable Intelligent Surfaces (RIS) to control the reflection and transmission of radio waves. Concurrently, we address propagation losses in high-frequency bands, such as millimeter waves (mmW), through the use of massive MIMO (mMIMO) and cost-effective hybrid beamforming (BF) techniques. However, when integrating NRNT and hybrid BF, traditional beam selection algorithms, such as the maximum RSRP criterion, may fail to select beams that facilitate multiple propagation paths (multipaths). To address this, we propose a novel method that enables beam selection towards previously un selected RIS directions. This is achieved by employing vectors from the kernel of the channel matrix, inclusive of analog beams between the base station (BS) and mobile station (MS), as precoding vectors on the BS side. This approach generates beams with nulls in the directions of already selected RIS paths, effectively enhancing multipath formation. We validate the efficacy of our proposed method through MIMO link-level simulations, demonstrating its potential for improving 6G network performance.

Digital twin technology, envisioned as a cornerstone of sixth-generation (6G) networks, demands robust capabilities for dynamic and real-time environmental reconstruction. Wireless sensing plays a pivotal role in enabling digital twin systems, yet significant challenges persist. High-resolution wireless imaging typically requires large array apertures, which are often accompanied by prohibitive deployment costs and implementation complexity. This paper addresses these limitations by proposing a user equipment (UE)-assisted synthetic aperture radar (SAR) imaging scheme. The proposed approach exploits UE mobility to emulate a synthetic aperture, enabling high-resolution imaging while reusing existing wireless communication infrastructure. Advanced image reconstruction algorithms, including back projection (BP) and Fourier transform (FT)-based methods, are employed to process echo signals and accurately reconstruct target scenes. To validate the feasibility of the scheme under realistic channel conditions, typical configurations aligned with current network deployments are adopted, and ray-tracing-based channel modeling is applied to representative local areas within urban environments such as Munich and Paris. Preliminary simulation results show that, with a carrier frequency of 3.5 GHz and a bandwidth of 100 MHz, the proposed scheme achieves meter-level resolution in reconstructing building envelopes and capturing distinct structural details. These findings demonstrate the feasibility, scalability, and cost-effectiveness of the UE-assisted SAR imaging framework, paving the way for practical wireless sensing and digital twin applications in future 6 G urban environments.

For 6G, the use of AI/ML is one of the key technologies and its application to the air interface is being widely studied. As an effective use of AI/ML for air interface, a deep learning-based pilotless transmission scheme has been proposed where AI/ML is applied to functions such as channel estimation, equalization and demodulation. Several papers have shown through simulation evaluations that the application of AI/ML to the air interface can improve spectral efficiency. However, in all AI/ML applications, it is important to verify how well they perform in real environments. In this paper, a proofof-concept for the deep learning-based pilotless transmission scheme is constructed and experimental results in real indoor environments are shown. The results of the conducted experiments at fixed and walking speeds verify that the scheme improves throughput performance by 6.5-18.2 %.

This paper proposes an internetwork interference coordination method using protected bands in the downlink of a network where high altitude platform station (HAPS) and terrestrial base stations (TBSs) coexist. When HAPS and TBS share the same transmission band, interference between them degrades the throughput for users, especially at cell edges. When the overall system bandwidth is divided into two non-overlapping bands, where HAPS and TBS use each of these bands, internetwork interference is avoided. However, this reduces the transmission bandwidth HAPS and TBS use, degrading the throughput of cell-interior users. To address this problem, the proposed method divides the overall system band into three parts. The first is a shared band used by both HAPS and TBS, and the other two are protected bands prepared exclusively for HAPS and TBS. By lowering the transmission power at the protected band of non-protected base stations, internetwork interference at the protected band of the target base station is mitigated while avoiding bandwidth loss in the conventional methods. We quantitatively demonstrate the improvement in system throughput achieved by the proposed method through computer simulations.

One of the possible methods for obtaining reduced latency is the use of neural network decoder (NND) on polar codes. However, the number of information bits K for one NND is limited, since the training complexity increases exponentially according to K. Therefore, concatenated NNDs are promising for practical information length. In the paper, we propose an iterative approach to optimize the information set for concatenated NNDs. Specifically, we first optimize the information set within same NND. We then optimize the information set between different NNDs. This process is iteratively performed until the block error rate (BLER) performance improvement is saturated. The simulation results show that the BLER performance is enhanced by using the proposed iterative approach.

The terminal-collaborated multiple-input multipleoutput (MIMO) reception technology can equivalently increase the number of receiving antennas by collaborating the receiving terminals. Using this technology, the spectrum efficiency of communication between the base station and the collaborating terminals can be increased. It is desirable to use high-frequency bands with high-speed and low-latency for collaborative communication. In this paper, we report the initial implementation results of a transmission experiment system for MIMO reception with terminal collaboration that uses the 25.9 GHz band for terminal-to-terminal communication, with the goal of future outdoor experiments. As a result, we show that the error rate performance is improved as the number of collaborating terminals is increased.

This paper presents the bit error rate (BER) performance for various quadrature amplitude modulation (QAM) schemes with low-density parity-check (LDPC) coding using pilot symbol assisted (PSA) - extended Kalman filter (EKF) phase-noise compensation (PNC) in the presence of strong phase noise (PN) for orthogonal frequency division multiplexing (OFDM) waveforms in sub-terahertz (sub-THz) bands. In the sub-THz bands, PN is a significant hardware impairment and comprises random Wiener PN and uncorrelated Gaussian PN. Hence, in this paper, we propose a compensation method for the correlated Wiener PN by using PSA-EKF PNC and to make use of the large coding gain from the LDPC code to mitigate the effect of the uncorrelated Gaussian PN in the sub-THz bands. Based on computer simulations, we show that the loss in the required received signal-to-noise ratio (SNR) at the BER of $10^{-6}$ compared to the case without PN is suppressed to less than 1.0 dB for both QPSK and 16QAM with the LDPC coding rate of $R=8 / 9$ for the power spectral density level of an uncorrelated Gaussian PN of $-105 \mathrm{dBc} / \mathrm{Hz}$ or lower. We also show that the 64QAM scheme achieves the BER of $10^{-6}$ by utilizing the LDPC coding gain at the cost of an increase in the required received SNR in the presence of strong PN in the sub-THz bands.

This paper presents detection probabilities for the physical layer cell ID (PCID) in the presence of a large carrier frequency offset (CFO) and strong phase noise (PN) for the New Radio downlink in sub-terahertz (sub-THz) bands. In the sub-THz bands, CFO is large due to the frequency error of the local oscillator in a set of user equipment. The PN power becomes large because of the increase in uncorrelated Gaussian PN with high frequency components due to a significantly wide transmission bandwidth. We compare the PCID detection probabilities of PCID detection methods that employ two promising CFO estimation methods. Computer simulation results show that the PCID detection method using fractional frequency offset (FFO) estimation based on the autocorrelation using the cyclic prefix waveform and joint integer frequency offset (IFO) estimation with the primary synchronization signal (PSS) attains a higher PCID detection probability than that using FFO estimation predicated on the cross-correlation of the PSS and joint IFO estimation with the secondary synchronization signal (SSS) sequence in the presence of a large CFO and strong PN in the sub-THz bands.

The use of high-frequency bands is becoming increasingly popular for achieving higher speed, lower latency, and higher capacity in wireless communication systems. One way to use high-frequency bands is to improve the transmission performance by linking neighboring terminals. However, it is recognized that high-frequency band wireless communications are susceptible to obstructions. In particular, the shadowing effect caused by the human body between neighboring terminals is a major problem, especially when high-frequency bands are used for terminal collaboration. This paper reports the comparison results of the shadowing effects measured in the super high frequency and ultra high frequency bands for a reciprocating motion pedestrian case. In this research, experiments are conducted in the electromagnetic anechoic chamber room to measure the effects of human shadowing in the $25.9 \mathbf{G H z}$ and 438 MHz bands. As a result, it was confirmed that the 25.9 GHz band has a shorter shadowed time and smaller shadowing gain than the 438 MHz band.

This paper proposes an autonomous decentralized user association method for the networks where the high-altitude platform station (HAPS) and conventional terrestrial base stations (BSs) are operated in the same coverage area. The throughput satisfaction rate of each user is defined as the ratio of the actual throughput to the target throughput. The purpose of the proposed method is to maximize the system-level throughput satisfaction rate, which is defined as a generalized mean of the throughput satisfaction rates of all users in the system coverage. The proposed method achieves optimal user association by iteratively performing an autonomous decentralized process that does not require complex cooperation among HAPS and terrestrial BSs. Computer simulation results show that the proposed method can increase the system-level throughput satisfaction rate compared to the conventional cell range expansion (CRE) method and that this increase is particularly significant when fairness among users in terms of the throughput satisfaction rate is emphasized.

The 6th generation mobile communication system (6G) is expected to realize ultrafast and high-capacity communications with a peak data rate of more than 100 Gbps and to contribute to the advancement of the Cyber-Physical System (CPS). In order to realize 100 Gbps ultrafast communications in 6G, the utilization of sub-terahertz bands up to 300 GHz band, which can utilize a dramatically wider signal bandwidth compared to 5G, is examined. We have developed a 6G simulator to clarify the feasibility of 100 Gbps ultrafast communications by utilizing the sub-terahertz band for the system. Existing 6G simulators can evaluate throughput when using the 100 GHz band in prepared scenarios such as shopping malls and factory, but not in specific real environments. To calculate throughput with high accuracy, it is necessary to estimate propagation characteristics with high accuracy in the real environment, and recently, raytrace simulation using a polygon model of a structure generated from point cloud data acquired in the evaluation environment has attracted attention as a highly accurate estimation method of propagation characteristics. Therefore, in order to evaluate the system performance in the real environment, we developed an advanced 6G simulator that can simulate and visualize the throughput based on the propagation characteristics obtained from raytrace simulation with a real environment model using point cloud data. In this paper, we describe an overview of the functions of this simulator and an example of performance evaluation.

One of the requirements for 6G in the 2030s is ultrahigh-speed communications exceeding 100 Gbps and large-capacity communications 10 times that of 5G. In order to realize these requirements, the use of sub-THz bands above 100 GHz is being considered for 6G. In this paper, we report on the measurement and evaluation results of propagation channel characteristics in an office environment in the 160 GHz band, which is one of the frequency bands under consideration for the sub-THz band in 6G. The measurements were made in our office environment, which is approximately 50 m x 16 m in size and 2.7 m in height from ceiling to floor. In this environment, OFDM signals with a bandwidth of 2 GHz were used at 7 LoS (Line-of-Sight) points and 12 NLoS (Non-Line-of-Sight) points, for a total of 19 points. An omni-antenna was used for the transmitter (Tx) at a height of 2 m and a horn antenna was used for the reciever (Rx) at a height of 1 m. The receiving antenna was rotated and measurements were made by SISO channel soundings. The propagation channel characteristics of the delay and angle profiles were obtained at distances from 8.5 m to 35.5 m between the Tx and Rx. The measurement results showed that at the LoS point, the direct wave and the reflected wave at the ceiling and the floor were confirmed, and that at the NLoS point, the reflected wave from the glass window was received more strongly than the reflected wave from the Tx direction. Furthermore, the delay spread in this environment was compared with the CDF at the LoS and NLoS points, and the results were almost the same.

This paper proposes a method to select a set of transmission reception points (TRPs) (TRP group) to be connected for each user equipment (UE) to maximize the system throughput defined based on the generalized mean of user throughput in downlink distributed multi-input multi-output (MIMO) system. As a realistic assumption, the TRP groups are assumed to be predetermined within the system coverage. The proposed method is realized by an autonomous decentralized control among UEs and TRP groups, which is favorable for implementation in a large-scale network. In the proposed method, each TRP group independently informs all UEs of supplementary information regarding the bandwidth allocated to newly connected UEs. Based on this information, each UE calculates a metric for selecting a TRP group and feeds back the metric value to the best TRP group showing the highest metric. Finally, each TRP group determines the UE to be newly connected, thus, to be handed over from other TRP groups based on the metrics reported by multiple UEs. By repeating the above process, the selection of a TRP group for all UEs that maximizes the system throughput is realized. Computer simulations quantitatively demonstrate the effectiveness of the proposed method compared to the conventional received signal power-based method.

5th-generation mobile communication system (5G) has already launched commercially since 2019 and has been introduced worldwide. 5G attracts much attention as a promising technology that can solve future industrial and social issues. Since industrial 5G use cases need not only high-speed and large-capacity communications, but also high-reliability and low-latency communications, higher requirements and performances are required compared to communication services for consumers. This paper introduces 5G experimental trials using millimeter-wave and sub-6 GHz bands in an actual manufacturing factory, which are conducted for further utilization of 5G in the industrial use cases. The experimental trials evaluate and compare some performances of 5G wireless transmission in 28 GHz and 3.7 GHz bands. In the trials, Nokia's 5G base station (BS) equipment was installed in the manufacturing area of OMRON Kusatsu Factory, and a commercial 5G mobile station (MS) was mounted on an autonomous mobile robot (AMR) moving at a low speed through the factory. Experimental results demonstrate both advantages and disadvantages in 28 GHz and 3.7 GHz bands and the effectiveness of 28 GHz compared to 3.7 GHz in the manufacturing factory.

This paper investigates a frequency block-dependent sleep control method of base stations (BSs) based on a deep Q-network (DQN) that tracks changes in user distribution aiming at the improvement in energy efficiency both in up and downlinks. Different from the conventional centralized control, which represents the entire system with a single action value function, the proposed method assumes that each BS is an agent and each BS has its own action value function, making the DQN-based BS activation control applicable to large-scale networks. To mitigate the negative impact of the exponential increase in the total number of actions due to the individual sleep control of all the frequency blocks for up and downlinks, we propose a divided model between uplink and downlink. Furthermore, a restricted action model is employed in the proposed method to accommodate the deep sleep state. Computer simulation shows the effectiveness of the proposed method by comparing the achievable performance of the proposed method using various DQN models with those of conventional probabilistic control method.

This paper investigates a peak-to-average power ratio (PAPR) reduction method for the massive multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) with uniform planar array (UPA) antennas. To accommodate the large variation in average transmit signal power among antennas in UPA scenario, we propose a new PAPR reduction method using average transmit signal power and the null space in the MIMO channel. First, the proposed method sufficiently reduces the PAPR at the antennas with large average transmit signal power by adding the peak cancellation (PC) signal without null space projection. After that, a compensating signal that cancels the interference caused by the PC signal at the receiver is added to the transmission signal of antennas with small average signal power. Finally, the proposed method removes the residual peak signals for all transmit antennas by the peak cancellation with channel-null constraint (PCCNC) method, which we previously reported. The proposed method allows the system to use all the transmit antennas for data transmission while suppressing the PAPR aggressively. Simulation results show that when employing a nonlinear power amplifier with UPA, the proposed method increases throughput compared to the conventional PCCNC and the method that does not use the null space in the MIMO channel.

6th-generation mobile communication system (6G) is expected to realize ultra-high-speed and large-capacity communications exceeding 100 Gbps as a peak data rate and contribute to advancement of Cyber-Physical System (CPS). In order to realize the 100 Gbps ultra-high-speed communication in 6G, the utilization of so-called sub-terahertz (THz) band from 100 GHz to 300 GHz band, which can exploit drastically larger signal bandwidth than 5G, is being investigated. We have developed a 6G system-level simulator (6G simulator) to clarify the feasibility of the 100 Gbps communication as a system by utilizing the sub- THz band. In our previous study, it has been confirmed that the 6G simulator can evaluate throughput performances using the 100 GHz band in two kinds of indoor environment scenarios that simulate shopping malls and factories, and that throughput exceeding 100 Gbps can be achieved in both scenarios. However, performance evaluations of the 6G simulator in an outdoor environment that requires a relatively wide area has never been studied. This paper presents an overview of a new developed 6G simulator introducing a distributed Multiple Input Multiple Output (MIMO) technology and performance evaluation results in a new outdoor urban scenario.

To establish a simulation technology for estimating and predicting radio propagation characteristics with short calculation time and high accuracy, we proposed a novel method named color images method (CIM) and showed its high performances at low frequency band of 1 GHz. However, the performances of the CIM in high frequency bands have not been clarified. In this paper, we compare estimated path loss results of an outdoor urban environment using the CIM with the measured results in high frequency bands of sub-6 GHz band and millimeter-wave to clarify the estimation performance of the CIM. The root mean squared errors (RMSEs) of the difference between the estimated results and the measured results are small about 8 dB for all frequencies. The calculation time is extremely short.

This paper proposes a method for configuring group of transmission reception points (TRPs) for cooperative transmission and a selection method of TRP group for each user equipment (UE) to increase the system throughput in the downlink of distributed multi-input multi-output (MIMO) system. Firstly, an overlapped TRP grouping (OTG) configuration is considered, where a part of TRPs can belong to multiple adjacent TRP groups. The OTG configuration can improve the transmission quality for UEs positioned at the border of coverage areas of neighboring TRP groups. In OTG, TRPs belonging to multiple groups split their bandwidth allocation among their TRP groups. We propose a method for this TRP group bandwidth allocation at the overlapped TRP to maximize the system throughput. Then, each UE calculates a metric for TRP group selection aiming at maximizing the system throughput and selects the connected TRP group based on this metric. The proposed TRP group bandwidth allocation and connected TRP group selection are realized through autonomous distributed control, making it applicable to large-scale networks with numerous TRPs and UEs. Computer simulations demonstrate the effectiveness of the OTG configuration and the proposed TRP group selection method compared to the received signal power-based conventional method.

This paper proposes a tone reservation (TR) method using common reserved tones among users for reducing the peak-to-average power ratio (PAPR) of transmit signals in the frequency division multiple access (FDMA) uplink. The proposed method utilizes a frequency block common to frequency-multiplexed users to transmit PAPR reduction signals. In general, TR does not cause interference from PAPR reduction signals in the data transmission band at the cost of the bandwidth loss due to the use of part of the transmission band exclusively for PAPR reduction. The proposed method reduces the bandwidth loss needed to transmit the PAPR reduction signal as the number of frequency-multiplexed users increases. This is achieved by using a common bandwidth for PAPR reduction among these users. Computer simulations considering the effect of nonlinear power amplifiers show quantitatively that the proposed method improves the packet error rate (PER) compared to the clipping and filtering (CF) based PAPR reduction method, especially when the number of multiplexed users is large.

Toward further enhancement of 5G and 6G, metasurface and reconfigurable intelligent surface (RIS) have been attracting much attention thanks to cost-effective and energy-efficient features. Experimental trials have been conducted to verify their effectiveness. In consideration of a legacy deployment, it is difficult to create a coverage area at the foot of a building in millimeter-wave (mmW) bands. Thus, for mmW coverage enhancement, we propose a novel solution that combines multiple transmissive metasurfaces (TMSs) with beamforming (BF) in a base station (BS). To verify the effectiveness of our proposed solution, experimental trials using a 28 GHz-band 5G BS and a user equipment were conducted. The BF function selectively aims the beam at one of five TMSs, and then TMS re-directs the radio wave to the foot area. This paper introduces an overview of the trials and shows received power and downlink throughput measured in the trials. It is verified that our solution can enhance the coverage area to the foot and achieve much higher throughput of approximately 1 Gbps than the legacy deployment.

In this paper, we propose a computationally-efficient method for user equipment (UE)-centric access point (AP) clustering in downlink cell-free multi-input multi-output (MIMO) systems using the regularized zero-forcing (RZF)-based beamforming (BF). The proposed method selects the set of APs used for each UE based only on the average path gain information between UEs and APs. Thus, it does not need a complex calculation of downlink BF matrix for AP clustering. Furthermore, we propose the AP clustering metric for the RZF-based BF, which takes into account the achievable BF gain and spatial interference levels for two extremes cases of RZF that is pure ZF and maximum ratio combining (MRC). This is achieved by utilizing the average path gain information regarding not only the target UE but also its neighbor UEs. This differentiates the proposed method from the conventional ones, and the proposed method can improve the system-level throughput performance. We assume a realistic partial channel state information scenario, and the computer simulation results show the effectiveness of the proposed method compared to the conventional approaches.

For 5G massive MIMO systems, hybrid beam-forming (HB) has been investigated thoroughly because it is composed of analog beamforming (AB) and digital beamforming (DB), and can reduce the number of baseband and RF circuits drastically. However, HB still has a major problem of pilot contamination and thus suffers from serious degradation of the channel estimation accuracy, when the same pilot signals are shared among multiple user terminals. To cope with this problem under time-varying channel conditions, we propose a method for adaptively predicting downlink (DL) channels while mitigating the pilot contamination. Considering time division duplex (TDD) multiuser MIMO communications, the proposed scheme firstly estimates an average angle of arrival (AoA) for each user’s cluster by applying multiple signal classification (MUSIC) to the received (Rx) pilot signals, which can reconstruct the channel frequency responses of the interfering users and then extracts the desired channel frequency responses from the combined ones. Since the desired channel frequency responses can be considerably extracted, the proposed scheme coarsely compensates for time-variation of the Rx signal by predicting the Doppler effect of each cluster. The residual time-variation can be tracked and removed using detected signals of all the streams in a decision-directed manner. Finally, the estimated total time-variation is used to predict the DL channel frequency response. Computer simulations demonstrate that the proposed scheme can accurately track the time-varying channels while mitigating the pilot contamination.

To establish a simulation technology for estimating and predicting radio propagation characteristics with short calculation time and high accuracy, we have proposed a novel color images method (CIM) based on processing colors images of walls of buildings which are seen from a transmission and reception points. However, the estimation performance of the proposed CIM has not been sufficiently clarified. In this paper, we compare the CIM with Wireless InSite (WI) employing a conventional ray tracing method (RTM) in terms of percentage of estimable reception points, estimation accuracy, and calculation time. Comparison results show that for the same percentage of estimable reception points, the average value and root mean squared error (RMSE) of path loss estimation errors of the CIM are -0.8 dB and 8.3 dB, respectively, and much smaller than those of the WI with 15.9 dB and 21.7 dB. Moreover, the calculation time of the CIM is 0.0089 times shorter than that of the WI in case of changing conditions of the transmission point.

The sixth generation (6G) wireless communication system requires extreme capacity and coverage, which can be achieved through the introduction of higher frequency and ultra-large-scale reconfigurable intelligent surface (RIS) array. This also enables the expansion of the near-field range to tens or even hundreds of meters. However, near-field beamforming presents different challenges than far-field beamforming, as it involves two parameters (angle and distance) rather than just angle. This requires a larger number of codewords for near-field beams and frequent beam switching due to the narrow beamwidth. To address these challenges, we propose an enhancement to the ring-type codebook (RTC) using non-uniform quantization of focal length and beamwidth control methods. Our proposed methods effectively reduce the beam management overhead and SNR fluctuation by 34% and 4dB respectively, while also improving the robustness of the beams. Overall, our proposed enhancement to the RTC can significantly improve the performance of near-field beamforming in the 6G wireless communication systems, enabling them to satisfy the extreme capacity and coverage requirements of the future.

Full-duplex communications are potential candidate for increasing the spectral efficiency for 5G mobile systems and beyond, which can transmit and receive simultaneously on the same carrier frequency. The key challenge is the self-interference (SI) caused by the coupling between the transmitter (TX) and its own receiver (RX). This paper proposes a new type of SI canceller based on the channel estimation between TX and its own RX through the use of a specific demodulation reference signal (DMRS) which is special type of physical layer signal. In this paper, we first show the configuration of the proposed SI canceller using the DMRS. Then, we demonstrate the BER performance of OFDM-based QPSK and QAM signals under static and Rician fading channels for SI, using link-level simulations. The simulation results indicate that the proposed SI canceller can sufficiently improve the BERs for OFDM-based QPSK and QAM signals.

The 6th generation mobile networks require large bandwidth on sub-Terahertz bands to provide more than 100 Gbps throughput. To provide enough beamforming gain to combat the path loss on high-frequency bands, large-scale antenna arrays should be used, which generate narrow and directive beams. It may require hundreds or thousands of beams to cover a cell and introduce challenges for the beam alignment during the initial access stage. As a result, both the overhead and latency of beam sweeping may become unacceptable. We propose the Hash-based beam alignment method to address this problem, including a Hash-based beam transmission scheme for the synchronization signal block (SSB) transmissions and a corresponding UE beam pair selection method. It utilizes multiple transceivers to transmit multiple beams simultaneously, where the beam order follows a pattern generated by a Hash function. Considering UE may only receive discrete SSBs randomly, we design a weighted voting algorithm for UE to fast select the optimal beam pair for the following data transmissions. Simulations demonstrate that the proposed methods can reduce about 50% of the beam alignment latency or find a better beam pair with about 3 dB higher beamforming gain given a limited searching time.

Reconfigurable intelligent surface (RIS) is a research hotspot for the fifth generation evolution (5GE) and the sixth generation (6G) wireless communication systems to extend high-frequency coverage. Due to the increase of the carrier frequency of the wireless communication system and the large scale of the RIS array, the near-field region is significantly expanded. Using the plane wave assumption-based discrete Fourier transform (DFT) codebook for the RIS near-field beamforming may result in a large signal-to-noise ratio (SNR) loss. Furthermore, for practical RIS with a large number of elements, it is more cost-effective to implement only discrete phase shifts with a small number of control bits for each element, for example, 1-bit for two-level (0 or$\pi$) phase shifts, which generates the mirror grating lobe with the same power as the main lobe and results in power leakage. In this paper, a ring-type codebook is proposed based on the Fresnel principle, which can achieve SNR enhancement as well as mirror grating lobe suppression for 1-bit beamforming in near-field region. Compared with DFT codebook, the proposed codebook improves the average throughput by 14% and achieves a main grating lobe suppression ratio of about 17.9dB.

Beamformer design for large-dimension reconfigurable intelligent surface (RIS) is investigated. The challenge arises from the expanded near-field of RIS where its beam-forming gain is, in general, distance-dependent due to near-field diffraction. To gain more insights into this effect, we first analyze the performance of conventional collimated beamforming using Fresnel integrals and compare it with that of focused beamforming. Then, a two-step beamforming scheme is proposed. The principle of the proposed scheme is to divide the RIS into multiple subarrays followed by steering the sub-beams of the individual subarrays to focus. Providing proper subarray size, the proposed scheme retains the low complexity of collimated beamforming and achieves a near-optimal performance, as shown both analytically and numerically.

Toward further enhancement of the fifth generation (5G) mobile communication system, referred to as 5G Evolution (5GE), more stable high data-rate transmission should be required even in high-mobility environments such as high-speed trains. By using 28 GHz-band experimental equipment with beamforming and beam tracking, a 5GE experimental trial had been conducted in the actual high-speed bullet train running at a speed of 283 km/h. However, it is important to evaluate transmission performances and robustness against the higher mobile speed in sub-6 GHz band in addition to millimeter-wave band. Thus, we conducted 4.85 GHz-band 5GE experimental trial aboard Shinkansen test train running at 360 km/h, which is the fastest experiment trial in Japan. This trial deployed two base station antennas along the Shinkansen railways, and a mobile station was mounted in the rolling stock of the Shinkansen test train. This paper introduces 4.85 GHz-band 5GE experimental equipment, an overview of the experimental trial, and measured transmission performances.

5G introduces a beamforming (BF) technology using massive MIMO. In further enhancement of 5G (5G evolution: 5GE), which will be actualized around 2025, performance improvement by digital precoding for hybrid BF and fully digital BF technologies can be expected. In addition, a cooperation technology of multiple base stations (BSs) is required to provide high data rate communications in millimeter wave bands, even when mobile stations (MSs) move at high speed. This paper describes 28 GHz-band experimental equipment that implemented a novel BS cooperation technology combined with digital BF. In outdoor experimental trials, measured downlink throughput is clarified, when three BS antennas are deployed in two kinds of directions and one MS moves at 90 km/h.

This paper proposes a novel estimation method of radio propagation characteristics in outdoor urban environments based on color images. The proposed method considers only direct waves and single scattered waves. In order to detect single scattering walls, each wall of buildings in an evaluation area is assigned to a different RGB color. We create two types of color images that are viewed from a transmission point and reception points. By analyzing and comparing the colors of these images, the visible colors from both transmission and reception sides corresponding to the single scattering walls are detected all at once. The numbers of pixels on the scattering walls with the visible colors are used to calculate a received power of the single scattering waves. Comparison with measurement results of path loss shows high estimation accuracy of the proposed method with root mean squared error of 7.9 dB, and its calculation time is extremely short.

In order to further utilize the high frequency band represented by millimeter wave, it is one of the issues to construct an economical indoor area. One of the solutions is to efficiently bring the radio waves from outdoor base stations to indoors. In this study, we have confirmed that the indoor radio wave strength can be significantly improved by using three elemental technologies: (1) radio wave transmission structure design technology (FSS technology), (2) metasurface technology (radio wave aggregation technology), and (3) relay stations.

5G introduces massive multiple-input multiple-output (M-MIMO) with analog beamforming (BF) in millimeter-wave (mmW) bands. 5G Evolution (5GE), which will be actualized around 2025, is expected to replace analog BF with hybrid or fully digital BF that can improve mmW-band downlink and uplink transmission performances. In addition, base station (BS) cooperation technologies are required for 5GE to provide stable and reliable cell-edge transmission in the mmW bands, even when mobile stations (MSs) travel at high speed. This paper introduces our developed 28 GHz-band trial equipment employing our proposed BS cooperation technologies with digital BF and distributed MIMO. It also shows evaluation results of outdoor experimental trials at two MSs' velocity of 90 km/h. Three M-MIMO antennas are deployed in the trials, and the BS cooperation selects one M-MIMO antenna instantly or performs BS cooperative transmission. It is verified that the BS cooperation can improve areal throughput and maintain high throughput in a wider area than non-BS cooperation.

To enhance 5G (5G Evolution) further, more stable high bit-rate transmission using high SHF bands including 28 GHz or EHF bands is provided even in high-mobility environments such as in high-speed trains. However, in future 5G use cases in the Shinkansen, a dynamic changes in the beam direction and a greater Doppler frequency shift may cause performance degradation in 28 GHz-band transmission. Thus, we conduct a 28-GHz band 5G experimental trial on an actual Shinkansen that runs at a maximum speed of 283 km/h. This paper introduces the world's first 28 GHz band 5G experimental system used in a 5G experimental trial in the Shinkansen travelling over 200 km/h. We detail the experimental configuration in which three base stations are deployed along the Tokaido Shinkansen railway with a mobile station located in the train. This paper also presents the downlink transmission performance measured in such a high-mobility environment. The experimental trial can achieve throughput exceeding 1.0 Gbps and consecutive handovers among the three BSs.

Commercial services in 5G have already started all over the world. The initial 5G introduces analog beamforming (BF) by Massive MIMO (M-MIMO) to compensate for the larger path-loss in millimeter-wave (mmW) bands such as 28 GHz. In 5G Evolution (5GE) that is expected to be realized for around 2025, the analog BF will be replaced with hybrid or fully digital BF to improve mmW-band downlink/uplink transmission performances. In addition, new radio access technologies are required for 5GE to provide stable and reliable cell-edge transmission in the mmW bands, even when mobile stations (MSs) travel in high mobility. This paper proposes two mmW base station cooperation technologies that are inter-baseband unit (inter-BBU) and intra-BBU cooperation for the hybrid BF. The former exploits two M-MIMO antennas in two BBUs connected to one central unit by limited-bandwidth fronthaul, and the latter cooperates two M-MIMO antennas connected to one BBU with Doppler frequency shift compensation. Computer simulations verify that the intra-BBU cooperation can achieve an excellent transmission performance in cases of two and four MSs moving at a velocity of 90 km/h.

The fifth-generation (5G) mobile communications system initially introduced massive multiple-input multiple-output (M-MIMO) with analog beamforming (BF) to compensate for the larger path-loss in millimeter-wave (mmW) bands. 5G Evolution (5GE), which should be actualized around 2025, will replace analog BF with hybrid or fully digital BF that improves the mmW-band downlink/uplink transmission performance. In addition, 5GE requires base station (BS) cooperation technologies to provide stable and reliable cell-edge transmission in the mmW bands, even when mobile stations (MSs) travel at high speed. This paper introduces our developed 28-GHz-band trial equipment employing a novel BS cooperation technology with digital BF to track the MS mobility. We present evaluation results of outdoor experimental trials at a MS velocity of 90 km/h. Two M-MIMO antennas are deployed in the trials, and the BS cooperation selects one M-MIMO antenna based on the maximum received power criterion of a reference signal transmitted by the MS. The trials clarify that the BS cooperation maintains the maximum throughput in a wider area than non-BS cooperation at 90 km/h.

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.

Fifth-generation mobile communication systems (5G) require significantly higher system capacity than 4G in order to accommodate rapidly increasing mobile data traffic. Cell densification is considered to be one of the most effective ways to increase the system capacity. However, severe inter-cell interference degrades the system capacity due to the increase of the line-of-sight (LOS) environment between the transmission point (TP) and the user equipment (UE). We have proposed large-scale coordinated multi-user multiple-input multiple-output (LSC-MU-MIMO), which is the combination of MU-MIMO and joint transmission from all the TPs connected to a centralized baseband unit, to increase the system capacity in dense cell environments. In this paper, we introduce our recent results of indoor experimental trials with up to sixteen TPs coordination and show that the system capacity is almost proportional to the number of TPs with LSC-MU-MIMO. We also study how to locate TPs in order to exploit potential performance of LSC-MU-MIMO. Both simulation and experimental results show that the throughput is significantly improved by deploying TPs to make at least a channel of TP LOS with a high probability. Therefore TPs are preferable to be located that UEs have at least one TP with LOS condition.

In this paper, we make a summary report of our 5G R&D activities for “High Capacity Technologies with Ultra High-Density Multi-Band and Multi-Access Layered Cells”, funded by the Ministry of Internal Affairs and Communications (MIC) in Japan. This national project consists of three research subjects: “Ultra high-density distributed antenna systems”, “Optical access technologies accommodating highly densified small cells” and “Multi-RAT system technologies for multi-band and multi-access layered cells”. We describe the research and development result for each subject.

The fifth-generation (5G) mobile communication system introduces the use of high frequency bands with very wide bandwidth for realizing super high bit rate more than 20 Gbps. To use such high frequency bands, Massive MIMO technologies by hybrid beamforming is a promising technology in terms of power consumption and implementation cost. Experimental trials of hybrid beamforming in 28 GHz band were already conducted in an indoor anechoic chamber and in an outdoor light-of-sight (LOS) environment. However, as MIMO takes advantage of multipath, it is important to evaluate the system performance in multipath-rich environment. It is also important to evaluate the system performance in non LOS (NLOS) environment, because high straightness of high SHF band can degrade its performance. To clarify these actual impacts, in multipath-rich room with some obstacles, we conducted two indoor downlink experimental trials: SU-MIMO and 8-user MU-MIMO at 28 GHz frequency band with 500 MHz bandwidth. The results of SU-MIMO experiments show more than 20 Gbps user throughput was achieved in the multipath-rich environment. We also confirmed that strong reflector improves the throughput when a user is in the NLOS environment. MU-MIMO experiments show the hybrid beamforming can improve both the system throughput and user fairness compared to analog beamforming.

In this paper, we summarize our 5G R&D achievements for "High-Data-Rate and Low-Power-Consumption Radio Access Technologies with Ultra Higher-Frequency-Band and Wider-Bandwidth Massive MIMO", funded by the Ministry of Internal Affairs and Communications (MIC), Japan. This Japanese national project consists of three research subjects: "Massive MIMO (massive-element antenna) and beam control technologies for low SHF bands", "Wideband Massive MIMO and beam control technologies for high SHF bands" and "Discovery technologies for terminals with ultra-low power consumption". We describe detailed research contents and achievements of our promoting 5G R&D project.

This paper focuses on multi-user detection in uplink Massive multiple-input multiple-output (MIMO) systems. As a low computational complexity signal detection algorithm, matched filter (MF)-based Gaussian belief propagation (MF-GaBP) using adaptively scaled belief (ASB) has been proposed. In addition, to suppress the negative impact of spatial correlation, an MMSE detector is introduced as the prior stage processing of MF-GaBP. Through computer simulations, this algorithm exhibits high performance even in high- spatially loaded Massive MIMO scenarios. However, in actual radio environments, there is a possibility to observe an extremely high channel correlation among some antenna elements or some users due to co-located polarization antenna elements or closely deployed mobile stations (MS). The highly correlated channel results in the performance degradation of MF-GaBP using ASB because of the low reliability of initial beliefs. This paper aims to verify the performances of MF- GaBP in the actual radio environments. The predetermined parameters in MF- GaBP are also adjusted according to actual environments for improving the convergence property of the iterative detection. To obtain propagation channel data for performance evaluation in the actual environments, an indoor experimental trial using Massive MMO equipment with 32 antenna elements was carried out under a condition that 16 MSs are closely deployed. Computer simulations using the obtained propagation channel data validate that simply applying the MMSE detector before MF-GaBP with ASB suppress an error floor in the bit error rate (BER) performance even in highly correlated channel.

Massive multiple-input multiple-output (MIMO) will be employed in the 5th generation mobile communications (5G). To implement the massive MIMO, hybrid beamforming (HB) is one of the most promising techniques, because HB is composed of the analog beamforming (AB) and the digital beamforming (DB) and thus can decrease the number of baseband and radio frequency (RF) circuits significantly. Furthermore, HB can reduce a much larger amount of cost and power consumption than the full DB (FDB) configuration, which requires one baseband and RF circuit for each antenna element. This paper proposes a scheme to estimate both phases for phase rotators in AB and a precoding matrix for DB so that HB can approximate the block diagonalization (BD), which can nullify interference from the undesired users and form efficient transmit beams. For time division duplex (TDD) multiuser MIMO communications, the proposed scheme estimates the FDB channel matrix in the uplink (UL) and the phase for the phase rotators of AB in the downlink (DL). After obtaining an equivalent channel matrix from both the estimated FDB channel matrix and the AB precoding matrix, the scheme estimates the DB precoding matrix in DL from the equivalent channel matrix so that HB can approximate BD in DL. Computer simulations demonstrate that HB employing the proposed scheme can achieve DL average bit error rate (BER) performances that are little inferior to those of FDB.

The fifth-generation (5G) mobile communication system has attracted much attention to provide various kinds of services and applications for 2020 and later. To verify the feasibility of high presence public viewing service, the world's first maritime 5G experiment is conducted in Windsurfing World Cup 2018. In the experiment, 5G is used for 4K video transmission from a ship to public viewing area at land. This paper briefly reports that downlink and uplink maritime transmission performance of 5G employing 28 GHz band Massive MIMO (Multiple Input Multiple Output) and the success of the public viewing trial.

Massive MIMO using beamforming (BF) is a promising technique for fifth-generation (5G) mobile communication system. In low SHF bands such as 3-6 GHz, fully digital Massive MIMO with digital BF can be easily introduced. Although it is expected that earlier 5G commercial services will launch in dense urban areas, 4.5 GHz-band 5G experimental trials have never been performed in such areas. Therefore, this paper presents some results of recent 4.5 GHz-band 5G experimental trials using digital-BF Massive MIMO in the dense urban area. It shows both coverage with single user and system throughput with four-user that are closely located in Tokyo, Japan.

Field trials of the fifth-generation (5G) mobile communication system using 28 GHz band at which almost 1-GHz band will be available have been performed all over the world. To realize large coverage at such a high frequency band, beamforming by Massive MIMO (Multiple Input Multiple Output) is necessary to compensate large path loss. Furthermore, beam tracking which adaptively changes beam direction according to user location is an important function to support user mobility. Authors already conducted high mobility trials at a velocity of 150 km/h using 28 GHz-band 5G prototypes implementing beam tracking. To support ultra-high mobility on a bullet train, in April 2018, we conducted outdoor trials using the specially customized vehicle with the velocity of up to 300 km/h. This paper reveals the latest downlink performance results of the trial.

This paper describes our activities related to massive multi input and multi output (MIMO) for use in 5G radio access network (RAN). To achieve capacity enhancement and low power consumption, Massive MIMO has drawn attention as a key technology. Our research focuses on utilizing Massive MIMO in the low super-high-frequency (SHF) band, which is expected to be used for 5G commercial bands relatively soon, and on enhancing spectrum efficiency by spatial multiplexing with a low-SHF-band Massive MIMO antenna. The present work introduces a prototype low-SHF-band Centralized-RAN (C-RAN) Massive MIMO System including a low-SHF-band Massive MIMO active antenna system (AAS) that has 64 antenna elements, reports on outdoor downlink (DL) multi-user (MU) MIMO and inter access point (AP) coordination evaluations with the prototype system, and presents the results of the evaluations. The results demonstrate that the prototype low-SHF-band C-RAN Massive MIMO system can increase RAN system capacity and improve cell edge performance.

Recently, the fifth-generation mobile communications system (5G) in higher frequency bands has been extensively studied. Massive MIMO is one of the promising technologies to dramatically increase spectral efficiency. The authors have been engaged on R&D into wideband massive MIMO and beam control technologies for high SHF bands. Recently we have developed a massive MIMO system capable of supporting 16-beam transmission at a 28 GHz band and have conducted its experimental trial. In this paper, we present recent indoor experimental trial results using 16-beam spatial multiplexing at 28 GHz with 500 MHz bandwidth. For a single-user MIMO scenario, it has been verified that the downlink throughput of 25.5 Gbps is reached in the anechoic chamber.

This paper describes our activities related to Massive Multi Input and Multi Output (MIMO) for use in a 5G radio access network (RAN). To achieve capacity enhancement and low power consumption, Massive MIMO has drawn attention as a key technology. Our research focuses on utilizing Massive MIMO in the low super-high-frequency (SHF) band, which is expected to be used for 5G commercial bands relatively soon, and on enhancing spectrum efficiency by spatial multiplexing with a low-SHF-band Massive MIMO antenna. The present work introduces a prototype low-SHF-band Centralized-RAN (C-RAN) Massive MIMO System including a low-SHF-band Massive MIMO Active Antenna System (AAS) that has 64 antenna elements, reports on outdoor and indoor downlink (DL) multi-user (MU) MIMO evaluations with the prototype system in both cases of 1 AAS unit and 2 AAS units, and presents the results of the evaluations. From the results, we confirmed that the prototype low-SHF-band C-RAN Massive MIMO system can optimize the performance with the minimum configuration of AAS units by changing the configuration appropriately depending on the environment.

This paper presents field experiments on the downlink throughput performance of a 5G centimeter wave radio access system employing higher rank MIMO in a shopping mall environment and a long corridor environment. The shopping mall has complex reflection paths within its non-line-of-sight areas. The long corridor environment exhibits line-of-sight propagation with many reflection paths. The results show that the maximum throughput of more than 4.8 Gbps in downlink is achieved with 400 MHz spectrum bandwidth in 20% of the course in the shopping mall environment. Moreover, the maximum Rank-8 transmission of more than 9.6 Gbps in downlink can be achieved in the long corridor environment.

The fifth-generation (5G) mobile communication system has attracted much attention to provide various kinds of mobile services and applications for 2020 and later. 28 GHz band is one of the most potent bands for 5G since super wide bandwidth of almost 1 GHz will be available. In the high frequency band, beamforming is mandatory to compensate large path loss. Hence beam tracking mechanism is important to support user mobility, especially users travelling with high speed car or train. In order to clarify the feasibility of the 28 GHz band for mobile communication, we have conducted super high mobility experimental trial targeting up to 150 km/h velocity, by using 5G Massive MIMO (Multiple Input Multiple Output) prototype with beam tracking mechanism. This paper reports results from the super high mobility experiment held in Fuji Speedway, Japan.

The fifth-generation (5G) radio access network will introduce the use of higher frequency bands with wider bandwidth and Massive MIMO technology for realizing super high bit rate and higher system capacity of more than 10 Gbps. In Massive MIMO using higher frequency bands, from the point of view of realization, as a novel method for hybrid beamforming (BF) that combines analog BF with digital precoding, joint processing of fixed analog BF and channel state information (CSI)-based precoding (FBCP) has been proposed. On the other hand, the user selection and rank adaptation algorithm are essential techniques for multi-user MIMO because the transmission performance of multi-user MIMO suffers from inter-user interference. However, in multi-user Massive MIMO, the computational complexity for user selection becomes large because MIMO channel matrix is massive. In this paper, we propose a 2-step user selection algorithm for high SHF wideband multi-user Massive MIMO using FBCP. It is shown by computer simulation that the proposed 2-step user selection algorithm can reduce the computational complexity by about 55 % while achieving almost the same throughput performance compared to conventional user selection.

In order to tackle rapidly increasing traffic, distributed Massive MIMO (DM-MIMO) has been proposed for fifth-generation (5G) mobile communication systems. DM-MIMO coordinates lots of distributed transmission points (TPs) that are located in ultra-high density (UHD) and that use an appropriate number of antenna elements. Furthermore, DM-MIMO with UHD-TPs can create user-centric virtual cells corresponding to user mobility. To deploy TPs for DM-MIMO, some key parameters such as the number of the distributed TPs, the number of antenna elements for each TP, and proper distance between TPs should be determined. Conventional works have shown such key parameters for DM-MIMO with UHD-TPs by computer simulations. However, these parameters have not been evaluated in actual deployment environments. We measure low-SHF- band (4.5 GHz) radio propagation channel in indoor and outdoor experiments of DM-MIMO with UHD-TPs. Then, by exploiting the experimental results, the appropriate numbers of antenna elements based on maximum system throughput criterion are shown for 5G DM-MIMO by computer simulations.

In fifth-generation mobile communication systems (5G), it is necessary to significantly increase system capacity compared with 4G in order to accommodate rapidly increasing mobile data traffic. Cell densification has been considered an effective way to increase system capacity. However, severe intercell interference degrades the system capacity due to the increase of the line-of-sight environment between the transmission point (TP) and user equipment (UE). We propose large-scale coordinated multi-user multiple-input multiple-output (LSC-MU-MIMO), which combines joint transmission from all the TPs connected to a centralized baseband unit and MU-MIMO. We have investigated the downlink performance of LSC-MU-MIMO via both computer simulation and field experiments and showed that LSC-MU-MIMO can significantly reduce severe inter-TP interference and improve the system capacity of high-density small cells. In this paper, we introduce our recent field experiment results where reception antennas of UEs are mounted on a vehicle driving at speeds of 5 km/h to 40 km/h. We also compare distributed TP deployment with localized TP deployment when fixing the total number of transmission antennas.

This paper describes our activities related to massive multiple-input multiple-output (MIMO) for use in 5G mobile communication networks. To achieve capacity enhancement and low power consumption for 5G wireless access systems, massive MIMO has drawn attention as one of the key technologies. Our research focuses on the utilization of massive MIMO in the low super-high-frequency (SHF) band, which is expected to be used for 5G commercial bands relatively soon, and on spectrum efficiency enhancement by spatial multiplexing with a low SHF band massive MIMO antenna. The present work introduces a prototype low SHF band massive MIMO antenna (including 64 antenna elements per unit), reports on indoor DL multi-user (MU) MIMO field trials with the massive MIMO antenna, and presents the results of the field trials. From the field trials, we confirmed the validity of the prototype low SHF band massive MIMO antenna to achieve high capacity.

This paper presents some results of experimental trial in high mobility environment for the 5th generation (5G) mobile communication systems using 28 GHz band. In order to tackle rapidly increasing traffic for 2020 and beyond, new radio access network for the 5G mobile communication systems will introduce the use of higher-frequency bands such as spectra higher than 10 GHz to achieve higher capacity and super high bit rate transmission of several tens of Gbps. The target of this experimental trial is to evaluate the effectiveness of using 28 GHz band with super-wide bandwidth of 800 MHz for 5G mobile communication systems. To compensate large path-loss in higher frequency, the beamforming based on Massive multiple-input multiple-output (MIMO) is one of promising techniques and can be combined with spatial multiplexing of multiple data streams to achieve much higher capacity. In addition, to support the mobility of mobile station (MS), beam tracking technique is important. In this paper, we show some results of the outdoor experiment of Massive MIMO beamforming combined with spatial multiplexing in high mobility environment with MS speed of up to 60 km/h by using prototype system with base station (BS) having 96-element array antenna and MS having smartphone-shape antenna with 8 elements. We also show that maximum throughput of 3.77 Gbps can be achieved with MS speed of 60 km/h by using beamforming with 2-stream multiplexing and beam tracking.

In the fifth generation mobile communications system (5G), it is expected to use millimeter wave (mmW) radio access with very wide frequency bandwidths of more than 1 GHz. To achieve good coverage and availability, high gain antennas or arrays are essential in order to compensate for the higher propagation loss experienced at mmW frequencies relative to current cellular bands. This paper presents the beamtracking performance and throughput performance of a 5G mmW Proof-of-Concept (PoC) system in field experiments conducted at up to 20 km/h vehicular speeds in outdoor line-of-sight (LOS) conditions. In addition, this paper recomposes the frame structure for low latency and evaluates latency performance in the vehicular experiments.

As the demand for higher transmission rates and spectral efficiency is steadily increasing, the research and development of novel mobile communication systems has gained momentum. This paper focuses on providing a comprehensive survey of research and development activities on fifth generation mobile communication systems reported in IEICE technical committee on radio communication systems. We try to survey a vast area of wireless communication systems and the developments that led to future 5G systems.

This paper presents the overview of the experimental trial for the 5th generation (5G) mobile communication systems based on the collaboration between Samsung Electronics and NTT DOCOMO. In order to tackle rapidly increasing traffic for 2020 and beyond, new radio access network for the 5G mobile communication systems will introduce the use of higher-frequency bands such as spectra higher than 10 GHz to achieve higher capacity and super high bit rate transmission of several tens of Gbps. The target of this experimental trial is to evaluate the effectiveness of using 28-GHz band with super-wide bandwidth of 800 MHz for 5G mobile communication systems. Firstly, the technical concept of this trial and the prototype design of the experimental system are presented. Massive multiple-input multiple-output (MIMO) technique is attracting attention to compensate for large path-loss in higher frequency bands. In this experimental trial, the beamforming (BF) based on Massive MIMO is introduced to the transmission using 28-GHz band. Some results of the indoor and outdoor experiments are also shown to evaluate the feasibility of using 28-GHz band with 800-MHz bandwidth.

5G radio access networks achieve large communication capacity with small control signal overheads by combining low frequency bands in macro cells with high frequency bands in small add-on cells. This paper addresses the challenges and solutions for 5G group mobility where rapidly moving platforms carry several different user devices. Entrance capacity and control signal load are also studied by link level and system level evaluations to verify the effectiveness of moving cell, an attractive possible solution.

The performance of 30 Gbps super high bit rate mobile communications is evaluated by computer simulation using channel data collected in 11 GHz band 24×24 MIMO outdoor propagation experiments, and the feasibility of 11 GHz band 30 Gbps transmission is verified. To achieve the super high bit rate mobile communications, 10 Gbps transmission using 11 GHz band 8×16 MIMO has been verified in outdoor transmission experiments. In addition, channel measurement and analysis have been conducted in 11 GHz band 24×24 MIMO radio propagation experiments. Although 24×24 MIMO transmission is expected to achieve a bit rate exceeding 30 Gbps, transmission experiments have not yet been performed due to the hardware limitations. In this paper, computer simulations based on 24×24 MIMO-OFDM eigenmode transmission are conducted by utilizing channel data measured using an 11 GHz band 24×24 MIMO channel sounder. This paper shows that throughput exceeding 30 Gbps is achieved in 11 GHz band mobile environments, and clarifies the requirements for the average signal-to-noise ratio, channel conditions, and accuracy of channel state information to achieve 30 Gbps throughput over a real 11 GHz band 24×24 MIMO channel.

In order to tackle rapidly increasing traffic, the 5th generation (5G) mobile communication system will introduce small cells using higher frequency bands with wider bandwidth to achieve super high bit rate transmission of several tens Gbps. Massive MIMO beamforming (BF) is one of promising technologies to compensate for larger path-loss in the higher frequency bands. Joint analog fixed BF and digital precoding has been proposed to reduce the cost of a Massive MIMO transceiver. However, the conventional scheme assumes the transmission of a few streams using well-known codebook-based precoding as the digital precoding, and both a selection method of the fixed BF weights and a digital precoder design, which are suitable for the super high bit rate transmission using multiple streams, have not been studied. This paper proposes a joint fixed BF and CSI-based precoding (called FBCP) scheme for the 5G Massive MIMO systems. FBCP first selects the analog fixed BF weights based on maximum total received power criterion, and then it calculates eigenmode (EM) precoding matrix exploiting CSI. This paper targets a 5G system achieving 20 Gbps in 20 GHz band as one example, and throughput performances of the proposed FBCP are evaluated by link level simulation and compared with those of the fixed BF and those of the EM precoding.

This paper proposes an approximate block diagonalization (BD) method that does not require feedback of channel state information (CSI) by using the second-order statistics of channels for multiuser MIMO mobile communications. On the assumption that an incidence angle (angle-of-arrival) distribution in the uplink channel is the same as an angle-of-departure distribution in the downlink channel, the proposed scheme estimates the autocorrelation matrix of the downlink channel from the average and variance of incidence angles in the uplink. Since the proposed scheme uses the estimate of the autocorrelation matrix of the downlink channel, it performs eigenvalue decomposition of the estimated autocorrelation matrix whereas the conventional BD precoding performs singular value decomposition of the channel matrix. To verify the effectiveness of the proposed scheme, computer simulations are conducted. It is shown that the proposed scheme, which does not need CSI feedback, is little inferior in the channel capacity to the conventional BD precoding that requires CSI feedback.

This paper presents detailed results of 11 GHz band outdoor transmission experiments employing 8 × 16 MIMO-OFDM for 10 Gbps super high bit rate mobile communications. Super high bit rate mobile communications have been studied for future mobile communications, and to verify the feasibility, outdoor transmission experiments were performed. A mobile station (MS) with 8 transmitter antennas traveled along a measurement course while a base station (BS) with 16 receiver antennas stored the received signals, and then the throughput performance levels were evaluated by demodulating them in an off-line mode. This paper introduces specifications and a hardware configuration for the 11 GHz band 8×16 MIMO-OFDM experimental system with a signal bandwidth of 400 MHz. In addition, it presents the SNR distribution and delay spread that are calculated from the received signals in the measurement course used in the outdoor experiments. The experimental results show that for 11.8 Gbps transmission with 8 streams, 64QAM, and the coding rate of 3/4, throughput of greater than 10 Gbps is achieved in an 11 GHz band outdoor mobile environment.

Joint signal detection and channel estimation based on the expectation-maximization (EM) algorithm has been investigated for orthogonal frequency-division multiplexing (OFDM) mobile communications over fast- fading channels. In order to make the signal detection suitable for the iterative EM-based receiver with multiple antennas, this paper proposes spatial removal from the perspective of a message- passing algorithm on factor graphs. The spatial removal performs the channel estimation of a targeted antenna by using detected signals that are obtained from the received signals of all antennas other than the targeted antenna. It can avoid the repetitive use of unreliable received signals for consecutive signal detection and channel estimation. Computer simulations under fast-fading conditions demonstrate that appropriate application of the spatial removal can exploit such a removal effect and the spatial diversity, resulting in improvement of the packet error rate (PER).

To maintain good transmission performance of orthogonal frequency-division multiplexing (OFDM) mobile communications over fast-fading channels, iterative maximum a posteriori (MAP) receivers that iterate channel estimation and signal detection on the basis of the expectation-maximization (EM) algorithm have been investigated. In order to make such a MAP receiver more robust against time-variant channels, this paper derives channel estimation with subcarrier removal from the perspective of a message-passing algorithm on factor graphs. The subcarrier removal estimates the channel frequency response by using the detected signals of all subcarriers other than that of a targeted subcarrier. This modification can avoid the repetitive use of incorrectly detected signals for the channel estimation. Computer simulations under fast-fading conditions demonstrate that the EM-based MAP receiver performing the subcarrier removal can improve the packet error rate (PER) drastically.

Super high bit-rate mobile communications have been investigated to achieve more than 10 Gbps bit-rate. This paper studies an experimental system that can achieve 5 Gbps throughput by using 11 GHz band radio frequency (RF) circuits and 4×4 MIMO-OFDM with 400 MHz wide bandwidth, of which feasibility is verified. The experimental system consists of (i) 11 GHz band RF circuits, (ii) FPGA boards with 800 MHz sampling DAC or ADC, and (iii) CPU boards that perform MIMO-OFDM transmission and reception processing in offline mode. The RF circuits employ a direct conversion method. Since IQ imbalance and phase noise in the RF circuits degrade transmission performance, this paper compensates for them by digital signal processing. The experimental system evaluates the transmission performance in 5 Gbps 4×4 MIMO-OFDM employing turbo detection, and it is shown that performance degradation of the experimental system to the simulation results can be limited to less than 1 dB by the compensation method for imperfection of the RF circuits. In addition, it is clarified that the turbo detection with two iterations can improve 6.0 dB at SNR to achieve 5 Gbps throughput.

This paper proposes a low-complexity signal detection algorithm for spatially correlated multiple-input multiple-output (MIMO) channels. The proposed algorithm sets a minimum mean-square error (MMSE) detection result to a starting point, and searches for signal candidates in multi-dimensions of the noise enhancement from which the MMSE detection suffers. The multi-dimensional search is needed because the number of dominant directions of the noise enhancement is likely to be more than one over the correlated MIMO channels. To reduce computational complexity of the multi-dimensional search, the proposed algorithm limits the number of signal candidates to O(NT) where NTis the number of transmit antennas. Specifically, the signal candidates, which are unquantized, are obtained as the solution of a minimization problem under a constraint that a stream of the candidates should be equal to a constellation point. Finally, the detected signal is selected from hard decisions of both the MMSE detection result and unquantized signal candidates on the basis of the log likelihood function. For reducing the complexity of this process, the proposed algorithm decreases the number of calculations of the log likelihood functions for the quantized signal candidates. Computer simulations under a correlated MIMO channel condition demonstrate that the proposed scheme provides an excellent trade-off between BER performance and complexity, and that it is superior to conventional one-dimensional search algorithms in BER performance while requiring less complexity than that of the conventional algorithms.

This paper proposes a new coherent detector that exploits the real zero (RZ) concept for signal recovery. The detector recovers the complex envelope consisting of in-phase (I) and quadrature (Q) components of linear quadrature amplitude modulation (QAM). The received signal before amplification is processed by RZ conversion, after which the converted signal can be amplified by a severely distorted amplifier. The RZ conversion combines the modulated received signal with a sinusoidal wave whose frequency is slightly outside the modulated frequency band, and the resultant signal is amplified to a saturated level. Next, the IQ detection applies the RZ concept to the IQ signal recovery. It utilizes a time to digital converter (TDC) for RZ timing estimation. A linear interpolation type TDC is introduced. Computer simulations demonstrate the effectiveness of the proposed detector. It is shown that error vector magnitude (EVM) of the recovered complex envelope is around -30 to -40 dB for a narrowband signal.

This paper proposes a precoding scheme for downlink multiuser MIMO-OFDM systems. The proposed precoding employs the minimum bit error rate (BER) criterion and estimates precoding matrices to minimize total BER of mobile stations by the steepest descent algorithm. As the cost function of the proposed scheme, an upper bound of the total BER is derived from the pairwise error probability (PEP) and is averaged with respect to channel state information (CSI) errors. Thus, the proposed scheme can be robust to imperfect CSI. Computer simulations under a frequency-selective fading condition demonstrate that the proposed precoder is more robust to the CSI errors than the zero-forcing (ZF) precoder, and that it is superior in BER to the conventional scheme.

This paper proposes an iterative receiver employing multiuser detection (MUD) and channel estimation for uplink MIMO-OFDM interleave-division multiple access (IDMA). IDMA can perform low-complexity MUD by employing a low-rate channel code and an individual interleaver for each user, and is expected to improve spectrum efficiency in mobile communications. However, an excellent channel estimator has not been proposed in the uplink MIMO-OFDM IDMA. This paper proposes the iterative receiver that repeats a joint processing of MUD and channel estimation by exploiting log-likelihood ratios of coded bits (bit LLRs) which channel decoders of all users provide. The receiver employs soft decision-directed channel estimation (SDCE) using the bit LLRs during data symbols, in addition to initial channel estimation (ICE) using known preambles. To improve an accuracy of the channel estimation, it performs tap selection (TS) that extracts only dominant paths from a channel impulse response estimated by ICE or SDCE. Moreover, this paper evaluates transmission performances of the proposed receiver, and shows that the receiver can achieve an excellent performance in 4-by-4 MIMO-OFDM IDMA with four users.

This paper proposes a peak-to-average power ratio (PAPR) reduction method that maintains an effect of linear precoding by block diagonalization (BD) for multiuser (MU) MIMO-OFDM systems. In the downlink MU-MIMO-OFDM, BD orthogonalizes multiple signals of all users by the linear precoding using channel state information, but the transmitted signal still has high PAPR due to OFDM characteristics. Therefore, BD suffers from waveform distortion caused by a nonlinear transmit power amplifier, especially when the number of transmit antennas is set large enough for the orthogonalization. This paper proposes BD transmission selected mapping (BD-SLM) that can reduce PAPR while maintaining the BD effect. BD-SLM performs the phase shift to modulation signals of all users before the linear precoding. From several phase sequences, it selects a phase sequence that minimizes the peak of the time-domain signals at all transmit antennas. Computer simulations demonstrate that BD-SLM can drastically reduce PAPR in 16 × 4 MIMO-OFDM with four users, and that it can alleviate the performance degradation even when the power amplifier causes nonlinear distortion.

This paper proposes methods that can reduce peak- to-average power ratio (PAPR) while maintaining performance improvement by linear precoding for the eigenmode MIMO- OFDM transmission. In the eigenmode MIMO-OFDM system, a transmitter performs linear precoding using channel state information (CSI), and thus excellent transmission performance can be achieved. However, the transmitted signal still has high PAPR because of OFDM characteristics. To reduce such PAPR, this paper enhances selected mapping (SLM) and partial transmit sequence (PTS) which are known as PAPR reduction methods for OFDM. The enhanced methods, which are referred to as eigenmode transmission SLM (EM-SLM) and eigenmode transmission PTS (EM-PTS), can be applied to the eigenmode MIMO- OFDM without any performance degradation. Computer simulations demonstrate that in 4times4 MIMO-OFDM, the proposed methods can drastically reduce PAPR, and thus they can alleviate performance degradation due to nonlinear amplification.

This paper proposes an iterative OFDM millimeterwave receiver employing low-complexity decision-directed phase noise compensation (DD-PNC) to alleviate degradation due to the phase noise. High bit-rate OFDM transceivers based on the single-chip Si RF-CMOS IC technology in the 60-GHz millimeter-wave band have been extensively studied for wireless personal area network (WPAN) systems, and the relatively large phase noise in the phase locked loop (PLL) synthesizer severely degrades transmission performance. The proposed OFDM receiver iterates DD-PNC and decision-directed channel estimation (DDCE) by exploiting the output of the channel decoder. DDPNC estimates the phase noise each sampling time by using the decoder output, and then it removes the estimate from a time-domain received signal. In addition, DDCE estimates a channel impulse response by using the compensated received signal. Computer simulations demonstrate that in the 64QAM modulation with the coding rate of 3/4, the proposed receiver with DD-PNC and DDCE can perfectly remove the phase noise of -85 dBc/Hz at 1 MHz offset, and that it can alleviate the degradation of the channel estimation due to the phase noise.

This paper applies iterative multiuser detection employing new channel estimation to multicarrier interleave- division multiple access (MC-IDMA), known as OFDM-IDMA, which is expected to improve spectral efficiency in mobile communications. The MC-IDMA transmitter uses both a low-rate channel code and an individual chip interleaver for each user. The MC-IDMA receiver, which this paper investigates, repeats the iterative multiuser detection and soft decision-directed channel estimation (SDCE) by exploiting log-likelihood ratios (LLRs) of the coded bits which the MAP-based channel decoders for all users provide. SDCE estimates channel impulse responses of all users by the LMS algorithm, which aims to minimize the mean squared error between the received signal and its replica. This paper investigates the performance of MC-IDMA employing SDCE and compares it with those of three MC-CDMA techniques. Computer simulations demonstrate that MC-IDMA employing SDCE outperforms time-spread MC-CDMA and frequency-spread MC-CDMA, and that it can achieve almost the same bit error rate performance as chip-interleaved MC-CDMA while requiring lower complexity.

This paper proposes low-complexity blind detection for orthogonal frequency division multiplexing (OFDM) systems with the differential space-time block code (DSTBC) under time-varying frequency-selective Rayleigh fading. The detector employs the maximum likelihood sequence estimation (MLSE) in cooperation with the blind linear prediction (BLP), of which prediction coefficients are determined by the method of Lagrange multipliers. Interpolation of channel frequency responses is also applied to the detector in order to reduce the complexity. A complexity analysis and computer simulations demonstrate that the proposed detector can reduce the complexity to about a half, although it suffers a loss of 2.5 dB in Eb/N0at BER of 10-3owing to the complexity reduction when the prediction order and the degree of polynomial approximation are 2 and 1, respectively, with the normalized Doppler frequency of 1.6 times 10-2.

This paper proposes a new channel estimation method for the expectation-maximization (EM) algorithm applied to an OFDM receiver. Conventional schemes using the EM algorithm assume the random walk model as a process equation of a channel impulse response, and thus have insufficient tracking ability to fading fluctuation over fast fading channels. To improve the tracking ability, the proposed channel estimation method employs the differential model having the first and higher order differentials of the channel impulse response with respect to time. Computer simulations under the fast fading condition demonstrate that the proposed method is much superior in packet error rate to the conventional schemes employing the random walk model.

This paper proposes a MIMO-OFDM transmission scheme employing subcarrier-block phase hopping that can reduce peak-to-average power ratio (PAPR) while simultaneously improving transmission performance with low numerical complexity. As a conventional scheme for achieving these objectives, a combination of the enhanced selected mapping (ESLM) and the subcarrier phase hopping for space division multiplexing (SPH-SDM) has been proposed. To reduce the complexity furthermore and to increase feasibility of hardware implementation, the proposed scheme enhances SPH-SDM by introducing subcarrier-block phase hopping based on the partial transmit sequence (PTS). It divides the overall subcarriers into blocks and rotates phases of the complex signals which IFFT generates from the subcarriers of each block. Computer simulations demonstrate that the proposed scheme can drastically reduce PAPR and improve the packet error rate (PER) in comparison with the standard MIMO-OFDM, and that it can achieve almost the same performance as SPH-SDM with ESLM while achieving half as low complexity as that of the conventional one.

This paper proposes a MIMO-OFDM transmission scheme that combines the subcarrier phase hopping for space division multiplexing (SPH-SDM) with an enhanced selected mapping (ESLM). SPH-SDM randomizes the subcarrier phases within an OFDM symbol and makes subcarrier components of the received signal uncorrelated in order to improve BER performance of a MIMO-OFDM coded system. The phase patterns are adjusted so that they can not only randomize the subcarrier phase hopping but also reduce the peak-to-average power ratio (PAPR). In contrast to conventional PAPR reduction techniques for OFDM such as the selected mapping (SEM), ESLM employs roughly quantized IFFT processing for selecting the phase pattern in order to reduce the numerical complexity, and modulates the pilot subcarriers so as to inform the receiver of the selected phase pattern. Computer simulation shows that SPH-SDM with ESLM can increase the frequency diversity gain by channel coding, while simultaneously reducing PAPR by a suboptimum phase pattern

This paper proposes a maximum likelihood detection (MLD) method for the differential space-time block code (DSTBC) in cooperation with blind linear prediction (BLP) of fast frequency-flat fading channels. The method, which linearly predicts the fading complex envelope, determines the linear prediction coefficients by the method of Lagrange multipliers, and does not use decision-feedback nor require information on channel parameters such as the maximum Doppler frequency in contrast to conventional ones. Computer simulations under a fast fading condition demonstrate that the proposed method with a proper degree of polynomial approximation can be superior in BER performance to a conventional method that estimates the coefficients by the RLS algorithm using a training sequence.

This paper proposes a new maximum likelihood OFDM receiver for multipath fading environment with large delay spread. The large delay difference over the guard interval (GI) duration of the OFDM signal generally results in intersymbol interference (ISI) between temporally adjacent symbols and interchannel interference (ICI). Both of them spread over a large number of subcarriers. The new OFDM receiver consists of equalizers for both ISI and ICI and an accurate channel impulse response estimator. Furthermore, it introduces a temporal-window technique for the FFT-window in order to suppress the ICI, which is very effective for reducing the complexity of ICI equalization by maximum likelihood sequence estimator (MLSE). Computer simulation is conducted in order to verify the performance of the proposed OFDM receiver, and its effectiveness is shown.