In this paper, we consider orthogonal time frequency space (OTFS) systems in the presence of fractional Doppler shifts. For OTFS systems, an embedded pilot-aided channel estimation (CE) method has been proposed. In this method, pilot and guard symbols are appropriately placed in the delay-Doppler (DD) domain, and thresholding is then applied to the received signal for CE. However, this method suffers from a significant degradation in CE accuracy due to inter-Doppler interference, which is caused by fractional Doppler shifts. To this end, we propose CE methods using the fast iterative shrinkage-thresholding algorithm (FISTA) and coordinate descent (CD) method, exploiting channel sparsity in the DD domain. Furthermore, these proposed methods reduce the size of the received signal and the measurement matrix by thresholding that takes advantage of channel sparsity in the delay domain. Computer simulations demonstrate that the proposed methods outperform the conventional threshold-based and sparse Bayesian learning-based CE methods.

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.

In this paper, we propose a new transmission scheme for grant-free non-orthogonal multiple access (GF-NOMA) based on orthogonal frequency division multiplexing (OFDM) in the presence of timing offsets. The proposed method utilizes differential phase-shift keying (DPSK) between subcarriers, eliminating the need for channel estimation and mitigating the degradation of data detection caused by timing offsets. Furthermore, the proposed method conducts active user detection based on the coordinate descent method, which offers a lower computational complexity compared to the conventional method based on the structured generalized approximate message passing (S-GAMP). Computer simulations demonstrate that the proposed method achieves a lower miss detection probability compared to S-GAMP and a lower bit error rate than the method using PSK.

This paper studies an uplink grant-free non-orthogonal multiple access (GF-NOMA) system using low-density signature orthogonal frequency division multiplexing (LDS-OFDM), where each user transmits non-orthogonal pilots and data symbols that are mapped onto a small number of subcarriers determined by the LDS sequence. After an initial estimation based on pilot symbols, our proposed receiver utilizes the low-density signal structure to improve active user detection, channel estimation, and multi-user detection accuracy. To achieve this, a birth-death drift process is applied to model the information exchange between two estimation modules for pilot and data symbols, and a corresponding side information (SI) calculation and denoiser design are proposed for SI-aided multiple measurement vector approximate message passing (MMV-AMP). Moreover, the asymptotic performance of the proposed SI-aided MMV-AMP is analyzed based on state evolution. Numerical results confirm that the proposed method outperforms state-of-the-art methods and that it can perform predictions using the derived state evolution.

This paper investigates a highly-efficient low-latency scheduling method in the round-trip wireless communication for feedback control. We propose scheduling metrics that consider both the instantaneous channel condition and temporal delay time of respective user equipments (UEs). We applying a- proportional fair criterion to the scheduling metric for instantaneous channel-aware scheduling in the fading channel to enhance the spectrum efficiency. Concurrently, the proposed metrics incorporate the delay-time awareness terms, preventing the round-trip delay (RTD) from exceeding the allowable maximum one as much as possible. Computer simulation results reveal that the proposed methods significantly reduce the outage probability of the RTD requirement compared to the conventional methods and accommodate much more UEs coexistence.

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.

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.

This paper investigates a beam selection method for the downlink massive multiple-input multiple-output (mMIMO) analog beamforming (BF) with non-orthogonal multiple access (NOMA) based on intra-beam superposition coding. The purpose of this study is to select a set of beams that have low mutual inter-beam interference and high NOMA gain by non-orthogonal multiplexing multiple users within each of the beams based only on the limited signal-to-noise power ratio (SNR) feedback of the best beam from each user which is assumed in 5G NR. The proposed method is based on our previously reported beam selection method using the angle-domain beam grouping (ADG), which utilizes a beam group that contains multiple neighboring-angle beams. The proposed ADG adaptively controls the configuration of beam groups so that the number of users reported the SNR for the edge beam of the beam group is minimized. This brings about a further reduction of inter-beam interference and an increase in the NOMA effect. Computer simulation results show that the proposed adaptive ADG improves the system-level throughput compared to the conventional methods.

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.

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.

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.

Cell-free multiple-input multiple-output (MIMO) is a promising distributed network architecture for 5G-and-beyond systems. This paper proposes an access point (AP) selection method that is specific to each user equipment (UE) in downlink cell-free MIMO. The APs are distributed throughout the system coverage area and know only a part of the instantaneous channel state information (CSI). As a beamforming (BF) method based on partial CSI, we use a layered partially non-orthogonal zero-forcing (ZF) method based on channel matrix muting, which is applicable to the case where different transmitting AP groups are selected for each UE under partial CSI conditions. The proposed clustering method initializes the transmitting AP group for each UE based on the conventional signal-to-leakage-and-noise-ratio based method and then updates them for all UEs in an iterative process based on the throughput considering the interference among the UEs. We show that the proposed method achieves higher average and worst-user throughput than those for conventional methods while minimizing the excessive increase in the number of transmitting APs per UE.

We propose a decentralized probabilistic method for the frequency block-dependent base station (BS) sleep control aiming at the improvement in energy efficiency both in up and downlinks. The energy efficiency is defined as the integrated system throughput which combines the user throughput levels of uplink and downlink per unit energy consumption at the BSs. The purpose of the proposed method is to reduce inter-cell interference and energy consumption of the BSs by adaptively performing the sleep control of each frequency block in the harmonized manner between the neighboring BSs based on the practical autonomous decentralized control for each BS. In the proposed method, two sleep states are defined for the BS: the micro-sleep state, in which switching between active and sleep states can be performed immediately, and the deep-sleep state, which takes time to switch but consumes less energy than the micro-sleep state. By adaptively choosing the sleep state of each frequency block, the energy efficiency of the proposed method is further improved. Computer simulations show that the proposed method, which adaptively switches between micro-sleep and deep-sleep, significantly improves the uplink and downlink energy efficiency by dividing the system band into approximately three frequency blocks.

We propose a new grant-free non-orthogonal multiple access (GF-NOMA) scheme, which makes full use both of the time and of the delay domains for further enhancement on low-latency and massive connectivity. The proposed GF-NOMA is based on a tailored signal model involving the delay-domain channel sparsity in conjunction with phase transition analyses for approximate message passing (AMP) algorithms. Moreover, the proposed design enables efficient and accurate estimation by appropriately designing message-passing rules. Simulation results are offered to demonstrate the superiority of the proposed GF-NOMA approach over the conventional scheme.

Grant-free non-orthogonal multiple access (GF-NOMA) systems suffer from the performance degradation of active user detection (AUD) due to carrier frequency offsets (CFOs) caused by imperfect frequency synchronization among users and a base station. Although a conventional method efficiently performs CFO estimation, AUD, and channel estimation (CE) by introducing extended pilot sequences based on discrete grids of the CFOs, the inevitable estimation error caused by off-grid CFOs significantly degrades the quality of the CE. This paper proposes a more accurate joint AUD and CE method by performing AUD and CFO estimation independently and iteratively. Computer simulations demonstrate that our proposed scheme can estimate the active users and the corresponding channel coefficients as accurately as the case with the perfect knowledge of the CFOs.

In this paper, we propose an autonomous decentralized user association method that maximizes the integrated system throughput, which is defined across multiple services. The quality of service (QoS) of each service is defined by the system throughput, which takes into account the trade-off between spectrum efficiency and fairness among users for each service. Then, the total quality of all services is defined by the integrated system throughput, which is calculated by the weighted generalized average of the system throughput levels of each service. The proposed method achieves the optimal user association that maximizes the integrated system throughput according to the distribution of base stations (BSs) and users in the system coverage based only on a small amount of control information broadcasted by each BS through autonomous decentralized control of each user terminal. Computer simulations assuming a heterogeneous network show that the proposed method increases the system throughput of each service and integrated system throughput compared to the cell range expansion (CRE) method, which is widely used in 4G and 5G mobile communication systems.

This paper proposes a repetition-based low-latency non-orthogonal multiple access (NOMA)-hybrid automatic repeat request (HARQ) method with adaptive termination for ultra-reliable low latency communications (URLLC). To reduce transmission delay, the proposed method retransmits packets in short transmission time intervals in advance and terminates packet retransmissions adaptively according to acknowledgement (ACK) feedback from the receiver. In addition, to reduce the throughput loss due to unnecessary retransmitted packets sent in the time until ACK feedback, retransmitted packets of the URLLC user are non-orthogonally multiplexed in the same channel with packets of other users based on NOMA. Two non-orthogonal multiplexing methods are compared: superposition coding (SPC) and joint modulation (JM). As for the receiver structure, a successive interference canceller (SIC) and complexity reduced maximum likelihood detection (R-ML) are investigated. We confirm that the proposed method using JM and R-ML provides the best improvement in the transmission delay time versus throughput trade-off based on computer simulations.

This paper proposes an efficient hybrid automatic repeat request (HARQ) method that reduces the reception error caused by excessively concentrated simultaneous packet transmissions from many users while achieving packet combining and resolution of random access identifier (RAID) collision during retransmission in a non-orthogonal multiple access (NOMA)-based random access system. The proposed method extends our previously reported hierarchical tree-structured RAID to accommodate random retransmission timing back-off. By separately defining a dedicated RAID set for retransmission at each back-off timing index, the proposed method reduces the reception error caused by the concentration of transmitting users while resolving RAID collisions and combining packets at the retransmission attempts. Computer simulation results show the effectiveness of the proposed method.

We started a new research project in the “advanced 5G” era that aims at accommodating various types of communications involving current and emerging services with different data flow-level quality requirements. In this paper, the objectives and the technical aspects of the research project are introduced. We propose an architecture based on a virtualized radio access network (vRAN) that enables adaptive control of equipment resources and location of functions in the vRAN environment in accordance with spatially and temporally changing communication demands. The seven planned research items that are essential for realizing the advanced 5G network are listed as follows: blockage prediction, new radio access technologies (RATs) and their implementations with software-defined radio (SDR), adaptive interference and resource control, integration of radio and fiber resource control, highly efficient access transmission control, adaptive placement of BS functions, and quality aware traffic pattern prediction.

We have recently proposed a massively concurrent non-orthogonal multiple access (MC-NOMA) scheme which, unlike other non-orthogonal multiple access (NOMA) schemes such as sparse-coded multiple access (SCMA) and pattern division multiple access (PDMA) that rely on sparsity, utilizes instead dense massively multiplexing with interference optimized via Frame Theory. Thanks to the properties of unit-norm tight frames (UNTFs) employed in the MC-NOMA scheme, all users can access all available resources at once but with the resulting interference collapsed and minimized, in a manner similar to that in sparsity-based NOMA. In this article we provide analytical evidence that MC-NOMA outperforms current NOMA schemes both in terms of sum-rate and spectral efficiency. To this end, we derive expressions for the achievable sum-rate and the spectral efficiency of MC-NOMA in a general context, and use the results to numerically validate the improvement potential of MC-NOMA schemes.