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 investigates a delay and channel-aware scheduling method in round-trip wireless communication for feedback control. We propose a simple uplink (UL) and downlink (DL) scheduling method that achieves a high spectrum efficiency while preventing the round-trip delay (RTD) from exceeding the allowable maximum one as much as possible. The proposed method derives the optimal metric for round-trip communication by aiming to minimize the risk of transmission failure based on the expected delay time of candidate UEs. The proposed method is simple and realistic since it reduces the number of parameters to be carefully tuned compared to the conventional a-proportional fair round-trip multiple deadlines (AP-RMD) method. Computer simulation results reveal that the proposed method accommodates many UEs while guaranteeing the required maximum RTD under the limited system bandwidth with fewer parameter adjustments than AP-RMD.

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 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 method for generating a peak cancellation (PC) signal vector considering the variance in the average signal power among transmitter antennas in the peak-to-average power ratio (PAPR) reduction method for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) signals using the null space in a MIMO channel. We first analyze the conditions for the PC signal vector to achieve a sufficient PAPR reduction effect after its projection onto the null space. The analysis indicates that the PC signal vector should be uncorrelated with the transmission signal vector as much as possible. Based on the analysis and the fact that reducing the difference in the average signal power among the transmitter antennas is beneficial for PAPR reduction, we propose a new method for generating a PC signal vector. The proposed PC signal vector is designed so that at the target timing, the signal power levels of all the transmitter antennas are restricted to be below the maximum power threshold while also being above the minimum power threshold. The newly introduced feature, i.e., controlling the signal power to be above the minimum power threshold, contributes to reducing the transmission power variance among antennas and increasing the PAPR reduction effect after the projection of the PC signal onto the null space in the MIMO channel. This is accomplished by reducing the correlation between the PC signal and data signal vectors. Computer simulation results show that the proposed method provides superior PAPR reduction characteristics compared to those for the conventional method.

In this paper, we propose a peak-to-average power ratio (PAPR) reduction method using the null space in a multiple-input multiple-output (MIMO) channel for downlink MIMO-orthogonal frequency division multiplexing (OFDM) signals with multiple-antenna amplify-and-forward (AF)-type relaying. Different from the conventional approach, the proposed method reduces the PAPR at the relay station (RS) using only the signal processing at the base station (BS). Mitigating the high PAPR of transmit MIMO-OFDM signals at the BS and at the RS transmitters is essential to achieve sufficient coverage enhancement from the RSs. On the other hand, performing complex signal processing for PAPR reduction at the AF-type RS is not possible in reality. In the proposed method, the RS does not require any signal processing for its PAPR reduction. The proposed method alternately repeats the generation of the signal that reduces the PAPR at the BS transmitter and the signal that reduces the PAPR at the RS transmitter, where both PAPR reduction signals are transmitted from the BS. Compared to the conventional method in which the PAPR at the RS transmitter is reduced using the signal processing at the RS utilizing the null space in the MIMO channel, the proposed method achieves almost the same PAPR reduction level at the BS and RS while requiring no signal processing for PAPR reduction at the RS.

In this paper, we investigate a method for activation/deactivation control of base stations (BSs) based on a deep Q-network (DQN) that tracks changes in user distribution in heterogeneous networks to increase downlink system throughput. Conventional BS activation control based on deep reinforcement learning (RL) assumes centralized control and represents the entire system with a single action value function. However, this method cannot be applied to dense heterogeneous networks with many BSs because the total number of actions grows exponentially. In the proposed method, each BS is an agent with its own action value function making the DQN-based BS activation control applicable to dense small-cell networks. Several definitions of states are compared from the viewpoints of the load required for acquiring the state variables and the achievable throughput performance. Computer simulations show that the proposed method achieves higher throughput compared to the conventional probabilistic control method and that it is adaptable to different system conditions.

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.

This paper proposes an efficient hybrid automatic repeat request (HARQ) method that simultaneously achieves packet combining and resolution of the collisions of physical channel identifiers (PCIDs) during retransmission in a non-orthogonal multiple access (NOMA)-based random access system for multi-packet reception. Here, the PCID functions as a separator for simultaneously received packets that use the same channel in NOMA. An example of this is a scrambling code used in 4G and 5G systems. Since users independently select the PCID, the decoding of received packets fails when multiple users select the same PCID. Random PCID reselection by each user when attempting retransmission can resolve the PCID collision; however, packet combining between the previous and retransmitted packets is not possible in this case because the base station receiver does not know the relationship between the PCID of the previously transmitted packet and that of the retransmitted packet. To address this problem, we propose employing novel hierarchical tree-structured PCID groups in which the PCID for the previous packet transmission has a one-to-one relationship to the set of PCIDs for retransmission. The proposed method resolves PCID collision at retransmission by randomly reselecting for each user a PCID from the dedicated PCID set from the previous transmission. Since the relationship between the PCIDs at the previous transmission and retransmission is known at the base station, packet combining is achieved simultaneously. Computer simulation results show the effectiveness of the proposed method.

Hybrid automatic repeat request (HARQ) is an essential technology that efficiently reduces the transmission error rate. However, for ultra-reliable low latency communications (URLLC) in the 5th generation mobile communication systems and beyond, the increase in latency due to retransmission must be minimized in HARQ. In this paper, we propose a non-orthogonal multiple access (NOMA)-based highly-efficient low-latency HARQ method for URLLC while minimizing the performance loss for coexisting services (use cases) such as enhanced mobile broadband (eMBB). In the proposed method, delay-sensitive URLLC packets are preferentially multiplexed with best-effort eMBB packets in the same channel using superposition coding to reduce the transmission latency of the URLLC packet while alleviating the throughput loss in eMBB. This is achieved using a weighted channel-aware resource allocator (scheduler). The inter-packet interference multiplexed in the same channel is removed using a successive interference canceller (SIC) at the receiver. This corresponds to non-orthogonal HARQ at the system level. Furthermore, the transmission rates for the initial transmission and retransmission are controlled in an appropriate manner for each service in order to deal with decoding errors caused by error in transmission rate control originating from a time varying channel. We show that the proposed method significantly improves the overall performance of a system that simultaneously provides eMBB and URLLC services.

In the 5th generation mobile communication system and beyond, the importance of relay transmission will increase in order to provide wide coverage for high-speed high-quality data transmissions using a higher carrier frequency band such as the millimeter wave band. Assuming multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) transmission as the baseline for 5G systems, mitigating its high peak-to-average power ratio (PAPR) not only at the base station (BS) but also at the relay station (RS) transmitters is essential to achieve sufficient coverage enhancement from the RSs by minimizing the transmission power backoff levels at the nonlinear power amplifier. In this paper, we propose applying our previously reported adaptive PAPR reduction method using null space in a MIMO channel for MIMO-OFDM transmission to a RS. In this study, we assume an amplify-and-forward (AF)-type RS with multiple antennas. Computer simulation results show that the proposed method significantly improves the PAPR-vs.-throughput performance compared to that for the conventional one thanks to the reduced interference levels from the PAPR reduction signal observed at the user equipment (UE) receiver.

In this paper, we propose an online probabilistic activation/deactivation control method for base stations (BSs) based on the temporal system throughput and activation states of neighbor BSs (cells) in heterogeneous networks. The conventional method iteratively updates the activation/deactivation states in a probabilistic manner at each BS based on the change in the observed system throughput and activation/deactivation states of that BS between past multiple consecutive discrete times. Since the BS activation control increases the system throughput by improving the tradeoff between the reduction in inter-cell interference and the traffic off-loading effect, the activation of a BS whose neighbor BSs are deactivated is likely to result in improved system performance and vice versa. The proposed method newly introduces a metric, which represents the effective ratio of the activated neighbor BSs considering their transmission power and distance to the BS of interest, to the update control of activation probability. This improves both the convergence rate of the iterative algorithm and the performance after convergence. Computer simulation results, in which the mobility of the user terminals is taken into account, show the effectiveness of the proposed method.

This paper proposes a transmitter filtering (TF) method for random access using physical channel identifier (PCID)-linked receiver beamforming (BF). The role of the transmitter filtering in this system is to reduce the delay spread of the multipath channel, reduce the multipath interference, and increase the received signal power gain. There is room for improvement in previous zero-forcing (ZF)-based and minimum mean squared error (MMSE)-based TF methods in terms of the trade-off between multipath interference and power gain in view of the actual channel estimation accuracy. The proposed method is characterized by the fact that the number of effective paths after TF operation is set to two. By increasing the number of effective paths from one to two and adaptively adjusting the power and phase difference between the two paths according to the actual channel, the proposed method improves the power gain compared to the ZF-based method and reduces the generation of many undetectable small paths compared to the MMSE method, thus reducing multipath interference. Computer simulations quantitatively show that the proposed method improves the error rate compared to that for the conventional method.

In random access with multi-packet reception for the 5th generation mobile communication system and beyond, the system prepares sufficient number of physical channel identifiers (PCIDs) in order to separate multiple user packet signals at the base station receiver. The base station receiver performs active user detection and channel estimation using preamble sequences, each of which has a one-to-one relationship with a PCID. As the number of PCIDs prepared in the system is increased, the computational complexity for active user detection and channel estimation becomes prohibitive. To address this problem, we propose three preamble sequence designs appropriate for random access achieving multi-packet reception. Assuming single-carrier transmission based on discrete Fourier transform (DFT)-spread orthogonal frequency division multiplexing (OFDM), the proposed methods construct the preamble sequence through repetition of the basic Zadoff-Chu (ZC) sequence, which is shorter than the length of one DFT block, with multiplication of the intra-DFT block orthogonal masking sequence. In this way, we reduce the computational complexity required for the correlation detection between the received signal and the preamble sequence. The cyclic-shifted ZC sequence-based intra-DFT block orthogonal masking mitigates the intra-DFT block interference caused by the multipath channel. Computer simulation results show quantitatively the effectiveness of the proposed methods.

This paper proposes a novel interleaver-linked receiver beamforming method for time division duplex (TDD)-interleave division multiple access (IDMA)-based random access. When the number of receiver antennas at the base station is large in a massive multiple-input multiple-output (MIMO) scenario, the channel estimation accuracy per receiver antenna at the base station receiver is degraded due to the limited received signal power per antenna from the user terminal. This results in degradation in the receiver beamforming (BF) or antenna diversity combining and user detection. In the proposed method, a unique receiver BF vector applied at the base station is linked to each of the M interleavers prepared by the system. The user terminal selects an appropriate pair comprising a receiver BF vector and an interleaver in advance based on the channel estimation results in the downlink assuming channel reciprocity in a TDD system. Therefore, the per-receiver antenna channel estimation for receiver BF is not necessary in the proposed method. Furthermore, we also propose an application of a transmitter channel equalization to the proposed method for effective channel shortening in order to increase the orthogonal preambles for channel estimation prepared for each interleaver. Computer simulations results show that the proposed method greatly improves the accuracy of user detection and channel estimation which results in reducing the error rate compared to the conventional method that performs channel estimation at each antenna in a massive MIMO environment.

We propose a non-orthogonal multiple access (NOMA)-based inter-base station (BS) cooperative scheduling method among terminals in multiple service channels to maximize integrated system throughput. The proposed method optimizes coordinated time/frequency-domain scheduling, which includes bandwidth and power allocation to terminals, with inter-BS cooperation using NOMA with successive interference canceller (SIC). The computational complexity and signaling overhead at the backhaul of the proposed method are expected to increase as the number of BS in cooperation increases. Therefore, we introduce the limited range in inter-BS cooperation. We also evaluate computationally-efficient suboptimal transmission power allocation strategies in addition to the optimal one. Numerical results show that the effectiveness of the proposed method is especially significant when the system prioritizes the fairness among terminals (including fairness among services). We also show that the appropriate range in inter-BS cooperation and use of the suboptimal transmission power allocation strategy maintain most from the potential gain by using the proposed method while the system complexity is effectively reduced.

In this paper, we investigate the user group selection method in multiuser multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) transmission when the adaptive peak-to-average power ratio (PAPR) reduction using the null space in a MIMO channel is applied. The user group to be spatially multiplexed is optimally selected by evaluating the throughput after the PAPR reduction process for all combinations of user candidates. However, the computational complexity of the optimal user group selection becomes prohibitive as the number of candidate users increases when performing the actual PAPR reduction process. To establish a computationally efficient user group selection method, we first discuss the relationship between the beamforming (BF) matrix and the achievable performance of the PAPR reduction using the null space in a MIMO channel. Based on the discussion, we propose a method that pre-selects user groups based on the BF matrix. By limiting the full assessment of the throughput evaluation after the PAPR reduction process to only the pre-selected user groups, the computational complexity of the proposed method is reduced. Computer simulation results show that the proposed method suppresses the loss in the PAPR-vs.-throughput performance while the computational complexity is significantly reduced.

In this paper, we propose a computational complexity-efficient algorithm called PCCNC for the adaptive peak-to-average power ratio (PAPR) reduction method using the null space in a multiple-input multiple-output (MIMO) channel for MIMO-orthogonal frequency division multiplexing (OFDM) signals. Based on our preliminary work, the proposed PCCNC in this paper newly introduces a parallel peak cancellation process at each iteration. By adding the multiple peak cancellation (PC) signals to the time-domain transmission signal vector simultaneously, the number of iterations in the iterative algorithm is reduced. The constraint so that the PC signal is transmitted only to the null space in the MIMO channel by beamforming (BF) makes interference from PC signal to the data streams not observed at the receiver side. Since the fast Fourier transform (FFT) and inverse FFT (IFFT) operations at each iteration are not required different from the conventional algorithm and thanks to the introduced parallel processing, the proposed PCCNC reduces the required number of iterations and total computational complexity compared to the conventional algorithm for achieving the same throughput-vs.-PAPR performance.

In this paper, we propose an extension to a previously reported complexity-efficient algorithm for adaptive peak-to-average power ratio (PAPR) reduction using the null space in a multiple-input multiple-output (MIMO) channel for MIMO-orthogonal frequency division multiplexing (OFDM) signals in a frequency-selective fading channel. We show that the effect of the adaptive PAPR reduction method using the null space in a MIMO channel is enhanced in a frequency-selective fading channel. We extend a previously reported peak cancellation (PC) signal-based algorithm (PCCNC) to accommodate the frequency-selective fading channel. In the newly proposed PCCNC, the PC signal vector is generated by multiplying basic time-domain pulse signal whose transmission band is limited to the respective frequency blocks with a beamforming (BF) vector directed at the null space of the MIMO channel in each frequency block. By adding the average PC signals of each frequency block to the transmitted signal vector in the time domain, PAPR reduction is achieved. We observe no interference imparted from the PC signal to the data streams on the receiver side when implementing a constraint limiting the transmission of the PC signal to only the null space in the MIMO channel using BF. We show that the throughput-vs.-PAPR performance of the proposed method is improved as the frequency-selectivity of the channel is increased and the proposed PCCNC successfully reduces the computational complexity compared to the conventional clipping and filtering (CF)-based algorithm.

We propose a decentralized probabilistic frequency-block activation control method for the cellular downlink. The aim of the proposed method is to increase the downlink system throughput within the system coverage by adaptively controlling the activation of each multiple frequency block at all base stations (BSs) to achieve inter-cell interference coordination (ICIC) and traffic load balancing effects. The proposed method does not rely on complicated inter-BS cooperation; it uses inter-BS cooperation only to share the temporal system throughput observed at the neighboring BSs. Based on the shared temporal system throughput information, each BS independently performs online activation control for the respective frequency blocks in a probabilistic manner, which autonomously achieves ICIC and load balancing effects among BSs. Computer simulation results, in which the mobility of the user terminals is taken into account, show the effectiveness of the proposed method.

We propose a novel non-orthogonal multiple access (NOMA)-based optimal multiplexing method for multiple downlink service channels to maximize the integrated system throughput. In the fifth generation (5G) mobile communication system, the support of various wireless communication services such as massive machine-type communications (mMTC), ultra-reliable low latency communications (URLLC), and enhanced mobile broadband (eMBB) is expected. These services will serve different numbers of terminals and have different requirements regarding the spectrum efficiency and fairness among terminals. Furthermore, different operators may have different policies regarding the overall spectrum efficiency and fairness among services. Therefore, efficient radio resource allocation is essential during the multiplexing of multiple downlink service channels considering these requirements. The proposed method achieves better system performance than the conventional orthogonal multiple access (OMA)-based multiplexing method thanks to the wider transmission bandwidth per terminal and inter-terminal interference cancellation using the successive interference canceller (SIC). Computer simulation results reveal that the effectiveness of the proposed method is especially significant when the system prioritizes the fairness among terminals (including fairness among services).

This paper proposes an extension to a previously reported low latency hybrid automatic repeat request (HARQ) method that uses early retransmission prior to channel decoding to perform multistage judgment at early-retransmission decision. The previous method mitigates the increased transmission latency resulting mainly from the delay time required for channel decoding in HARQ by requesting early retransmission before the channel decoding process is completed based on the channel state information (CSI) obtained before channel decoding. However, the early-retransmission decision may include error that results in unnecessary retransmission and consequently throughput loss. To address the potential throughput loss due to the decision error in the early-retransmission request, we use multistage decision in which the number of bits conveyed by the early-retransmission packet is controlled dependent on the measured CSI before channel decoding. By setting the number of retransmitted coded bits to a low value when the observed channel state seems on the verge of performing early retransmission, we reduce the throughput loss when the early retransmission is not necessary since the initial packet is eventually channel decoded correctly. Simulation results show that the proposed extension further improves the achievable tradeoff between the transmission latency and throughput, which contributes to achieving ultra-reliable low latency communications (URLLC).

We propose an online probabilistic activation control of base stations (BSs) considering both uplink and downlink system throughput. The proposed method is established by extending previous work on probabilistic BS activation control in which only the downlink system throughput is taken into account for maximization. In the proposed method, the total system throughput is defined to include both the uplink and downlink system throughput. The proposed method tries to find a set of activation states of all BSs in a decentralized manner to maximize the defined total system throughput. More specifically, each BS independently and iteratively updates the activation probability based on the time variation of the network-level total system throughput and the transition results of activated or deactivated states at that BS. We show the effectiveness of the proposed method quantitatively based on computer simulations.

This paper investigates various interference canceller structures appropriate for interleave division multiple access (IDMA)-based random access in order to support massive machine-type communications (mMTC) in the fifth generation (5G) mobile communication system. To support massive connectivity in the uplink, a grant-free and contention-based multiple access scheme is essential to reduce the control signaling overhead and transmission latency. To suppress the packet loss due to collision and to achieve multi-packet reception, non-orthogonal multiple access (NOMA) with interference cancellation at the base station receiver is essential. We use IDMA and compare various interference canceller structures such as the parallel interference canceller (PIC), successive interference canceller (SIC), and their hybrid from the viewpoints of the error rate and decoding delay time. Based on extensive computer simulations, we show that IDMA-based random access is a promising scheme for supporting mMTC and the PIC-SIC hybrid achieves a good tradeoff between the error rate and decoding delay time.

The fifth generation (5G) mobile communication system is expected to support various wireless communication services such as massive machine-type communications (mMTC), ultra-reliable low latency communications (URLLC), and enhanced mobile broadband (eMBB). These services will serve different numbers of terminals and have different requirements regarding the spectrum efficiency and fairness among terminals. Furthermore, different operators may have different policies regarding the overall spectrum efficiency and fairness among services. We propose a novel channel-dependent dynamic frequency bandwidth allocation method among service channels to maximize the defined system throughput that integrates the system performance of all services considering the above requirements. The proposed method jointly optimizes the time/frequency-domain scheduling and frequency bandwidth allocation among service channels in terms of maximizing the integrated system throughput. Since the proposed method can enhance the multi-user (multi-terminal) diversity gain by performing channel-aware scheduling across the terminals of all services, the integrated system throughput can be further improved compared to a semi-static frequency bandwidth allocation method among service channels previously reported by our group. Extensive computer simulation results show the effectiveness of the proposed method under various system operation scenarios.

In this paper, we investigate adaptive settings for the initial interpolation vector in our previous iterative time-domain compression method for multiple-input multiple-output (MIMO) channel state information (CSI). The iterative MIMO-CSI compression method is based on time-domain CSI compression with frequency-domain CSI bundling and interpolation for all combinations of transmitter and receiver antennas. Previously, the method for generating the initial interpolation vector, which reuses the actual frequency-domain CSI variation, was proposed in order to take into account the periodical channel variation in the frequency domain. However, in this method, the appropriate length interpolation vector is dependent on the delay spread of the channel. In order to address this problem, this paper proposes adaptively controlling of the length of the interpolation vector based on the correlation of the CSI vector in the frequency domain. The numerical results show that the proposed method further improves the accuracy of the compressed CSI or enhances the CSI compression capability of the previous method, which assumes a fixed length for the interpolation vector.

This paper proposes a low latency hybrid automatic repeat request (HARQ) method that uses the channel state information before channel decoding to address the increased transmission latency resulting mainly from the delay time required for the channel decoding in HARQ. We focus on applying the proposed method to ultra-reliable low latency communication. The proposed method reduces the transmission latency by requesting early retransmission before the channel decoding process is completed. We consider two metrics for deciding the early retransmission request: (1) mutual information (MI) calculated from the signal-to-noise ratio of the received packet and (2) the a priori log-likelihood ratio (LLR) of the coded bits obtained from the signal detection results of the received packet. Simulation results show that the proposed method successfully reduces the transmission latency at the cost of a very small loss in throughput. Between these two metrics, the MI-based method achieves a slightly better tradeoff between the throughput and transmission latency than the LLR-based method.

In this paper, we investigate transmitter beamforming (BF) control based on the firefly algorithm (FA) in the massive multiple-input multiple-output (MIMO) downlink with a per-antenna power constraint with the aim of maximizing the variously defined system throughput. In the massive MIMO downlink, transmitter BF is essential to achieve a beamforming (power) gain and/or a reduction in the inter-user interference in a multiuser MIMO case based on the channel state information (CSI). Massive MIMO requires a large number of transmitter power amplifiers dedicated to each transmitter antenna and the allowable transmission power per power amplifier (thus per transmitter antenna) is severely limited. Therefore, it is necessary to perform transmission BF considering the limitation on the transmission power per antenna, which is in general a difficult problem to resolve. In this paper, we apply our recently reported BF control method using the FA under various definitions of the system throughput considering the tradeoff between the frequency efficiency and user fairness. By setting the BF matrix as the position for the firefly and the obtainable metric for the system throughput maximization as the brightness of the firefly, we apply the FA to the BF control. We evaluate the performance of the BF control using the FA when the system throughput is the arithmetic mean, geometric mean, harmonic mean, and minimum user throughput based on computer simulations and compare the results to those for the case using conventional zero forcing (ZF)-based BF.

The combination of massive multiple-input multiple-output (MIMO) with beamforming (BF) and orthogonal frequency division multiplexing (OFDM) signaling achieves highrate data transmission with a wide coverage area thanks to the high BF gain. The drawback to this transmission scheme is its high peak-to-average power ratio (PAPR). A high PAPR results from the transmission power variation among transmission antennas due to BF in addition to the multicarrier-based OFDM signaling. This paper investigates the performance of our previously reported adaptive PAPR reduction method using the null space in a MIMO channel in eigenmode massive MIMO-OFDM signals. The adaptive PAPR reduction method mitigates the degradation in data throughput due to the interference from the PAPR reduction signal generated by the clipping and filtering (CF) algorithm by restricting the transmission of the PAPR reduction signal to only the null space of the MIMO channel. In a massive MIMO scenario, since the number of spatial streams is generally much less than that of the transmitter antennas, the effect of the adaptive PAPR reduction method is expected to be enhanced due to the increase in the dimensions of the null space in the MIMO channel. Computer simulation results show that the adaptive PAPR reduction method is effective especially when (i) the number of transmitter antennas is large, (ii) the required PAPR is low, and (iii) the signal-to-noise ratio (SNR) is high.

In the fifth generation mobile communication systems, various wireless communication services such as massive machine-type communications (mMTC), ultra-reliable low latency communications (URLLC), and enhanced mobile broadband (eMBB) will be supported. The physical channels (service channels) dedicated to these respective wireless communication services will be multiplexed in a shared system frequency band and these services will have different requirements regarding spectrum efficiency and fairness among terminals. Therefore, appropriate frequency bandwidth assignment to each service channel considering the service requirement, number of terminals, and their channel conditions for all wireless communication services is important to maximize the system performance. In this paper, we define different forms of system throughput for the respective service channels considering different service requirements. Based on a definition of the integrated system throughput that combines the system throughput levels of all service channels, this paper proposes the optimum frequency bandwidth allocation method among service channels to maximize the integrated system throughput. Computer simulation results show the effectiveness of the proposed method.

In this paper, we propose an improvement to our previously reported online probabilistic transmission power control method for base stations (BSs) in heterogeneous networks where low transmission-power pico-BSs are overlaid onto a high transmission-power macro-BS. The previously reported method requires only a single metric to be exchanged among BSs, which is a measure of system throughput at each cell. The previous method adaptively controls the activation probability of each BS individually, depending on the time variation in the system throughput and the temporal transmission power states of each BS. As a result, the system throughput is enhanced due to appropriately reduced inter-cell interference. However, since the previous method updates the transmission power of each BS with a small step size at each iteration, there is room for further improvement in the convergence rate and ability to track the change in user distribution. The proposed new algorithm defines a limited set of candidate transmission power levels. The application probability of each candidate transmission power level is updated based on the time variation in the system throughput and the temporal transmission power states of each BS. As a result, the performance after convergence and the ability to track the change in user distribution is improved compared to the previous method. Computer simulation results show the effectiveness of the proposed method compared to conventional approaches.

This paper proposes a jointly optimized user association and time/frequency-domain scheduling method that maximizes the downlink system throughput defined based on (p, α)-proportional fairness. A previously reported user association method by members of our research group maximizes the downlink system throughput considering the base station (BS) distribution, user distribution, and the average channel conditions between each of the BSs and user terminals. However, the overall performance of the user association and subsequent time/frequency-domain scheduling conducted at each BS against the connected users taking into account the instantaneous channel variation due to multipath fading is not sufficient, although this approach can be actualized without relying on inter-BS cooperation. The proposed method assumes the centralized radio access network (C-RAN) architecture and achieves joint optimization of the user association and time/frequency-domain scheduling with regard to the achievable system throughput defined based on (p, α)-proportional fairness. Computer simulation results show that the proposed method significantly enhances the achievable system throughput compared to the user association method based on the average channel conditions, especially when the number of pico BSs is large and the system throughput prioritizes the fairness among users.

In the massive multiple-input multiple-output (MIMO) downlink, transmitter beamforming (BF) to achieve a beamforming (power) gain and/or a reduction in inter-user interference in a multiuser MIMO case based on the channel state information (CSI) at the transmitter side is essential. Massive MIMO requires a large number of transmitter power amplifiers dedicated to each of the transmitter antennas, and the allowable transmit power per power amplifier (thus per transmitter antenna) is severely limited. Therefore, it is necessary to perform transmission BF considering the limitation on the transmit power per antenna. In this paper, we consider BF control using the firefly algorithm (FA) assuming the per-antenna transmit power constraint aiming at maximizing the system throughput defined as log-sum user throughput (equivalently the geometric mean user throughput) corresponding to proportional fairness (PF). The FA is a kind of swarm intelligence and by mimicking the behavior of fireflies that move toward neighboring fireflies based on the distance and brightness of the neighboring fireflies, a multimodal optimization solution is obtained efficiently. By setting the BF matrix as the position of the firefly and the obtainable metric for the system throughput maximization as the brightness of the firefly, we apply the FA to the BF control. We evaluate the performance of BF control using the FA from the viewpoint of the geometric mean user throughput based on computer simulations.

Recently, non-orthogonal multiple access (NOMA) has gained more attention as a candidate multiple access scheme for future radio access. A major reason for this is that NOMA has the potential to achieve a more favorable tradeoff between system efficiency and user fairness than orthogonal multiple access (OMA), which is adopted in the 4th generation mobile communication systems such as LTE and LTE-Advanced. In this paper, we focus on downlink cellular systems. This paper gives an overview on the principle of NOMA and recent investigations. Several issues for implementing NOMA in real systems are also presented.

Power consumption of network (NW) equipment has been rapidly increasing; therefore it is necessary to build a resource-efficient NW. Router virtualization, which involves dynamically re-allocating virtual routers to physical resources as server virtualization, is becoming more common as a method of using NW equipment effectively and robustly. Edge routers which are gateways of core NWs should be virtualized because they have many functions and resources just as servers do. A metro NW is a wide area layer-2 aggregation NW that connects each user's residential gateway to edge routers. To achieve edge router virtualization, the metro NW must trace dynamic edge router re-allocation by changing the route of each Ethernet flow. Therefore, we previously proposed a virtual layer-2 switch architecture with scale-out control that can improve route control performance to trace dynamic virtual router re-allocation to use metro NW equipment effectively and robustly. The routes are controlled in parallel flow-by-flow on this architecture. When edge router failure occurs, the controller must change the routes of all flows passing through the failed edge router. On the other hand, load imbalance of this route change occurs among parallel processes. If we can distribute this load evenly, we can decrease resources for the controller deployed in advance. Therefore, in this paper we propose two re-allocation methods for allocating flows to parallel processes according to the virtual router re-allocation. We evaluated the methods through simulation and showed that they can evenly distribute load among parallel processes not only in large-scale metro NWs, but also in data center NWs, which have recently become an important type of large-scale layer-2 NW.

This paper proposes a proportional fair-based joint optimization method for cell association and the bandwidth ratio of protected radio resources exclusively used by pico base stations (BSs) for inter-cell interference coordination (ICIC) in heterogeneous networks where low transmission-power pico BSs overlay onto a high transmission-power macro BS. The proposed method employs an iterative algorithm, in which the cell association for a given bandwidth ratio of protected radio resources and the bandwidth ratio control of protected radio resources for a given cell association are alternately repeated for convergence. For cell association, we use our previously reported decentralized iterative cell association method based on the feedback information of each individual user assisted by a small amount of broadcast information from the respective BSs. Based on numerical results, we show that the proposed method adaptively achieves optimal cell association and bandwidth ratio control of protected radio resources, which maximizes the geometric mean user throughput within the macrocell coverage area. The system throughput for the proposed method is compared to that for conventional approaches to show the performance gain.

This paper proposes a joint control method for cell association and transmission power control of the pico base stations (BSs) in heterogeneous networks (HetNet) with inter-cell interference coordination (ICIC). The optimization criterion is proportionally fairness. The proposed method employs an iterative algorithm that alternately repeats the cell association for a given transmission power for all BSs and the transmission power control of the respective BSs for a given cell association for convergence. The proposed method assumes a decentralized approach and does not require inter-BS cooperation, which is an advantageous feature for real implementation. For cell association, we use our previously reported decentralized iterative cell association method, which achieves optimal cell association from the proportionally fairness viewpoint. The transmission power control step tries to increase the proportional fair metric taking into account the potential inter-cell interference. Based on numerical results, we show that the proposed method adaptively achieves appropriate cell association and transmission power control for a given user distribution within the macrocell coverage area, which contributes to increasing the geometric mean user throughput.

This paper comprehensively compares our two previously reported inter-cell interference coordination (ICIC) methods based on frequency block-dependent coordinated transmission power control (TPC) in non-orthogonal access with a successive interference canceller (SIC) in the cellular uplink. First ICIC method uses different target signal-to-noise ratios (SNRs) and compensation factors in fractional TPC (FTPC) among frequency blocks, which are coordinated among neighboring cells. Second ICIC method uses TPC based on inter-cell interference power in addition to the conventional received signal power-based FTPC, where the allowable inter-cell interference power among multiple frequency blocks within the system bandwidth are coordinated among neighboring cells. Based on computer simulation, we show that ICIC using either of our previously reported TPC methods enhances the system-level throughput compared to the conventional universal frequency reuse (UFR) without ICIC and fractional frequency reuse (FFR) since the proposed ICIC methods alleviate the degradation in multiuser diversity and the non-orthogonal user multiplexing effect compared to the conventional FFR, which achieves a similar ICIC effect. Between the two previous ICIC methods, the ICIC method using TPC based on inter-cell interference power achieves the best throughput performance.

This paper proposes a downlink transmission power control method using protected and non-protected bands, which are assumed in the inter-cell interference coordination (ICIC) strategy, in downlink heterogeneous networks. The proposed method requires inter-base station (BS) cooperation. However, the proposed method can be implemented by sharing only a single metric among BSs, which can contribute to a realistic implementation. In the proposed method, cooperating BSs share the tentative worst user throughput that is observed in the protected band within the cooperating cells. With this information, the transmission power of the protected band of each BS is controlled so that the worst user throughput within a macrocell coverage area is maximized. Different from our previously reported transmission power control method, the proposed method fully utilizes the remaining BS transmission power in the non-protected band. Therefore, the proposed method maintains the average user throughput at nearly the same level as that without transmission power control, while the worst user throughput is significantly improved. Thus, the proposed method enhances the user fairness within a macrocell coverage area. Multi-cell simulation evaluation shows that the proposed method yields advantageous throughput performance compared to conventional approaches.

This paper proposes a one-tap frequency-domain transmitter filtering method that assumes channel knowledge at the transmitter for turbo coded single-carrier transmission with our previously reported frequency-domain puncturing at the transmitter and turbo equalization at the receiver. In coded single carrier transmission in a frequency-selective fading channel, inter-symbol-interference (ISI) severely degrades the error rate at the receiver. Furthermore, frequency-domain puncturing causes additional ISI for the sake of increased number of coded bits to be transmitted. The proposed one-tap frequency-domain filtering method of the transmission signal at the transmitter reduces the ISI or increases the frequency diversity to increase the received signal power. In order to take into account the ISI reduction at the receiver due to turbo equalization, the proposed method is designed with the ISI level as a parameter. Simulation results show that by appropriately setting the ISI-level parameter according to the number of iterations in the turbo equalization, the proposed method reduces the error rate compared to the case without transmitter filtering. We also show the effectiveness of frequency-domain puncturing compared to the conventional time-domain puncturing with the number of receiver antennas as a parameter.

This paper extends our previously proposed non-orthogonal multiple access (NOMA) scheme to the base station (BS) cooperative multiple-input multiple-output (MIMO) cellular downlink for future radio access. The proposed NOMA scheme employs intra-beam superposition coding of a multiuser signal at the transmitter and the spatial filtering of inter-beam interference followed by the intra-beam successive interference canceller (SIC) at the user terminal receiver. The intra-beam SIC cancels out the inter-user interference within a beam. This configuration achieves reduced overhead for the downlink reference signaling for channel estimation at the user terminal in the case of non-orthogonal user multiplexing and also achieves the applicability of the SIC receiver in MIMO downlink. The transmitter beamforming (precoding) matrix is controlled based on open loop-type random beamforming using a block-diagonalized beamforming matrix, which is very efficient in terms of the amount of feedback information from the user terminal. Simulation results show that the proposed NOMA scheme with block-diagonalized random beamforming in BS cooperative multiuser MIMO and the intra-beam SIC achieves better system-level throughput compared to orthogonal multiple access (OMA), which is assumed in LTE-Advanced. We also show that BS cooperative operation along with the proposed NOMA in particular achieves a high cell-edge user throughput gain which implies better user fairness and universal connectivity.

This paper investigates a beamforming control method in base station (BS) cooperative multiuser multiple-input multiple-output (MIMO) transmission assuming a block-diagonalized unitary beamforming matrix. More specifically, we comprehensively compare open-loop type random (opportunistic) beamforming control to codebook-based closed-loop type beamforming control. The proportional fair (PF)-based scheduling is assumed, and the number of users, codebook size, and delay time of the feedback signal from the user terminals are taken into account in the comparison. Based on numerical results, codebook-based beamforming control with a large codebook size achieves higher system throughput than random beamforming when the number of users is small and the feedback delay time is short. However, we show that when the number of users is large and the feedback delay time is long, random beamforming achieves higher system-level throughput than codebook-based beamforming. This implies that the transmission-rate control with random beamforming is more robust against channel variation than that with codebook-based beamforming. These results suggest that random beamforming, which requires a lower feedback overhead than the codebook-based beamforming, is a promising beamforming strategy in future radio access.

We investigate the influence of channel estimation error on the achievable system-level throughput performance of non-orthogonal access with successive interference cancellation (SIC) in the cellular downlink. The channel estimation error in non-orthogonal access causes residual interference in the SIC process, which decreases the achievable user throughput. Furthermore, the channel estimation error causes error in the transmission rate control for the respective users, which may result in decoding error at not only the destination user terminal but also other user terminals for SIC. However, we show that by using a simple transmission rate back-off algorithm, the impact of the channel estimation error is effectively alleviated and non-orthogonal access with SIC achieves clear average and cell-edge user throughput gains relative to orthogonal access similar to the case with perfect channel estimation.

Power consumption of network equipment has been rapidly increasing, so it is therefore necessary to build a resource efficient network (NW). Router virtualization, which involves dynamically reallocating virtual routers to physical resources as a server virtualization, is becoming more common as a way to use network equipment effectively and robustly. Especially, edge router which is gateway of core network should be virtualized, because edge routers have many functions and resources just as servers do. A metro NW is wide area layer-2 aggregation NW that connects each user's residential gateway to edge routers. To achieve edge router virtualization, the metro NW must trace the dynamic edge router reallocation by changing a route of each Ethernet flow. Furthermore, access/metro NWs occupy a large proportion of the total NW power consumption, so logically centralized control architecture suits to the metro NW to change routes flexibly because it can use resources effectively by avoiding resource deployment in many locations. However, it is not scalable. When many Ethernet flows need to be accommodated in a metro NW, it can not change routes of the flows quickly enough. Therefore, we propose a virtual wide area layer-2 switch architecture with scale-out control that can improve the route control performance even in a worst case edge router node failure by processing in parallel flow-by-flow, by considering route information, and by rebalancing the load between parallel processes dynamically. We evaluated the proposed architecture through theoretical analysis and experiments with prototype. The results showed that the proposed architecture can increase the number of flows accommodated in a metro NW.

We propose an enhancement to the power allocation among multiple beams in our previously reported non-orthogonal access with random beamforming and intra-beam successive interference canceller (SIC) for cellular multiple-input multiple-output (MIMO) downlink. By equally allocating transmission power among multiple beams, the average user throughput within a cell can be enhanced due to the effective use of spatial multiplexing. The throughput of users at the cell edge may be improved by allocating non-equal transmission power among beams since the inter-beam interference is decreased and the received signal power is increased from the viewpoint of the main beam to which most of the transmission power is allocated. Aiming at further enhancement of the system efficiency and cell-edge user experience in our previous non-orthogonal access scheme, we propose coordinated frequency block-dependent inter-beam power allocation and inter-frequency block power allocation based on fractional frequency reuse (FFR) for inter-cell interference coordination. Based on simulation results, we show that the proposed power allocation method effectively enhances the tradeoff between the average and cell-edge user throughput.

We investigate non-orthogonal access with a successive interference canceller (SIC) in the cellular multiple-input multiple-output (MIMO) downlink for systems beyond LTE-Advanced. Taking into account the overhead for the downlink reference signaling for channel estimation at the user terminal in the case of non-orthogonal multiuser multiplexing and the applicability of the SIC receiver in the MIMO downlink, we propose intra-beam superposition coding of a multiuser signal at the transmitter and the spatial filtering of inter-beam interference followed by the intra-beam SIC at the user terminal receiver. The intra-beam SIC cancels out the inter-user interference within a beam. Furthermore, the transmitter beamforming (precoding) matrix is controlled based on open loop-type random beamforming, which is very efficient in terms of the amount of feedback information from the user terminal. Simulation results show that the proposed non-orthogonal access scheme with random beamforming and the intra-beam SIC simultaneously achieves better sum and cell-edge user throughput compared to orthogonal access, which is assumed in LTE-Advanced.

This paper proposes an extension to our previously reported simple decentralized cell association method for heterogeneous networks, where low transmission-power pico or femto base stations (BSs) overlay onto a high transmission-power macro BS, to accommodate the effects from a fading channel and proportional fair (PF)-based scheduling. The focus of this investigation is on the downlink and the purpose of the cell association is to increase the minimum user throughput (worst user throughput). In the proposed method, appropriate cell association for all users within a cell is achieved in an iterative manner based on the feedback information of each individual user regarding the throughput gain by connecting a new BS assisted by a small amount of broadcast information from the respective BSs regarding the allowable resource allocation and multiuser diversity gain. The proposed method does not require cooperation between BSs. The proposed method can be applied to the case with inter-cell interference coordination (ICIC) through the use of protected radio resources exclusively used by the pico/femto BSs. Based on numerical results, we show that the proposed method adaptively achieves better cell association for all users according to the user location distributions compared to the conventional cell range expansion (CRE) method. The advantage of the proposed method over CRE is further enhanced in an ICIC scenario.

This paper investigates the influence of channel state information (CSI) error at the transmitter on the achievable throughput of our previously reported layered partially non-orthogonal block diagonalization (BD) precoding method with adaptive interference admission control (AIAC) for downlink multiuser (MU) multiple-input multiple-output (MIMO) transmission employing multiple base station (BS) cooperation. The impacts of the CSI estimation error at the user terminal and feedback delay are separately examined and they are compared to conventional approaches. Computer simulation results reveal that the layered BD with AIAC is more robust against CSI error at the transmitter than full BD with full CSI feedback, which implies the effectiveness of the layered BD with AIAC method in practice.

This paper presents a non-orthogonal multiple access (NOMA) concept for cellular future radio access (FRA) towards the 2020s information society. Different from the current LTE radio access scheme (until Release 11), NOMA superposes multiple users in the power domain although its basic signal waveform could be based on the orthogonal frequency division multiple access (OFDMA) or the discrete Fourier transform (DFT)-spread OFDM the same as LTE baseline. In our concept, NOMA adopts a successive interference cancellation (SIC) receiver as the baseline receiver scheme for robust multiple access, considering the expected evolution of device processing capabilities in the future. Based on system-level evaluations, we show that the downlink NOMA with SIC improves both the capacity and cell-edge user throughput performance irrespective of the availability of the frequency-selective channel quality indicator (CQI) on the base station side. Furthermore, we discuss possible extensions of NOMA by jointly applying multi-antenna/site technologies with a proposed NOMA/MIMO scheme using SIC and an interference rejection combining (IRC) receiver to achieve further capacity gains, e.g., a three-fold gain in the spectrum efficiency representing a challenging target for FRA.

This paper proposes an enhancement to a previously reported adaptive peak-to-average power ratio (PAPR) reduction method based on clipping and filtering (CF) for eigenmode multiple-input multiple-output (MIMO) - orthogonal frequency division multiplexing (OFDM) signals. We enhance the method to accommodate the case with adaptive modulation and channel coding (AMC). Since the PAPR reduction process degrades the signal-to-interference and noise power ratio (SINR), the AMC should take into account this degradation before PAPR reduction to select accurately the modulation scheme and coding rate (MCS) for each spatial stream. We use a lookup-table-based prediction of SINR after PAPR reduction, in which the interference caused by the PAPR reduction is obtained as a function of the stream index, frequency block index, clipping threshold for PAPR reduction, and input backoff (IBO) of the power amplifier. Simulation results show that the proposed PAPR reduction method increases the average throughput compared to the conventional CF method for a given adjacent channel leakage power ratio (ACLR) even when we assume AMC.

This paper proposes a bi-directional signal detection and decoding method for our previously proposed hybrid automatic repeat request (HARQ) scheme using superposition coding (SPC-HARQ). The SPC-HARQ scheme multiplexes the initial and retransmission packets within the same frequency band using superposition coding to improve the transmission efficiency at retransmission. Since the superposition coding in the SPC-HARQ can be viewed as packet-level convolutional coding, the receiver can detect and decode the series of received packets in the reverse order of the time slots in addition to the forward order, that is, bi-directional detection and decoding. Simulation results show that the proposed bi-directional signal detection and decoding in the SPC-HARQ improves the throughput performance compared to when using the conventional HARQ and SPC-HARQ with forward decoding only.

This paper investigates the application of a previously proposed adaptive peak-to-average power ratio (PAPR) reduction method based on clipping and filtering (CF) to block diagonalization-based multiuser multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) transmission. Assuming that the total number of receiver antennas is less than that for the transmitter antennas, we introduce virtual streams used only for PAPR reduction and the precoding matrix for these virtual streams, where the virtual streams do not impart interference to the actual data streams. Since the interference caused by the clipping operation is concentrated in the virtual streams and consequently restricts the interference to the actual data stream, the proposed method can significantly improve the sum capacity compared to the conventional CF method for a given PAPR. Simulation results show the capacity gain using the proposed method compared to the conventional method.

This paper investigates a precoding method in downlink multiuser multiple-input multiple-output (MIMO) transmission employing multiple base station (BS) cooperation where the sets of user equipment basically feedback the instantaneous channel state information (CSI) only to the nearest BS, but the users near the cell edge additionally feedback the instantaneous CSI to the second nearest BS among the cooperating BSs. The proposed precoding method is categorized into multi-cell processing, in which the transmission information sent to a user is shared by the cooperating BSs in order to utilize fully the degrees of freedom of the spatial channel, and is based on block diagonalization of the channel matrix. However, since some elements of the channel matrix are unknown, we allow inter-user interference to users with limited instantaneous CSI feedback. More specifically, in the proposed method, the elements corresponding to the unknown instantaneous CSI in the overall channel matrix are muted by setting these elements to zero, and the standard generation procedure of the block diagonalization precoding matrix is performed on the muted channel matrix. The proposed method efficiently utilizes the degrees of freedom of the spatial channel compared to the case with full orthogonal transmission at the cost of increased inter-user interference. Simulation results show that the proposed method decreases the transmission power for a given capacity requirement compared to the case with full orthogonal transmission.

This paper investigates the enhancement of cell-edge user throughput by using non-orthogonal access with a successive interference canceller (SIC) in the cellular uplink compared to orthogonal access, which is widely used in 3.9 and 4G mobile communication systems. Since both the total user throughput and cell-edge user throughput are important in a real system, we compare the cell-edge user throughput, while subject to the same total user throughput. The optimum resource allocation such as bandwidth and transmission power for each user is assumed for the respective access schemes based on the instantaneous channel conditions. Based on the evaluation results, we show that non-orthogonal access with a SIC can significantly enhance the cell-edge user throughput (thus user fairness) compared to the orthogonal access while achieving the same total user throughput in the context of the cellular uplink, which can yield some insights regarding the direction of the wireless access scheme for the systems beyond IMT-Advanced.

This paper proposes a channel-dependent adaptive spreading code selection method in downlink multicarrier code division multiple access (MC-CDMA). The proposed method is compared to the conventional MC-CDMA and orthogonal frequency division multiple access (OFDMA). First we show that the signal-to-noise and interference power ratio (SINR) after minimum mean squared error (MMSE)-based equalization is represented as a weighted average of the instantaneous SNR of the respective subcarriers to which the spread symbols are mapped, where the weighting factor is determined by the respective chip power of the spreading sequence. Based on this, we propose a method for channel-dependent adaptive spreading code selection in which the minimum SINR among code multiplexed users is maximized. The simulation results show that the proposed adaptive spreading code matrix selection increases the minimum SINR at the outage probability of 10-2 by approximately 1 dB compared to the OFDMA with channel-dependent subcarrier allocation (frequency-domain scheduling).

In this paper, in order to achieve the available spatial, temporal and frequency diversities, and also make the system implementation feasible for high speed orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) multiplexing, a novel LSTF transmitter architecture with multiple channel encoders is proposed with each independently coded information sub-stream being threaded in the three-dimensional (3-D) space-time-frequency transmission resource array. The performance of a receiver consisting of a maximum-likelihood detector with QR decomposition and M-algorithm (QRM-MLD) is exploited, by employing irregular low-density parity-check (LDPC) code as the channel code. The threaded distribution of each coded information sub-stream in the proposed LSTF makes it to achieve the spatial, temporal and frequency diversities the same as the conventional single-encoder LSTF where coding is applied across the whole information stream, and simulation results show that the performance of the proposed multiple-encoder LSTF architecture is very close to that of the conventional single-encoder LSTF. However, due to the use of multiple parallel encoders/decoders with a shorter codeword length, the proposed LSTF architecture has much lower hardware processing speed and complexity than the conventional LSTF.

We consider a precoding method in downlink multiuser multiple-input multiple-output (MIMO) transmission with multiple base station (BS) cooperation where some of the instantaneous channel state information (CSI) is not available at the BSs. The proposed precoding method is based on block diagonalization of the channel matrix. However, since some elements of the channel matrix are unknown, we allow a partially non-orthogonal transmission. More specifically, we allow inter-user interference to users with limited instantaneous CSI feedback from the channel where the instantaneous CSI of that user is not obtained at the BSs. The other inter-user interferences are set to zero based on the block diagonalization of the channel matrix. The proposed method efficiently uses the degree of freedom of the spatial channel compared to the case with full orthogonal transmission at the cost of increased inter-user interference to the users with limited instantaneous CSI feedback. Simulation results show that the proposed method increases the capacity for a given transmission power or decreases the transmission power for a given capacity requirement compared to the case with full orthogonal transmission.

This paper presents a new peak-to-average power ratio (PAPR) reduction method based on clipping and filtering for precoded orthogonal frequency division multiplexing (OFDM)-multiple-input multiple-output (MIMO) transmission. While the conventional clipping and filtering method adds roughly the same interference power to each of the transmission streams, the proposed method restricts the interference power applied to the streams that experience good channel conditions. Since the sum capacity is dominated by the capacity of the streams that experience good channel conditions and the interference caused by the PAPR reduction process severely degrades the achievable capacity of these streams, the proposed method can significantly improve the achievable sum capacity compared to the conventional clipping and filtering method for a given PAPR. Simulation results demonstrate the capacity gain when using the proposed method compared to that when using the conventional method.

This paper investigates a tradeoff between the peak-to-average power ratio (PAPR) and channel capacity of the proposed PAPR reduction method based on clipping and filtering (CF) for precoded multiple-input multiple-output (MIMO)- orthogonal frequency division multiplexing (OFDM) transmission in a frequency-selective faded channel. While the conventional CF method adds roughly the same interference power to each of the transmission streams, the proposed method restricts the interference power applied to the streams that experience good channel conditions. Since the sum capacity is dominated by the capacity of the streams that experience good channel conditions and the interference caused by the PAPR reduction process severely degrades the channel capacity of these streams, the proposed method can significantly improve the sum capacity compared to the conventional CF method for a given PAPR.

This paper presents a frequency-domain puncturing method for turbo codes. Conventional rate control for turbo codes is achieved by time-domain puncturing where some of the coded bits (parity bits) are punctured to increase the coding rate. The proposed puncturing method is done in the frequency domain. After applying the discrete Fourier transform (DFT) to the transmission coded symbols, some of the frequency components (spectrum) are punctured to increase the coding rate. The difference from the conventional time-domain puncturing is that the frequency-domain puncturing can transmit all the coded bits. Therefore, the coding gain is expected to increase. However, the frequency-domain puncturing operation causes inter-symbol interference (ISI). Therefore, we propose applying frequency-domain turbo equalization based on a soft canceller followed by the minimum mean squared error filter (SC/MMSE) at the receiver to mitigate the ISI. The simulation results show that the additional use of the frequency-domain puncturing can improve the bit error rate (BER) and frame error rate (FER) compared to pure time-domain puncturing.

This paper considers applying hybrid automatic repeat request (HARQ) to the predetermined-rate multicast data channel. The major differences in the HARQ design between multicast data and normal unicast data come from the fact that the multicast data assumes a predetermined data-rate using a given transmission frequency bandwidth and power, and multiple independent acknowledgement (ACK) and negative-acknowledgement (NAK) feedback from the mobile terminals. We present a new HARQ method based on superposition coding appropriate for the predetermined-rate multicast data. Computer simulation results reveal that the throughput performance is improved by using the proposed HARQ based on the superposition coding especially when there are many mobile terminals and the required data rate is relatively high.

The long-term evolution (LTE), which was specified as the release 8 specification in the 3rdgeneration partnership project (3GPP), will achieve broadband services with the peak data rate of 100 Mbps in the downlink. This paper presents the measured throughput performance and power consumption performance of the implemented demodulation and decoding LSI for 2-by-2 MIMO-OFDM in the Rel-8 LTE downlink. The implemented LSI comprises FFT timing detection, a 2048-point FFT, a channel estimation filter, maximum likelihood detection (MLD) based signal detection, a channel deinterleaver, and a turbo decoder with the max-log-MAP (maximum a posteriori) algorithm. The most prominent feature of the implemented LSI is MLD based signal detection with low power consumption and a low required received signal-to-noise power ratio (SNR) for 2-by-2 MIMO multiplexing. Experiments show that the implemented LSI achieves the measured throughput of 100 Mbps at the average received SNR of 22 dB with a low power consumption level of approximately 40 mW with 16QAM and the channel coding rate of 8/9.

This paper proposes the optimum radio link connection (RLC) using remote radio equipment (RRE) in a heterogeneous network in which various types of cell configurations exist with different cell sizes and downlink transmission power levels for LTE (Long-Term Evolution)-Advanced. The instantaneous received signal-to-interference plus noise power ratio (SINR) and the user throughput performance employing the proposed RLC methods are investigated using system-level simulations. From the simulation results, we show that the proposed independent RLC method between the downlink and uplink and the proposed multipoint RLC method based on coordinated multipoint transmission/reception achieve a higher user throughput both in the downlink and uplink compared to the conventional common RLC method. Therefore, the proposed independent and multipoint RLC methods are very promising candidates to improve the user throughput particularly near the cell edge in a heterogeneous network for LTE-Advanced.

This paper investigates the optimum radio parameters, i.e., numerology, in layered orthogonal frequency division multiple access (OFDMA) for long-term evolution (LTE)- Advanced. Similar to the layered OFDMA concept, it was agreed at the 3GPP RAN WG1 meeting that the entire transmission bandwidth in LTE-advanced should comprise multiple frequency blocks called component carriers to achieve a wider system bandwidth than that for the current LTE (Rel-8 LTE). We investigate the optimum radio parameters in the frequency domain including the need for an inter-component carrier guard band (GB). In the simulation evaluations, we set the parameters for the receiver filter assuming the worst case that satisfies the minimum requirement specified in Rel-8 LTE. Simulation results show that the loss in the average received signal energy per symbol-to-noise power spectrum density ratio (Es/No) satisfying the required block error rate compared to the case without an inter-component carrier GB is negligible irrespective of the modulation scheme and coding rate. As a result, we conclude that the inter-component carrier GB is unnecessary and can be minimized to 19 subcarriers in the downlink while retaining the same number of subcarriers within a component carrier as that for Rel-8 LTE.

This paper presents a new maximum likelihood detection (MLD)-based signal detection method for orthogonal frequency division multiplexing (OFDM) multiple input multiple output (MIMO) multiplexing with frequency domain spreading and code multiplexing. The proposed MLD reduces the computational complexity by utilizing signal orthogonalization based on QR decomposition of the product of the channel and spreading code matrices. Simulation results show that when the spreading factor and number of code multiplexing are 16, the proposed MLD reduces the average received signal energy per bit-to-noise spectrum density ratio (Eb/N0) for the average packet error rate (PER) of 10"2 by approximately 12 dB compared to the conventional minimum mean squared error (MMSE)-based filtering for 4-by-4 MIMO multiplexing (16QAM with rate-3/4 Turbo code is assumed).

Multi-carrier based radio access schemes have been widely accepted as one of the PHY candidates in many standards, for their capabilities not only to combat multi-path propagation, but also to support multiple user access aided by multi-user scheduling. The performance of multi-user wireless system with multi-user scheduling depends on the fluctuation of independent channel response across different users. However, the channel selectivity may be limited in some channel conditions, e.g., light-of-sight (LOS) or spatial correlation, which may limit the multiuser diversity gain. Multi-degree random cyclic delay diversity (MD-RCDD) (Zhengang Pan et al., 2006) scheme is introduced to obtain opportunistic beamforming gain with intensified frequency fluctuation of channel response. However, the available delay sets in MD-CDD are randomly generated, not fully utilizing the spatial resources. Therefore, multi-degree adaptive cyclic delay diversity (MD-ACDD) scheme is proposed in this paper to further optimize the system performance, in which the available delay sets are channel-dependent, and generated based on the feedback information of all users. Moreover, the detailed working procedure and overhead analysis are presented, and simulation results show that proposed scheme can achieve significant performance gain compared with conventional MD-RCDD.

This paper proposes layered orthogonal frequency division multiple access (OFDMA) radio access for LTE (long-term evolution)-advanced to achieve higher-level system requirements than those in Release 8 LTE called Evolved UMTS terrestrial radio access (UTRA) and UMTS terrestrial radio access network (UTRAN). Layered OFDMA comprises layered transmission bandwidth assignment according to the required data rate, a layered control signaling structure, and support for layered environments in which an adaptive multi-access scheme with hybrid single-carrier and multicarrier based radio access is applied. layered OFDMA radio access will support all functionalities provided in Release 8 LTE and its enhancement. Key radio access techniques such as fast inter-cell radio resource management that take advantage of remote radio equipment (RRE) aiming at inter-cell orthogonality, multi-antenna transmissions with more antennas, and coverage enhancing techniques are used to achieve a high level of capacity and cell-edge spectrum efficiency.

This paper presents indoor and field experimental results on extremely high-speed packet transmissions of 4.92 Gbps in a 100 -MHz channel bandwidth, i.e., the frequency efficiency of approximately 50 bit/second/Hz, in the downlink OFDM radio access. The required received signal-to-noise power ratio (SNR) is less than 30dB, which is almost the upper limit in cellular environments near a cell site even with a light channel load, with the aim of application to future universal broadband packet radio access. We apply 12-by-12 MIMO multiplexing using the maximum likelihood detection employing QR decomposition and the M-algorithm (QRM-MLD) with adaptive selection of the surviving symbol replica candidates (ASESS),64QAM data modulation, and Turbo coding with the coding rate of R = 8/9 to achieve an extremely high frequency efficiency level. In the field experiments conducted in the Yokosuka Research Park (YRP) district of Yokosuka city, we show that the measured throughput of 4.92 Gbps is achieved at the average received SNR per receiver antenna of approximately 28.0 and 28.5 dB when the respective receiver antenna separation is 40 cm (6.2lambda) and 10 cm (1.5lambda) and the transmitter antenna separation is 70 cm (10.9lambda) at the average speed of 10 km/h under non-line-of-sight (NLOS) conditions.

This paper proposes the application of closed-loop type antenna switching transmit diversity (ASTD) associated with frequency domain channel-dependent scheduling to single-carrier frequency division multiple access radio access in the evolved UTRA (UMTS terrestrial radio access) uplink. In the proposed scheme, virtual resource units (RUs) are assigned to the optimum transmitter antenna in the spatial domain, while they are also assigned to the optimum frequency band (i.e., physical RUs) in the frequency domain according to the propagation channel conditions of each user. The reference signal overhead is the same as that for single-antenna transmission and the impact on the hardware implementation in the user equipment is slight since only one RF chain is necessary. Simulation results show that the gain of the closed-loop type ASTD is approximately 1.2-1.5 dB at the 1% level in the cumulative distribution function, assuming unequal antenna gain, i.e., the difference in the average transmission power of 3 dB and the fading correlation between transmitter antennas of 0.3 when frequency domain channel-dependent scheduling is applied with hybrid ARQ and soft-combining.

This paper presents the optimum channel coding and multiplexing schemes for Layer 1 (Ll)/Layer 2 (L2) control signals for OFDM based radio access in the Evolved UTRA downlink. Simulation results show that separate channel coding of downlink-related L1/L2 control signals is superior to joint coding among simultaneous accessing user equipments (UEs), from the viewpoint of the required total symbol energy of the L1/L2 control signals. The results also show that time division multiplexing (TDM) based multiplexing associated with transmission time interval (TTI)-by-TTI frame format control employing a control channel format indicator (CCFI) achieves almost the same cell throughput as that for frequency division multiplexing (FDM). Therefore, considering the advantages of TDM such as the possibility of a power saving effect through micro-sleep operation and a short control loop delays for scheduling, adaptive modulation and coding, and hybrid ARQ, we conclude that TDM based multiplexing of L1/L2 control signals with a shared data channel is superior to using FDM based multiplexing.

This paper proposes a handover method that uses a combination of bicast and forwarding (BIFO) of IP packets to achieve a short handover delay time and a low amount of IP packet traffic in the backhaul for future IP-based radio access networks (RANs). To validate the effect of the proposed BIFO method, we implemented prototype experimental equipment comprising a RAN_access router (RAN_AR), base stations (BSs) associated with a radio control server (RCS), and user equipment (UE) including a controller. The experimental results show that the BIFO achieves a shorter handover delay time than when using only the conventional bicast method or forwarding method by taking advantage of the respective merits of bicast and forwarding. The results also confirm that the BIFO method achieves the handover delay time in the control plane of approximately 10 - 20 msec.

For orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) multiplexing in high speed wireless transmission under broadband fading channel, a novel coded layered space-time-frequency (LSTF) architecture is proposed, which can achieve the available spatial, temporal and frequency diversity, and make the system implementation easy for high data rate transmission. In this novel architecture, each independently coded layer is threaded in the three-dimensional space-time-frequency transmission resource array. Channel estimation based on a time-multiplexed pilot channel is employed. The convolutional code is used as the constituent code, and the iterative receiver structure consisting of a linear minimum-mean-square-error soft-interference- cancellation (LMMSE-SIC) detector and the soft-input soft-output (SISO) maximum a posteriori (MAP) decoders is adopted. Simulation results show that the proposed LSTF architecture can considerably outperform the LSTF (i.e., LSTF-b) in which each independently coded layer is associated with a permanently assigned antenna,, and get almost the same performance as the LSTF (i.e., LSTF-a) where coding is applied across the whole information stream. Since its structure consists of multiple parallel lower-speed encoders/decoders with shorter codeword length, the proposed LSTF architecture can be much more easily implemented than the LSTF-a. Moreover, four iterations for joint detection and decoders are sufficient, and the percentage of the pilot channel energy in one packet should be in the range of [16%, 24%] to provide near optimum performance.

Single-Carrier (SC)-FDMA based radio access in the evolved UTRA uplink that is necessary for downlink scheduling, link adaptation, hybrid ARQ (automatic repeat request) with soft-combining, and for uplink feedback control signaling. We propose SC-based multiplexing of L1/L2 control signaling with a shared data channel for a set of user equipment (UE) that transmits both an uplink shared data channel and L1/L2 control channel within the same sub-frame. We also propose an L1/L2 control signaling multiplexing scheme without uplink data transmission that takes advantage of frequency hopping using multiple exclusively-assigned time-frequency resources to achieve the frequency diversity effect. Computer simulation results show that the proposed SC-based multiplexing of the L1/L2 control channel with the shared data channel achieves a higher user throughput than multicarrier-based multiplexing. The results also show that the proposed L1/L2 control signaling multiplexing scheme for the UE without uplink data transmission achieves both high quality reception through large frequency diversity and an increase in coverage through full use of the transmission time duration compared to the conventional schemes.

This paper presents the optimum primary- synchronization channel (P-SCH) and secondary-SCH (S-SCH) sequences for the SCH based on criteria to achieve fast initial and neighboring cell search time performance in the Evolved UTRA (E-UTRA) downlink. Simulation results show that the P-SCH sequence has a slight influence on the achievable cell search time, and that the application of multiple sequences, i.e., three P-SCH sequences, is beneficial in achieving fast cell search time performance particularly for the neighboring cell search. The results also show that the Walsh-Hadamard sequence or cyclic-shifted orthogonal sequence is suitable for the S-SCH sequence to indicate the cell ID group. Furthermore, it is shown that the combination of a permutation of the orthogonal S-SCH and P-SCH-specific scrambling sequences is advantageous in suppressing the cross-correlation between S-SCHs that collide and in achieving a fast cell search time particularly for the neighboring cell search in inter-node B synchronous operation.

This paper presents a performance comparison of the initial cell search time and neighboring cell search time using hierarchical and non-hierarchical synchronization channel (SCH) structures in order to establish the optimum SCH structure in the evolved UTRA downlink. Computer simulation results show that in a 19-cell configuration, the cell search time at the 90% cumulative distribution function using the hierarchical SCH structure is less than half that using the non-hierarchical SCH structure in a neighboring cell search under low signal-to-interference plus noise power ratio conditions, although both structures achieve almost the same cell search time in the initial cell search. This is due to the cross-correlation based SCH symbol timing detection in the hierarchical SCH structure, which is affected less by noise than auto-correlation based detection in the non-hierarchical SCH structure. Thus, we conclude that the hierarchical SCH structure is more appropriate than the non-hierarchical SCH structure based on the cell search time performance especially in the neighboring cell search.

This paper proposes a group-wised reference signal (RS) for channel quality indicator measurement in frequency domain channel-dependent scheduling employing adaptive control of the transmission bandwidth in the single-carrier frequency division multiple access-based evolved UMTS terrestrial radio access uplink. In the proposed method, fluctuation in the interference power imparted to the surrounding cells is suppressed to a low level, and both the user throughput for a user equipment (UE) near the cell edge and the cell throughput are improved compared to the conventional uniform RS allocation due to reductions in the error when selecting the optimum UE in channel-dependent scheduling and selecting the modulation and coding scheme in the adaptive modulation and channel coding. Simulation results show that the user throughput at the 5% cumulative distribution function value when employing the proposed group-wised sounding RS allocation with the adaptive transmission bandwidth is increased by approximately 75% and 65% for the inter-site distance of 500 and 1732 m, respectively, compared to that using a uniform RS allocation, while achieving almost the same level of cell throughput.

This paper presents indoor and field experimental results on extremely high-speed packet transmissions of 4.92 Gbps in a 100-MHz channel bandwidth, i.e., the frequency efficiency of approximately 50 bit/second/Hz, in the downlink orthogonal frequency division multiplexing (OFDM) radio access. The required received signal-to-noise power ratio (SNR) is less than 30 dB, which approaches the almost upper limit in cellular environments near a cell site even with a light channel load, with the aim of the application to future universal broadband packet radio access. We apply 12-by-12 MIMO multiplexing using the maximum likelihood detection employing QR decomposition and the M-algorithm (QRM-MLD) with adaptive selection of the surviving symbol replica candidates (ASESS), 64QAM data modulation, and turbo coding with the coding rate of R = 8/9 to achieve an extremely high frequency efficiency level. In the field experiments conducted in the Yokosuka Research Park (YRP) district of Yokosuka city, we show that the measured throughput of 4.92 Gbps is achieved at the average received SNR per receiver antenna of approximately 28.0 and 28.5 dB when the receiver antenna spacing is 40 cm (6.2lambda), and 10 cm (1.5lambda) at the average speed of 10 km/h, respectively.

In the evolved UTRA (UMTS terrestrial radio access) uplink, single-carrier frequency division multiple access (SC-FDMA) radio access was adopted owing to its advantageous low peak-to-average power ratio (PAPR) feature, which leads to wide coverage area provisioning with limited peak transmission power of a set of user equipment (UE). This paper proposes orthogonal pilot channel generation using the combination of FDMA and CDMA in the SC-FDMA-based evolved UTRA uplink. In the proposed method, we employ distributed FDMA transmission for simultaneous accessing users with different transmission bandwidths, and employ CDMA transmission for simultaneous accessing users with identical transmission bandwidths. Moreover, we apply a code sequence with a good auto-correlation property such as a constant amplitude zero auto-correlation (CAZAC) sequence employing a cyclic shift to increase the number of sequences. Simulation results show that the average packet error rate (PER) performance using an orthogonal pilot channel with the combination of FDMA and CDMA in a six-user environment, i.e., four users each with a 1.25-MHz transmission bandwidth and two users each with a 5-MHz transmission bandwidth, employing turbo coding with the coding rate of R = 1/2 and QPSK and 16QAM data modulation coincide well with those in a single-UE environment with the same transmission bandwidth. We show that the proposed orthogonal pilot channel structure using the combination of distributed FDMA and CDMA transmissions and the application of the CAZAC sequence is effective in the SC-FDMA-based evolved UTRA uplink.

In the evolved UTRA (UMTS terrestrial radio access) downlink, orthogonal frequency division multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference and flexible accommodation of different spectrum arrangements. This paper presents the optimum adaptive modulation and channel coding (AMC) scheme for simultaneously assigned resource blocks (RBs) for the same user when frequency and time domain channel-dependent scheduling is used in the downlink OFDMA radio access with single-antenna transmission. Theoretical analyses and simulation results show that, irrespective of the application of power adaptation to RB-dependent modulation, the improvement in the achievable throughput of the RB-dependent modulation scheme compared to that for the RB-common modulation scheme is slight, i.e., 4 to 5%. In addition, the number of required control signaling bits in the RB-dependent modulation scheme becomes greater than that for the RB-common modulation scheme. Therefore, the authors conclude that the RB-common modulation and channel coding rate scheme is preferred, when multiple RBs of the same coded stream are assigned to one user in the case of single-antenna transmission.

This paper proposes physical channel structures and a cell search method for OFDM based radio access in the evolved UTRA (UMTS Terrestrial Radio Access) downlink, which supports multiple scalable transmission bandwidths from 1.25 to 20 MHz, in the proposed physical channel structures, the central sub-carrier of the OFDM signal is located on the frequency satisfying the 200-kHz raster condition regardless of the transmission bandwidth of the cell site. Moreover, the synchronization channel (SCH) and broadcast channel (BCH), which are necessary for cell search, are transmitted from the central part of the entire transmission spectrum with a fixed bandwidth. In the proposed cell search method, a user equipment (UE) acquires the target cell in the cell search process in the initial or connected mode employing the SCH and possibly the reference signal, which are transmitted from the central part of the given transmission bandwidth. Alter detecting the target cell, the UE decodes the common control information through the BCH, which is transmitted at the same frequency as the SCH, and identifies the transmission bandwidth of the cell to be connected. Computer simulations show the fast cell search time performance using the proposed SCH structure and the cell search method.

For high speed orthogonal frequency division multiplexing (OFDM) plus multiple-input multiple-output (MIMO) multiplexing, a new coded layered space-time-frequency (LSTF) architecture with iterative signal processing at the receiver is proposed, where each independent codeword is threaded in the three-dimensional (3D) space-time-frequency transmission resource array. The iterative receiver structure is adopted consisting of a joint minimum-mean-square-error soft-interference-cancellation (MMSE-SIC) detector and the maximum a posteriori (MAP) convolutional decoders. Simulation results show that the proposed LSTF architecture can achieve almost the same performance as the LSTF (i.e., LSTF-a) where coding is applied across the whole information stream. However, due to its structure of multiple parallel lower speed encoders/decoders with shorter codeword length, the proposed LSTF architecture can be more easily implemented than the LSTF-a.

Abstract This paper describes the concept of Super 3G, the long‐term evolution (LTE) of 3G mobile communications systems. The objectives, technical proposals, the 3rd Generation Partnership Project (3GPP) standardization activities, and recent research activities on 4G radio access technologies are also described. Copyright © 2007 John Wiley & Sons, Ltd.

This paper proposes an intra-node B orthogonal pilot channel structure employing a sector-specific orthogonal sequence and cell-specific (node B-specific) scrambling code for orthogonal frequency division multiplexing (OFDM)-based radio access in the E-UTRA downlink. This paper also proposes a sophisticated orthogonal pilot channel employing code division multiplexing (CDM) to separate sectored beams and distributed frequency division multiplexing (FDM) to separate streams (transmission antennas) in MIMO channel transmissions simultaneously. Successive simulation results verify the effectiveness of the proposed orthogonal pilot channel structures using distributed FDM among multiple streams and CDM between sectored beams. Furthermore, the simulation results also show that the average packet error rate (PER) performance using the proposed intra-node B orthogonal pilot channel is significantly improved compared to that of the conventional structure employing a random sequence, particularly for 16QAM and 64QAM modulations under high intra-node B interference-to-desired signal power ratio conditions

This paper compares the user throughput and the sector throughput performance using fast sector selection (FSS) with muting and simultaneous transmission with soft-combining for the intra-node B macro diversity in the downlink OFDM based radio access. System-level simulations show that FSS with muting achieves an approximately 10% higher user throughput for intra-node B macro diversity user equipment (UE) compared to simultaneous transmission with soft-combining, while maintaining almost the same user throughput performance for one-link connection UE assuming the equal served traffic model. Therefore, we conclude that FSS with muting is a more promising candidate than simultaneous transmission with soft-combining as the intra-node B macro diversity scheme in the downlink OFDM based radio access

This paper presents the optimum resource block (RB) bandwidth for frequency and time domain channel-dependent scheduling based on system-level simulations in the downlink orthogonal frequency division multiplexing (OFDM) based radio access. The simulation results clarifies that the optimum RB bandwidth is within the range of approximately 150 to 375 kHz assuming typical power delay profile models in a cellular system such as typical urban, pedestrian-B, and vehicular-A modes, from the viewpoint of the multiuser diversity effect gained in frequency domain channel-dependent scheduling. The results also show that considering the overhead loss by the control signaling bits for scheduling, adaptive modulation and coding and hybrid ARQ with packet combining, the optimal RB bandwidth is in the range of approximately 375 to 750 kHz

This paper proposes frequency and time domain channel-dependent scheduling employing an adaptive transmission bandwidth for the pilot channel for channel quality indication (CQI) measurement in the uplink single-carrier (SC)-FDMA radio access. In the proposed scheme, a high multiuser diversity effect due to frequency domain channel-dependent scheduling is gained by assigning a wide pilot transmission bandwidth to user equipments (UEs) near a cell site, whereas the influence of the CQI measurement error is mitigated by assigning a narrow pilot transmission bandwidth to UEs near the cell boundary at the cost of utilizing multiuser diversity in the frequency domain. System simulation results elucidate that the cell throughput using the adaptive pilot transmission bandwidth with the predetermined threshold, THSINRof -6 dB is increased by approximately 600 kbps compared to that using a fixed 1.25-MHz pilot transmission bandwidth, i.e., only time domain channel-dependent scheduling, owing to an increasing multiuser diversity effect in the frequency domain. We also show that the user throughput using the adaptive pilot channel transmission bandwidth scheme at a 5% cumulative distribution function is increased by approximately 35 and 30% compared to that using a 5-MHz transmission for 16 and 32 UEs per cell, respectively, for the inter-site distance of 500 m

We propose a pseudo-inverse minimum mean squared error (MMSE) based QR decomposition (QRD) M-algorithm (QRD-M) for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithm can effectively mitigate the detrimental effect of MIMO channel ill-conditions on the bit error rate (BER) performance. As a result, the number of retained branches (M value) can be reduced to half of the original QRD-M algorithm in [1] in order to achieve the same performance for both the uncoded and coded systems. The detection complexity is therefore greatly reduced.

This paper presents field experimental results on real-time 1-Gbps packet transmission using antenna-dependent adaptive modulation and channel coding (AMC) with 4-by-4 MIMO multiplexing using maximum likelihood detection employing QR decomposition and the M-algorithm (QRM-MLD) with adaptive selection of the surviving symbol replica candidates (ASESS) in the downlink orthogonal frequency division multiplexing (OFDM) radio access. The field experiments, which are conducted at the average moving speed of 30 km/h, show that the real-time packet transmission of greater than 1 Gbps in a 100-MHz channel bandwidth (i.e., 10 bits/second/Hz) is achieved at the average received signal-to-interference plus background noise power ratio (SINR) of approximately 13.5 dB using 16QAM modulation and turbo coding with the coding rate of 8/9, The results also show that the antenna-dependent AMC is very beneficial to improve the measured throughput in a real propagation channel environments, where the average received power associated with each transmitted signal varies with the difference of greater than approximately 6 dB. Finally, we show that the measured throughput of greater than 1 Gbps is achieved employing antenna-dependent AMC at the probability of approximately 98% in a measurement course, where the maximum distance from the cell site was approximately 300 m with the respective transmitter and receiver antenna spacings of 1.5 m and 40 cm with the total transmission power of 10 W

This paper is focused on the study of layered space-time-frequency (LSTF) architectures with channel coding for orthogonal frequency division multiplexing (OFDM) multiple-input multiple-output (MIMO) multiplexing system in high speed wireless transmission under frequency-selective fading channel. In order to achieve the available spatial, temporal and frequency diversity, and at the same time make the system implementation feasible for ultra high data rate OFDM MIMO multiplexing, we propose a new LSTF architecture with each independently coded layer being threaded in the three-dimensional space-time-frequency transmission array. List sphere decoding is used for MIMO multiplexing detection, and the irregular low-density parity-check code (LDPC) is chosen as the channel code. Three construction rules are adopted to generate high coding rate irregular LDPC codes with low encoding complexity and little performance loss. Simulation results show that the proposed LSTF architecture can significantly outperform the LSTF assuming each independently coded layer being transmitted through a permanently assigned antenna, and get almost the same performance as the LSTF applying coding across the whole information stream. On the other hand, due to its structure of multiple parallel lower-speed codecs with shorter codeword length, the proposed LSTF architecture is more realizable for ultra high data rate transmission than the LSTF applying coding across the whole information stream

This paper presents experimental results on real-time packet transmission of greater than 1 Gb/s using 4-by-4 multiple-input-multiple-output (MIMO) multiplexing and maximum-likelihood detection (MLD)-based signal detection with a decreased level of computational complexity in orthogonal frequency-division multiplexing (OFDM) radio access. We apply our previous algorithm called adaptive selection of surviving symbol replica candidates (ASESS) based on the maximum reliability in MLD employing QR decomposition and the M-algorithm (QRM-MLD) to reduce the extremely high level of computational complexity in the conventional MLD. The experimental results using multipath fading simulators are in good agreement with the computer simulation results. The loss in the required received signal energy per bit-to-background noise power spectrum density ratio (Eb/N0) is suppressed to within approximately 1-2 dB. Therefore, through experiments, we demonstrate that the QRM-MLD employing ASESS is very beneficial in reducing the influence of hardware implementation loss, as well as in decreasing the required received Eb/N0. We further show that the extremely high-speed real-time packet transmission of greater than 1 Gb/s in a 100-MHz channel bandwidth (i.e., 10 bit/s/Hz) is achieved at the average received Eb/N0per receiver antenna of approximately 12 dB using 16QAM modulation and turbo coding with the coding rate of 8/9 in 4-by-4 MIMO multiplexing

This paper proposes adaptive control of the number of surviving symbol replica candidates, Sm(m denotes the stage index), based on the minimum accumulated branch metric of each stage in maximum-likelihood detection employing QR decomposition and the M-algorithm (QRM-MLD) in orthogonal frequency-division multiplexing with multiple-input-multiple-output (MIMO) multiplexing. In the proposed algorithm, Smat the mth stage (1lesmlesNt, Ntis the number of transmission antenna branches) is independently controlled using the threshold value calculated from the minimum accumulated branch metric at that stage and the estimated noise power. We compared the computational complexity of QRM-MLD employing the proposed algorithm with that of conventional methods at the same average packet error rate assuming the information bit rate of 1.048 Gb/s in a 100-MHz channel bandwidth (i.e., frequency efficiency of approximately 10 bit/s/Hz) using 16QAM modulation and turbo coding with the coding rate of 8/9 in 4-by-4 MIMO multiplexing. Computer simulation results show that the average computational complexity of the branch metrics, i.e., squared Euclidian distances, of the proposed adaptive independent Smcontrol method is decreased to approximately 38% that of the conventional adaptive common Smcontrol and to approximately 30% that of the fixed Smmethod (Sm=M=16), assuming fair conditions such that the maximum number of surviving symbol replicas at each stage is set to Mcirc=16

The paper proposes an adaptive selection algorithm for surviving symbol replica candidates based on the maximum reliability in ML detection with QR decomposition and M-algorithm (QRM-MLD) for OFCDM MIMO multiplexing. In the proposed algorithm, symbol replica candidates newly-added at each stage are ranked for each surviving symbol replica from the previous stage using multiple quadrant detection. Then, branch metrics are calculated only for the limited number of symbol replica candidates with high reliability based on an iterative loop in increasing order of the accumulated branch metrics from the candidate with the minimum one. Simulation results show that the computational complexity of the proposed algorithm from the viewpoint of the number of real multiplications is reduced to approximately 1/6 and 1/1900, respectively, compared to that of the original QRM-MLD and that of the conventional MLD with squared Euclidian distance calculations for all symbol replica candidates, assuming the identical achievable average block error rate (BLER) performance in 4-by-4 MIMO multiplexing with 16QAM data modulation. The results also show that 1-Gbps throughput is achieved at the E/sub b//N/sub 0/ per receiver antenna of approximately 9 dB using the adaptive selection algorithm in QRM-MLD associated with 16QAM modulation and turbo coding with coding rate of 8/9, assuming a 100-MHz bandwidth for a twelve-path Rayleigh fading channel (rms delay spread, 0.26 /spl mu/sec; maximum Doppler frequency, 20 Hz).

This paper proposes likelihood function generation of complexity-reduced maximum likelihood detection with QR decomposition and M-algorithm (QRM-MLD) suitable for soft-decision turbo decoding and investigates the throughput performance using QRM-MLD with the proposed likelihood function in multipath Rayleigh fading channels for orthogonal frequency and code division multiplexing (OFCDM) with multiple-input multiple-output (MIMO) multiplexing. Simulation results show that by using the proposed likelihood function generation scheme for soft-decision turbo decoding following QRM-MLD in 4-by-4 MIMO multiplexing, the required average received signal energy per bit-to-noise power spectrum density ratio (E/sub b//N/sub o/) at the average block error rate (BLER) of 10/sup -2/ at a 1-Gbps data rate is significantly reduced compared to that using hard-decision decoding in OFCDM access with 16 QAM modulation, the coding rate of 8/9, and 8-code multiplexing with a spreading factor of 8 assuming a 100-MHz bandwidth. Furthermore, we show that by employing QRM-MLD associated with soft-decision turbo decoding for 4-by-4 MIMO multiplexing, the throughput values of 500 Mbps and 1 Gbps are achieved at the average received E/sub b//N/sub o/ of approximately 4.5 and 9.3 dB by QPSK with the coding rate of R = 8/9 and 16 QAM with R = 8/9, respectively, for OFCDM access assuming a 100-MHz bandwidth in a twelve-path Rayleigh fading channel.

This paper presents field experimental results on an adaptive antenna array-beam forming (AAA-BF) receiver with that uses simultaneous estimations of received timings and direction of arrivals (DOAs) of Rake combining paths employing directive beam reception based on a pilot channel-assisted two-dimensional power profile in the reverse link for broadband DS-CDMA packet wireless access. Laboratory experimental results elucidate that path-independent weight generation is superior to path-common weight generation based on the implemented AAA-BF receiver in a multipath Rayleigh fading channel with up to eight paths. The required average received signal energy per bit-to-background noise power spectrum density ratio (E/sub b//N/sub 0/) using path-common antenna weight generation is increased according to the increase in the angle spread among paths, /spl sigma/, since the antenna gain toward the DOA of each path is reduced. Nevertheless, the increase in the antenna gain according to the number of antennas is achieved by using path-independent antenna weight generation regardless of the /spl sigma/ value up to 20 degrees. Furthermore, the field experimental results show that the required average received signal to interference plus background noise power ratio (SINR) at the average block error rate (BLER) of 10/sup -2/ employing path-independent weight generation is reduced by approximately 3.5 dB compared to that using path-common weight generation for a large r.m.s. angle spread such as 9 degrees. In the field experiments, the required average received SINR at the average BLER of 10/sup -2/ using the AAA-BF receiver with path-independent antenna weight generation is reduced by approximately 6.0 dB compared to that with one-antenna reception, regardless of the angle spread values.

This paper proposes maximum likelihood detection with QR decomposition and the M-algorithm (QRM-MLD) sophisticatedly combined with the multipath interference canceller (MPIC) based on multipath interference (MPI) replica generation exploiting two-dimensional minimum mean squared error (MMSE) for multiple-input multiple-output (MIMO) multiplexing in broadband DS-CDMA. Simulation results clarify that the proposed QRM-MLD coupled with MPIC exhibits excellent signal separation capability by sufficient MPI suppression effect through a three-stage MPIC comprising two-dimensional MMSE, followed by QRM-MLD associated with soft-decision Turbo decoding. We also show that the throughput values of 200,250, and 300 Mbps (corresponding frequency efficiency values are 5.0, 6.0, and 7.5 bits/sec/Hz, respectively) are achieved at the average received signal energy per bit-to-noise power spectrum density ratio (E/sub b// N/sub 0/) of approximately 6.0,9.5, and 13.0 dB by QPSK with the coding rate of R = 8/9, 8PSK with R = 3/4, and 8PSK with R = 8/9, respectively in 4-by-4 MIMO multiplexing, for broadband DS-CDMA assuming a 40-MHz bandwidth in a six-path Rayleigh fading channel with the maximum Doppler frequency of 20 Hz.

This paper investigates the practical effects of low-rate turbo coding and its soft-decision decoding considering the influence of path timing detection, channel estimation, and pre-decoder level adjustment for broadband packet DS-CDMA in the reverse link in a real multipath Rayleigh fading channel. Field experiments are conducted in an urban environment using the implemented broadband DS-CDMA transceiver at the average vehicular speed of approximately 30 km/h. The experimental results elucidate that accurate path timing detection and channel estimation of each packet frame are achieved by taking advantage of the dedicated pilot channel for the use of low-rate turbo coding (i.e., actual spreading factor value of data channel is nearly one). We clarify that by employing turbo coding with the coding rate of R = 1/8, the required average received signal energy per bit-to-interference plus background noise power spectrum density ratio (E/sub b//I/sub o/) at the average packet error rate (PER) of 10'2 is decreased by approximately 1.0 to 1.5 dB compared to that with R = 1/2. Furthermore, we show that the additional improvements by reducing R to 1/16 are small assuming the same chip rate and employing two-branch antenna diversity reception associated with coherent Rake combining for broadband DS-CDMA with a 40-MHz bandwidth.

We recently proposed broadband packet wireless access based on variable spreading factor (VSF)-orthogonal frequency and code division multiplexing (OFCDM) in the forward link and variable spreading and chip repetition factors (VSCRF)-CDMA in the reverse link for the systems beyond IMT-2000. This paper presents experimental evaluations on the throughput performance of that proposed broadband packet wireless access scheme associated with adaptive modulation and channel coding (AMC) in a multipath Rayleigh fading channel employing the implemented base station (BS) and mobile station (MS) transceivers. The implemented OFCDM transceiver verifies the effectiveness of the combination of the OFCDM and AMC including QPSK and 16QAM data modulations with a fast round trip delay of 6 msec for improving the throughput performance by exploiting the frequency diversity effect in a broadband channel. The experimental results show that the throughput performance of 100 and 200 Mbps employing AMC is achieved at the average received signal energy per symbol-to-background noise power spectrum density ratio (E/sub s//N/sub 0/) of approximately 7.5 and 19 dB, respectively, for 15-code multiplexing for the spreading factor of SF=16 in the OFCDM forward link with a 100-MHz bandwidth, in a six-path fading channel with the fading maximum Doppler frequency of f/sub D/=20 Hz. Furthermore, we elucidate that the peak throughput of 20 Mbps is achieved at the average received E/sub s//N/sub 0/of approximately 9 dB by employing QPSK data modulation with R = 1/2 for three-code multiplexing for SF= 4 in a four-path fading channel in the broadband CDMA reverse link with a 40-MHz bandwidth.

This paper experimentally investigates the throughput performance employing hybrid automatic repeat request (ARQ) with a packet combining algorithm, i.e.. incremental redundancy and Chase combining, from the viewpoints of the achievable throughput satisfying the required residual packet error rate (PKR), coupled with real-time adaptive modulation and channel coding (AMC) operation, for variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) broadband wireless access in a forward-link multipath fading channel. Since the implemented base station (BS) and mobile station (MS) transceivers can achieve a high peak throughput of greater than 100 Mbps, the time diversity effect by packet combining or code combining in hybrid ARQ is thoroughly examined associated with the frequency diversity effect over a broadband channel and antenna diversity reception. The experimental results demonstrate that even in a six-path Rayleigh fading channel, incremental redundancy is superior to Chase combining under high Doppler frequency conditions such as f/sub D/ = 140 Hz, especially when the number of maximum retransmissions is small, because the larger coding gain after code combining is effective in compensating for packet errors due to inappropriate modulation and coding scheme (MCS) selection caused by the tracking error of the AMC loop for the fast fading variations. Furthermore, it is elucidated that the throughput values of 7 Mbps and 10 Mbps (achieved at the average received signal energy per symbol-to-background noise power spectrum density ratio (E/sub s/M/sub 0/) of approximately -5 dB) satisfying the required PER of below 10/sup -4/ are achieved when the maximum number of retransmissions is approximately 3 and 7, respectively, in the six-path fading channel with f/sub D/ = 20 Hz.

This paper proposes a three-step cell search algorithm for a multi-carrier (MC)-CDMA broadband packet wireless access system with a 50-100 MHz bandwidth and evaluates the cell search time performance in a numerous-multipath fading channel. To achieve fast cell search, we introduce a continuously transmitted synchronization channel (SCH), which is multiplexed into sub-carriers with equal sub-carrier spacing and scrambled by the code common to all cell sites, and a common pilot channel (CPICH), which is continuously-transmitted in both the time and frequency domains and scrambled by a cell specific scrambling code (CSSC). In the proposed three-step algorithm, fast Fourier transform (FFT) window timing and packet frame timing are successively detected by using a guard interval and SCH and then, the CSSC of the target cell is identified by CPICH. Furthermore, in the proposed method, since the CSSC identification is performed for several candidates of the FFT window and packet frame timings, the best cell having the least path loss is precisely obtained despite the channel load, ie, the number of dedicated traffic channels (DTCH) in each cell. Computer simulation results show that by using the three-step cell search algorithm, the cell detection probability of 90% is achieved within approximately 77 msec (44 msec) when the number of DTCH with the information bit rate of 2 Mbit/s of each cell is distributed with the average of 20 and the standard deviation of 15 (0), when the FFT symbol-correlation of the SCH is averaged over N/sub PST//sup SCH/ = 20 (4) frames in the second step and the CPICH symbol-correlation is coherently averaged during N/sub CST//sup CPICH/ = 16 (10) symbols along the time axis and N/sub CSF//sup CPICH/ = 8 sub-carriers along the frequency axis CS-F in the third step.

Based on laboratory experiments, the paper investigates the multiuser interference suppression effect of the coherent adaptive antenna array diversity (CAAAD) receiver. An optical fiber feeder is employed in the intermediate frequency (IF) stage of a base transceiver station (BTS), aiming at the practical use of adaptive antenna array beamforming techniques in the W-CDMA air interface. We employ a configuration in which the optical fiber conversion, i.e., electrical-to-optical (E/O) or O/E conversion, is performed on a received signal amplified by an automatic gain control (AGC) amplifier in the IF stage in order to abate the impact of the noise component generated by the E/O (O/E) converters. We first show by computer simulation the superiority of the optical fiber conversion in the IF stage to that in the radio frequency (RF) stage from the viewpoint of reducing the bit error rate (BER) performance. Furthermore, experimental results elucidate that the loss in the required transmit signal energy per bit-to-background noise power spectrum density ratio (E/sub b//N/sub 0/) of the implemented CAAAD receiver at the average BER of 10/sup -3/ employing the optical fiber feeders in the IF stage compared to that with coaxial cables is within a mere 0.2 dB for conditions of 6 antennas, 3 users, 2-path Rayleigh fading channel model, and the ratio of the target signal energy per bit-to-interference power spectrum density ratio (E/sub b//I/sub 0/) of the desired user to that of the interfering users for fast transmission power control is /spl Delta/E/sub b//I/sub 0/ = -15 dB.

This paper compares by computer simulation the throughput performance of a multipath interference canceller (MPIC) and that of a sliding-window chip equalizer (SWCE) in high-speed downlink packet access (HSDPA) in a multipath fading channel. We also propose new MPIC and SWCE configurations that suppress not only multipath interference (MPI) of the downlink shared channel (DSCH) in its own cell but also the multiuser interference (MUI) of the DSCH from a neighboring interference-dominant cell. Computer simulation results elucidate that when the MPIC or the SWCE which suppresses the MPI of the DSCH in only its own cell is employed, the MPIC achieves throughput greater than that of the SWCE ranging from a lower (i.e., the lower average received E/sub b//N/sub 0/) to a higher throughput such as 4.8 Mbit/s, which is achieved by the modulation and coding scheme (MCS) of 16QAM with coding rate R = 1/2. In an even higher throughput region, achieved using the MCS of 16QAM with R = 3/4, the SWCE exhibits superiority over MPIC. Furthermore, we also clarify that with the MPIC and the SWCE with neighboring-cell MUI suppression, the SWCE achieves better throughput compared to the MPIC.

This paper proposes a multipath interference canceller (MPIC) associated with orthogonal code-multiplexing that achieves much higher peak throughput than 2 Mbit/s with adaptive data modulation for high-speed packet transmission in the wideband direct sequence-code division multiple access (W-CDMA) forward link, and evaluates its throughput performance by computer simulation. The simulation results elucidate that sufficient multipath interference (MPI) suppression is achieved by a four-stage MPIC with 6-12 orthogonal code-multiplexing using one iterative channel estimation with pilot and decision feedback data symbols and further that the interference rejection weight control according to the number of observed multipaths is effective in improving the throughput. It is also demonstrated that MPIC exhibits a superior MPI suppression effect to a chip equalizer in the lower received signal energy per bit-to-background noise spectrum density (E/sub b//N/sub 0/) channel around 0-3 dB owing to the successive channel estimation at each stage. We show that the maximum peak throughput using MPIC is approximately 2.1 fold that without MPIC in a two-path and three-path Rayleigh fading channel and that the peak throughput of 8.0 Mbit/s is achieved using 64 QAM data modulation in a two-path fading channel within a 5 MHz bandwidth. Furthermore, the required average E/sub b//N/sub 0/ for satisfying the same throughput with MPIC is decreased by more than 2.0 dB.

A three-stage coherent multistage interference canceller (COMSIC) employing pilot symbol-assisted (PSA) channel estimation for replica generation of multiple access interference (MAI) is implemented and its performance in the presence of frequency selective multipath fading is experimentally evaluated by a multipath fading simulator. A fast transmission power control (TPC) method suitable for COMSIC is also proposed, in which the signal-to-interference plus background noise power ratio (SIR) at the matched filter (MF) based RAKE receiver is measured to achieve a short TPC delay and the target signal-to-interference ratio value is compensated by an outer loop so that the measured block error rate (BLER) is equal to the prescribed target value. The experimental results show that as expected the COMSIC satisfactorily reduces the MAI even when the number of active users is equal to the spreading factor in a multipath fading environment, and thus, improves the bit error rate (BER) performance in a multiuser environment. The results also show that the proposed fast TPC method with a two-slot delay associated with COMSIC works satisfactorily and the combination of COMSIC and fast TPC significantly decreases the transmission power of a mobile station (required transmission power of a mobile station with COMSIC at the average BER of 10/sup -3/ is decreased by approximately 2.0 (3.0) dB compared with the MF-based RAKE receiver with (without) antenna diversity reception). This extends the cell coverage, battery life, and increases the system capacity in the reverse link.

This paper proposes a new parallel-type coherent multistage interference canceller (COMSIC) coupled with multiple-antenna reception (MA-COMSIC) and evaluates the capacity increase in a multipath fading channel in the DS-CDMA reverse link, in order to improve further the channel estimation accuracy and increase the space diversity effect. Simulation results show that the MA-COMSIC with a two block-wise antenna configuration, in which each block has 3 antennas (we denote as (B, D) = (2, 3)), achieved a lower required transmit E/sub b//N/sub 0/ than that with (B, D) = (1, 6) except for extreme cases such as when the number of simultaneous users K exceeds 120 corresponding to K/Pg = 250% (where Pg is the processing gain of 48 = 16/spl times/3). Consequently, even when K = 120, the increase in the required transmit E/sub b//N/sub 0/ of MA-COMSIC with (B, D) = (2, 3) from that in a single-user case is only 1.0 dB owing to the significant improvement in the channel estimation accompanied by the diversity effect. We also elucidate that even when the angle spread among constituent waves in the same path is /spl sigma//sub /spl Lambda/S/ = 5 degrees, the loss of the required transmit E/sub b//N/sub 0/ with (B, D) = (2, 3) for the ideal fading correlation case is suppressed to within 0.5 dB.

This paper proposes a new fast transmit power control algorithm for forward link inter-cell site diversity, called site independent diversity transmit power control (hereafter SIDTPC) and compares the effect of decreasing the total transmit signal energy per bit-to-background noise power spectrum density ratio (E/sub b//N/sub 0/) of all cell sites for three methods: SIDTPC, site selection diversity transit power control (SSDT), and conventional TPC during inter-cell site diversity (hereafter CTPC) in the W-CDMA forward link. The SIDTPC method differs from SSDT in that each cell site transmits a dedicated physical channel (DPCH) with different transmit power, which is adaptively weighted according to the path loss between each cell site and a mobile station (MS), that is to say, with larger (smaller) transmit power for the cell having the smaller (larger) path loss, resulting in a decreased total transmit power similar to the SSDT method, while maintaining a superior feature which enables SIDTPC to be less sensitive to the reception errors of the control bits sent via the reverse link. Simulation results demonstrated that when the traffic ratio of voice service to data service is 1:1, the required total average transmit E/sub b//N/sub 0/, at the average block error rate (BLER) of 10/sup -2/ for 50% of MSs using the SIDTPC method is decreased by approximately 0.4 (0.5) dB and 2.0 (0.2) dB compared to those with the CTPC and SSDT methods, respectively, for Doppler frequency, f/sub D/, of 5 (80) Hz.

In order to increase the link capacity in the wideband direct sequence code division multiple access (W-CDMA) reverse link, employing a parallel-type coherent multistage interference canceller (COMSIC) is more practical than using a serial (successive)-type due to its inherent advantage of a short processing delay, although its interference suppression effect is inferior to the serial-type. Therefore, this paper proposes a parallel-type COMSIC with iterative channel estimation (ICE) using both pilot and decision-feedback data symbols at each canceling stage. Computer simulation results show that the required average transmit E/sub b//N/sub 0/ for obtaining the average BER of 10/sup -3/ using the three-stage parallel-type COMSIC with ICE using pilot and decision-feedback data symbols after forward error correction (FEC) decoding (number of iterations was 2 at each stage) was decreased by approximately 2.1 dB compared to a COMSIC with channel estimation using only pilot symbols, with the spreading factor (SF) of 16 and when the number of active users was 20 (thereby the capacity normalized by the processing gain (Pg) becomes 42%). The results also show that the required average transmit E/sub b//N/sub 0/, loss of the parallel-type COMSIC using ICE at the average BER of 10/sup -3/ compared to the serial-type was negligible (i.e., within 0.2 dB) due to the significantly improved channel estimation and decreased data decision error, while achieving a much shorter processing delay irrespective of the number of active users.

The combined effect of coherent RAKE combining using the weighted multislot averaging (WMSA) channel estimation filter and closed-loop fast transmit power control (TPC) in the 4.096 Mchip/s direct sequence code division multiple access (DS-CDMA) mobile radio reverse link is experimentally evaluated. The WMSA channel estimation filter utilizes periodically transmitted pilot symbols (four pilot symbols are time-multiplexed in each 40-symbol time slot). Its observation period is extended to 2-K slots in order to improve the accuracy of the channel estimation. The fast TPC is based on the measurement of signal-to-interference plus background noise ratio (SIR) using pilot symbols. Laboratory experiments show that the use of the K=2 WMSA channel estimation filter reduces the required E/sub b//I/sub 0/ at the average BER of 10/sup -3/ by approximately 0.5 dB compared to use of the linear interpolation filter, and that the required E/sub b//I/sub 0/ is minimized when the SIR measurement interval is M=10 symbols (one slot TPC delay). It was also clarified that SIR-based TPC works satisfactorily when two users with different information data rates, i.e., SF, independently employ fast TPC. Field experimental results obtained in an area nearby Tokyo showed that the average BER of 10/sup -3/ is achieved at the target E/sub b//I/sub 0/ per antenna of approximately 2.5 dB by using four-finger branch RAKE and two-branch antenna diversity. Although the target E/sub b//I/sub 0/ to achieve same BER, when there is one interfering user with a fourfold greater transmit power than that of the desired user that independently employs fast TPC, is almost the same as that in the single-user case, the mobile transmit power is increased by 1.0-2.0 dB due to the increased MAI. These results indicate that the combination of coherent RAKE combining and fast TPC works well in practical multipath fading channels.

A 3-stage coherent multistage interference canceller for DS-CDMA mobile radio was implemented and its performance in a frequency selective multipath fading channel is experimentally evaluated by using a multipath fading simulator. The implemented interference canceller applied pilot symbol-assisted channel estimation for data decision and replica generation of multiple access interference (MAI). The laboratory experiment demonstrated that, as is expected, the coherent multistage interference canceller can reduce the MAI to satisfactory levels in a multipath fading environment and thus, improving the bit error rate (BER) performance under multiuser environment.