This paper proposes block-wise resource block (RB)-level distributed OFDMA transmission with ND-block division in order to obtain the frequency diversity effect even for low-rate traffic (here ND indicates the number of virtual RBs within one physical RB) in Evolved UTRA downlink. More specifically, we propose a constraint rule such that distributed transmission is multiplexed into a different physical RB from that of localized transmission in order to achieve the same resource assignment and independent decoding between the distributed and localized transmissions. Based on the proposed rule, a virtual RB for distributed transmission is segmented into ND blocks with the size of 1/ND of the original virtual RB. Then, the ND virtual blocks with the size of 1/ND are mapped together into each ND physical RB in a distributed manner, resulting in a large frequency diversity effect. Numerical calculations show that the block-wise RB-level distributed transmission can reduce the number of control signaling bits required for resource assignment compared to the subcarrier-level distributed transmission scheme, which provides the best performance. Moreover, a system-level simulation shows that the loss in the cell throughput employing the block-wise RB-level distributed transmission compared to that using the subcarrier-level transmission is only within 3-4% when the channel load is 0.5 and 1.0, i.e., the maximum loss is 3-4% at approximately 90% in the cumulative distribution function (CDF).

This paper proposes applying the Layered Orthogonal Frequency Division Multiple Access (OFDMA) radio access scheme and its radio access techniques to LTE (Long-Term Evolution)-Advanced to satisfy its system requirements, which are much stricter than those of the Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN). Layered OFDMA comprises layered transmission bandwidth assignment (bandwidth is assigned to match the required data rate), a layered control signaling structure, and support for layered environments for both the downlink and uplink. Especially in the uplink, an adaptive multi-access scheme with hybrid single-carrier and multicarrier based radio access is applied. Layered OFDMA radio access will support all the functionalities specified in Release 8 LTE and later enhancements. Key radio access techniques such as fast inter-cell radio resource management that takes advantage of remote radio equipment (RRE) so as to realize inter-cell orthogonality, multi-antenna transmission with more antennas, and coverage enhancing techniques are used to achieve a high level of capacity and cell-edge spectrum efficiency.

This paper presents the optimum physical random access channel (PRACH) structure in terms of the number of control signaling bits accommodated and the transmission bandwidth based on the link budget in order to satisfy the coverage requirement for the single-carrier (SC)-FDMA based E-UTRA uplink. First, we present the design concept of the PRACH structure considering the purposes of the random access procedure in the E-UTRA. Simulation evaluations including a system-level simulation show that a PRACH comprising a 0.5-msec preamble sequence can convey a 6-bit control signal at the cell edge when the inter-site distance (ISD) is 500 m under full channel load conditions with one-cell frequency reuse. It is also shown, however, that a PRACH longer than one-sub-frame, e.g., 1.0 msec, is necessary to support the ISD of 1732 m assuming the same conditions. We also show that the best transmission bandwidth for the PRACH is approximately 1.08-4.5 MHz from the viewpoint of the misdetection probability, and a 1.08-MHz transmission bandwidth is suitable considering other aspects such as flexible resource assignment in the time domain and a small number of options in the transmission bandwidth.

This paper proposes an outer loop control method of fast transmit power control (TPC) for high-quality data transmission such as that with the average bit error rate (BER) of 10-6 in serial concatenated channel coding combining convolutional (inner) and Reed-Solomon (outer) codings for DS-CDMA mobile radio. In the proposed method, the outer loop control is performed based on the measured intermediate block error rate (BLER) value after inner-channel decoding. Since the number of block errors after inner-channel decoding is much greater than that of the final output after outer channel decoding, fast tracking performance of the sudden changes in the propagation conditions such as the number of multipaths and fading Doppler frequency, i.e., moving speed of the mobile station, is achieved. The experimental results clarify that the measured BLER after outer channel decoding is accurately controlled to almost a constant value from the low to high fading maximum Doppler frequency of up to 480 Hz, and that the measured BER after outer channel decoding is within the range of one-order of magnitude of the antenna diversity reception (meanwhile, the target SIR value after Rake combining varied with the range of 2.5 dB).

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 a 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 a novel random access identifier (RAID)-linked receiver beamforming method for time division duplex (TDD)-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 active RAID detection. The purpose of the proposed method is to achieve accurate active RAID detection and channel estimation with a reasonable level of computational complexity at the base station receiver. In the proposed method, a unique receiver BF vector applied at the base station is linked to each of the M RAIDs prepared by the system. The user terminal selects an appropriate pair comprising a receiver BF vector and a RAID in advance based on the channel estimation results in the downlink assuming channel reciprocity in a TDD system. Therefore, per-receiver antenna channel estimation for receiver BF is not necessary in the proposed method. Furthermore, in order to utilize fully the knowledge of the channel at the user transmitter, we propose applying transmitter filtering (TF) to the proposed method for effective channel shortening in order to increase the orthogonal preambles for active RAID detection and channel estimation prepared for each RAID. Computer simulation results show that the proposed method greatly improves the accuracy of active RAID detection and channel estimation. This results in lower error rates than that for the conventional method performing channel estimation at each antenna in a massive MIMO environment.

This paper investigates the cell search time performance of our previously proposed three-step cell search method in a two-cell site environment by laboratory and field experiments supporting asynchronous cell site operation, which is one of the most notable features of wideband direct sequence code division multiple access (W-CDMA) mobile communications. The cell search methods used in the paper are based on the ongoing third generation partnership project (3GPP), in which our original scheme was refined with respect to several points in order to reduce the complexity of the receiver. The experimental results demonstrate that the method achieves the fast cell search time of less than one second in real multipath-fading channels. The cell search is accomplished in less than approximately 700 msec at 90% of the detection probability when 4.7% and 0.5% of the total transmit power of a cell site is assigned to the common pilot channel (CPICH) and synchronization channels (SCHs), respectively, in a two-cell site environment. We also elucidate that the cell search time at the detection probability of 90% using time switched transmit diversity (TSTD) is decreased by approximately 100 msec compared to that without TSTD in low-mobility environments such as the average vehicular speed of 5 km/h with a transmit power assignment of the CPICH of 4.7%.

This paper proposes a multipath interference canceller (MPIC) for orthogonal code multiplexed channels in the W-CDMA forward link and evaluates by computer simulation the improvement in BER performance owing to the multipath interference (MPI) suppression effect obtained by the MPIC. The simulation results show that a one-stage MPIC, which removes the MPI from the common pilot channel (PICH), common control channel (CCH), and synchronization channel (SCH), achieves a sufficient MPI suppression effect, and that the required received Eb/N0 of the traffic channel (TCH) at the average BER of 10-3 using the MPIC for the common channels is decreased by approximately 6.5 dB compared to that with a matched filter (MF)-based Rake receiver (the transmit power ratio of each common channel to TCH: ΔPICH/TCH=0 dB, ΔCCH/TCH=5 dB, ΔSCH/TCH=3 dB, without fast transmit power control (TPC) and antenna diversity reception). We also show that by using MPIC, the required transmit Eb/N0 at the average BER of 10-3, when the ratio of the target Eb/I0 of the 9-interfering users to desired user is ΔInt/Des=6 dB with fast TPC, is increased by only approximately 0.6 dB compared to that when ΔInt/Des=0 dB. This implies that the preferential MPI suppression from high-rate TCHs that abates the increase in complexity in a mobile terminal is effective in increasing the link capacity in the forward link.

This paper proposes a computational complexity-reduced algorithm for an adaptive peak-to-average power ratio (PAPR) reduction method previously developed by members of our research group that uses the null space in a multiple-input multiple-output (MIMO) channel for MIMO-orthogonal frequency division multiplexing (OFDM) signals. The proposed algorithm is an extension of the peak cancellation (PC) signal-based method that has been mainly investigated for per-antenna PAPR reduction. This method adds the PC signal, which is designed so that the out-of-band radiation is removed/reduced, directly to the time-domain transmission signal at each antenna. The proposed method, referred to as PCCNC (PC with channel-null constraint), performs vector-level signal processing in the PC signal generation so that the PC signal is transmitted only to the null space in the MIMO channel. We investigate three methods to control the beamforming (BF) vector in the PC signal, which is a key factor in determining the achievable PAPR performance of the algorithm. Computer simulation results show that the proposed PCCNC achieves approximately the same throughput-vs.-PAPR performance as the previous method while dramatically reducing the required computational cost.

In this paper, we propose applying our previously reported adaptive peak-to-average power ratio (PAPR) reduction method using null space in a multiple-input multiple-output (MIMO) channel for orthogonal frequency division multiplexing (OFDM) signals to the downlink MIMO amplify-and-forward (AF) relaying transmission. Assuming MIMO-OFDM transmission, mitigating its high 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 study, we assume an AF-type RS with multiple antennas. In the proposed method, the BS suppresses the PAPR of the transmitted signal through adaptive PAPR reduction utilizing the null space of the integrated overall MIMO channel that combines the channel between the BS and RS and the channel between the RS and a set of user equipment (UE). However, the PAPR of the received signal at each RS antenna is increased again due to the MIMO channel between the BS and RS. The proposed method reduces this increased PAPR at the AF-type RS transmitter by PAPR reduction processing that utilizes the null space in the MIMO channel between the RS and UE. Since the in-band PAPR reduction signal added at the RS transmitter is transmitted only in the null space of the MIMO channel between the RS and UE, interference at the UE receiver is mitigated. 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 UE receiver.

This paper presents experimental evaluations of the effect of time diversity obtained by hybrid automatic repeat request (HARQ) with soft combining in space and path diversity schemes on orthogonal frequency division multiplexing (OFDM)-based packet radio access in a downlink broadband multipath fading channel. The effect of HARQ is analyzed through laboratory experiments employing fading simulators and field experiments conducted in downtown Yokosuka near Tokyo. After confirming the validity of experimental results based on numerical analysis of the time diversity gain in HARQ, we show by the experimental results that, for a fixed modulation and channel coding scheme (MCS), time diversity obtained by HARQ is effective in reducing the required received signal-to-interference plus noise power ratio (SINR) according to an increase in the number of transmissions, K, up to 10, even when the diversity effects are obtained through two-branch antenna diversity reception and path diversity using a number of multipaths greater than 12 observed in a real fading channel. Meanwhile, in combined use with the adaptive modulation and channel coding (AMC) scheme associated with space and path diversity, we clarify that the gain obtained by time diversity is almost saturated at the maximum number of transmissions in HARQ, K ' = 4 in Chase combining and K ' = 2 in Incremental redundancy, since the improvement in the residual packet error rate (PER) obtained through time diversity becomes small owing to the low PER in the initial packet transmission arising from appropriately selecting the optimum MCS in AMC. However, the experimental results elucidate that the time diversity in HARQ with soft combining associated with antenna diversity reception is effective in improving the throughput even in a broadband multipath channel with sufficient path diversity.

This paper investigates the influence of channel estimation error on the precoding matrix selection and signal detection for MIMO multiplexing using precoding in the downlink OFDM radio access. In a simulation, we assume codebook-based unitary precoding and signal detection that employs a minimum mean squared error (MMSE) based interference suppression filter. The simulation results clarify the effect of the channel estimation error with respect to the precoding matrix selection and signal detection from the viewpoints of the number of streams, i.e., rank order, the number of transmit antennas, the modulation scheme and channel coding rate, and codebook size, i.e., quantization for precoding matrix feedback information.

This paper proposes efficient single-carrier (SC) based multiplexing schemes for Layer 1 (L1)/Layer 2 (L2) control signals in SC-FDMA radio access using DFT-Spread OFDM in the Evolved UTRA uplink. L1/L2 control signals are necessary for key packet access techniques such as downlink scheduling, link adaptation, hybrid automatic repeat request (ARQ) with soft combining, and for uplink feedback control signals. We first propose a SC-based multiplexing scheme for L1/L2 control signals within a shared data channel for a set of user equipment (UE) that transmits both an uplink shared data channel and L1/L2 control signals within the same subframe. We also propose a multiplexing scheme for L1/L2 control signals without uplink data transmission that takes advantage of intra-subframe frequency hopping (FH) using multiple exclusively-assigned time-frequency resource blocks (RBs) to obtain a frequency diversity gain. Furthermore, we propose an orthogonal CDMA-based multiplexing scheme using cyclic shifts of a constant amplitude zero auto-correlation (CAZAC) sequence for L1/L2 control signals from different UEs within the same narrowband time-frequency RB. Computer simulation results show that the proposed SC-based multiplexing scheme for the L1/L2 control signals within the shared data channel achieves a higher user throughput than a multicarrier-based multiplexing scheme. The results also show that the proposed multiplexing scheme for the L1/L2 control signals that takes advantage of the intra-subframe FH for the UE without uplink data transmission achieves high quality reception through large frequency diversity gain. Furthermore, we show that the proposed cyclic-shift based orthogonal CDMA multiplexing is effective in the multiplexing of multiple L1/L2 control signals from different UEs within the same RB.

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

This paper presents our investigation of non-orthogonal multiple access (NOMA) as a novel and promising power-domain user multiplexing scheme for future radio access. Based on information theory, we can expect that NOMA with a successive interference canceller (SIC) applied to the receiver side will offer a better tradeoff between system efficiency and user fairness than orthogonal multiple access (OMA), which is widely used in 3.9 and 4G mobile communication systems. This improvement becomes especially significant when the channel conditions among the non-orthogonally multiplexed users are significantly different. Thus, NOMA can be expected to efficiently exploit the near-far effect experienced in cellular environments. In this paper, we describe the basic principle of NOMA in both the downlink and uplink and then present our proposed NOMA scheme for the scenario where the base station is equipped with multiple antennas. Simulation results show the potential system-level throughput gains of NOMA relative to OMA.

Router virtualization is becoming more common as a method that uses network (NW) equipment effectively and robustly similar to server virtualization. Edge routers, which are gateways of core NWs, should be virtualized because they have many functions and resources just as servers do. To virtualize edge routers, a metro NW, which is a wide area layer-2 NW connecting each user's residential gateway to edge routers, must trace dynamic edge router re-allocation by changing the route of each Ethernet flow. Therefore, we propose a scalable centralized control architecture of a virtual layer-2 switch on a metro NW to trace virtual router re-allocation and use metro NW equipment effectively. The proposed scalable control architecture improves the centralized route control performance by processing in parallel on a flow-by-flow basis taking into account route information even in the worst case where edge routers fail. In addition, the architecture can equalize the load among parallel processes dynamically by using two proposed load re-allocation methods to increase the route control performance stably while minimizing the amount of resources for the control. We evaluate the scalability of the proposed architecture through theoretical analysis and experiments on a prototype and show that the proposed architecture increases the number of flows accommodated in a metro NW. Moreover, we evaluate the load re-allocation methods through simulation and show that they can evenly distribute the load among parallel processes. Finally, we show that the proposed architecture can be applied to not only large-scale metro NWs but also to data center NWs, which have recently become an important type of large-scale layer-2 NW.

This paper proposes a new cell-specific scrambling code (CSSC) assignment method and a fast cell search algorithm in the forward link for Orthogonal Frequency and Code Division Multiplexing (OFCDM) wireless access that are suitable for a system incorporating coexisting isolated and cellular cells. In the proposed method, one or some CSSC groups and thereby the CSSCs belonging to the CSSC groups are exclusively assigned to isolated cells. By detecting the best CSSC assigned to an isolated cell with higher priority than the cellular cells, the best cell including the isolated cell obtaining the minimum path loss can be detected far faster than by using the conventional cell search method, which employs uniform CSSC assignment. Computer simulation results show that by using the proposed cell search method together with the exclusive CSSC assignment to the isolated cells, the isolated-cell detection probability of approximately 90% is achieved at the cell boundary after the cell search time of 10 msec, while corresponding detection probability using conventional CSSC assignment is approximately 80% without notifying the user equipment of the cell type and its CSSC information of the surrounding cells via the broadcast channel, at the average received signal energy per bit-to-noise power spectrum density ratio (Eb/N0) of 10 dB for the common pilot channel (CPICH) in the cellular cells, when the transmission power ratio of the CPICH to the packet data channel (PDCH) for a one-code channel is RCPICH = 9 dB in a 20-cell layout model.

To further increase the capacity of the DS-CDMA reverse-link, this paper investigates the effectiveness of interference rejection weight control (IRWC) for the pilot symbol-assisted coherent multistage interference canceller (PSA-COMSIC) using recursive channel estimation (RCE). First, a bit error rate (BER) expression of the serial (successive) and parallel type hard decision multistage interference canceller (MSIC) with IRWC using Gaussian approximation for multiple access interference (MAI) are presented for no fading channels. It is theoretically shown that IRWC is effective in mitigating the interference replica generation error in hard decision MSIC. Next, the BER performance of PSA-COMSIC using IRWC in a multipath Rayleigh fading channel when channel coding is applied is evaluated by computer simulations. The BER performance and capacity are evaluated not only for the conventional serial and parallel types but also for serial/parallel (S/P) hybrid type and non-linear/linear (N/L) hybrid type schemes, both of which are effective in significantly reducing the demodulation processing delay. The simulation results demonstrate that, in interference-limited channels where the back ground noise is negligible, the capacity of serial type PSA-COMSIC using IRWC is about 10% higher than that without IRWC. It is also found that if we can accept a slight capacity degradation compared to the serial type PSA-COMSIC, S/P hybrid schemes are effective in reducing the demodulation processing delay.

This paper proposes a 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. The focus of this investigation is on the downlink and the purpose of cell association is to achieve better user fairness, in other words, to increase the minimum average user throughput (worst user throughput). In the proposed method, an appropriate cell association for all users within a cell is achieved in an iterative manner based on the feedback information of each individual user assisted by a small amount of broadcast information from the respective BSs. The proposed method does not require cooperation between BSs. Furthermore, the proposed method is applicable to cases of inter-cell interference coordination (ICIC) between macro and pico/femto BSs 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 proposes a new user association method to maximize the downlink system throughput in a cellular network, where the system throughput is defined based on (p,α)-proportional fairness. The proposed method assumes a fully decentralized approach, which is practical in a real system as complicated inter-base station (BS) cooperation is not required. In the proposed method, each BS periodically and individually broadcasts supplemental information regarding its bandwidth allocation to newly connected users. Assisted by this information, each user calculates the expected throughput that will be obtained by connecting to the respective BSs. Each user terminal feeds back the metric for user association to the temporally best BS, which represents a relative increase in throughput through re-association to that BS. Based on the reported metrics from multiple users, each BS individually updates the user association. The proposed method gives a general framework for optimal user association for (p,α)-proportional fairness-based system throughput maximization and is especially effective in heterogeneous cellular networks where low transmission-power pico BSs overlay a high transmission-power macro BS. Computer simulation results show that the proposed method maximizes the system throughput from the viewpoint of the given (p,α)-proportional fairness.