To increase the spectral utilization efficiency of a wireless link, multiple-input multiple-output (MIMO) systems can be employed to transmit several data streams in parallel at the same time and on the same frequency but from different transmit antennas. However, at the receiver side multi-stream detection is needed. In this paper, ordered successive MMSE detection (OSD) is considered as a low-complexity detection scheme. OSD's main computational cost lies in computing the nulling weights that correspond to each stage of successive detection. In this paper, we develop an efficient semi-adaptive approach to generate MMSE weights. This semi-adaptive approach efficiently combines two approaches: channel estimates-based direct matrix inversion weights generation (direct approach) and Recursive Least Squares (RLS) algorithm-based weights generation (adaptive approach). Although the direct approach alone performs better than the adaptive approach, it is more complex for updating weights within the tracking mode. On the other hand, the adaptive approach alone is less complex in updating weights within the tracking mode, but converges slowly within the training mode. Our combined semi-adaptive approach effectively offsets these disadvantages. We demonstrate, through computer simulations, that the semi-adaptive approach can achieve the BER of the direct approach in slow time-varying MIMO channels, while its computational complexity is less than or comparable to that of the adaptive approach.

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.

This paper investigates a downlink non-orthogonal multiple access (NOMA) combined with single user MIMO (SU-MIMO) for future LTE (Long-Term Evolution) enhancements. In particular, we propose practical schemes to efficiently combine NOMA with open-loop SU-MIMO (Transmission Mode 3: TM3) and closed-loop SU-MIMO (Transmission Mode 4: TM4) specified in LTE. The goal is also to clarify the performance gains of NOMA combined with SU-MIMO transmission, taking into account the LTE radio interface such as frequency-domain scheduling, adaptive modulation and coding (AMC), and NOMA specific functionalities such as, multi-user pairing/ordering, transmit power allocation and successive interference cancellation (SIC) at the receiver side. Based on computer simulations, we evaluate NOMA link-level performance and show that the impact of error propagation associated with SIC is marginal when the power ratio of cell-edge and cell-center users is sufficiently large. In addition, we evaluate NOMA system-level performance gains for different granularities of scheduling and MCS (modulation and coding scheme) selection, for both genie-aided channel quality information (CQI) estimation and approximated CQI estimation, and using different number of power sets. Evaluation results show that NOMA combined with SU-MIMO can still provide a hefty portion of its expected gains even with approximated CQI estimation and limited number of power sets, and also when LTE compliant subband scheduling and wideband MCS is applied.

Multiple-input multiple-output (MIMO) systems can improve the spectral efficiency of a wireless link, by transmitting several data streams simultaneously from different transmit antennas. However, at the receiver, multi-stream detection is needed for extracting the transmitted data streams from the received signals. This letter considers ordered successive detection (OSD) for multi-stream detection. OSD consists of several stages, and at each stage only one data stream is chosen to be detected among the remaining streams according to a specified ordering metric. OSD has been formulated using both the zero forcing (ZF) and minimum mean square error (MMSE) criteria. This letter clarifies the reason behind the superiority of OSD using the MMSE criterion to OSD using the ZF criterion through the investigation of the relation between their ordering metrics. For uncorrelated MIMO channels, we show that both ordering metrics yield the same performance for OSD using either ZF or MMSE criterion. Accordingly, the superiority of OSD using the MMSE criterion to OSD using the ZF criterion is clarified to be a direct result of the inherent superiority of MMSE nulling to ZF nulling, and to be independent of the ordering operation. Performance comparisons of OSD and maximum likelihood detection are also given for modulation schemes of different sizes.

Multiple-input multiple-output (MIMO) wireless systems can realize large spectral efficiency and high performance communication links. For wideband transmissions, the combination of MIMO systems with orthogonal frequency division multiplexing (OFDM) has recently attracted a lot of attention and it is well known as MIMO-OFDM. In MIMO-OFDM systems, the overlapped signals over each subcarrier need to be separated by the receiver. For fading coefficients based receivers, the performance of the receiver depends largely on the accuracy of the estimated channel. Especially, when the channel varies with time, accurate channel tracking is needed. Conventional optimized channel tracking has large computational complexity because large matrix inverse computation is required. In order to reduce the complexity, a simplified channel tracking scheme assuming PSK modulation has been considered. However, such a simplified scheme is found to suffer from large performance degradation when applied to multi-level QAM modulation. In this paper, we derive a new simplified but improved channel tracking scheme for MIMO-OFDM systems that can be applied to both PSK and multi-level QAM modulation. The performance and the complexity of the proposed scheme are evaluated with comparisons to conventional schemes.

Currently, many operators worldwide are deploying Long Term Evolution (LTE) to provide much faster access with lower latency and higher efficiency than its predecessors 3G and 3.5G. Meanwhile, the service rollout of LTE-Advanced, which is an evolution of LTE and a “true 4G” mobile broadband, is being underway to further enhance LTE performance. However, the anticipated challenges of the next decade (2020s) are so tremendous and diverse that there is a vastly increased need for a new generation mobile communications system with even further enhanced capabilities and new functionalities, namely a fifth generation (5G) system. Envisioning the development of a 5G system by 2020, at DOCOMO we started studies on future radio access as early as 2010, just after the launch of LTE service. The aim at that time was to anticipate the future user needs and the requirements of 10 years later (2020s) in order to identify the right concept and radio access technologies for the next generation system. The identified 5G concept consists of an efficient integration of existing spectrum bands for current cellular mobile and future new spectrum bands including higher frequency bands, e.g., millimeter wave, with a set of spectrum specific and spectrum agnostic technologies. Since a few years ago, we have been conducting several proof-of-concept activities and investigations on our 5G concept and its key technologies, including the development of a 5G real-time simulator, experimental trials of a wide range of frequency bands and technologies and channel measurements for higher frequency bands. In this paper, we introduce an overview of our views on the requirements, concept and promising technologies for 5G radio access, in addition to our ongoing activities for paving the way toward the realization of 5G by 2020.

This paper presents results of outdoor experiments conducted in the 39-GHz band. In particular, assuming mobile communications such as the fifth generation mobile communications (5G) and beyond, we focus on achieving 1Gbit/s or greater throughput at transmission distances exceeding 1km in the experiments. In order to enhance the data rate and capacity, the use of higher frequency bands above 6GHz for mobile communications is a new and important technical challenge for 5G and beyond. To extend further the benefits of higher frequency bands to various scenarios, it is important to enable higher frequency bands to basically match the coverage levels of existing low frequency bands. Moreover, mobility is important in mobile communications. Therefore, we assume the 39-GHz band as a candidate frequency for 5G and beyond and prepare experimental equipment that utilizes lens antenna and beam tracking technologies. In the experiments, we achieve the throughput values of 2.14Gbit/s at the transmission distance of 1850m and 1.58Gbit/s at 20-km/h mobility. Furthermore, we show the possibility of achieving high throughput even under non-line-of-sight conditions. These experimental results contribute to clarifying the potential for the 39-GHz band to support gigabit-per-second class data rates while still providing coverage and supporting mobility over a coverage area with distance greater than 1km.

For multiple-input multiple-output (MIMO) precoded transmission that has individual constraints on the maximum power of each transmit antenna or a subset of transmit antennas, the transmit power optimization problem is a non-linear convex optimization problem with a high level of computational complexity. In this paper, assuming the use of the interior point method (IPM) to solve this problem, we propose two efficient techniques that reduce the computational complexity of the IPM by appropriately setting its parameters. Based on computer simulation, the achieved reductions in the level of the computational complexity are evaluated using the proposed techniques for both the fairness and the sum-rate maximization criteria assuming i.i.d Rayleigh fading MIMO channels and block diagonalization zero-forcing as a multi-user MIMO (MU-MIMO) precoder.

Ordered successive MMSE detection (OSD) can achieve a good tradeoff between performance and complexity for multiple-input multiple-output (MIMO) wireless systems where different data streams are transmitted and received simultaneously over several equal number of transmit and receive antennas. However, over time-varying MIMO channels, the performance of OSD is degraded with realistic channel estimation. In this paper, we introduce two different approaches in order to improve the performance of OSD. First, we propose to improve the accuracy of the generated nulling weights by using a modified noise variance that takes into consideration the weights update lag error besides to the noise and channel estimation errors. Second, we introduce a new iteration approach in order to improve the decisions made by OSD on transmitted streams. The proposed iterative scheme, named as backward iterative detection, exploits the tentative decisions on lately detected streams in order to mainly improve the decision on the firstly detected stream that limits the overall performance. Simulation results of combining both approaches show significant performance improvements of OSD.

The fifth generation mobile communications (5G) systems will need to support the ultra-reliable and low-latency communications (URLLC) to enable future mission-critical applications, e.g., self-driving cars and remote control. With the aim of verifying the feasibility of URLLC related 5G requirements in real environments, field trials of URLLC using a new frame structure are conducted in Yokohama, Japan. In this paper, we present the trial results and investigate the impact of the new frame structure and retransmission method on the URLLC performance. To reduce the user-plane latency and improve the packet success probability, a wider subcarrier spacing, self-contained frame structure, and acknowledgement/negative acknowledgement-less (ACK/NACK-less) retransmission are adopted. We verify the feasibility of URLLC in actual field tests using our prototype test-bed while implementing these techniques. The results show that for the packet size of 32 bytes the URLLC related requirements defined by the 3GPP are satisfied even at low signal-to-noise ratios or at non-line-of-sight transmission.

This paper proposes applying random (opportunistic) beamforming to base station (BS) cooperative multiuser multiple-input multiple-output (MIMO) transmission. This proposal comprises two parts. First, we propose a block-diagonalized random unitary beamforming matrix. The proposed beamforming matrix achieves better throughput distribution compared to the purely random unitary beamforming matrix when the average path loss determined by distance-dependent loss and shadowing loss is largely different among transmitter antennas, which is true in BS cooperative MIMO. Second, we propose an online update algorithm for a random beamforming matrix to improve the throughput compared to the purely random and channel-independent beamforming matrix generation, especially when the number of users is low. Different from conventional approaches, the proposed online update algorithm does not increase the overhead of the reference signal transmission and control delay. Simulation results show the effectiveness of the proposed method using a block-diagonalized random unitary beamforming matrix with online updates in a BS cooperative multiuser MIMO scenario.

Non-orthogonal multiple access (NOMA) utilizing the power domain and advanced receiver has been considered as one promising multiple access technology for further cellular enhancements toward the 5th generation (5G) mobile communications system. Most of the existing investigations into NOMA focus on the combination of NOMA with orthogonal frequency division multiple access (OFDMA) for either downlink or uplink. In this paper, we investigate NOMA for uplink with single carrier-frequency division multiple access (SC-FDMA) being used. Differently from OFDMA, SC-FDMA requires consecutive resource allocation to a user equipment (UE) in order to achieve low peak to average power ratio (PAPR) transmission by the UE. Therefore, sophisticated designs of scheduling algorithm for NOMA with SC-FDMA are needed. To this end, this paper investigates the key issues of uplink NOMA scheduling such as UE grouping method and resource widening strategy. Because the optimal schemes have high computational complexity, novel schemes with low computational complexity are proposed for practical usage for uplink resource allocation of NOMA with SC-FDMA. On the basis of the proposed scheduling schemes, the performance of NOMA is investigated by system-level simulations in order to provide insights into the suitability of using NOMA for uplink radio access. Key issues impacting NOMA performance are evaluated and analyzed, such as scheduling granularity, UE number and the combination with fractional frequency reuse (FFR). Simulation results verify the effectiveness of the proposed algorithms and show that NOMA is a promising radio access technology for 5G systems.

This paper proposes a novel method for adaptively controlling the admission of interference to users in our previously proposed layered partially non-orthogonal block diagonalization (BD) precoding method for downlink multiuser multiple-input multiple-output (MIMO) transmission that employs cooperation among multiple base stations (BSs). The proposed method is applicable when some of the instantaneous channel state information (CSI) feedback between the user equipment and the respective BSs is missing if the path loss between the user equipment and BS is higher than a predetermined threshold. The proposed method suppresses the loss in the transmitter diversity (beam forming) gain caused by the perfect nulling of inter-user interference in BD. By allowing the inter-user interference from a link that has a high average path loss, the overall throughput performance of simple BD is enhanced. We show that the combination of layered transmission that restricts the set of BSs used for the signal transmission and adaptive control of interference admission significantly increases the throughput of BS cooperative multiuser MIMO with partial CSI feedback.