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.

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 enables the use of the SIC receiver in the 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 than orthogonal multiple access (OMA), which is assumed in LTE-Advanced. We also show that BS cooperative operation along with the proposed NOMA further enhances the cell-edge user throughput gain which implies better user fairness and universal connectivity.

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.

In order to enhance the fifth generation (5G) mobile communication system further toward 5G Evolution, high bit-rate transmission using high SHF bands (28GHz or EHF bands) should be more stable even in high-mobility environments such as high speed trains. Of particular importance, dynamic changes in the beam direction and the larger Doppler frequency shift can degrade transmission performances in such high frequency bands. Thus, we conduct the world's first 28 GHz-band 5G experimental trial on an actual Shinkansen running at a speed of 283km/h in Japan. This paper introduces the 28GHz-band experimental system used in the 5G experimental trial using the Shinkansen, and then it presents the experimental configuration in which three base stations (BSs) are deployed along the Tokaido Shinkansen railway and a mobile station is located in the train. In addition, transmission performances measured in this ultra high-mobility environment, show that a peak throughput of exceeding 1.0Gbps and successful consecutive BS connection among the three BSs.

The fifth-generation (5G) mobile communications system initially introduced massive multiple-input multiple-output (M-MIMO) with analog beamforming (BF) to compensate for the larger path-loss in millimeter-wave (mmW) bands. To solve a coverage issue and support high mobility of the mmW bands, base station (BS) cooperation technologies have been investigated in high-mobility environments. However, previous works assume one mobile station (MS) scenario and analog BF that does not suppress interference among MSs. In order to improve system performance in the mmW bands, fully digital BF that includes digital precoding should be employed to suppress the interference even when MSs travel in high mobility. This paper proposes two mmW BS cooperation technologies that are inter-baseband unit (inter-BBU) and intra-BBU cooperation for the fully digital BF. The inter-BBU cooperation exploits two M-MIMO antennas in two BBUs connected to one central unit by limited-bandwidth fronthaul, and the intra-BBU cooperates two M-MIMO antennas connected to one BBU with Doppler frequency shift compensation. This paper verifies effectiveness of the BS cooperation technologies by both computer simulations and outdoor experimental trials. First, it is shown that that the intra-BBU cooperation can achieve an excellent transmission performance in cases of two and four MSs moving at a velocity of 90km/h by computer simulations. Second, the outdoor experimental trials clarifies that the inter-BBU cooperation maintains the maximum throughput in a wider area than non-BS cooperation when only one MS moves at a maximum velocity of 120km/h.

In order to realize the higher bit rates compared for the fifth-generation (5G) mobile communication system, massive MIMO technologies in higher frequency bands with wider bandwidth are being investigated for 5G evolution and 6G. One of practical method to realize massive MIMO in the high frequency bands is hybrid beamforming (BF). With this approach, user selection is an important function because its performance is highly affected by inter-user interference. However, the computational complexity of user selection in multi-user massive MIMO is high because MIMO channel matrix size excessive. Furthermore, satisfying user fairness by proportional fairness (PF) criteria leads to further increase of the complexity because re-calculation of precoding and postcoding matrices is required for each combination of selected users. To realize a fair and low-complexity user selection algorithm for multi-user massive MIMO employing hybrid BF, this paper proposes a two-step user selection algorithm that combines PF based user selection and chordal distance user selection. Computer simulations show that the proposed two-step user selection algorithm with higher user fairness and lower computational complexity can achieve higher system performance than the conventional user selection algorithms.

We propose an efficient hybrid automatic repeat request (HARQ) method that simultaneously achieves packet combining and resolution of the collisions of random access identifiers (RAIDs) during retransmission in a non-orthogonal multiple access (NOMA)-based random access system. Here, the RAID 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 a RAID from the candidate set prepared by the system, the decoding of received packets fails when multiple users select the same RAID. Random RAID reselection by each user when attempting retransmission can resolve a RAID 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 RAID of the previously transmitted packet and that of the retransmitted packet. To address this problem, we propose a HARQ method that employs novel hierarchical tree-structured RAID groups in which the RAID for the previous packet transmission has a one-to-one relationship with the set of RAIDs for retransmission. The proposed method resolves RAID collisions at retransmission by randomly reselecting for each user a RAID from the dedicated RAID set from the previous transmission. Since the relationship between the RAIDs 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.