Published papers

One of key technologies in the fifth generation mobile communications is a distributed antenna system (DAS). In the DAS inter-user interference effects on its spectrum efficiency as distributed antennas are located closely. Round-robin (RR) scheduling is known as a scheme that achieves a trade-off between computational complexity and spectrum efficiency. In this research user equipment (UE) allocation scheme for RR scheduling are proposed. The proposed scheme determines the phases of UE allocation sequence that is pre-determined for low complexity. Numerical results obtained through computer simulation have shown that maximum selection that sequentially searches the phase with the maximum tentative throughput realizes the largest spectrum efficiency next to full search. The system throughput increases as the number of UEs increases in 2-user RR scheduling.

In this paper, user scheduling with beam selection for fulldigital massive multi-input multi-output (MIMO) is proposed. Inter-user interference (IUI) can be canceled by precoding such as zero-forcing at a massive MIMO base station if ideal hardware implementation is assumed. However, owing to the non-ideal characteristics of hardware components, IUI occurs among multiple user terminals allocated on the same resource. Thus, in the proposed scheme, the directions of beams for allocated user terminals are adjusted to maximize the total user throughput. User allocation based on the user throughput after the adjustment of beam directivity is then carried out. Numerical results obtained through computer simulation show that when the number of user terminals in the cell is two and the number of user terminals allocated to one resource block (RB) is two, the throughput per subcarrier per subframe improves by about 3.0 bits. On the other hand, the fairness index (FI) is reduced by 0.03. This is because only the probability in the high throughput region increases as shown in the cumulative distribution function (CDF) of throughput per user. Also, as the number of user terminals in the cell increases, the amount of improvement in throughput decreases. As the number of allocated user terminals increases, more user terminals are allocated to the cell-edge, which reduces the average throughput.

The fifth generation mobile communication systems are expected to support the Internet-of-Things (IoT) applications. One of the problems in massive connections is the assignment of pilot sequences. For supporting a large number of the IoT devices, non- orthogonal sequences have to be assigned and a receiver in a base station needs to estimate channel impulse responses (CIRs) by solving a joint optimization problem. In this paper, a new channel estimation scheme for the IoT devices is proposed. The proposed scheme is based on a repeating weighted boosting search algorithm and modifies the generation process of candidate CIR vectors for faster convergence in search iterations. Instead of calculating the sum of weighted candidate CIR vectors, the proposed scheme generates a candidate CIR vector that is assumed to be closer to the global optimum. In this case, the proposed scheme improves the convergence rate as compared to the conventional scheme though the mean square errors (MSE) of the solutions is worse. If the number of subcarriers is limited, the MSE of the estimated CIR vectors with the proposed algorithm is equivalent to that with the conventional scheme. Thus, the proposed scheme is less complex and suitable for massive connections. Numerical results obtained through computer simulation have shown that the proposed scheme achieves faster convergence by about 17-35% as compared to the conventional scheme.

© 2018 IEEE. The multiuser massive MIMO (multi-input multi-output) system using the low-resolution analog-to-digital converters (ADCs) in the uplink is to realize low cost and power consumption demands, which, however, reduce the signal quality. Beamforming as a signal processing technique can supply high throughputs in the MIMO systems. In this paper, we investigate the robustness of low-resolution ADCs against the interference among the different streams in the uplink and the different users in downlink systems respectively, and propose the maximum ratio transmission - block diagonalization (MRT-BD) scheme as a kind of beamforming to evaluate the effects of low-resolution ADCs in the full digital massive MIMO system. The preliminary conclusion is that the MRT-BD scheme achieves our anticipation to make the rate achievable, even more than it in the infinite-resolution (no quantization) of BD case can be approached using ADCs with only a few bits of resolution.

Copyright © 2018 The Institute of Electronics, Information and Communication Engineers. This paper applies minimum mean square error (MMSE) interference rejection followed by joint maximum likelihood detection (MLD) to a receiver in a distributed antenna network (DAN).DANreceivers capture not only the desired signals, but also the interference signals from nearby uncoordinated antennas. For the overloaded signal situation, nonlinear detection schemes such as joint MLD can be applied to the received signals. However, the amount of metric calculations in joint MLD increases exponentially with the number of signal streams. Therefore, MMSE interference rejection followed by MLD detection is proposed. The proposed scheme reduces the complexity by a factor of 1=2M (NT -1) where NT is the number of interference signals with 2MQAM modulation. The effect of residual interference after the MMSE interference rejection is evaluated. Numerical results obtained through computer simulation and experiment show that the performance of the proposed scheme is about 4:0 dB worse at a bit error rate (BER) of 10-3 than that of the joint MLD while its complexity is four times lower for QPSK signal streams. The BER performance degradation can be suppressed to about 2:5 dB by adjusting the value of the coefficient in the MMSE matrix.