It is important to improve a cell-edge throughput of next generation mobile communication systems. Frequency reuse schemes such as three-cell reuse or fractional frequency reuse are suitable for achieving this goal. Another candidate is multi-link transmission; signals on different sub-carriers from adjacent base stations are received by a mobile. However, the orthogonality of these signals can collapse if a frequency offset between adjacent base stations is excessive; this loss triggers adjacent-channel interference. This paper proposes an interference canceller to solve this problem and confirms the effectiveness of the method through numerical analysis and computer simulations.

In next-generation mobile communications, it is important to improve the throughput of the cell edge as well as that of the whole cell. Multi-link transmission from two adjacent BSs has been studied for improving the throughput at the cell edge in OFDM-based cellular systems, which are major candidates for next-generation mobile communication systems. In multi-link transmission, an MS at a cell edge receives signals from both adjacent BSs by orthogonally multiplexing those signals in the frequency domain. Therefore, the cell-edge MS can utilize the frequency and power resources of both adjacent BSs, which improves the cell-edge throughput. However, when the received timing difference between the signals from both BSs exceeds the maximum permissible value, adjacent-channel interference is caused by the collapse of the orthogonality. In this paper, to resolve this issue, we propose a novel timing-offset interference canceller. To clarify the performance of the proposed canceller, this paper evaluates its performance with respect to the residual interference power and the BER by computer simulation.

Long Term Evolution (LTE) system, which is specified in the 3rd Generation Partnership Project (3GPP) Release 8 and employs downlink multiple input multiple output (MIMO) transmission, is drawing attention as a promising next generation cellular mobile radio system due to its high spectral efficiency compared to the current High-Speed Packet Access (HSPA) system. The authors performed a field trial of an LTE system that complies with 3GPP Release 8 in Kitakyushu-city, Fukuoka, Japan, as a specified ubiquitous district project promoted by the Ministry of Internal Affairs and Communications of Japan. This paper first summarizes the field trial. Next, it describes the overview of the field trial system and reports the field experiment results on the downlink 22 MIMO wireless transmission. Finally, it compares the field experimental results to laboratory experimental results obtained with a hardware channel simulator using the channel model based on Recommendation ITU-R P.1816.

In CDMA mobile cellular systems, wireless quality is improved by soft handoff techniques. However, it requires to hold multiple channels of cells, which is likely to increase call blocking at wired channels. It is therefore necessary to consider the entire system including the wired and wireless portions of systems for investigating an effectiveness of the soft handoff. In this paper, we also clarify the effect of interference power from mobile stations that are not in the soft handoff because of lack of wired channels. In the analysis, we model three-way soft handoff which has not been considered in past researches. We also show the effect of a call admission control to wireless quality.

As a candidate for the transmission technology of next generation mobile communication systems, time-domain spreading MC-CDMA systems have begun to attract much attention. In these systems, data and pilot symbols are spread in the time domain and code-multiplexed. To combat fading issues, we need to conduct channel estimation by using the code-multiplexed pilot symbols. Especially in next generation systems, frequency bands higher than those of current systems, which raise the maximum Doppler frequency, are expected to be used, so that a more powerful channel estimation method is expected. Considering this, we propose a channel estimation method for highly accurate channel estimation; it is a combination of a two-dimensional channel estimation method and an impulse response-based channel estimation method. We evaluate the proposed method by computer simulations.

Network-listening-based synchronization is recently attracting attention as an effective timing synchronization method for indoor small-cell base stations as they cannot utilize GPS-based synchronization. It uses only the macro-cell downlink signal to establish synchronization with the overlaying macro cell. However, the loop-back signal from the small-cell base station itself interferes with the reception of the macro-cell downlink signal in the deployment of co-channel heterogeneous networks. In this paper, we investigate a synchronization method that avoids loop-back interference by muting small-cell data transmission and shifting small-cell transmission timing. Our proposal enables to reduce the processing burden of the network listening and mitigate the throughput degradation of the small cell caused by the data-transmission mutation. In addition to this, the network-listening system enables the network listening in dense small cell deployments where a large number of neighboring small cells exist. We clarify the performance of our proposal by computer simulations and laboratory experiments on actual equipment.

High-altitude platform stations (HAPSs) are recognized as a promising technology for coverage extension in the sixth generation (6G) mobile communications and beyond. The purpose of this study is to develop a HAPS system with a coverage radius of 100km and high capacity by focusing on the following two aspects: array antenna structure and user selection. HAPS systems must jointly use massive multiple-input multiple-output (mMIMO) and multiuser MIMO techniques to increase their capacity. However, the coverage achieved by a conventional planar array antenna is limited to a circular area with a radius of only tens of kilometers. A conventional semi-orthogonal user selection (SUS) scheme based on the orthogonality of channel vectors achieves high capacity, but it has high complexity. First, this paper proposes a cylindrical mMIMO system to achieve an ultra-wide coverage radius of 100km and high capacity. Second, this paper presents a novel angle-based user selection (AUS) scheme, where a user selection problem is formulated as a maximization of the minimum angular difference between users over all user groups. Finally, a low-complexity suboptimal algorithm (SA) for AUS is also proposed. Assuming an area with a 100km radius, simulation results demonstrate that the proposed cylindrical mMIMO system improves the signal-to-interference-plus-noise ratio by approx. 12dB at the boundary of the area, and it achieves approx. 1.5 times higher capacity than the conventional mMIMO which uses a planar array antenna. In addition, the results show that the proposed AUS scheme improves the lower percentiles in the system capacity distribution compared with SUS and basic random user selection. Furthermore, the computational complexity of the proposed SA is in the order of only 1/4000 that of SUS.

The layered cell configuration, in which a large number of small cells are set in a macro-cell coverage area, is attracting much attention recently as a promising approach to handle the rapidly increasing mobile data traffic. In this configuration, cells of various sizes, from macro to small, are placed in various locations, so that the variation in the number and the distribution of the users among cells becomes much wider than in conventional macro-cell homogeneous networks. Therefore, even in the layered cell configuration, the users in the cell with many users and low received signal quality may experience low throughput especially at cell edge. This is because such users experience both low spectral efficiency and few radio resources. In order to resolve this issue, a lot of techniques have been proposed such as load balancing and cooperative multi-point transmission. In this paper, we focus on scheduling priority control as a simple solution that can also be used in combination with load balancing and coordinated multi-point transmission. We propose an adaptive scheduling priority control scheme based on the congestion and user distribution of each cell and clarify the effect of the proposed method by computer simulations.