High-altitude platform stations (HAPSs) are expected to provide ultrawide-coverage areas and disaster-resilient networks from the stratosphere at around 20 km by installing wireless equipment on HAPS. Because their altitude is much lower than that of communications satellites, HAPSs can provide mobile communications services directly to smartphones, which are commonly used in terrestrial networks, such as fourth generation Long Term Evolution. Considering the widespread nature of mobile broadband communications and the importance as a backup line in case of disaster, HAPSs are expected to provide a large capacity in the future. A cellular system with single-cell frequency reuse using multiple cells similar to terrestrial mobile communications should be introduced to achieve such a capacity. The number of cells that a HAPS can accommodate ranges from 1 to more than 100, depending on unmanned aerial vehicle (UAV) ability. By contrast, the optimal cell configuration, which depends on the number of available cells, has not been clarified in previous research. In this paper, we propose an optimization method for the cell configuration for HAPS mobile communications using a genetic algorithm, which can be generally applied regardless of the number of cells and can clarify the optimal cell configuration. Although many cells are required to achieve gigabit-class HAPS mobile communications, the heightened power consumption due to the large number of cells is a critical problem for UAVs. Thus, we also investigate the reduction of the total transmission power and demonstrate the feasibility of energy-efficient gigabit HAPS mobile communications with wide coverage.
Inter-Cell Interference Coordination (ICIC) is attracting attention recently. In ICIC, cell-edge throughput can be improved by preventing BSs from transmitting signals (hereafter, muting BSs), and the information exchange among BSs is little because each UE is only served by a BS at any instant. However, when a BS is muted, no radio resource is allocated to UEs belonging to the muted BS while UEs belonging to other BSs enjoy high cell-edge throughput. Therefore, there is a possibility that overall cell performance may degrade. To prevent this, we propose a multi-BS cooperative interference control method. The basic concept of the proposed method is that the muting is triggered only when the total throughput of the cooperation area is increased by the muting compared to the total throughput possible without muting. The proposed method makes it possible to increase cell-edge throughput without degrading overall cell performance. We also propose a way to realize this interference control on practical systems. First, we propose a way to realized it on 3GPP Release 8 LTE systems. In the proposed interference control, it is important to estimate throughput (SINR) values with and without muting appropriately. We propose to utilize feedback signals defined in LTE such as Channel Quality Indicator (CQI) and Received Signal Received Power (RSRP) to achieve the accurate throughput estimation. Furthermore, we propose to realize the proposed interference control on a distributed sector configuration using optical fiber systems such as Radio over Fiber (RoF) or Remote Radio Head (RRH). With this configuration, it is possible to achieve ICIC with less burden of information exchange. Especially with three sector configuration, it is possible to achieve "inter-cell" cooperation with "inter-sector" cooperation, which can be easily implemented.
Multiple base station cooperation techniques have been attracting much attention for the improvement in cell-edge throughput recently. In 3GPP, such techniques are referred to as CoMP and studied actively. Joint transmission is a promising technique in CoMP. In CoMP JT, previous studies have mainly focused on intra-eNB CoMP because it is relatively easy to implement. The intra-eNB CoMP JT in combination with optical fiber systems such as RRH or RoF can realize throughput improvement at cell edge. However, the number of RRHs being able to be connected to the same eNB is usually limited to a few because of the signal-processing capability of eNB. Therefore, CoMP JT can be used only within the cells connected to the same eNB, which makes it impossible to use CoMP JT between at any cell border. To enable all cell-edge UEs enjoy the merit of CoMP JT, CoMP JT based on a distributed cooperation approach using inter-eNB interface such as X2 interface has been proposed. In the distributed cooperation, CoMP JT can be realized in a distributed manner, so that CoMP JT can be used at any cell border. However, the previous studies focused on only concepts or evaluation by computer simulations. To verify the feasibility and its effect with real system, we developed a prototype system of CoMP JT realized on a distributed cooperation approach using inter-eNB interface. The technical details to realize it is shown in this paper. We also conducted laboratory and field experiments and demonstrated its feasibility. Also, we confirmed that drastic throughput improvement at cell edge can be realized with the real system.
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