Kenji Hoshino

A Field Trial of Multi-Cell Cooperative Transmission over LTE System

Multi-cell cooperative transmission is attracting much attention. This technology improves the cell-edge throughput since multiple base stations cooperate with each other in transmitting signals to the cell-edge user. One basic implementation of multi-cell cooperative transmission is inter-sector cooperation, in which a base station with three sectors uses its own equipment to realize multi-cell cooperative transmission. We extended inter-sector cooperation with our proposal of the sector configuration in which three sectors belonging to the same base station equipment are located at different sites linked by optical fiber. With the sector configuration, users located at cell edges between sites can benefit from the inter-sector cooperation, while users located in areas between sectors can enjoy the merit of inter-sector cooperation via the conventional sector configuration. Our previous studies clarified that inter-sector cooperation with the proposed distributed sector configuration provides superior throughput performance compared to the conventional sector configuration. To clarify the effectiveness of the inter-sector cooperation on the distributed sector configuration, we implemented this function on 3GPP Release 8 LTE-compliant equipment and conducted a field trial with the equipment in Kitakyushu, Fukuoka Prefecture, Japan on March, 2010. In the field trial, we implemented the distributed sector configuration by using RoF and evaluated the multi-cell cooperative transmission. The results of the field trial clarify that multi-cell cooperative transmission can improve cell-edge throughput dramatically.

Field Experiment of CoMP Joint Transmission over X2 Interface for LTE-Advanced

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.

Improving Throughput by Multi-Cell Coordinated Vertical Plane Beam Control with Pre-Coding

Cellular mobile communication systems reuse the same frequency channel in geographically distant areas to improve the spectrum efficiency. However, co-channel interference among cells remains a critical issue. Mitigating co-channel interference leads to a further improvement in spectrum efficiency. One technique for mitigating co-channel interference is antenna beam tilting, which controls the vertical plane beam of base station antennas. In cellular mobile communication systems, antenna beam tilting is generally used by setting fixed beams at each base station. If the vertical plane beam pattern is changed dynamically for each mobile station, it is possible to significantly improve the SINR (Signal- to-Interference plus Noise Ratio), which drastically improves the spectrum efficiency. We propose here a vertical plane beam control method that uses pre-coding; the vertical plane beam pattern for each mobile station is easily set by controlling the phase of the baseband signal. The proposed method can be easily implemented with little or no modification of the antenna configuration at the base station. This system is effective for improving not only cell edge capacity but also overall cell capacity at the same time. A computer simulation shows that the spectrum efficiency of the proposed system is approximately twice that of conventional antenna beam tilting.

Throughput Improvement by Power Reallocation in Multi-Cell Coordinated Power Control

Multi-cell coordinated power control coordinates transmit-power allocation of tens or hundreds of cells to control mutual interferences among them, which leads to the improvement of spectral efficiency. In the coordinated power control, transmit powers are determined so as to satisfy the minimum required SINRs of mobile terminals. As a result, most of the base stations usually have room to increase their transmit powers to improve spectral efficiency. In this paper, we propose two methods: a scaling method and a linear programming method. These methods reallocate transmit powers to increase spectral efficiency without decreasing the number of mobile terminals satisfying their minimum required SINRs. We evaluated the proposed methods by computer simulations and confirmed that the linear programming method improves spectral efficiency by 40-percent.

Field Evaluations on a Prototype System of Cooperative Multi-Cell MIMO Transmission for Asynchronous Inter-Site Base Station Networks

For next generation cellular mobile communication systems, cooperative multi-cell MIMO transmission (cooperative MIMO) technologies, in which multiple base stations (BSs) coordinate their wireless transmission, have recently been receiving considerable attention due to their potential cell-boundary throughput improvement. However, there are few studies that provide field evaluation of cooperative MIMO in real radio-propagation environments. To evaluate cooperative MIMO in a real field, BSs located at different sites should be synchronized to each other with high accuracy in asynchronous inter-site BS networks. We develop a prototype of an experimental system by using a GPS (Global Positioning System)-based inter-BS synchronous controller. We conduct field experiments on two cooperative MIMO transmission schemes: (i) cooperative MIMO-SDM based on space division multiplexing and (ii) Cooperative MIMO-SFBC based on space frequency block coding. We confirm that cooperative MIMO technologies improve the cell-boundary throughput in real radio-propagation environments. This paper describes the developed field experimental system and its field evaluation results. It also shows the effectiveness of cooperative MIMO wireless transmission at the cell-boundary.

A Study on Frequency Offset Interference Canceller for Multi-Link Transmission in OFDM Systems

It is important to improve the cell-edge throughput of the next generation mobile communication systems. Frequency reuse schemes such as 3-cell reuse or fractional frequency reuse are suitable for achieving this goal. Another candidate is multi- link transmission; it receives signals on different sub-carriers from adjacent base stations. However, the orthogonality of these signals can collapse if the frequency offset between the 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 with computer simulations.

Throughput improvement by precoding-based vertical plane beam control and cooperative MIMO transmission

Cellular mobile communication systems reuse the same frequency channel in geographically distant areas to improve the spectrum efficiency. However, co-channel interference among cells remains a critical issue. Mitigating co-channel interference leads to a further improvement in spectrum efficiency. To reduce the co-channel interference we proposed vertical plane beam control using precoding and cooperative MIMO transmission. Vertical plane beam control, which controls the vertical plane beam of base station antennas with the use of precoding, can improve overall cell capacity since SINR (Signal-to-Interference plus Noise Ratio) at each mobile station in the coverage area can be significantly improved. In cooperative MIMO transmission, adjacent base stations coordinate to transmit signals to cell-edge mobile stations. This technique can improve cell edge capacity, especially in low SINR areas. If the two techniques are used simultaneously, a synergetic effect can be expected; improved capacity in both the whole cell and cell edge. We propose here a method that combines vertical plane beam control with cooperative MIMO transmission. A computer simulation shows that the spectrum efficiency of the proposed system is approximately 2 times higher for the whole cell and 3 times higher for cell edge users than the conventional system.

QoS-Guaranteed Multi-Cell Coordinated Power Control Considering Base Station Cooperative Transmission

To further increase the spectrum efficiency of cellular mobile communication systems, it is important to control co-channel interference among cells. To control co-channel interference and improve spectrum efficiency over the whole cell, we proposed QoS-guaranteed multi-cell coordinated transmit power control, which limits the communication of some users and fully guarantees the QoS for the remaining users. On the other hand, to further increase the spectrum efficiency, it is also important to address the deterioration of communication quality at the cell edge. As one of the techniques for improving the communication quality at the cell edge, we have studied adjacent base station cooperative transmission in which simultaneously transmits signals from both own base station and adjacent base station to the cell-edge mobile terminal. In this paper, we propose a novel scheme that combines the adjacent base station cooperative transmission with the our proposed QoS- guaranteed multi-cell coordinated transmit power control in order to increase not only cell capacity but also cell edge capacity at the same time; its impact on overall performance was clarified.

Network Coordinated Inter-Cell Interference Control Using Horizontal-Plane Beamforming on Small Cells in 3D Cell Structure

3D cell structure, where small cells are three- dimensionally deployed on macro cells, is considered as one of the effective cell structures to accommodate data traffic growing rapidly in recent years. In this cell structure, inter-cell interference among small cells is one of the major issues. As a technique for mitigating inter-cell interference among small cells, we propose a network coordinated base station beamforming where small-cell base stations, applying horizontal-plane beam control, coordinate their beams by network coordination. We also clarify the throughput performance of the proposed method under a multi-cell environment considering a realistic cell structure using computer simulation.

Analytical Results of Field Experiment on Precoding-Based Vertical Plane Beam Control for LTE-Advanced Systems

Most current cellular mobile communication systems employ 1-cell frequency reuse to achieve high spectrum efficiency. With 1-cell frequency reuse, the co-channel interference among cells remains a critical barrier to achieving further improvements in the spectrum efficiency. Vertical plane beam control at the base station is one approach to mitigating the co-channel interference. The conventional method of vertical plane beam control, antenna beam tilting, is widely used. In antenna beam tilting, the tilt angle is generally fixed for each base station. If the vertical plane beam can be changed dynamically, it becomes possible to significantly improve the reception environment. Given this background, we have proposed precoding- based vertical plane beam control. In the proposed method, the vertical plane beam pattern for each mobile station is easily changed dynamically by just the precoding operation without changing the conventional antenna configuration drastically. To demonstrate the feasibility and performance of the proposed method, we develop a prototype system, in which the proposed scheme is implemented on an LTE- based system, and conduct a field experiment. This paper overviews the prototype system and the field experiment results. They clarify that the proposed method is effective in mitigating interference level to the neighbor cells as well as improving the desired signal level in the serving cell. We also conduct a theoretical analysis and confirm that the experimental results well match the theoretical results.