Sho Nabatame

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.

An Experimental Evaluation on Network-Listening Based Synchronization with Loop-Back Interference Avoidance

In deploying co-channel heterogeneous networks, which use the same frequency band for both macro and small cells, it is essential to avoid mutual interference. This can be achieved by enhanced Inter-Cell Interference Coordination (eICIC), in which some of the downlink transmitted signals are muted in order to reduce the interference from a macro-cell base station to small cells or vice versa. Because interference control in eICIC is conducted in the time domain, accurate timing synchronization is required between macro and small cells. Network-listening-based synchronization is recently attracting attention as an effective timing synchronization method even for indoor small-cell base stations, which 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. To solve this issue, we proposed a synchronization method that avoids loop-back interference and clarified its synchronization accuracy by computer simulations. In this paper, we propose an advanced synchronization- timing detection algorithm that improves the tolerance of loop-back self-interference. Furthermore, we prototype the proposed synchronization method and clarify its feasibility and performance by computer simulations and laboratory experiments on actual equipment. Moreover, we propose a control architecture for network-listening based synchronization that enables accurate synchronization even in dense small cell deployments where neighboring small-cell interference degrades the accuracy of macro-cell signal detection.

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.

An Experimental Study on Network-Listening Based Synchronization with Loop-Back Self-Interference Canceller

The overlaid cell structure, in which a large number of small cells are deployed in a macro-cell coverage area, is considered to be a promising approach in order to accommodate rapidly increasing mobile data traffic. In the overlaid cell structure, eICIC (enhanced Inter-Cell Interference Coordination) is an essential technique to avoid the interference between the macro cells and the small cells when co- channel deployment is used. Because eICIC conducts interference coordination in the time domain, accurate timing synchronization is required between the macro cells and the small cells. We have proposed a network-listening based synchronization method, which enables accurate synchronization even indoors without receiving GPS signal. In the proposed method, a small-cell eNB receives a macro- cell downlink synchronization signal and establishes synchronization with the macro-cell eNB. However, we need to update synchronization periodically and receive a macro-cell signal even while there is a loop-back interference signal from the small-cell eNB itself. In order to solve the problem, we have proposed a method to mute the transmission and avoid loop-back self-interference. Although this works well, it is an issue that the spectral efficiency is deteriorated by the signal stopping. In this paper, we propose a synchronization method that cancels loop-back self-interference without stopping the transmission of small-cell eNB and clarify the effect of the proposed method by computer simulations and laboratory experiments using the first-ever prototype system.

Inter-cell interference coordination by horizontal beamforming for 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 beamforming where small-cell base stations, applying horizontal beamforming, 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.

Experimental Evaluations of Coordinated Interference Control for Co-Channel Overlaid Cell Structure

The overlaid cell structure, in which a large number of small cells are deployed in a macro-cell coverage area, is considered to be a promising approach in order to accommodate rapidly increasing mobile data traffic. When co-channel deployment is used in the overlaid cell structure, the interference between macro and small cells becomes an issue and it needs to be solved in order to increase system capacity. eICIC is an effective technique to control the co-channel interference. In eICIC, the interference from a macro- cell eNB to small cells or vice versa can be avoided by stopping some parts of the downlink signal transmission. However, its distributed approach to decide the amount and the position of the muted parts cannot increase spectral efficiency effectively as the number of cells increases in the system. In order to solve this issue, we propose the coordinated interference control system based on a centralized approach. We also develop a prototype of the proposed system and clarify the feasibility and the throughput performance through laboratory and field experiments.

Field Experiment on Precoding-Based Vertical Plane Beam Control for LTE 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 as a critical barrier to achieving further improvement in the spectrum efficiency. Vertical plane beam control at base station is one approach to mitigating the co-channel interference. As a conventional method of vertical plane beam control, antenna beam tilting has been widely used. In the antenna beam tilting, the tilt angle is generally fixed per base station. If the vertical plane beam can be changed dynamically for each mobile station, it is 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 (phase control of digital baseband signal) operation without changing conventional antenna configuration drastically. To demonstrate the feasibility and performance of the proposed method, we developed a prototype system, in which the proposed scheme is implemented on an LTE-based system, and conducted a field experiment. This paper overviews the prototype system and field experiment results. Measured results 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.