Atsushi Nagate

A study on channel estimation methods for MC-CDMA systems

In this paper, we propose a novel channel estimation method for time-domain spreading multi-carrier code division multi-carrier code division multiple access (MC-CDMA) systems. In time-domain spreading MC-CDMA systems, code-multiplexed pilot symbols are transmitted consecutively in both time and frequency domains. In our proposed method, weighted pilot symbols in both domains are coherently added to improve signal-to-noise ratio (SNR) of pilot symbols in channel estimation. However, the channel estimation quality degrades when the weights of pilot symbols are not selected appropriately according to the changes of propagation environments. We propose a method which the optimum weighting factors for pilot symbols are adaptively selected according to the changes of propagation environments, by measuring maximum Doppler frequency for the time domain and delay spread for the frequency domain at MS (mobile station). We evaluate our proposed method by computer simulation, and clarify improvement of channel estimation accuracy.

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.

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.

Design of Control Architecture for Downlink CoMP Joint Transmission with Inter-eNB Coordination in Next Generation Cellular Systems

CoMP (Coordinated Multiple Point transmission) is being discussed in 3GPP (3rd Generation Partnership Project) to improve the cell edge performance as well as the total cell throughput of mobile systems. In this paper, we focus on the control architecture to realize downlink CoMP JT (joint transmission) with inter eNB (evolved NodeB) coordination. We have designed the control architecture with a distributed approach. Since user data is simultaneously transmitted from multiple eNBs to a UE (User Equipment), the schedulers of the coordinated eNBs need to negotiate on resource allocation for CoMP UEs over a backhaul network. This process will be a heavy burden due to the delay of the backhaul network and the complexity of scheduling algorithm that has to take into account a dynamic multi-UE and multi-cell scenario. We propose a simple and practical resource allocation method by which the scheduler in each eNB can make scheduling calculation independently without any resource contention. We also implement the proposed schemes as a real system for evaluation.

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.

A study on channel estimation methods for time-domain spreading MC-CDMA systems

In time-domain spreading MC-CDMA systems, which are candidates for the 4G system, 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. In this paper, 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.

Field Experiment of Multi-BS Cooperative Transmission Control over X2 Interface for LTE/LTE-Advanced

The multiple base station cooperation approach has been attracting much attention recently. In 3GPP, this approach is referred to as CoMP. The promising techniques in CoMP are joint transmission and dynamic cell selection. They improve cell-edge UE throughput through the cooperation of eNBs, however, the signal-processing burden or the technical difficulties to realize the accurate synchronous transmission is high. As an alternative for improving cell-edge throughput, cooperative transmission control has been proposed, in which signal transmission is conducted from only the serving cell while transmission from its neighbor cell is stopped. This technique can mitigate the signal-processing burden or the technical difficulties of CoMP while achieving sufficient throughput improvement. However, the previous studies were based on centralized cooperation with the use of an optical fiber system such as RRH or RoF. Therefore, cooperation is possible only within the eNBs connected to the same central signal processing unit. To avoid this limitation, we propose cooperative transmission control based on a distributed cooperation approach that uses an inter- eNB interface such as X2. It allows cooperative transmission control to be realized at any cell border. We also propose the control algorithm to start and end the cooperation properly, which is a key part to improve cell-edge throughput effectively on the distributed approach. We also develop a prototype system to demonstrate the feasibility and the performance of the proposal. Through laboratory and field experiments, we show that the proposal works well with real equipment and that cell-edge UE throughput performance can be improved drastically.

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.

A simple network-listening based synchronization for small cells in LTE-Advanced

The layered cell structure, in which a large number of small cells overlay each macro cell to increase network capacity is attracting much attention. In the layered cell structure, eICIC is essential 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. Although GNSS-based or packet-based timing synchronization is effective in many cases, GNSS-based synchronization cannot be used indoors and the packet-based approach may not work depending on the backhaul network configuration. Network-listening based synchronization is attracting much attention recently as an alternative to the above methods. It uses just the macro-cell signal to realize accurate synchronization. However, the loop-back signal from the small-cell eNB itself interferes the reception of the macro-cell signal. In this paper, we propose a simple synchronization method that eliminates loop-back interference, and clarify that it achieves accurate synchronization by computer simulations.