Manabu Mikami

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.

Iterative MIMO Signal Detection with Inter-Cell Interference Cancellation for Downlink Transmission in Coded OFDM Cellular Systems

MIMO/OFDM is receiving considerable attention for next generation mobile radio communication systems. These systems require improved throughput performance to users at cell-edges. In our previous study, we introduced an enhancement of MLD (Maximum Likelihood signal Detection) algorithm, in which desired and interference signals are jointly detected, to improve the cell-edge throughput in the downlink of single- cell-frequency-reuse MIMO/OFDM cellular systems. In order to achieve higher cell-edge throughput in coded OFDM cellular systems, this paper proposes an iterative signal detection with ICI cancellation based on an enhancement of MAP (Maximum A posteriori Probability) algorithm in which signal path search space includes not only desired signal space but also interference signal space. Its transmission characteristics are evaluated by computer simulation and it is confirmed that the proposed iterative signal detection can achieve higher ICI cancellation and improve BLER (BLock Error Rate) and throughput performance more than the conventional MLD-based signal detection.

A Downlink Transmission Method for OFDM Cellular Systems with Inter-Cell Interference Cancellation Using Simplified MLD based on MMSE QRD-M Algorithm

For next generation mobile radio communication, one-cell-frequency-reuse OFDM systems are attracting considerable attention. In such systems, it is necessary to improve the throughput performance of users at the cell-edge. This paper introduces an effective signal detection method with ICI canceller for OFDM cellular systems. The proposed method is an enhancement of a simplified MLD (Maximum Likelihood Detection) in which the signal path search space includes not only the desired signal space but also the interference signal space. This algorithm uses the MMSE QR decomposition of the virtual MIMO channel matrix and the M-algorithm in order to reduce the computational complexity of the original MLD. Its transmission performance is evaluated by computer simulation and it is confirmed that the signal detection that the proposed method can improve the throughput performance of the MS at the cell-edge in fully-loaded OFDM cellular systems. This paper also clarifies the tolerance of the frequency offset difference between local oscillators in the adjacent base stations for the proposed signal detection method.

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 Cell Identification Performance Improvement in Co-Channel Heterogeneous Cellular Networks with Cell Range Expansion

Recently, data traffic has been explosively increasing in cellular mobile communication systems due to the prevalence of smart phones. A co-channel heterogeneous cellular network overlaid with pico-cells (HetNet), in which pico-cell base stations (BSs) with low transmission power are placed at the hot spot area of a macro-cell base station with a high transmission power, has been receiving attention in order to deal with the increasing traffic. In general cellular mobile communication systems, synchronization signal (SS) symbols for the initial serving-cell identification and for the neighboring-cell identification at user equipments (UEs) are periodically transmitted from base stations (BSs). For HetNet, Cell Range Expansion (CRE), in which a bias value is used for the reference signal received power from pico-cell BSs, has been investigated as a promising approach to offload the traffic into the pico-cell from the macro- cell. Unfortunately, there is an issue that the cell identification delay at pico-cell area increases with the CRE bias value, since larger CRE bias value suffers from larger interference from macro-cell base stations (Macro-BSs) at the received SS symbols transmitted from pico-cell base stations (Pico-BSs) in the pico-cell area expanded by CRE. This paper investigates the cell- identification performance improvement possible with transmission power coordination of the SS symbols for HetNet with CRE. Then, it evaluates the cell-identification characteristics of a cell search operation based on the hierarchical SS structure of 3GPP LTE/LTE-Advanced, and also clarifies the effectiveness of the transmission power coordination of the SS symbols in terms of the pico-cell identification for HetNet with CRE.

R&D activities for 5G in IEICE technical committee on radio communication systems

As the demand for higher transmission rates and spectral efficiency is steadily increasing, the research and development of novel mobile communication systems has gained momentum. This paper focuses on providing a comprehensive survey of research and development activities on fifth generation mobile communication systems reported in IEICE technical committee on radio communication systems. We try to survey a vast area of wireless communication systems and the developments that led to future 5G systems.

A Prototype System for Evaluating Multi-Cell Cooperative Transmission in Asynchronous Mobile Radio Networks

Mobile communication services are now expected to support the speeds offered by fixed lines. To fill this demand, a critical goal is to improve user throughput performance especially at the cell-edge, even in the LTE (Long Term Evolution)-Advanced system. Multi-cell cooperative transmission is a promising technology and has been extensively studied. This technology requires highly accurate synchronization between multiple BSs. However, the future mobile network will consist of asynchronous networks (e.g., those controlled by IP protocols) and stable synchronization is not possible with existing techniques. We have been studying synchronization schemes for asynchronous networks to realize multi-cell cooperative transmission. In this paper, we present a prototype of a multi-cell cooperative transmission system that combines our synchronization scheme with radio equipment. Experiments confirm its synchronization accuracy, and the resulting wireless transmission performance in the field evaluation indicates our synchronization scheme provides a practical and effective approach.

An Inter-Cell Interference Cancellation Scheme with Multi-Cell Coordinated Scheduling for Downlink of MIMO/OFDM Cellular Systems

Next generation cellular mobile communication systems require cell-edge throughput improvement as well as performance improvement of peak throughput and system throughput. Recently, coordinated multi-cell MIMO technologies have been receiving considerable attention due to their cell-edge throughput improvement. Unfortunately, these technologies have difficulty in improving both cell-edge throughput and system throughput, simultaneously, under fully-loaded traffic conditions. For the simultaneous improvement of cell-edge and system throughput, inter-cell interference (ICI) cancellation has been investigated as a promising approach. In our previous studies, downlink transmission schemes with ICI canceling signal detection base on joint detection were proposed for MIMO/OFDM cellular systems. However, the ICI canceling signal detection is too complex for mobile stations to implement since the MS receiver requires information of the transmission scheme used by the interference signals such modulation and precoding. This paper proposes an ICI cancellation scheme by combining multi-cell coordinated scheduling and a simple linear interference canceling algorithm for the downlink transmission of MIMO/OFDM cellular systems. This paper evaluates the transmission performance of the proposed scheme, and clarifies its effectiveness.

Investigation on Inter-Cell Interference Cancellation Scheme for Small-Cell User Equipments in Heterogeneous Networks Employing Cell Range Expansion

Recently, data traffic is explosively increasing in cellular mobile communication systems due to the prevalence of smart phones. A heterogeneous cellular network overlaid with small-cells to a macro-cell (HetNet), in which small-cell base stations (BSs) with a low transmission power are placed at the hot spot area of a macro-cell base station with a high transmission power, has been receiving attention in order to deal with the increasing traffic. For HetNet, Cell Range Expansion (CRE), in which a bias value is used for the reference signal received power (RSRP) from small-cell BSs, has been investigated as a promising approach to offload the traffic into the small-cell from the macro-cell. However, User Equipments (UEs), located in the small-cell area expanded by CRE, suffers large inter-cell interference (ICI) from the macro-cell BSs for the case of co- channel HetNet in which the same carrier frequency is used between macro-cells and small-cells. Hence, it is required for co-channel HetNet to mitigate ICI. In order to improve the downlink user throughput characteristics for co-channel HetNet employing large CRE bias value, this report investigates a successive interference cancellation (SIC) technique of codeword level SIC. The codeword level SIC (CWSIC) realizes a large ICI mitigation effect employing the output of the channel decoder for interference signals. Simulation results confirm that CWSIC based on soft replica effectively mitigate the ICI from the macro-cell BSs, compared to CWSIC based on hard replica.