Daigo Ogata

Cell Edge Throughput Improvement by Base Station Cooperative Transmission Control with Reference Signal Interference Canceller in LTE System

Inter-cell interference coordination (ICIC) is attracting attention recently. This is a technique mainly improving cell-edge throughput by coordinating scheduling and signal transmission of multiple BSs. Different from joint transmission (JT), in which the desired signals for a user equipment (UE) are simultaneously transmitted from multiple BSs, the burdens of information exchange among BSs and its attendant signal processing are mitigated in ICIC because it requires only the exchange of scheduling and control information. In ICIC, the cell-edge throughput is improved by adaptively preventing BSs from transmitting signals (in other words, muting BSs) that would otherwise impose strong interference on the cell- edge UEs in the neighbor cells. Although this improves cell-edge throughput, it may degrade cell overall performance because no radio resource is assigned to UEs belonging to the muted BS. To resolve this issue, we first propose a scheduling method, which enables muting only when the throughput improvement (cell-edge UE) obtained by muting is superior to the total throughput possible without muting. Also, when considering applying this ICIC technique to current commercial systems such as LTE, it should be noted that reference signal, also known as pilot signal, is always transmitted regardless of muting because reference signal is a common signal used for channel estimation. To solve this issue, we also propose a reference signal interference canceller, in which the UE cancels the reference signal being transmitted from the neighbor BS. We evaluate the performance improvement by computer simulations and confirm that cell-edge throughput is dramatically improved by introducing the proposed canceller. We also propose a control algorithm that activates the canceller only when throughput improvement is possible. A basic lab experiment was also conducted to confirm the effect of the proposed interference canceller with 3GPP Rel-8 LTE-compliant equipment.

Experimental Evaluation of Reference Signal Interference Canceller for Multi-BS Cooperative Transmission Control in LTE

Inter-cell interference coordination (ICIC) is attracting attention recently. In ICIC, mutual interference among cells are coordinated to mitigate strong interference to UEs, while the exchange of data signal is not necessary among eNBs, which results in lower signal processing and networking burdens compared to coordinated multi-point (CoMP) transmission. In ICIC, the interference from an eNB to UEs in surrounding cells can be mitigated by stopping signal transmission (muting) on some time or frequency radio resources. However, even when the data signal transmission is stopped, common signals such as reference signal (RS) continue to be transmitted in LTE systems. Therefore, UEs still receive the residual interference from the RS, which degrades the performance. To solve this issue, we proposed an RS interference canceller. Because the RS is a common signal, the signal can be detected and removed (cancelled) by any UE. We evaluated the effect of the canceller by computer simulation and clarified that it yields improved performance in the simple single-input single-output (SISO) antenna configuration. In this paper, we extended the canceller to the multiple-input multiple-output (MIMO) antenna configuration. We also propose a frequency-domain interference canceller, which drastically reduces signal processing cost without degrading performance. Furthermore, we implement the proposed canceller on 3GPP Release 8 LTE UE and conduct an experimental evaluation. The results demonstrate that the proposed canceller gives high performance improvement even with actual equipment.

Network-Listening Based Synchronization with Loop-Back Interference Avoidance for Small Cells in LTE-Advanced

The overlaid cell structure, in which a large number of small cells are deployed in a macro-cell coverage area, is attracting much attention recently as a promising approach to cope with rapidly increasing mobile data traffic. In the overlaid cell structure with co-channel deployment, in which the same frequency band is used in both macro and small cells, it is essential to avoid the mutual interference. The interference avoidance can be achieved by enhanced Inter-Cell Interference Coordination (eICIC), in which some parts of the downlink transmitted signals are muted in order to reduce the interference from a macro-cell eNB to small cells or vice versa. Because the 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 regardless of the location of small-cell base stations or their backhaul network configuration. It uses just the macro-cell downlink signal to realize accurate synchronization. However, the loop-back signal from the small-cell eNB itself interferes the reception of the macro-cell downlink signal. In this paper, we propose a synchronization method that avoids the loop-back interference and clarify its synchronization accuracy by computer simulations. We also extend the method to incorporate the effect of time and antenna diversities and verify the performance.

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.

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.

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.