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Hitachi America, Ltd.Hitachi America, Ltd.

Hitachi America, Ltd., Research & Development

Efficient Operation of Dense Small Cell Networks

Overlaying small cells onto the legacy macro cell network is a well-established method of boosting cellular coverage as well as capacity. For LTE Release-12 and beyond, the focus is on deploying and optimizing very dense small cell networks, where issues such as interference coordination and frequent handover become critical. WSRL is actively involved in several aspects of efficient small cell operation at both PHY and higher layers, such as interference mitigation via small cell ON/OFF switching, robust over-the-air synchronization, coordinated resource allocation over non-ideal backhaul links, dynamic optimization of uplink/downlink TDD configurations, and seamless mobility by exploiting dual connectivity and RRC diversity. Upcoming areas of study will focus on how real-time small cell traffic conditions can be efficiently monitored and optimized by the core network, using enhanced self-organizing network mechanisms for fault diagnosis and avoidance.

Full Dimension MIMO

Full-dimensional multiple input multiple output (FD-MIMO) is a practical implementation of massive MIMO in cellular systems. FD-MIMO relies on an active two-dimensional antenna grid at the base stations and harvests the benefits of both azimuth and elevation beamforming. To make the best out of the use of elevation beamforming and FD-MIMO, new three-dimensional channel models are currently being discussed in 3GPP Rel. 12. WSRL is actively involved in the standardization activities of FD-MIMO taking into account the three-dimensional placement of users and base stations. WSRLs’ research further covers several aspects of FD-MIMO concepts and applications. Areas of research range from analyzing the gains from vertical sectorization and multi-user vertical beamforming, to efficient multi-user precoding and FD-MIMO codebook design. WSRL’s research on FD-MIMO focuses in general on enhancing the performance of cellular networks, bringing together massive MIMO and small cell networks.

Inter-eNodeB Coordination

Base Station coordination technology also known as coordinated multipoint transmission reception (CoMP) can dramatically enhance the performance of cellular systems by coordinating the transmissions across several base stations and thus mitigating the interference. There are multiple ways to coordinate the base stations such as coordinated scheduling and beamforming and joint processing of UE data from multiple base stations. WSRL has worked extensively on 3GPP Release 10 and 11 topics such as precoders design, user selection, scheduling and resource allocation for the base station and feedback and receiver design at the UE side. With advent of Release 12, the area of work has been extended to proposing effective CoMP technologies when the base stations are connected via non-ideal backhaul.

Inter-Cell Interference Coordination

Inter-Cell Interference Coordination (ICIC) is an important feature introduced in LTE Release 8, which allows two neighboring cells to coordinate their transmissions and interference in frequency domain via X2 backhaul interface. ICIC techniques were further enhanced in LTE-Advanced Releases 10 and 11 allowing for subframe-level interference coordination in time domain. WSRL has done extensive work in this area including topics such as Almost Blank Subframe (ABS), Reduced Power ABS, Cell Range Expansion (CRE), Common Reference Signal Interference Cancellation (CRS-IC), and weak cell detection. Since Release 12, our focus has been on efficient interference coordination mechanisms for dense small cell deployment scenarios.