Hyoungju Ji

UE's role in LTE advanced heterogeneous networks

Deployment of low-power nodes such as picocells, femtocells, and relay nodes within macrocell coverage is seen as a cost-effective way to increase system capacity and to equip wireless WANs with the ability to keep up with the increasing demand for data capacity. These new types of deployments are commonly referred to as heterogeneous networks and are currently receiving significant attention in industry. However, simple deployment of low-power nodes can lead to underutilization of air-interface resources due to the relatively small footprint of the lowpower nodes or service outage in the case of femto cells with restricted access. Time-domain interference management techniques by the configuration of almost blank subframes, introduced in LTE Rel-10 standards, allow the removal of most of the interference from the dominant interfering nodes. This mechanism enables cell biasing or cell range extension of weak cells, thereby maximizing the incremental gain provided by the deployment of low-power nodes. The configuration of ABS changes the interference conditions seen by the user equipment and therefore requires corresponding resource-specific measurements and feedback at the UE. In this article, we provide an overview of LTE Rel-10 resource specific radio link monitoring, radio resource management, and channel state information feedback procedures. Also, we provide evaluation results to show that UE receivers, in the detection of weak cells and removal of interference in demodulation of control and data channels, play a critical role in realizing the full potential that the deployment of heterogeneous networks can offer.

Overview of Full-Dimension MIMO in LTE-Advanced Pro

Multiple-input multiple-output (MIMO) systems with a large number of base station antennas, often called massive MIMO, have received much attention in academia and industry as a means to improve the spectral efficiency, energy efficiency, and processing complexity of next generation cellular systems. The mobile communication industry has initiated a feasibility study of massive MIMO systems to meet the increasing demand of future wireless systems. Field trials of the proof-of-concept systems have demonstrated the potential gain of the Full-Dimension MIMO (FD-MIMO), an official name for the MIMO enhancement in the 3rd generation partnership project (3GPP). 3GPP initiated standardization activity for the seamless integration of this technology into current 4G LTE systems. In this article, we provide an overview of FD-MIMO systems, with emphasis on the discussion and debate conducted on the standardization process of Release 13. We present key features for FD-MIMO systems, a summary of the major issues for the standardization and practical system design, and performance evaluations for typical FD-MIMO scenarios.

Evolution beyond LTE-advanced with Full Dimension MIMO

In this paper, we study the overall development of Full Dimension MIMO (FD-MIMO) technology in the context of next generation evolution towards B4G and 5G cellular systems. FD-MIMO relies on a large number of antennas placed in a 2-dimensional antenna array panel at base stations (BSs). Utilizing the 2-dimensional antenna array, FD-MIMO simultaneously transmits to a large number of mobile stations (MSs) effectively realizing high order multi-user MIMO (MU-MIMO) well beyond what is possible today. This paper discusses the research and standardization effort in 3GPP to incorporate the features and performance benefits of FD-MIMO in the next evolution of LTE standards. In order to demonstrate the potential performance benefits of FD-MIMO, system level evaluation results are provided using the 3GPP evaluation methodology. Evaluation results show that FD-MIMO can provide vast improvement over conventional LTE networks.

Dynamic resource adaptation in beyond LTE-A TDD heterogeneous networks

Dynamic downlink and uplink resource adaptation in TDD (Time Domain Duplex) systems by flexibly changing the ratio of time slots for downlink and uplink transmissions is studied under Heterogonous Network (HetNet) scenarios. The time-scale of the resource adaptation and the interference management between downlink and uplink transmissions from different cells are important factors in determining the achievable level of resource efficiency, user experience and energy saving in TDD systems. In this paper, we apply the dynamic downlink and uplink resource adaptation in multi-cell macro-pico scenarios with co-channel interference. The performance is evaluated in system level for LTE-Advanced TDD configurations. Reconfiguration rates of 10 msec and 640 msec are considered as the time-scale of the dynamic resource adaptation. In addition, the time domain inter-cell interference coordination between the macro cells and the pico cells is applied for handling the high level of interference from macro cells to pico cells in the downlink multi-cell dynamic simulations.

Effect of 3-dimensional beamforming on full dimension MIMO in LTE-Advanced

Large-scale antenna system has attracted significant attention in the wireless industry and academia area in the past few years as a candidate technology for the next evolution towards beyond 4th generation cellular systems. It can utilize a large number of antennas placed in a two-dimensional (2-D) antenna array panel for realizing spatially separated transmission links to a large number of mobile stations (MSs). The arrangement of these antennas on a 2-dimensional active antenna array allows the extension of spatial separation to the vertical domain as well as the traditional horizontal domain with the user-specific 3-dimensional beamforming. Utilizing such spatial separation capability, a base station (BS) can realize spatial multiplexing to a large number of MSs under 3-dimensionally spread channel environment. In this paper, we study the effect of user-specifica beamforming using both veritcal and horizontal direction on the large-scale antenna evolution of LTE system: Full Dimension MIMO. Evaluation results with various antenna configurations show while FD-MIMO provides significant improvement over conventional LTE systems, the size and spacing of active array within 2-D antenna structure need to be carefully selected to maximize FD-MIMOs full potentials in different cell deployments.

Interference avoidance and coordination for small cells in B4G cellular networks

Base station density in modern cellular networks is continuously increasing in order to cope with the exponential increase in mobile data traffic. Cellular networks which used to rely on macro cells for both system coverage and data transfer are now turning to small cells at an increasing rate. Small cells can complement macro cell dominant cellular networks by providing a cost effective means for offloading mobile data traffic. As in macro cells, communication performance in small cells is determined in large part by the inter-cell interference. Compared to macro cells, inter-cell interference can be much more severe for small cells due to their clustered deployment in urban hotspot areas. This paper discusses how inter-cell interference can be effectively managed in small cells with the use of a diverse range of interference avoidance/coordination techniques.

Adaptive Unlicensed Band Access for Downlink Cellular Networks

Licensed band communication systems have recently begun trying to utilize unlicensed bands. Due to lack of coordination, however, this spectrum utilization results in strong interference to other unlicensed band equipment (UBE). This problem highlights the need for coexistence scenario that will prevent UBE performance degradation. In this paper, we propose a new access control mechanism for downlink cellular networks using the unlicensed band. First, we analyze the impact of the cellular networks on the UBE outage performance. Based on the analysis, we then obtain a decision threshold for the unlicensed band access control. In the proposed control mechanism, the decision threshold is adaptively controlled by considering the density of the UBE. The numerical results demonstrate that the proposed access control mechanism guarantees the UBEs target performance under various network circumstances.

3-Dimensional channel characteristics on 2-dimensional active array antennas

Geometry-based stochastic channel models have been developed and refined over time by a number of research groups such as 3GPP, 3GPP2, ITU and the WINNER initiative. Traditional channel model is a two-dimensional (2-D) channel model, where an elevation angle of each signal path is always assumed to be zero. While such an approach is acceptable for evaluating performance of systems with horizontally placed linear antenna arrays, modeling of elevation angles is necessary when evaluating a massive MIMO or FD-MIMO technology utilizing a 2-D antenna array. In 3GPP, a study has been finalized the details and the following key parameters are takes into account the wireless channel propagation in the elevation direction as well as the azimuth direction; height dependent pathloss generation, correlation of large scale parameters, and statistical distribution of elevation components. This paper discusses spatial channel characteristics of vertical domain as well as horizontal domain under different 2-D active array configuration. Impact on separate and full channel measurement and feedback for horizontal and vertical antennas also discusses. Furthermore, we recommend the design principle of codebook for 2-D active array which impacts on system performance.