Joon-Young Cho

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.

Cooperative communication technologies for LTE-advanced

The LTE-Advanced (LTE-A) system is currently under development to allow for significantly higher spectral efficiency and data throughput than LTE systems. In a wireless system based on orthogonal frequency division multiplexing (OFDM) with frequency reuse factor one such as LTE, the achievable cell spectral efficiency is often limited by the inter-cell interference or coverage shortage of base stations. Hence in LTE-A, coordinated multi-point (CoMP) transmission/reception (a.k.a. multi-cell MIMO or base station cooperation) and relaying technologies are being introduced to clear these major performance hurdles. In this paper, overall picture of cooperative communication technologies being discussed in LTE-A systems including CoMP and relaying is presented, together with considerations on system design.

Coordinated multi-point transmission in heterogeneous networks: A distributed antenna system approach

With the frequency reuse approaching one and the networks becoming more and more heterogeneous, cooperative multi-point MIMO has become a key candidate technology in the design of future wireless networks and is nowadays an active research topic in the academia and the industry. In 3GPP LTE-Adv. Rel. 11, a new study item on Coordinated Multi-Point transmission and reception (commonly denoted as CoMP) has been initiated in December 2010 and is expected to be one of the major features of Rel. 11. A particularly interesting deployment scenario is the distributed antenna system (DAS) where cooperative transmission is applied between distributed transmission points belonging to a common cell. In this paper, we discuss the pros and cons of distributed antenna systems versus classical heterogeneous networks like picocells, the appropriate cooperative schemes and the design challenges. We confirm the benefits of such technology through system level performance evaluations compliant with LTE-Adv.

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.

Interference mitigation in heterogeneous cellular networks of macro and femto cells

We investigate the impact of co-channel deployment of femtocells on the existing macro-cellular systems considering the techniques to resolve the laud neighbor problem. There are several approaches to this aim, e.g., femtocell power control, interference coordination, opening access to femtocells and so on. Among them, the coordinated scheduling, including power control, and their impact will be the main focus of this work. In the context of 3GPP-LTE, we examine under various operational scenarios the performance in terms of the average and 5% worst user throughput, a useful measure of fairness among users, both for femto and macro cells. Although recent studies have shown that co-channel femtocell has minor impact on the macrocell performance in average sense, non-negligible percentage of users may lose their opportunity to get satisfactory data service and, hence, our focus will be put more on 5% worst users.