Laetitia Falconetti

Dynamic cell association for downlink sum rate maximization in multi-cell heterogeneous networks

In this work, we consider a heterogeneous network consisting in several macro nodes and pico nodes. Our goal is to associate users, belonging to this network, to one of the nodes, while maximizing the sum rate of all users. We also want to analyze the load balancing achieved by this association. Therefore, we develop a new theoretical framework to study cell association for the downlink of multi-cell networks and derive an upper bound on the achievable sum rate. We propose a dynamic cell association heuristic, which achieves performance close to optimal. Finally, we verify our results through numerical evaluations and implement the proposed heuristic in an LTE simulator to demonstrate its viability.

Distributed Uplink Macro Diversity for Cooperating Base Stations

Nowadays cellular systems operate with frequency reuse one, where adjacent cells use the same frequency band. Use equipments (UEs) located at cell edge are mostly affected by the resulting co-channel interference. In addition, cell edge UEs suffer from their weak carrier signal strength. This paper proposes a new method to increase the performance of cell edge UEs by means of information exchange between base stations (BSs). A BS serving a cell edge UE requests support from a co-channel BS. The supporting BS transfers demodulated or decoded bits received from the cell edge UE back to the serving BS. The serving BS then combines the information. The concept of cooperative BSs described in this paper is based on a request-response mechanism and does not require a central control node. Performance evaluation by means of simulation shows the capability of BS cooperation applied to 3GPP long-term evolution (LTE) in terms of user throughput and emphasizes the trade-off in terms of increased backhaul requirement due to BS-BS communication.

Cell association in small heterogeneous networks: Downlink sum rate and min rate maximization

This paper considers the problem of associating users, in an heterogeneous network, to either a macro node or a pico node within a tightly coordinated cell cluster. We introduce a new theoretical framework to model this problem for the downlink and derive upper bounds for achievable sum rate and minimum rate using convex optimization. Further we propose heuristics, consisting in dynamic cell association, enabling to achieve performance close to the upper bounds. Finally we implement these heuristics in an LTE simulator and show the potential of such dynamic cell association for a small LTE network.

Licensed-Assisted Access LTE: coexistence with IEEE 802.11 and the evolution toward 5G

LAA is a new operation mode of LTE in the unlicensed spectrum, which will be featured in LTE Release 13. Under LAA, licensed carriers will be aggregated with unlicensed carriers in order to opportunistically enhance downlink user throughput while still offering seamless mobility support. In order to coexist with WiFi, some of the new functionalities required of LAA LTE include a mechanism for channel sensing based on listen-before-talk, discontinuous transmission on a carrier with limited maximum transmission duration, and multicarrier transmission across multiple unlicensed channels. This article presents a detailed overview of the design agreements for LAA, the impact of unlicensed spectrum operation on the LTE physical layer architecture, and the scope of additional enhancements beyond LTE Release 13. A range of simulations for indoor and multicarrier scenarios show that fair coexistence between LAA and WiFi can be achieved, and that deployment of LAA can provide a boost in WiFi performance.

System architecture and coexistence evaluation of licensed-assisted access LTE with IEEE 802.11

Licensed-assisted access (LAA) is a new operation mode of Long-Term Evolution (LTE) in the unlicensed spectrum currently under study in the 3GPP standardization forum. In order to coexist with Wi-Fi, some of the new functionalities required of LAA LTE include a mechanism for clear channel assessment based on listen-before-talk (LBT), discontinuous transmission on a carrier with limited maximum transmission duration, and dynamic frequency selection (DFS) for radar avoidance in certain frequency bands. This paper presents a detailed overview of the impact of unlicensed spectrum operation on the LTE physical layer architecture, such as downlink physical channel design, scheduling, and radio resource management. System-level simulation results are then presented for indoor and outdoor scenarios, and show that fair coexistence between LAA and Wi-Fi can be achieved and that deployment of LAA can provide a boost in Wi-Fi performance.

Energy efficiency in heterogeneous networks

An attractive approach to meet increasing traffic demands is to densify existing cellular networks with low power nodes. This creates a heterogeneous network. In this paper we analyse the impact of such a densification on the network energy consumption and the possibilities it offers to enhance the network energy efficiency. In a heterogeneous network, the user performance can be significantly improved. This performance increase leads to a shorter transmission time of most data packets, creating longer idle time in the network nodes. In this work, we introduce two node sleep modes operating on a fast and intermediate time scale respectively, in order to exploit short and longer idle periods of the nodes. Our results point out that the total energy consumption of the heterogeneous network featuring the node sleep modes can be maintained at a similar level as the one of the reference network, while the user performance remains superior. At very high traffic load we show that the heterogeneous network can even be more energy-efficient. This means that it is actually possible to increase end-user performance and decrease energy consumption at the same time.

Design and Evaluation of Licensed Assisted Access LTE in Unlicensed Spectrum

Licensed assisted access (LAA) is a new feature for 3GPP LTE systems to operate in the unlicensed spectrum. Under LAA, licensed carriers will be aggregated with unlicensed carriers in order to opportunistically enhance user throughput, while still offering seamless mobility and outdoor coverage. In order to coexist with other technologies in the unlicensed bands, several new functionalities for LAA LTE have been introduced, including longterm channel selection, short-term channel sensing based on listen-before-talk (LBT), and discontinuous transmission on a carrier with limited maximum transmission duration. In this article, we present research findings behind the designs for LAA systems. We discuss the impact of several parameters of the LAA LBT framework on the channel access opportunities of LAA, and its coexistence performance toward co-channel networks based on extensive system-level simulation results. The investigation covers both single-channel as well as multi-channel operation and coexistence scenarios. In addition to the finalized designs for downlink LAA operations in Release 13, our findings in this article further shed light on the uplink LAA operations to be introduced in Release 14.

Codebook based Inter-Cell Interference Coordination for LTE

Current cellular systems suffer from co-channel interference, when simultaneous transmissions in other cells use the same physical resources. Schemes involving cooperation between Base Stations (BS) are currently discussed for further releases of the 3GPP Long Term Evolution (LTE) system as a promising method to mitigate co-channel interference for users located at cell-edge. The BS cooperation approach proposed in this paper applies to the Downlink (DL) of cellular systems and can be considered as an expansion of Inter-Cell Interference Coordination (ICIC) techniques to the spatial domain. Cooperating cells coordinate their antenna weights to reduce interference caused to selected users. This scheme uses an extension of the implicit DL channel feedback mechanism defined in current Frequency Division Duplex (FDD) cellular systems that is based on predefined codebooks of antenna weights. Performance evaluations by means of simulation show the capability of codebook based ICIC applied to LTE in terms of mean and cell-edge user throughput.

Practical energy-aware cell association for small cell deployment

In this work, we consider a heterogeneous network consisting of macro and pico base stations. Our goal is to associate users to one of the base stations,while minimizing the energy consumption of the network. Moreover, the algorithm should consider constraints of a real system and thus, only include the necessary complexity that helps reducing the energy consumption. Therefore, we first formulate the energy minimization problem in three different manners, each one including a different level of modeling details. The solution of each problem is analyzed to identify the important factors to consider when designing an energy-optimized cell association algorithm. Based on this analysis, we then develop a practical energy-aware cell association heuristic. Finally, the developed algorithm is evaluated using complex system simulations. Results show that significant energy savings are achievable by the developed algorithm compared with the conventional cell association in cellular networks. Copyright

Smart Small Cell Wake-Up Field Trial: Enhancing End-User Throughput and Network Energy Performance

Field trial measurements of small cell sleep mode with three different wake-up solutions are presented in this paper. A small heterogeneous LTE test network consisting of one macro base station and four small cells was deployed in Bundang, South Korea. Two UEs, one stationary and one moving, were used in the field trial. The studied solutions were (1) uplink interference based stand-alone small cell activation; (2) macro- assisted load-based small cell activation; and (3) macro-assisted load-based and timing advance-based small cell activation. Solution 1 was evaluated in a scenario where the same frequency band is used in the macro and small cell layers and the results show that Solution 1 provides an average reduction in energy consumption of 10% as well as a user throughput increase of 24% compared to a reference case without small cell sleep mode. For solutions 2 and 3 different frequency bands were used for the small cells and the macro layer and here the energy reduction gains were 16% and 23% respectively while the increase in user throughput was 28% and 51% respectively.