Luis Guilherme Uzeda Garcia

Carrier aggregation for LTE-advanced: functionality and performance aspects

Carrier aggregation is one of the key features for LTE-Advanced. By means of CA, users gain access to a total bandwidth of up to 100 MHz in order to meet the IMT-Advanced requirements. The system bandwidth may be contiguous, or composed of several non-contiguous bandwidth chunks that are aggregated. This article presents a summary of the supported CA scenarios as well as an overview of the CA functionality for LTE-Advanced with special emphasis on the basic concept, control mechanisms, and performance aspects. The discussion includes definitions of the new terms primary cell (PCell) and secondary cell (SCell), mechanisms for activation and deactivation of CCs, and the new cross-CC scheduling functionality for improved control channel optimizations. We also demonstrate how CA can be used as an enabler for simple yet effective frequency domain interference management schemes. In particular, interference management is anticipated to provide significant gains in heterogeneous networks, envisioning intrinsically uncoordinated deployments of home base stations.

Autonomous component carrier selection: interference management in local area environments for LTE-advanced

Low-power base stations such as femtocells are one of the candidates for high-data-rate provisioning in local areas, such as residences, apartment complexes, business offices, and outdoor hotspot scenarios. Unfortunately, the benefits are not without new challenges in terms of interference management and efficient system operation. Due to the expected large number of user-deployed cells, centralized network planning becomes impractical, and new scalable alternatives must be sought. In this article we propose a fully distributed and scalable solution to the interference management problem in local areas, basing our study case on LTE-Advanced. We present extensive network simulation results to demonstrate that a simple and robust interference management scheme, called autonomous component carrier selection, allows each cell to select the most attractive frequency configuration; improving the experience of all users and not just the few best ones, while overall cell capacity is not compromised.

5G small cell optimized radio design

The 5th generation (5G) of mobile radio access technologies is expected to become available for commercial launch around 2020. In this paper, we present our envisioned 5G system design optimized for small cell deployment taking a clean slate approach, i.e. removing most compatibility constraints with the previous generations of mobile radio access technologies. This paper mainly covers the physical layer aspects of the 5G concept design.

B4G local area: High level requirements and system design

A next generation Beyond 4G (B4G) radio access technology is expected to become available around 2020 in order to cope with the exponential increase of mobile data traffic. In this paper, research motivations and high level requirements for a B4G local area concept are discussed. Our suggestions on the design of the B4G system as well as on the choice of its key technology components are also presented.

Fixed Frequency Reuse for LTE-Advanced Systems in Local Area Scenarios

LTE-Advanced systems, which aim to provide high data rate wireless services, have received world-wide researching interests nowadays. In this paper, the performance of fixed frequency reuse with different reuse factors is studied in LTE-Advanced systems. Performance is measured in terms of both average cell throughput and cell edge user throughput. It is found that a properly chosen reuse factor with respect to cell size (which leads to different level of inter-cell interference), can offer up to 30% gain in average cell throughput and much higher gain for cell-edge user throughput in Local Area (LA). This high gain from frequency reuse makes it attractive for future LTE-Advanced systems.

Self-Organizing Coalitions for Conflict Evaluation and Resolution in Femtocells

The recent introduction of carrier aggregation in LTE-Advanced enables new possibilities in designing frequency domain interference reduction and management schemes. These methodologies are of extreme interest in the case of dense and uncoordinated deployments of femtocells. In such scenarios, dense deployment of cells coupled with the scarcity of frequency resources may lead to a potentially disruptive amount of interference, which severely affects the performance of the system. This contribution presents a novel method inspired by graph and coalitional game theories. The proposed algorithm consists of a set of distributed and scalable rules for building coalitions; these rules essentially resolve the conflicts among avid femtocells competing for a limited amount of resources. The proposed scheme has been designed by targeting localized reconfigurations, thus avoiding reconfiguration storms in the network. Furthermore, the rules governing the resource redistribution ensure overall system performance improvements while maintaining a certain degree of fairness among the competing nodes. Simulation results prove the effectiveness of the proposed method.

IEEE 802.11 Networks: A Simple Model Geared Towards Offloading Studies and Considerations on Future Small Cells

WiFi is the prevalent wireless access technology in local area deployments and is expected to play a major role in a mobile operators data offloading strategy. As a result, having simple tools that are able to assess the offloading potential of IEEE 802.11 networks is vital. In this paper, we propose a simple closed-form solution to calculate down- and uplink throughput values per user under full-buffer traffic when small WiFi cells are used to offload macrocells. Extensive measurement campaigns and simulation results demonstrate that there is an excellent quantitative match between analytical model and data despite the simplicity of the former. Finally, in light of our observations we discuss some of the fundamental technological limitations that may have a significant impact on the future of small cells.

On the Impact of Explicit Uplink Information on Autonomous Component Carrier Selection for LTE-A Femtocells

The increasing popularity of the femtocell concept has revamped the interest in dynamic interference coordination techniques for dense and uncoordinated deployments of low- power home base stations. One of the proposed schemes for 4G OFDMA femtocells is known as Autonomous Component Carrier Selection (ACCS). ACCS capitalizes on the carrier aggregation framework of LTE-Advanced to curb inter-cell interference levels. Albeit being exclusively based on downlink information, previous contributions attested the effectiveness of the scheme in the uplink as well. This paper extends the initial argumentation by including uplink information into the component carrier selection process. We assess and discuss the uplink performance of two proposed variants of ACCS via extensive system level simulations. The striking conclusion based on the results is that the mere addition of uplink information, which is difficult to estimate in the real world, does not provide substantial performance improvements.

On Open versus Closed LTE-Advanced Femtocells and Dynamic Interference Coordination

Low-power home base stations, also known as femtocells, are one of the strong candidates for high data rate provisioning in indoor environments. Unfortunately, the benefits are not without new challenges in terms of interference management and efficient system operation. In this paper we take a closer look at several aspects associated with the deployment of LTE-Advanced home eNBs under two different access policies: closed subscriber group (CSG) and open subscriber group (OSG). Our results are derived from extensive downlink system level simulations. We limit our scope to dense-urban deployment of femtocells assuming dedicated carriers, i.e. no interference to/from the macro layer. Particularities of each access mode are discussed under different hard frequency re-use configurations. Our results indicate that an OSG deployment is indeed able to cut short the lower end of the SINR distribution when universal frequency re-use is employed. However, when other re-use configurations are considered, OSG no longer guarantees improved SINR conditions. In addition, we present additional results for the autonomous component carrier selection (ACCS) concept introduced in earlier contributions, providing strong suggestions that the scheme yields attractive performance benefits independently of the access policy selected by the operator. Finally, we point out that uplink results including realistic power control settings need to be considered before definitive conclusions can be safely drawn.

Enhanced Uplink Carrier Aggregation for LTE-Advanced Femtocells

In this paper we investigate uplink carrier aggregation in the context of dense residential deployments of LTE-Advanced Femtocells. Previous work in the literature based on macro-cells suggested considering UE power limitations to infer which UEs should be allowed to employ multiple component carriers. However, due to the very small radius of femtocells, UEs are not expected to become power-limited at all. We propose a decentralized scheme with limited signalling requirements that incorporates power control information, not only to guide the UE-specific carrier selection procedure, but also to capitalize on the inherent power spectral density and path loss differences in order to minimize inter-cell interference. We present a series of system level simulation results which provide strong evidence that our scheme delivers substantial gains in terms of coverage compared to existing solutions without sacrificing the SINR.