Alvaro Valcarce

OFDMA femtocells: A roadmap on interference avoidance

OFDMA femtocells have been pointed out by the industry as a good solution not only to overcome the indoor coverage problem but also to deal with the growth of traffic within macrocells. However, the deployment of a new femtocell layer may have an undesired impact on the performance of the macrocell layer. The allocation of spectrum resources and the avoidance of electromagnetic interference are some of the more urgent challenges that operators face before femtocells become widely deployed. In this article a coverage and interference analysis based on a realistic OFDMA macro/femtocell scenario is provided, as well as some guidelines on how the spectrum allocation and interference mitigation problems can be approached in these networks. Special attention is paid to the use of self-configuration and self-optimization techniques for the avoidance of interference.

Access control mechanisms for femtocells

Femtocells are a solution that helps to reduce the capital and operational expenditure of a mobile network while enhancing system coverage and capacity. However, the avoidance of interference is still an issue that needs to be addressed to successfully deploy a femtocell tier over existing macrocell networks. Moreover, interference is strongly dependent on the type of access control, which decides if a given user can or cannot connect to the femtocell. In this article the existing access methods for femtocells together with their benefits and drawbacks are explained. A description of the business model and technical impact of access methods in femto/macro networks is also provided. Finally, the need for hybrid access methods and several models are presented.

Interference avoidance and dynamic frequency planning for WiMAX femtocells networks

Femtocells have been recently proposed as a potential good solution to increase not only indoor radio coverage, but also system capacity. In this paper, a framework for radio coverage prediction and system level simulation for WiMAX macrocell/femtocell scenarios is presented. Furthermore, the feasibility of the co-channel deployment of WiMAX femtocell in an existing WiMAX macrocell network is investigated, and a method for interference avoidance based on DFP (dynamic frequency planning) is proposed. The resulting impact of DFP in a macrocell/femtocell scenario compared with other frequency assignment strategies is analyzed. Experimental evaluations carried out using our framework show the boost in the system capacity when using DFP and femtocells.

Access methods to WiMAX femtocells: A downlink system-level case study

Over the last two years, GSM and UMTS femtocell access points have been proposed as a solution to the poor indoor coverage problem experienced in certain areas. Research on these devices has shown that femtocells will not only increase indoor system coverage, but also system capacity. Femtocells will allow new services and business models to be offered to indoor users. Almost parallely, the WiMAX standard has emerged as a potential candidate technology for the future wireless networks. WiMAX femtocells are currently under development and will therefore play an important role in the world of indoor broadband wireless access. However, several aspects of this new technology, such as the access method and interference avoidance techniques play a crucial role in the amount of interference caused to co-channel deployed macrocells. This paper provides a framework for the study of WiMAX macro-femtocell hybrid scenarios. An in-depth description of the necessary radio coverage prediction and system-level simulation for this kind of scenarios is introduced. Simulations and numerical results for two different types of access methods (public and private) in the downlink are also presented.

Intracell handover for interference and handover mitigation in OFDMA two-tier macrocell-femtocell networks

There are two main access policies (open and closed) to Femtocell Access Points (FAPs), being closed access the customers favorite. However, closed access is the root cause of crosstier interference in cochannel deployments of two-tier networks (i.e., macrocells and femtocells). Further, the effect of this problem is remarkably serious in the downlink of outdoor users not subscribed to any femtocell. Open access has been considered as a potential solution to this problem. However, this increases signaling in the network due to the elevated number of HandOvers (HOs) that mobile users have to perform. Therefore, this paper proposes an interference avoidance technique based on the use of Intracell HandOvers (IHOs) in Orthogonal Frequency Division Multiple Access (OFDMA) femtocells. It is shown that a proper combination of IHO and power control techniques reduces the outage probability for nonsubscribers compared with that of closed and open access. In addition, the impact of several network parameters such as the femtocell penetration is also considered in the analysis.

Limited access to OFDMA femtocells

Femtocells are a promising solution for the provision of high indoor coverage and capacity. Furthermore, OFDMA-based femtocells have proven to be highly versatile when dealing with cross-layer co-channel interference thanks to the allocation of frequency subchannels. However, concerns still exist related to the impact of the different access methods to femtocells in an overlayed network. Femtocells based on a Closed Subscribers Group, where only device owners are allowed connectivity introduce severe interference to macrocell users. On the other hand, Open Access femtocells where any user can connect, does not bring many advantages to the femtocell owner. In this paper, an intermediate access method based on a limited access is proposed. The performance of the model is evaluated throughout system-level simulations and it is shown that limited access contributes to seriously reduce cross-layer interference while guaranteeing a minimum performance to the femtocell subscribers.

Applying FDTD to the coverage prediction of WiMAX femtocells

Femtocells, or home base stations, are a potential future solution for operators to increase indoor coverage and reduce network cost. In a real WiMAX femtocell deployment in residential areas covered by WiMAX macrocells, interference is very likely to occur both in the streets and certain indoor regions. Propagation models that take into account both the outdoor and indoor channel characteristics are thus necessary for the purpose of WiMAX network planning in the presence of femtocells. In this paper, the finite-difference time-domain (FDTD) method is adapted for the computation of radiowave propagation predictions at WiMAX frequencies. This model is particularly suitable for the study of hybrid indoor/outdoor scenarios and thus well adapted for the case of WiMAX femtocells in residential environments. Two optimization methods are proposed for the reduction of the FDTD simulation time: the reduction of the simulation frequency for problem simplification and a parallel graphics processing units (GPUs) implementation. The calibration of the model is then thoroughly described. First, the calibration of the absorbing boundary condition, necessary for proper coverage predictions, is presented. Then a calibration of the material parameters that minimizes the error function between simulation and real measurements is proposed. Finally, some mobile WiMAX system-level simulations that make use of the presented propagation model are presented to illustrate the applicability of the model for the study of femto- to macrointerference.

Empirical Indoor-to-Outdoor Propagation Model for Residential Areas at 0.9–3.5 GHz

This letter introduces analytical expressions for the modeling of path loss and shadow fading in residential indoor-to-outdoor scenarios. The formulas have been calibrated using channel power measurements at the radio frequencies of common cellular systems and are thus suitable for channel modeling in femtocell networks. The expressions presented here can be used as a simple propagation model in system-level simulators (SLS), as well as for comparison to other models. Furthermore, its compact formulation simplifies its use for theoretical studies of two-tier networks, while its empirical nature strengthens its validity.

A GPU approach to FDTD for radio coverage prediction

The benefits of using Finite-Difference alike methods for coverage prediction comprise highly accurate electromagnetic simulations that serve as a reliable input for wireless networks planning and optimization algorithms. These algorithms usually require several thousands of iterations in order to find the optimal network configuration, so to obtain results within reasonable computation times, the applied propagation models must be as fast as possible. In this study an implementation-oriented analysis on the suitability of using Graphics Processing Units (GPU) to perform Finite-Difference Time-Domain simulations is carried out. We believe that the recently released Compute Unified Device Architecture (CUDA) technology has opened the door for computational intensive algorithms such as FDTD to be considered for the first time as a precise and fast propagation model to predict radio coverage.

A New Trend in Propagation Prediction

Owing to its direct applicability in solving problems of the telecommunications industry, propagation prediction for a long time has been an important area of research and development. Because of the increasing complexity of wireless networks, growing number of smaller cells, and higher intercell interference, software tools that aid in network optimization are necessary. There fore, in the study of particular environments, where wireless networks are deployed, deterministic propagation models play an important role. Deterministic models can predict the path loss in a given scenario through the simulation of the main propagation phenomena such as reflections and diffractions. The most commonly used deterministic solutions for radio coverage prediction are ray models, which model electromagnetic (EM) waves through optical laws. However, since their appearance nearly 40 years ago, there has been an increasing interest in using other techniques such as finite-difference EM methods. These methods [the most well known is finite-difference time-domain (FDTD) method] solve Maxwells equations on a discrete space time grid. They can provide highly accurate results, provided there are accurate boundary conditions, since the reflections, diffractions, and scattering effects are implicit. This contrasts with ray-based methods that must account explicitly for a limited number of rays. This also explains why Yees FDTD methods, have been and are still widely used today, especially in confined environments such as for antenna design that involves reasonable grid sizes.