Wireless Femto-Relays: A New Model for Small Cell Deployments

Wireless Femto-Relays: A New Model for Small Cell Deployments

Nikolaos Nomikos (Department of Information and Communication Systems Engineering, University of the Aegean, Samos, Greece), Prodromos Makris (Computer Technology Institute, Kazantzaki, Greece), Dimitrios N. Skoutas (Department of Information and Communication Systems Engineering University of the Aegean, Samos, Greece), Demosthenes Vouyioukas (Department of Information and Communication Systems Engineering University of the Aegean, Samos, Greece) and Charalambos Skianis (Department of Information and Communication Systems Engineering, University of the Aegean, Samos, Greece)
DOI: 10.4018/ijwnbt.2015010104


The continuous increase in mobile data traffic creates the need for radical innovations in the mobile broadband system design. Heterogeneous networks (HetNets) deployment paradigm is an emerging research trend considered as the most significant aspect for meeting the mobile data challenge, mainly by proposing spectral efficiency enhancements and cell capacity gains from an overall system perspective. Two are the main challenges that dense small cell networks are facing: a) various types of interference mainly caused by unplanned deployments, and b) limited and Quality of Service (QoS)-unreliable wired backhaul links. In this paper, the authors propose a model where femtocells' and relays' complementary characteristics are effectively exploited into femto-relays. More specifically, the authors describe four main functionalities of this model: a) interference protection, b) two-fold backhaul alternatives (i.e. wired and wireless backhaul), c) opportunistic femto-relay selection, and d) full-duplex in-band relaying. To evaluate the anticipated technical impact of the proposed model, targeted simulation results are presented for a campus topology where femto-relays are compared with classic femtocells. Finally, related open issues, such as signaling overheads management, context-aware resource management, and business logic aspects are presented, with emphasis on the femto-relay's real market penetration opportunities in the mid and long-term future.
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1. Introduction

The future of cellular networks is bound to be as much challenging for researchers and operators as exciting for users. An increasing number of users is equipped with smart devices, while network providers are struggling to satisfy the users’ need for ubiquitous communication. In order to improve the network’s efficiency, there is a shift from the traditional static cell deployments towards multi-tier architectures, where small cells are overlaid by larger ones (Shu-Ping Yeh, Talwar, Geng, Himayat & Johnsson, 2011). Heterogeneous network (HetNets) topologies include distributed antenna systems connected through fiber or microwave technologies with macro base stations (BS), picocells which are miniaturized BSs, relays which route data through multi-hops and femtocells (Lopez-Perez, Guvenc, De La Roche, Kountouris, Quek & Zhang, 2011)). The latter are installed by users, i.e. individuals in a plug-and-play fashion or enterprises with the help of IT professionals. From the users’ perspective, the increased indoor coverage can provide services of high QoS within the boundaries of their local network. On the other hand, the adoption of femtocell technology by users leads to a considerable decrease in capital and operational expenditures (CAPEX/OPEX) for cellular operators and better traffic load balancing (Saquib, Hossain, Long & Dong, 2012).

To achieve these gains, certain detrimental factors introduced by the multi-tier architecture need to be tackled. A number of studies indicate that the interference arising between the tiers of the network (cross-tier interference), as well as the one among the same tiers (co-tier interference), constitute the main challenge threatening the efficient integration of femtocells in the cellular network (Shu-Ping Yeh et al., 2011, Lopez-Perez D. et al., 2011, Xia, Chandrasekhar & Andrews, 2010). Another major challenge that arises due to the femtocell architecture is the limited and QoS-unreliable wired backhaul. A recent release by Small Cell Forum (Small Cell Forum, 2013) has analyzed various wireless technologies that can facilitate the deployment of femtocells either in areas where there is no xDSL infrastructure or when increased QoS requirements cannot be achieved by the wired backhaul. In contrast to Universal Mobile Telecommunication System (UMTS) deployments, the increased capacity in the Long Term Evolution (LTE) access link determines that the wired backhaul is the bottleneck in the end-to-end communication between the core network and the mobile terminal (MT). Another factor that imposes constraints on the wired backhaul is the existence of various technologies which may share its capacity. As Wi-Fi service may be provided through the same xDSL connection used by LTE femtocells, resource partitioning further reduces the QoS experienced by the femtocell’s users. In terms of spectral efficiency, this bottleneck leads in wasting precious spectral and temporal resources, as the access link is not efficiently utilized. More specifically, the latter is not inherently equipped to provide delay resiliency and thus operators need to coordinate HetNets backhaul carefully for the realization of an optimal mixture of both wireless and wired backhaul technologies (Lopez-Perez D. et al., 2011, Saquib et al., 2012).

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