1 Abstract—This paper summarizes the outcomes of the 5 th International Workshop on Femtocells held at King’s College London, UK, on the 13 th and 14 th of February, 2012. The workshop hosted cutting-edge presentations about the latest advances and research challenges in small cell roll-outs and heterogeneous cellular networks. This paper provides some cutting edge information on the developments of Self-Organizing Networks (SON) for small cell deployments, as well as related standardization supports on issues such as carrier aggregation (CA), Multiple-Input-Multiple-Output (MIMO) techniques, and enhanced Inter-Cell Interference Coordination (eICIC), etc. Furthermore, some recent efforts on issues such as energy-saving as well as Machine Learning (ML) techniques on resource allocation and multi-cell cooperation are described. Finally, current developments on simulation tools and small cell deployment scenarios are presented. These topics collectively represent the current trends in small cell deployments. Index Terms—Small cell, standardization, carrier aggregation, MIMO, CoMP, eICIC, SON, green, backhaul, cognition, game theory, learning, modeling, propagation, simulation, stochastic geometry, deployment. I. INTRODUCTION UE to the proliferation of smart mobile devices and innovative mobile data services, telecommunication systems are facing increasing demands for ubiquitous heterogeneous broadband mobile communications. It has been predicted that the volume of wireless data will exceed that of wired data by 2015. In order to realize the enormous data capacity and meet user Quality of Service (QoS) requirements, while keeping operators’ costs low, low-power low-cost small cells operating in licensed spectrum, have been widely considered as the most promising solution. Small cells include femtocells, picocells, metrocells and microcells, in order to increase the cell coverage and capacity. Due to their low cost and easy deployment, small cells provide a viable and cost- effective way to improve cellular coverage, capacity and applications for homes, enterprises, as well as for metropolitan and rural areas. Driven by their attractive features and potential advantages, the development and deployment of small cells have gained tremendous momentum in the wireless industry and research communities in recent years. Small cells have also attracted the attention of standardization bodies, e.g., the 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE) and LTE-Advanced. However, it is worth noting that the successful rollout and operation of small cells are still facing significant technical challenges and issues. The aim of this paper is to summarize the outcomes and discussions of the 5 th International Workshop on Femtocells held at King’s College London, UK, on the 13 th and 14 th of February, 2012. The workshop hosted cutting-edge presentations about the latest advances and research challenges in small cell roll-outs and heterogeneous cellular networks. The rest of the paper is organized as follows. In Section II, the standardization activities regarding small cells within 3GPP are reviewed. In Section III, the self-organization features and related techniques of small cells are discussed. Section IV presents 3GPP LTE-Advanced small cell Carrier Aggregation (CA) techniques and performance evaluation. In Section V, enhanced Inter-Cell Interference Coordination (eICIC) is proposed for small cells and the related performance is evaluated through simulations. In Section VI, Multiple-Input-Multiple-Output (MIMO) techniques are Small Cell Deployments: Recent Advances and Research Challenges Zubin Bharucha, DOCOMO Eurolabs, Germany, Emilio Calvanese, CEA-LETI, France, Jiming Chen, Ranplan Wireless Network Design Ltd, UK Xiaoli Chu, University of Sheffield, UK, Afef Feki, Bell-Labs, France, Antonio De Domenico, CEA-LETI, France, Ana Galindo-Serrano, CTTC, Spain, Weisi Guo, University of Sheffield, UK, Raymond Kwan, Ubiquisys, UK, Jimin Liu, Ranplan Wireless Network Design Ltd, UK, David López-Pérez, King’s College London, UK, Massod Maqbool, SAGEMCOM, France, Ying Peng, Datang Telecom Group, China, Samir Perlaza, Princeton University, USA, Guillaume de la Roche, Mindspeed Technologies, France, Serkan Uygungelen, DOCOMO Eurolabs, Germany, Alvaro Valcarce, Triagnosys, Germany, and Jie Zhang, University of Sheffield, UK D