Motivation. The last decades brought an exponential increase in mobile traffic volume. This will continue and a 1000-fold increase by 2020 has been forecasted. Small-cells promise to provide the required data rates through an increased spatial utilisation of the spectrum.<br/>Problem statement. Due to strong inter-cell interference, small-cell deployments will require a high degree of coordination as offered by centralised processing. Furthermore, heterogeneous backhaul solutions will be used to connect small-cells and core network. So far, access and backhaul are individually designed and therefore not optimised. In order to support centralised processing and a heterogeneous backhaul, challenges on access and backhaul must be simultaneously tackled.<br/>Approach. iJOIN introduces the novel concept RAN-as-a-Service (RANaaS), where RAN functionality is flexibly centralised through an open IT platform based on a cloud infrastructure. iJOIN aims for a joint design and optimisation of access and backhaul, operation and management algorithms, and architectural elements, integrating small-cells, heterogeneous backhaul, and centralised processing. Additionally to the development of technology candidates across PHY, MAC, and the network layer, iJOIN will study the requirements, constraints, and implications for existing mobile networks, specifically 3GPP LTE-A.<br/>Results. iJOIN will design new network operation and management algorithms in the context of RANaaS, show their implications on 3GPP LTE, and evaluate the derived technologies with respect to four quantitative key objectives:<br/>1) system throughput2) energy-efficiency3) cost-efficiency4) utilisation-efficiency<br/>iJOIN will further impact<br/>1) the research community by identifying new challenges,2) business opportunities through new concepts for implementing mobile networks, and3) standardisation through strong industry participation of all major stakeholders.