## 1 Introduction
The main obstacle of mediation approaches is overcoming the heterogeneity of the user modeling data [10]. In [10], the authors suggest bridging the gaps between both approaches. Among other future research directions, they recommend the use of domain thesauri and ontologies and in particular the use of WordNet for identifying synonyms and enhance reasoning and mediation of user modeling data. They also see the need to develop flexible mechanisms that will allow applying ontologies in different domains, situations and constrains. In this paper, we address the interoperability problem between heterogeneous sources and the evolutivity problem trying to overcome the limitations of both approaches. We present a dynamic user profile structure based in Simple Knowledge Organization for the Web (SKOS) to provide knowledge representation for ubiquitous user model, and we propose a two-tier matching strategy for concept schemas alignment to enable user modeling interoperability. The process of concept alignment provides articulation between profile suppliers and consumers establishing the mappings from individual concepts of the sources with the corresponding ubiquitous user model concepts. This process of concept alignment consists in two steps, an element level matching and a structure level matching. In addition to enable interoperability, this process determines the evolution in time of the ubiquitous user model deciding when a new concept must be added. The main contributions of our approach, the semantic representation of the ubiquitous user model and the process of concept alignment, will allow dealing with syntactic and semantic heterogeneity with the least intervention of profile and provide a semantic representation for the user model that adapts itself to new profile suppliers and consumers. Our proposed solution provides flexibility overcoming the limitations of a static user model ontology, and the semantic schema matching that defines semantic concept mappings is done automatically unlike other authors approaches that establish the mappings manually or semi automatically. We present a ubiquitous user model to enhance the understanding of the user and provide more proactive and personalized services through sharing and reusing profile information with semantic web technologies. We give an example (Section 5.2) to illustrate how reusing and sharing profile information from heterogeneous sources can be done with the mediation of our ubiquitous user model and the process of concept alignment. This example considers the scenario using information from different user profiles to deal with overweight and obesity. In this scenario, static, semi-static and dynamic information is necessary, and heterogeneous sources are involved: social network applications, personal health records, specialized health sites and web applications, FOAF and personal devices with attached sensors. We specifically reuse the ubiquitous user model information to automatically personalize a web service populating its input parameters. For our experimentation, the ubiquitous user modeling ontology based in SKOS was set up with real data of Facebook, FOAF and one profile of a specialized web application to monitor person's diet and physical activity of one user. We gathered information about training sessions with Polar devices and automatically established the pertinent mappings to enable the interoperability with the mentioned sources. The process of concept alignment also identified useful concepts from the user model that can satisfy a provider's web service input parameters requirements and personalize its instantiation. The ubiquitous user model is part of an ongoing project to develop a user-adaptive system based on a distributed architecture to exchange, merge and share profile information contained in existing heterogeneous user models. The paper is organized as follows: in Section 2, we present an overview of the main characteristics of the ubiquitous user model and heterogeneous sources considered.