The recent O-RAN specifications promote the evolution of RAN architecture by
function disaggregation, adoption of open interfaces, and instantiation of a
hierarchical closed-loop control architecture managed by RAN Intelligent
Controllers (RICs) entities. This paves the road to novel data-driven network
management approaches based on programmable logic. Aided by Artificial
Intelligence (AI) and Machine Learning (ML), novel solutions targeting
traditionally unsolved RAN management issues can be devised. Nevertheless, the
adoption of such smart and autonomous systems is limited by the current
inability of human operators to understand the decision process of such AI/ML
solutions, affecting their trust in such novel tools. eXplainable AI (XAI) aims
at solving this issue, enabling human users to better understand and
effectively manage the emerging generation of artificially intelligent schemes,
reducing the human-to-machine barrier. In this survey, we provide a summary of
the XAI methods and metrics before studying their deployment over the O-RAN
Alliance RAN architecture along with its main building blocks. We then present
various use-cases and discuss the automation of XAI pipelines for O-RAN as well
as the underlying security aspects. We also review some projects/standards that
tackle this area. Finally, we identify different challenges and research
directions that may arise from the heavy adoption of AI/ML decision entities in
this context, focusing on how XAI can help to interpret, understand, and
improve trust in O-RAN operational networks.Comment: 33 pages, 13 figure