  This paper presents a survey of the state of the art in research related to
handovers employing software defined networking (SDN) enabled architectures,
serving multiple coexisting radio access technologies. As the industrial
roll-out of cellular services continues to evolve, it brings with it the
coexistence of various IP based networks such as 5G NR, LTE, WiFi, Satellite,
and IoT networks. This coexistence of different radio access technologies
presents researchers with the opportunity to use these technologies
interchangeably to address the longstanding challenges associated with network
and traffic management. Advances in software defined technologies including SDN
enables handover and interoperability schemes that utilize the principles of
SDN to improve the handover performance and achieve interconnection between
these heterogeneous networks. This paper explores such advanced SDN enabled
architectures for radio access networks, offloading techniques, and
implementation approaches. Finally, the challenges and shortcomings of the SDN
based handover optimization approaches are discussed, and a few future research
paths are laid out.
