Analysis and Recommendations for a Universal Interworking Function (UIWF) for Mission Critical LMR & LTENetworks, Final Report [open pdf - 1MB]
From the Executive Summary: "Good progress is being made in the 3rd Generation Partnership Project (3GPP) in standardizing an Interworking Function (IWF) interface on the Mission Critical Push To Talk (MCPTT) side to support Land Mobile Radio (LMR)/Long Term Evolution (LTE) interoperability. Presently 3GPP Release 15 (LTE) and ATIS/TIA (Project 25 [P25] LMR) work is in progress, which is likely to result in providing some level of open-standard LMR to LTE capabilities. The 3GPP standard will likely address the major requirements National Public Safety Telecommunications Council (NPSTC) documented. However, the development for the 3GPP IWF interface protocol will not be complete until late 2019. In addition to commercialization time, this means equipment implementing the 3GPP IWF will not likely be broadly available until 2021 at the earliest."
U.S. Department of Homeland Security: http://www.dhs.gov/