Filter bank multi-carrier (FBMC) is a candidate modulation scheme for 5G cellular mobile broadband networks. A specific domain where FBMC offers a clear advantage over other multicarrier solutions is the efficient occupancy of underutilized and fragmented spectrum. This is due to the rich spectral containment of the FBMC technology, which guarantees superior interference protection to the primary coexisting transmissions. These features of FBMC could play an important role in the economic delivery of 5G services in licensed and unlicensed bands. This article focuses on the public safety domain where existing professional mobile radio (PMR) users plan to add broadband services at the 400 MHz band, aiming to occupy the spectral holes left by current narrowband transmissions. In this respect an agile SDR broadband downlink FBMC system, aimed at exploiting unused licensed PMR spectrum, was developed and experimentally validated. The FBMC frame structure shared key similarities with the LTE specification. Two different SDR design methodologies were used to build the real-time baseband prototype. The level of interference protection offered by a broadband FBMC system to a coexisting primary narrowband PMR transmission was practically demonstrated and compared to that of an equivalent LTE system. This was made feasible by evaluating the performance of a PMR terminal under different mobile channels and FBMC waveform configurations. The applicability of this work to other 5G spectrum cohabitation scenarios is discussed. Finally, the article highlights the need to extend the SDR design paradigm in order to tackle the challenges of real-time baseband processing in 5G broadband cellular systems.