Ericsson recently published a thoughtful review article, “5G Radio Access,” by Stefan Parkvall, et. al. Their paper properly begins with an assessment of the use cases that they target with 5G; categorized under overlapping demands for extended broadband, Mission-critical Machine Type Communications (MTC) and Massive MTCs as illustrated in their figure below.
They join the consensus view that “additional spectrum for mobile wireless communications is one of the most critical [aspects of 5G].” They point out that the ITU World Radiocommunication Conference for WRC-15 will likely focus on allocations below 6.5 GHz, but they expect that to provide multi-Gbps data rates effectively frequencies above 10 GHz will be needed but which will not be addressed by the ITU until WRC-19 (to be held in 2019). This clarifies their sense of the timeline for the development of mm-wave support for 5G, particularly when one considers that it tends to take several years to go from the ITU recommendation of WRC to the national allocations, license grants and standardization. 5G’s mm-wave capability will arrive after 2020, as portrayed in the following figure:
Critically, they state, “Using one radio-interface structure to address such a wide range of frequencies is probably not the best approach.” This suggests that 5G might consist of main line evolution of LTE below a few GHz and a frequency addendum in the mm-wave frequency bands with lower complexity, beam forming with massive number of base station antenna elements and used only for improved performance for short range access, or for backhaul.
The paper is worth reading, and an excellent overview of the challenges of addressing the targeted use cases.
 “5G radio access,” June 18, 2014, in Ericsson Review, 2014 · 6, by Erik Dahlman, Gunnar Mildh, Janne Peisa, Joachim Sachs, Yngve Selén and Stefan Parkvall. Available on-line at: http://www.ericsson.com/res/thecompany/docs/publications/ericsson_review/2014/er-5g-radio-access.pdf