The way to look at generations of mobile wireless baseband technologies is by understanding the different dimension of spectrum the technology accesses. Multiple standards can be built around a new generation of mobile baseband technology. Looking at arbitrary groupings and arbitrary performance requirements is useful for spectrum alignment and adherance to the upper end of performance goals, but viewing the way the technology leverages spectrum leads to the simplest and clearest way to look at generations.
1G: frequency division
2G: + time division
3G: + code division (information-related rather than physical)
4G: + amplitude (without code division, fr + t + amplitude to form orthogonal division)
5G: + spacial division
Judging by majority of the proposed technologies for 5G, it is apparent that the space within each cell will be reused through 3D spatial separation using massive MIMO and 3D beamforming as well as other technologies. Some of these are existing technologies for 4G that are used to strengthen RF signals, but now they will be pre-coded into the baseband to be part of the new generation of mobile baseband technology. The spectrum will be much more efficiently used this way, and then on top of that much larger channels will be used. These larger channels will be found in the 10 to 100 GHz range. The range will be much less than is common today, but the point is to use small cells anyway, which also reuse spectrum from a network topology level through cell division. Smaller cells are required anyway in order to optimally use massive MIMO and to have devices use more than one BTS at a time.
Nothing is set in stone yet, but this seems to be the way it is headed. The actual waveform and even many of these details are still up for debate. One of the technologies being mentioned is so different, it may even wind up being the core technology behind 6G, though it can certainly be applied to today's generations as well.
