- - Adding channels. This is what MIMO spatial multiplexing (SM) is about;
- - Increasing the used spectrum bandwidth. There is a lot of spectrum at high frequencies, with new challenges, and one option for 5G is to use this;
- - Increase the signal to noise ratio. This is what beam-forming is about.
Having more MIMO SM layers (i.e. concurrent channels) is not practical. The complexity of a MMSE decoder isO(L^3) with L the number of layers, so it gets ugly quickly. Today MIMO SM is typically limited to 2 layers in practice, with 4 likely coming and 8 the practical limit (and that may not be so practical really...).
Using very high frequencies (above 10 GHz) gives access to a lot of free spectrum, but the higher one go the lower the reach for a given power budget. To compensate for the high attenuation this is coupled with massive multi-antennas, the talk for 5G is 64 to 256. This is split between a few very costly MIMO SM layers and the rest for cheap beam-forming. So for example 256 antennas would behave like four 64 patches BF antennas for 4 layers MIMO. Of course with that many antennas and RF transceiver you have to compromise in cost and quality. So it's a lot of poor receive chains, vs. a few very high quality ones today. But there's still the potential to gain overall.
It has challenges though: it will still be for small cells (low reach) and rather low mobility (the beam steering cannot track high speed mobiles, plus small cells don't work wall for highly mobile devices: too many handovers). But because most people are low speed and the places where capacity is most needed are urban centers where small cells are ok, it still can be a win.
But as one can see, high speed 5G won't be universal like 4G is. By this I mean that 4G can (and will) completely replace 2G and 3G in time, while this high frequencies / massive BF 5G could only complement 4G is high density urban places, but will never be suitable for lower density parts (rural) where 4G would stay.
And then there's the elephant in the room: a lot of the improvements in telecoms have been riding on Moore's law. With the scaling problems that start now to be more openly discussed, how much more processing power we can use for 5G and what the users are prepared to pay (cost and power) for all these improvements are interesting questions.