Is space division multiplexing really multiplexing?
Space Division Multiplexing sounds like snake-oil, but it's worth treating it as a topic for study, as there are costs and benefits when applying it.

It's defined as pushing several independent channels of data over a shared medium. That encompasses both the 'is it really doing something?' of parallel wires, and the very clever 'how do they do that?' of MIMO, multiple input multiple output radio transmission.

If you have multiple well-isolated coaxial cables, then you would be right to insist that this was in fact a totally obvious way to move signals, and that there are in fact multiple physical channels available, one signal per channel.

If the channels are not well isolated however, say four unscreened twisted pairs in a common jacket (ethernet), then you need to start thinking about crosstalk between them. This may need some channel coding to protect against errors induced between the channels. This is insignificant at 10Mbit/s, but needed at 1Gbit/s.

The extreme of channels not well isolated from each other is the airspace between a transmitter (Tx) and a receiver (Rx). You would be forgiven for thinking that even if you had several Tx antennae and several for Rx, you would still only have one usable channel between Tx and Rx. 

If the antennae are far enough apart, a quarter wavelength is sufficient, and if they are in a multipath environment, then each sees a different transmission path. Sometimes it's slightly different, sometimes it's very different, depending on where the filing cabinets and buildings are along the RF path. Consider our ethernet example, and the transmission space between the sockets. Each of the four physical channels mostly guides the EM waves along a diff pair of wires, this is what distiguishes the four channels. But it's only mostly, the four channels are not totally separate. Similarly, each RF path is influenced by the different arrangement of filing cabinets and buildings that each RF spatial channel sees.

Now these spatial channels cannot be accessed as easily as the four channels in an ethernet cable, they are not distinguishable enough for that. However, they can be accessed by beamforming, matrixing several logical channels of data with various phase shifts and weights to all the available antennae at the Tx, and then inverse matrixing them at the Rx. This is the magic of MIMO. With 4 antennae at each end, it's often possible to get 3x as much data as you could with a single antenna if the multipath is interesting enough, and usually 2x as much.