I don't know much about electronics, but happened upon the Wikipedia article for Multiplexing, which defined it as:

a method by which multiple analog or digital signals are combined into one signal over a shared medium

Shortly after are listed different types of multiplexing, among them Space-division multiplexing:

the use of separate point-to-point electrical conductors for each transmitted channel

An example given was a stereo audio cable.

But it seems to me that if you have seperate physical streams of data (conductors in this case)- you haven't really 'multiplexed' your streams- there are still two seperate streams of data on two seperate physical conductors.

It might just be terminology, but I'm curious if there's something fundamental I'm missing.

  • \$\begingroup\$ I think your argument is valid in that the wikipedia article is weakly connected (a problem with multiple authors) if taken as a whole. One could reasonably argue that all of the listed "types" of multiplexing are either space or time. The wikipedia article even confesses that "Orbital angular momentum" may be a form of space-division multiplexing. \$\endgroup\$ – st2000 Jul 6 '19 at 12:47

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.

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  • \$\begingroup\$ Okay, I'd not heard of this before and this is fascinating. \$\endgroup\$ – Hearth Jul 6 '19 at 11:42

I'm inclined to agree with you. The opening paragraph of the Multiplexing article defines multiplexing thus:

In telecommunications and computer networks, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium.

The stereo example appears to do the opposite. It takes the one set of stereo data, splits it into two analog streams and recombines them at the loudspeakers into the shared medium of air.

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Space division multiplexing does indeed have two live streams and a full implementation has two transmitters and two receivers but only one source and one destination.

Critically, only one of the streams ends up at the destination and the two transmitted streams are from the same source.

This method was (perhaps still is) used for telephone services over microwave links especially in the southwest USA (it was too difficult to lay cables and besides, that is oil country).

The systems I worked with (early 80s) had a bit error rate monitor on each receiver and had a predictor (based on an old MC6800) that would pre-emptively switch receivers when the current receiver BER was increasing too far (10^^-6 was required for voice channels).

The real engineering is to take those channels and be able to perform a switchover without losing the framing information as the free space paths could vary by up to about 30 feet or so (an antenna that is quite high on a tower will move around somewhat).

I designed an update to a hitless switch (that could change sources that were at different locations in the bitstream by using a pair of shift registers and a clocked XOR to shift the effective data so that at the output of the switch the data streams were aligned at the same bit.

Keep in mind that framing was done at multiple levels:

4 * DS1 (1.544 Mb/sec nominal) -> DS2. DS1 is also known as a T1 line and is composed of 24 voice channels plus framing bits when used as such.

7 * DS2 (each at 6.312 Mb/sec nominal) -> DS3 at 44.736 Mb/sec nominal

2 * DS3 -> final output at 90.148 Mb/sec nominal

Note also that as each of the DS1 sources was relatively asynchronous, there were extra challenges for wrapping them into a single stream.

The reason the bit rates at each higher level are higher than the number of channel raw data rates is due to bit stuffing overhead and order wire implementations (the order wire allowed maintenance staff to plug into the racks and make calls from the rack rather than routing elsewhere).

If I define multiplexing as taking data from one of two sources, then space division really is multiplexing for this scenario.

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