Is MIMO equivalent to link aggregation?

Question

Is MIMO the wireless version of link aggregation in wired networks?

I'm not asking if MIMO is a similar link aggregation protocol like LACP or PAgP. What I'm asking is, if it is the same concept as bonding multiple links to act as one channel.

Let's say for example, we have a pair of 2 x 2 MIMO transmitter and receiver. If the transmitter is to send two packets to the receiver, does it mean packet 1 will be sent through antenna 1 and packet 2 will be sent through antenna 2 simultaneously, and the receiver will receive packet 1 through antenna 1 and packet 2 through antenna 2 simultaneously?

If yes, how do the two signals not interfere with each other? Is it because they are orthogonal signals? Also, is OFDM always required in MIMO because it is the only way to generate orthogonal signals?

If no, then how does MIMO increase capacity? What is the main point if MIMO?

Answer 1

MIMO is not the same as link aggregation.

How does MIMO work?

Imagine you have two radio channels that are physically separated, a Tx/Rx antenna pair here, and another pair 'over there'. The two channels will be essentially independent, even if they use the same frequency, and you'll be able to transmit a total of 2x the information that you could on a single channel.

Now bring those antenna pairs closer together. You can deal with interference between them by matrixing the signals, so that receiver A might get 90% of Tx A's signal and 10% of Tx B's. As long as you know the channel response, you can put the correct coefficients into the matrix.

Now co-site the antennae, but make sure that they are separated by at least 0.5 wavelengths. This means that if the path between Tx and Rx is sufficiently full of scatterers (cars, filing cabinets, people, window frames), their fading will be independent. Conceptually this means that each antenna 'sees' a different RF channel to the others. Once this complicated channel has been 'sounded' by a test signal, and assuming the that the channel characteristics stay steady for at least enough time to transmit a few frames, the signals can be matrixed onto both channels by the transmitter, and dematrixed by the receiver. You don't get 2x speedup with 2 antennae, 50% would be reasonable going, and it depends on having a sufficiently 'busy' RF environment between transmitter and receiver, line of sight does not work at all well for MIMO.

Answer 2

Firstly, whenever people talk about MIMO, it is good to clarify what they mean.

There is a narrower meaning of MIMO, where multiple streams of data can be transmitted in the same band simultaneously, provided the channel conditions support it. This is probably your meaning. It is sometimes called by other names like spatial multiplexing to avoid ambiguity.

There is also a broader meaning of MIMO - just having multiple antennas at both the transmitter and receiver - where the multiple antennas are may be used for various purposes, including beam forming or transmit diversity, besides spatial multiplexing.

So, assuming we are talking about MIMO as in spatial multiplexing, it can be said to be analogous to link aggregation in wireline in the sense of transmitting multiple streams of data in parallel in the same band simaltaneously. However, it works very differently from the wireline case, as more sophisticated signal processing is needed in the wireless case. They are just independent "pipes" in parallel, but signal processing is needed to give you something like that, in a loose sense.

If the channel conditions are not good (low SINR for example), it may be better to go with other forms of MIMO like beam forming. Even if the channel conditions are ok, the amount of multipath also makes a difference. The rank of the channel is an indication of how many streams of data you may be able to transmit simultaneously in the same band.

MIMO is different from OFDM, to answer your other question too. The separation of the different streams of data is not related to OFDM.

And your other question, if the rank of the channel is sufficient, you can indeed get higher capacity than with SISO.