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I am reading some research articles on wireless communication where they talk about beamforming techniques and their differences in Multiple antenna Tx-Rx systems (MIMO). It says analog beamforming can only support one stream( Or one user) at a time while digital beamforming can support multiple streams and users.

Can someone clearly explain why is this the case?

To ask more directly, why cannot I create beams in multiple directions carrying different information streams using an array of antennas while using analog beamforming?

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  • \$\begingroup\$ Not exactly. I guess the question is applicable to any general scenario though. \$\endgroup\$ – Jyotish Robin Sep 30 '18 at 17:31
  • \$\begingroup\$ Could you add some context, like the articles in question? \$\endgroup\$ – Dmitry Grigoryev Oct 8 '18 at 7:32
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Beam forming is, at its base, nothing more than combining the signals from different sources so as to maximize the signal and minimize the noise. This is done by having the antennas physically separated so that there is a delay between reception at all antennas. Then you introduce electrical delays to line the received signals up again. In that way, the signal adds up and is amplified by the number of receiving antennas. The noise doesn't add the same way, so the final sum has a better signal to noise ratio than the received signal from any single antenna.

The term "beam forming" comes from the idea that this summing effect acts much like having a single directional antenna with high gain pointed at the source. The directionality of an antenna is called a "beam." Since you are making a directional antenna from non-directional antennas, you are forming a beam: beam forming.

The direction the beam points is determined by the delays between the different antennas.

In an analog system, you have a set of antennas with fixed delays - your beam points in one particular direction.

You could build an analog system where you split off the signal from each antenna into multiple delays, and sum them together to get multiple beams. I think the losses and unwanted delays of the needed splitters and amplifiers would reduce the usefulness of such a system to the point where it isn't worth the effort.

A digital system is different in that you can vary the delays in software. There are no losses, so you can make as many beams as your processor can handle in realtime.

Digital systems incorporate algorithms whereby the base station tries to improve the communication with the clients by aiming beams at them. The time delays at which the signal from the clients arrive at the base station can be used to calculate the delays needed to aim the beam. Basically, the base station determines the direction to the client and aims one of its beams in that direction.

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  • \$\begingroup\$ Thanks! could you clarify the following: Say if I have 2x2 array of antennas, and if i have a total of 4 phase shifters each one feeding one antenna element in the array. So can I say this structure will generate a beam with a specific directionality determined by the phase shift values? \$\endgroup\$ – Jyotish Robin Sep 30 '18 at 17:44
  • \$\begingroup\$ Yes. Exactly. Though, you can probably do it with just three phase shifts. A delay of zero is still a delay. \$\endgroup\$ – JRE Sep 30 '18 at 17:48
  • \$\begingroup\$ Thanks again! I have still some pending confusions . "A digital system is different in that you can vary the delays in software. There are no losses, so you can make as many beams as your processor can handle in realtime." I am not able to clearly understand the last part of this. I understand that I can easily slide the phase delays using a digital system. But even in digital, at any given time point, there are only 4 (or 3) phase shifters that feed my antenna array elements. So won't it be a single directional beam at any instant? NB: It may sound stupid, but i am confused :( \$\endgroup\$ – Jyotish Robin Sep 30 '18 at 17:57
  • \$\begingroup\$ No phase shifters in a digital system at all. All the phase shifting is done in software. So, pretty much any delay you need can be reached. You only have a limited number of antennas, and that dictates the maximum improvement in the signal to noise ratio. The amount of processing you can do dictates how precisely you can aim the beam, and that determines how much of the signal to noise ratio improvement you can really achieve. \$\endgroup\$ – JRE Sep 30 '18 at 18:02
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    \$\begingroup\$ You digitize the signals in the digital system. You have 4 data streams. You can apply as many different sets of 4 delays to those data sets as you want , so making as many beams as you want to. \$\endgroup\$ – JRE Sep 30 '18 at 18:26

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