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I've been working on an assignment about the IEEE 802.11 standards and I've had some trouble with one aspect of a question. I need to explain the differences between various features of the different 802.11 standards (a, b, g, n, etc). I've been able to find most of the details online, especially through official IEEE documents.

What I've been unable to find any information on is the different "kinds of antennas" used by each standard and how they differ from one another. I know that 802.11n introduced MIMO and thus allowed the use of multiple antennas, and that antennas can be either directional or omnidirectional, but all 802.11 standards seem to be omnidirectional. I don't know what is actually different between the antennas themselves as specified by the IEEE standards.

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    \$\begingroup\$ Look into "Beam Forming" and "Phased Antenna Arrays". MIMO and Diversity antenna systems like these will often use multiple omnidirectional antenna elements but these become directional when used in an array. \$\endgroup\$ – Wossname May 15 '18 at 10:27
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As pointed out by Marcus in the previous answer, the IEEE standards do not prescribe a type of antenna that has to be employed. Based on the functionality, one would pick an appropriate antenna.

For instance, standards 802.11a/b/g do not employ MIMO, and function through the simple use of an omnidirectional antenna that radiates energy in all directions (this depends on the radiation pattern, only an idealistic isotropic antenna can actually do this). These standards could also employ multiple antennas, wherein selection combining or maximal ratio combining are carried out to improve reliability.

Coming to newer standards such as 802.11n/ac, MIMO is used to exploit spatial diversity. In this case, multiple antennas are not used to simply combine the signals as earlier, but to transfer more data (or redundant data for reliability) across the various streams that are created by the use of multiple antennas. Again, omnidirectional antennas can be used, since there is no explicit need for directivity towards a particular station.

Another new standard is 802.11ad, which functions precisely through directionality. Directional antennas such as a horn antenna or a phased array would be used to steer beams towards particular users in order to improve the link budget. By operating at 60 GHz, antennas arrays would have smaller footprints, allowing us to pack in more antenna elements, which in turn would allow better beam-steering, lower beamwidths, and higher gains.

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Communication standards usually, and especially 802.11, do not specify antennas. So, there's no "standard antennas".

Which antenna you use depends on your use case; and in the case of WiFi and similar technologies, using highly directive antennas makes no sense, since you can't know in which direction your communication partner is, seeing that usually at least one part of a link is somewhat mobile, and seeing that the WiFi environment often relies on non-line-of-sight links.

Antenna design usually is a relatively experience-heavy field, which not only incorporates the physics of the wave you're working with (that being, almost exclusively, carrier frequency range, and maybe device impedance), but also things like package geometry, cost, weight and size restrictions, aesthetical and ergonomic properties. The standard only defines carrier frequencies/bandwidths. Therefore, it would be illogical to define what antenna to use (you might be misunderstanding what an antenna does, maybe?).

So, your question is inanswerable.

Regarding MIMO: For MIMO to yield a gain, you'll either need the ability to steer a beam (in which case you wouldn't want overly directive antennas in the first place), or to have diversity gain, i.e. uncorrelated channel realizations on individual antennas (and using antennas that are very directive doesn't make much sense here, either, since you might be getting uncorrelated channels easier that way, but also get a high per-antenna outage/deep fade probability, which is the opposite of what you'd want).

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