Why can't WiFi work as full duplex while 3G and 4G can

[Hussam-Bay]

Hello guys,

I'm still not finding the right answer to my question: Why can't WiFi work as full-duplex while 3G and 4G can?

APs have multiple antennas so the question is: why can't we add two antennas to an AP and each antenna will work on a different channel to support Full-duplex. 

[BrechtSchamp]

Good question. I think it has to do with the fact that there are many more frequencies available in the cell world and that it's a licensed medium.

[Uberseehandel]

Wireless doesn't sense collisions, so full duplex is doooooooooommmmmmmmmmmeeeeeeeeeeeeeeeedddddddddddddddd

[ww]

@Uberseehandel basically  he  means 2x half duplex 🙂

[Uberseehandel]

because without collision detection, everything goes to hell.

[antonis_sp · Accepted solution]

Not only Wi-Fi cannot work as full-duplex, but also two or more devices cannot transmit or receive traffic simultaneously.

Unlike 3G/4G, Wi-Fi uses unlicensed frequencies in the spectrum, which simply means you do not have to pay for using them.

Not only frequencies used for WiFi are quite limited, but also other technologies use the same frequencies (bluetooth, zigbee, radar systems). On top of that, there is also a lot of interference on these frequencies (microwave ovens, wireless cameras).

Long story short, at any time WiFi devices have to make absolutely sure that a certain frequency is "free" from other transmitions before they can even send one packet to each other.

Even MU-MIMO can only allow a few devices to transmit or receive at the same time (still one-way).

[NolanHerring]

**I'm not a cellular guy by any stretch of the imagination, so what typing below may not be accurate. This is also me just dumping thoughts onto a keyboard so some of this might be scattered.

So first let's all get on the same page with the fact that cellular technically is not Full-Duplex either (yet). It uses mechanism's that allow it to mimic/emulate a full-duplex setup, but it is still wireless the same way 802.11 (Wi-Fi) is wireless and transmissions sent over the same frequency will cause interference/cancellation issues. Echo cancellation (which you'll see in my links below) is the new hype that in theory could make cellular actually full-duplex, if it actually works outside of a lab.

Here is a short overview of how it functions:
https://www.youtube.com/watch?v=6J5kbTjVaCQ

Here are some links that will help as well:
https://en.wikipedia.org/wiki/Channel_access_method
https://en.wikipedia.org/wiki/Duplex_(telecommunications)

802.11 operates in the unlicensed spectrum, where as cellular (3G/4G/5G etc.) operates in licenses frequencies that are essentially 'controlled' by the provider. Think of that setup sort of like an airport 'tower' controlling how planes land and take-off etc.

802.11 is the wild west with no 'tower' so the clients (AP included) have to figure it out themselves with an arbitration process (lots of overhead). Due to that initial constraint, it suffers from the 'one person at a time' issue inherent. However there are improvements in this field (MU-MIMO which again is more of a unicorn that anything else), but more importantly the introduction of OFDMA (hey look...a 4G cellular technology for wi-fi !!!) with Wi-Fi 6 (802.11ax).

This is a complicated question with decades of history and I am 100% certain that if we could take the knowledge we have now, and go back in time, that the 'Wi-Fi' we have today would be much difference and far better planned and more efficient. We have Wi-Fi6E coming out soon (giving wifi the use of 6GHz spectrum), and OFDMA which essentially is taking a 20MHz channel (and 40 and 80) and 'slicing it up' so that multiple clients can talk at the same time. OFDMA is sort of making the AP the 'tower' in this setup (very crude explanation but you get the idea). Most wireless frames are tiny, and really do not need 80MHz channels with unlimited power raaaaaawr, so we can absolutely benefit from this the way cellular has.

[Uberseehandel]

Attend an ITU conference and you might be less sanguine about existing WiFi usage. Manufacturers have a lot invested in current technology. They will have to be dragged screaming into the 802.11ad world. There is a lot of uninformed prejudice about what is possible in the ad portion of the spectrum. As a consumer, one has to ask why the manufacturers are still trying to sell us technology that has a poor future.