How do WiFi and 4G Works with Many* users?

ksec

Senior member
Mar 5, 2010
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I was reading on MU-MIMO and something has strike me,

*Correct me if I am wrong.
On WiFi with SU-MIMO, a Single Devices used all the antenna possible to achieve maximum possible speed. So if one devices only has 2x2 ( Phone and Tablet ) with a 3x3 Router, it will use the best reception of the 2 Antenna possible from the Router. I was told if there are multiple clients connecting, the router will share its resources. So on SU-MIMO, a 2x2 802.11 ac allows a theoretical max of 867Mbps. I was told the amount of concurrent users are limited by the computation power of the Router CPU, since every users connection required some calculation. So if there were 20 users, each will get a theoretical max of ~43.3 Mbps. Taking a real world Wireless speed get 50% speed reduction and 20% multiple user overhead, we are getting ~17 Mbps per user.

This is very slow, and in a real world scenario with much higher interference it is likely to be even slower.

So how do enterprise AP from Ubnt and Rackus or Aruba handle this? I read on their spec they also only have 4x4 Antenna max, much like any consumer hardware except may be with much more powerful CPU and software. But wouldn't the theory still be the same? With 40 concurrent users we are talking about ~8 Mbps.
( And Steve Jobs telling people to get off the WiFi in Apple Event suddenly pops up in my head )

802.11ac Wave 2 with MU-MIMO was supposed to solve this, with better utilization of Antenna, But i read this more like help then solved. On the new Standard allows a maximum of 8x8 antenna, 80+80 / 160 Mhz, bringing a possible speed of 6.77Gbps, a similar 40 concurrent user will only get 67.7Mbps. And I guess there will likely be additional overhead from wider channels and MU-MIMO.

Edit: And i just find out MU-MIMO is for Download only......

So I guess, we found out now WiFi isn't as fast as we thought ( and it was never fast enough to begin with anyway ), especially in a multi user environment.

So how did 4G mobile network handle this? Surely Mobile Operator have to handle loads that are much higher then WiFi? And why we dont have Enterprise AP that gets 16 or 32 antenna?
 
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azazel1024

Senior member
Jan 6, 2014
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CDMA as well as TDMA are the common schemes. cellular also uses sector antennas. Also multiple channels. A carrier might be using 20MHz LTE and 150Mbps, but at the site, they might be using 60MHz of spectrum. So some clients would be some channels, others on other channels with alternating channel usage on the different antennas. That combined with each sector antenna might only cover 30 degrees. So one tower might reasonably have 1800Mbps or more of bandwidth to share out to its users.

On MU:MIMO, most wifi traffic is from basestation to client anyway.

On your "I guess additional overhead for wider channels", no, there is less overhead, not more. MU:MIMO overhead is very minimal, just some control frames, <1% overhead IIRC.

50% real world speed just depends. Far from a base station and you might be getting 1% of the speed, close and you might get 75%.

11ac is still evolving, a lot on the hardware side of things. The Intel 7260ac, the BEST I have seen can hit about 76% usable throughput on 2.4GHz 11n close to a newer router, which is basically the max theoretical. That other 24% is "lost" due to error correction overhead (FEC). 802.11ac actually has a slightly higher maximum theoretical usable bandwidth, because it implements low parity error checking, which is around 14-16% overhead IIRC, rather than the ~24% of FEC.

The BEST I have seen with an Intel 7260ac to a really awesome 802.11ac basestation is right around 56% useable bandwidth. The 7265ac is supposedly 20% faster than the 7260ac (I haven't tested it), because of better radios and components on it. That would mean it should hit around 65-68% usable bandwidth, much closer to best theory. I'd imagine future 802.11ac clients will be even better.

80MHz is hard. As are the modulation rates being used (256QAM). Newer firmware, newer radios, newer amplifiers, etc. all help with this.

On the enterprise side, the reason they used 4:4 streams, even on some older 802.11n base stations is that most of those enterprise APs utilized beam forming (beam forming was optional in 802.11n, support is mandatory in 802.11ac). The more streams you have for beam forming, the better the possible signal gain. Also more radio chains/streams = greater signal gain, up to the limits of the FCC emissions caps.

A perk is, you can use lower power radios, which also means cleaner amps. The more power you put through a radio/amp, the more distortion you get. Which means you can't actually drive as high a signaling rate (or else you lose packets from corruption). So a 4:4 router at the FCC emissions limits is likely to have a much cleaner signal than a 2:2 router, also at the signal limits. Or it can use much cheaper radios and amps than that 2:2 router.

Oh, for MU:MIMO, the only overhead is you lose one stream to allow it to work. So a 4:4 MU:MIMO router is limited to 3:3 speed, which operating in MU:MIMO. That means feeding one 2:2 and one 1:1 client at full speed. Or it could feed a single 4:4 client at full speed. Or 3 1:1 clients at full speed.

In general a dozen Mbps is more than enough for almost all reasonable applications. No, it does not allow fast file transfer, but it certainly allows a typical H.264 compressed 1080p video stream with some headroom. If you need fast file transfers, you either need to be operating in a low user count environment, or else you need to plug the darned thing in to a wired network.

Even in most corporate situations, there are not a lot of times that one AP is feeding 40 users and NEEDS super high bandwidth. Most users are not hitting an AP heavy all at once. Most are doing light needs, like email, web browsing, etc. Where all they need are short bursts of 2-8Mbps and then back to idle connections. Unless in a conference room, you probably are not going to have 40 clients on one AP.

With our IT deployment, even in most of the cubicle farm areas, we probably have a one AP to around 30-40 cubicles (around abouts 1 per 2,000sq-ft). All of the desktop workstations are wired and all of the laptops use wired docking stations. So the entire load is only when a laptop is undocked or BBs. Most of the large conference rooms have their own AP and the smaller ones aren't likely to have more than a dozen people sitting in them.

Also as a work environment, this generally isn't 40 people sitting in a conference room all streaming 4k video on their work laptops.

The really tough environments are places like malls with free wifi, or hotels. Especially the later, unless you get in to really dense deployments, you often only have 1-2 APs per floor trying to feed possibly 12-20 rooms and you are MUCH more likely to have multiple people in each room. You are also MUCH more likely to have those people doing high demand things like streaming video. And you also have a setup with lots of walls where 5GHz isn't going to penetrate well to take advantage of 802.11ac.