I don't think anyone is trying to do simultanious receive of different stations.
That is an incredibly difficult thing to do right.
MU-MIMO is aimed at haivng the AP transmit to multiple stations at the same
time. For the typical browser/streaming use, this traffic is FAR larger than the
traffic from the stations to the AP. As such, it is worth focusing on optimizing
this direction.
While an ideal network may resemble a wired network without guides, I don't
think it's a good idea to think about wifi networks that way.
The reality is that no matter how good you get, a wireless network is going to
have lots of things that are just not going to happen with wired networks.
1. drastic variations in signal strength.
On a wired network with a shared buss, the signal strength from all stations
on the network is going to be very close to the same (a difference of 2x would
be extreme)
On a wireless network, with 'normal' omnidirctional antennas, the signal drops
off with the square of the distance. So if you want to service clients from 1 ft
to 100 ft away, your signal strength varies by 1000 (4 orders of magnatude),
this is before you include effects of shielding, bounces, bad antenna alignment,
etc (which can add several more orders of magnatude of variation)
The receiver first normalized the strongest part of the signal to a constant
value, and then digitizes the result, (usually with a 12-14 bit AD converter).
Since 1000x is ~10 bits, the result of overlapping tranmissions can be one
signal at 14 bits, and another at <4 bits. This is why digital processing isn't
able to receive multiple stations at the same time.
2. 'hidden transmitters'
On modern wired networks, every link has exactly two stations on it, and both
can transmit at the same time.
On wireless networks, it's drastically different. You have an unknown number
of stations (which can come and go without notice).
Not every station can hear every other station. This means that they can't
avoid colliding with each other. In theory you can work around this by having
some central system coordinate all the clients (either by them being told when
to transmit, or by being given a schedule and having very precise clocks). But
in practice the central system doesn't know when the clients have something to
say and so in practice this doesn't work as well (except for special cases like
voice where there is a constant amount of data to transmit)
3. variable transmit rates and aggregation
Depending on how strong the signal is between two stations, you have different
limits to how fast you can transmit data. There are many different standard
modulations that you can use, but if you use one that's too fast for the signal
conditions, the receiver isn't going to be able to decode it. If you use one
that's too slow, you increase the probability that another station will step on
your signal, scrambling it as far as the receiver is concerned. We now have
stations on the network that can vary in speed by 100x, and are nearing 1000x
(1Mb/sec to 1Gb/sec)
Because there is so much variation in transmit rates, and older stations will
not be able to understand the newest rates, each transmission starts off with
some data being transmitted at the slowest available rate, telling any stations
that are listening that there is data being transmitted for X amount of time,
even if they can't tell what's going on as the data is being transmitted.
The combination of this header being transmitted inefficiently, and the fact
that stations are waiting for a clear window to transmit, means that when you do
get a chance to transmit, you should send more than one packet at a time. This
is something Linux is currently not doing well, qdiscs tend to round-robin
packets without regard to where they are headed. The current work being done
here with the queues is improving both throughput and latency by fixing this
problem.
You really need to think differently when dealing with wireless network. The
early wifi drivers tried to make them look just like a wired network, and we
have found that we just needed too much other stuff to be successful whith that
mindset.
The Analog/Radio side of things really is important, and can't just be
abstracted away.
David Lang
On Sun, 26 Jun 2016, Bob McMahon wrote:
Is there a specific goal in mind? This seems an AP tx centric proposal,
though I may not be fully understanding it. I'm also curious as why not
scale in spatial domain vs the frequency domain, i.e. AP and STAs can also
scale using MiMO. Why not just do that? So many phones today are 1x1, some
2x2 and few 3x3. Yet APs are moving to 4x4 and I think the standard
supports 8x8. (I'm not sure the marginal transistor count increase per
each approach.) On the AP tx side, MuMIMO is also there which I think is
similar to the DAC proposal.
I'm far from a PHY & DSP expert, but I think the simultaneous AP receive is
the most difficult issue per the power variations, aka SNR. Each of the
mobile device energies is affected per inverse square law (unless some form
of wave guide is used.) Hence wi-fi use of time domain slots prior to
transmit (no longer simultaneous in time.) Particularly needed for devices
that communicate with one another. Unfortunately, "collocated"
energy/information sources must honor this TDM even when not communication
with one another per tragedy of the commons. (Agreed there is no such
thing as a collision so let's redefine it to mean that the receiver is
unable to receive per RF energy "confusion", still a tragedy of the
commons. I don't know what drives the limits to DSP decode that could
minimize or eliminate this tragedy.)
At the end of the day, would the ideal network would resemble a wired
ethernet (including ethernet switch fabric) without the waveguides (or
wires/fibers)? From that perspective, here are some thoughts to the goals
o) TX op/access to transmit driven to zero. (Collision avoidance isn't
nearly as good as instantaneous collision detect in this context, though
"collision" should replaced with "confusion")
o) RX confusion detection time propagated to stop offending TX(s) driven
to zero
o) Support of different encodings (e..g phy rates) pushes towards virtual
output queueing prior (queuing at the transmitter)
o) Power per bit xfered towards zero or driven per cost & energy density
of batteries. Note: Atomic batteries not allowed per humans being involved.
o) Transistor count (chip cost) per bit moved driven by Moore's law and
those economics
o) Reduce "collision/confusion" domain (less STAs per AP) ideally to zero
Just some thoughts off the top of my head. Please do comment and correct.
Thanks in advance for the discussion.
Bob
On Fri, Jun 24, 2016 at 2:24 PM, <dpr...@reed.com> wrote:
Without custom silicon, doing what I was talking about would involve non
standard MAC power management, which would require all devices to agree.
David Lang's explanation was the essence of what I meant. the transmission
from access point on multiple channels is just digital addition if the DACs
have enough bits per sample. to make sure that the signals to the AP are
equalized, just transmit at a power that makes that approximately true...
which means a power amp with at most 30 dB of dynamic gain setting. typical
dynamic path attenuation range (strongest to weakest ratio) among stations
served by an AP is < 20 dB from my past experiments on well
operating-installtions, but 25 can be seen in reflection heavy
environments.-----Original Message-----
From: "David Lang" <da...@lang.hm>
Sent: Fri, Jun 24, 2016 at 1:19 am
To: "Bob McMahon" <bob.mcma...@broadcom.com>
Cc: "Bob McMahon" <bob.mcma...@broadcom.com>,
make-wifi-f...@lists.bufferbloat.net, "cerowrt-devel@lists.bufferbloat.net"
<cerowrt-devel@lists.bufferbloat.net>
Subject: Re: [Make-wifi-fast] more well funded attempts showing market
demandfor better wifi
well, with the kickstarter, I think they are selling a bill of goods.
Just using the DFS channels and aggregating them as supported by N and AC
standards would do wonders (as long as others near you don't do the same)
David Lang
On Thu, 23 Jun 2016, Bob McMahon wrote:
Date: Thu, 23 Jun 2016 20:01:22 -0700
From: Bob McMahon
To: David Lang
Cc: dpr...@reed.com, make-wifi-f...@lists.bufferbloat.net,
"cerowrt-devel@lists.bufferbloat.net"
Subject: Re: [Make-wifi-fast] more well funded attempts showing market
demand
for better wifi
Thanks for the clarification. Though now I'm confused about how all the
channels would be used simultaneously with an AP only solution (which is
my
understanding of the kickstarter campaign.)
Bob
On Thu, Jun 23, 2016 at 7:14 PM, David Lang wrote:
I think he is meaning when one unit is talking to one AP the signal
levels
across multiple channels will be similar. Which is probably fairly true.
David Lang
On Thu, 23 Jun 2016, Bob McMahon wrote:
Curious, where does the "in a LAN setup, the variability in [receive]
signal strength is likely small enough" assertion come? Any specific
power numbers here? We test with many combinations of "signal strength
variability" (e.g. deltas range from 0 dBm - 50 dBm) and per different
channel conditions. This includes power variability within the spatial
streams' MiMO transmission. It would be helpful to have some physics
combined with engineering to produce some pragmatic limits to this.
Also, mobile devices have a goal of reducing power in order to be
efficient
with their battery (vs a goal to balance power such that an AP can
receive simultaneously.) Power per bit usually trumps most other
design
goals. There market for battery powered wi-fi devices drives a
semi-conductor mfg's revenue so my information come with that bias.
Bob
On Thu, Jun 23, 2016 at 1:48 PM, wrote:
The actual issues of transmitting on multiple channels at the same time
are quite minor if you do the work in the digital domain (pre-DAC).
You
just need a higher sampling rate in the DAC and add the two signals
together (and use a wideband filter that covers all the channels).
No RF
problem.
Receiving multiple transmissions in different channels is pretty much
the
same problem - just digitize (ADC) a wider bandwidth and separate in
the
digital domain. the only real issue on receive is equalization - if
you
receive two different signals at different receive signal strengths,
the
lower strength signal won't get as much dynamic range in its samples.
But in a LAN setup, the variability in signal strength is likely small
enough that you can cover that with more ADC bits (or have the MAC
protocol
manage the station transmit power so that signals received at the AP
are
nearly the same power.
Equalization at transmit works very well when there is a central AP
(as
in
cellular or normal WiFi systems).
On Thursday, June 23, 2016 4:28pm, "Bob McMahon" >>>
bob.mcma...@broadcom.com>
said:
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An AP per room/area, reducing the tx power (beacon range) has been my
approach and has scaled very well. It does require some wires to
each
AP
but I find that paying an electrician to run some quality wiring to
things
that are to remain stationary has been well worth the cost.
just my $0.02,
Bob
On Thu, Jun 23, 2016 at 1:10 PM, David Lang wrote:
Well, just using the 5GHz DFS channels in 80MHz or 160 MHz wide
chunks
would be a huge improvement, not many people are using them (yet),
and
the
wide channels let you get a lot of data out at once. If everything is
within a good range of the AP, this would work pretty well. If you
end
up
needing multiple APs, or you have many stations, I expect that you
will
be
better off with more APs at lower power, each using different
channels.
David Lang
On Thu, 23 Jun 2016, Bob McMahon wrote:
Date: Thu, 23 Jun 2016 12:55:19 -0700
From: Bob McMahon
To: Dave Taht
Cc: make-wifi-f...@lists.bufferbloat.net,
"cerowrt-devel@lists.bufferbloat.net"
Subject: Re: [Make-wifi-fast] more well funded attempts showing
market
demand
for better wifi
hmm, I'm skeptical. To use multiple carriers simultaneously is
difficult
per RF issues. Even if that is somehow resolved, to increase
throughput
usually requires some form of channel bonding, i.e. needed on both
sides,
and brings in issues with preserving frame ordering. If this is just
channel hopping, that needs coordination between both sides (and
isn't
simultaneous, possibly costing more than any potential gain.) An
AP
only
solution can use channel switch announcements (CSA) but there is a
cost to
those as well.
I guess don't see any break though here and the marketing on the
site
seems
to indicate something beyond physics, at least the physics that I
understand. Always willing to learn and be corrected if I'm
misunderstanding things.
Bob
On Wed, Jun 22, 2016 at 10:18 AM, Dave Taht
wrote:
On Wed, Jun 22, 2016 at 10:03 AM, Dave Taht
wrote:
https://www.kickstarter.com/projects/portalwifi/portal-turbocharged-wifi?ref=backerkit
"Portal is the first and only router specifically engineered to
cut
through and avoid congestion, delivering consistent,
high-performance
WiFi with greater coverage throughout your home.
Its proprietary spectrum turbocharger technology provides access
to
300% more of the radio airwaves than any other router, improving
performance by as much as 300x, and range and coverage by as
much as
2x in crowded settings, such as city homes and multi-unit
apartments"
It sounds like they are promising working DFS support.
It's not clear what chipset they are using (they are claiming
wave2)
-
but they are at least publicly claiming to be using openwrt. So I
threw in enough to order one for september, just so I could
comment
on
their kickstarter page. :)
I'd have loved to have got in earlier (early shipments are this
month
apparently), but those were sold out.
https://www.kickstarter.com/projects/portalwifi/portal-turbocharged-wifi/comments
--
Dave Täht
Let's go make home routers and wifi faster! With better software!
http://blog.cerowrt.org
--
Dave Täht
Let's go make home routers and wifi faster! With better software!
http://blog.cerowrt.org
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