[Cerowrt-devel] wifi over narrow channels

Tue Oct 14 08:21:43 EDT 2014

David -

1) you are right that LBT fails because propagation does not allow a transmitter to hear the stations that might be transmitting on the same channel. But LBT need not be the optimal approach for the general idea of packet  multiplexing. A better decentralized approach is rts/cts. Think about it. Start with that idea and improve it... I'm leaving it there as a learning moment. You'll learn more by improving it than by having me tell you.

2) co channel separation was hard before computing and sampling became good. It is no longer hard unless you handicap your thinking by thinking it is hard.

3) spread spectrum is the first non- narrowband modulation technique. It is far from the best.  Consider very wideband wavelet coding... 

4) subdivision of space time is wasteful in the extreme. Why try to create walls when the space around the walls becomes useless? The denser the walls in space time and frequency the more waste... 

Radio is not the sin or cos function.  There is nothing in either Maxwell or QED  that invokes Fourier. Nothing.

On Oct 14, 2014, David Lang <david at lang.hm> wrote:
>The problem is that you really have trouble with different transmitters
>sharing 
>one channel over a wide area. By having more channels and smaller areas
>that 
>each one covers you end up with better coverage overall, less
>interference and 
>greater throughput.
>
>If it was really possible to have different transmitters talking at the
>same 
>time on the same channel, this wouldn't be a problem, but we really
>can't in 
>practice [*], so it is a problem.
>
>The key problems show up because you can have two mobile stations on
>opposite 
>sides of the AP that can't hear each other, but can hear the AP just
>fine. When 
>they both transmit, the AP can't listen to both at once (it's
>technically 
>possible if the two signals are very close to the same strength, but if
>one is 
>significantly more powerful than the other, the weaker one gets lost in
>the 
>noise)
>
>So in practice, you are better off with more APs on different channels
>with the 
>APs connected together than you are with fewer, faster channels
>covering a 
>larger area per channel.
>
>Things like a fixed "listen before transmit" wait time just emphisise
>this 
>because they mean that as the bit rate of the channel goes up, you are
>wasting 
>a higher percentage of the capability when you are transmitting the
>same amount 
>of data.
>
>If you wait 4ms and then transmit for 28ms, you are wasting 1/8 of the
>channel 
>bandwidth, but if you can transmit the same data in 12ms, you are
>wasting 1/4 of 
>the channel bandwidth.
>
>Now, shorter transmissions do mean that there is less chance that
>someone else 
>who you can't hear will transmit, but having that other station on a
>different 
>channel talking to an AP that's closer to it will do even better.
>
>
>[*] In theory, spread spectrum transmission allows you to have
>different 
>stations talking on the same wideband channel without interfering with
>each 
>other.
>
>In practice there are a few problems with this.
>
>1. the different sets of transmissions need to have different
>transmission 
>patterns (keys), and these patterns need to be distributed to all the
>stations 
>using a particular key.
>
>2. not all keys are equally good, especially when combined with other
>keys in 
>use, so coordination of the keys is needed.
>
>3. timing on all the different radios in the network is critical so
>that they 
>all start looking at the same time. This is something that is very hard
>to get 
>in practice without some external timekeeping being provided (some
>equipment 
>would use the broadcast TV sync timing before it went digital for
>example)
>
>4. You still have the problem of not being able to transmit and receive
>at the 
>same time.
>
>5. The RF signal gets amplified by a variable amount to make the
>resulting 
>signal a constant strength before it's decoded. The RF signal from
>different 
>transmitters is falling off at roughly the cube of the distance, and
>the 
>decoders have 12-14 bits of resolution. If a more distant station is 
>transmitting at the same time as a nearby station, this makes the
>distant staton 
>unreadable because it's signal ends up being represented by too few
>bits in the 
>decoder.
>
>David Lang
>
>On Thu, 9 Oct 2014, dpreed at reed.com wrote:
>
>> Wideband is far better for scaling than narrowband, though.  This may
>seem 
>> counterintuitive, but narrowband systems are extremely inefficient. 
>They 
>> appeal to 0/1 thinking intuitively, but in actual fact the wider the
>bandwidth 
>> the more sharing and the more scaling is possible (and not be
>"balkanization" 
>> or "exclusive channel negotiation").
>> 
>> Two Internets are far worse than a single Internet that combines
>both. 
>> That's because you have more degrees of freedom in a single network
>than you 
>> can in two distinct networks, by a combinatorial factor.
>> 
>> The analogy holds that one wide band is far better than two disjoint
>bands in 
>> terms of scaling and adaptation. The situation only gets better
>because of the 
>> physics of "multipath", which creates more problems the more
>narrowband the 
>> signal, and when the signal is a single frequency, multipath is
>disastrous.
>> 
>> The same is true if you try to divide space into disjoint "channels"
>(as 
>> cellular tries to).
>> 
>> So in the near term, narrowband wifi might be a short-term benefit,
>but 
>> long-term it is 180 degress away from where you want to go.
>> 
>> (the listen-before-talk protocol in WiFi is pragmatic because it is
>built into 
>> hardware today, but terrible for wideband signals, because you can't
>shorten 
>> the 4 usec. pre-transmit delay, and probably need to lengthen it,
>since 4 
>> usec. is about 1.25 km or 0.8 miles, and holds 40 bits at 10 Mb/s, or
>4000 
>> bits at 1 Gb/sec).
>> 
>> Either for distance or for rate, the "Ethernet MAC+PHY" was designed
>for short 
>> "coax" or "hub" domains. Its not good for digital wireless Internet,
>except 
>> for one thing: it is based on distributed control that does not
>require any 
>> advance planning.
>> 
>> If you want to improve open wireless, you have to a) go wide, b)
>maintain 
>> distributed control, c) get rid of listen-before-talk to replace it
>with a 
>> mixture of co-channel decoding and propagation negotiation.  Then you
>can beat 
>> cellular trivially.
>> 
>> I wish I could attract investment away from the "short term" WiFi
>thinking, 
>> but in the last 15 years, I've failed.  Meanwhile WiFi also attracts
>those 
>> people who want to add bufferbloat into the routers because they
>don't 
>> understand congestion control.
>> 
>> Sad.
>>
>>
>> On Wednesday, October 8, 2014 6:14pm, "Dave Taht"
><dave.taht at gmail.com> said:
>>
>>
>>
>>>
>https://www.usenix.org/system/files/conference/nsdi12/nsdi12-final142.pdf
>>> 
>>> I've had 5mhz channels working in the ath9k at various points in
>>> cerowrt's lifetime. (using it for meshy stuff) After digesting most
>of
>>> the 802.11ac standard I do find myself wishing they'd gone towards
>>> narrower channels rather than wider.
>>> 
>>> The netgear x4 defaults to a 160mhz wide channel. :sigh:
>>> 
>>> The above paper has some nifty ideas in it.
>>> 
>>> --
>>> Dave Täht
>>> 
>>> https://www.bufferbloat.net/projects/make-wifi-fast
>>> _______________________________________________
>>> Cerowrt-devel mailing list
>>> Cerowrt-devel at lists.bufferbloat.net
>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>>
>
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