OLPC hardware: what if there was an SDR modem / chipset?

david at lang.hm david at lang.hm
Mon Jan 25 11:24:52 EST 2010

On Mon, 25 Jan 2010, Luke Kenneth Casson Leighton wrote:

> i've been doing some research and found a couple of companies with SDR
> R.F. front-end ICs.  one is 40nm and is so tiny that it will only cost
> about $2, mass-produced.  also thanks to being in 40nm, the speed of
> the (SoC / embedded) ARM9 core is so fast that it's perfectly capable
> of handling multiple protocols.  as you're no doubt aware,

are you sure that your $2 SDR chip is going to be able to do a protocol as 
complex as wifi? that will take quite a bit of computing power (digital 
signal processing isn't something you do efficiantly on the ARM9 core), so 
it may push you from the $2 chip to a $10 chip (what do you think the 
existing wifi chipsets are?)

> Software-Defined Radio has been "full of promise" for quite some time
> as "The" low-cost transceiver option, but it's only recently that the
> speed of embedded ICs has gone up enough and the geometry small enough
> to bring the cost down into the affordable range.
> key to making SDR "work" is of course having the software :)  but,
> i've found a company who already have GSM through EDGE; there's
> http://openbts.org and also of course there's the gnu-radio project
> which has produced part of 802.11, amongst other things.  ( but,
> remember: the nice thing about the 2.4ghz and 5ghz bands is that you
> don't _actually_ need to do 802.11, you can in fact just use the
> entire set of bands to do absolutely anything you want.  and, with
> SDR, you _could_ do anything you want).

but if you don't do 802.11a/b/g/n you won't be able to talk to other 

> the neat thing about SDR is that the _same_ solution replaces:
> * a WIFI chipset ($10)
> * a GSM chipset ($12 lowest i've found in 100k+ volumes)
> * or a 3G chipset ($30 lowest i've heard about in mass-volume)
> * a GPS chipset ($6 and that's again an SDR solution,
>   you need a DSP to translate; $12 for dedicated chipset)
> * a WIMAX chipset (haven't even looked this up, but estimate $20)
> * a DVB TV chipset (approx $5 and again that's an SDR solution)
> * an FM Radio chipset (don't know its cost, don't honestly care!)
> i repeat.  all those can be replaced with _one_ i repeat _one_ single
> solution, costing roughly... $12, if that.

while the chip can decode all the different protocols you mention below, 
the problem is that these cover many different frequency ranges, and 
getting the RF portion of the device (including antennas) to handle all 
those different bands on one device, let alone at one time, is far more 
complex than you imagine (and would you really want a device that could be 
wifi, OR GPS OR.... but not more than one at a time?)

it's bad enough if you are just needing to receive all these different 
frequencies, but if you have to transmit on them it takes even more work 
(several of your items are receive only, but the critical ones require 
transmit capability)

you also ignore licensing and regulatory approval. That actually amounts 
to a significant portion of some of the chipsets. you can avoid that by 
eliminating those items from your list, but that also reduces the 
attractivness of the solution.

David Lang

> issues which need to be resolved:
> * paying for a minimum of 8 40nm 10in wafers (appx 5000 ICs per wafer)
> @ $0.50 ea, running the test vectors @ $0.75 ea, packaging @ $0.50 ea
> it works out roughly at $2.00 times 40,000.
> * creating the PCB with RF MEMS filters
> (http://en.wikipedia.org/wiki/RF_MEMS) and other assorted components,
> coax connectors for antenna, the remaining components are going to be
> somewhere around $10.
> * license compliance and Certification in the countries in which the
> final modem is deployed (remember that if frequencies other than
> 2.4ghz or 5ghz @ greater than 100mW are used or 400mW in Hong Kong
> then licenses are NOT required)
> even with these issues to be costed out and resolved, i wanted to ask:
> * is the incredible low-cost and flexibility of SDR worth pursuing?
> * is the current Marvell 88688 "proprietary firmware" 802.11 blob
> _that_ acceptable / accepted?
> * is the possibility of being able to run an XO up a pole (or placed
> at the top of any tall building) and have it _be_ the GSM or WIMAX
> base station for an entire town or village and the surrounding
> countryside for miles around the kind of thing that is attractive to,
> and useful to the aims of the OLPC project, or not?
> also - one thing that also would help to have an answer to : if
> answers above turn out to be resounding "yes", what's next?  who makes
> the decision?
> l.
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