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 From:  Jim Thompson <jim at netgate dot com>
 To:  "Alex M" <radiussupport at lrcommunications dot net>
 Cc:  "Monowall Support List" <m0n0wall at lists dot m0n0 dot ch>
 Subject:  Re: [m0n0wall] 802.11 draft N miniPCI cards for ALIX
 Date:  Fri, 25 Jan 2008 08:50:46 -1000
On Jan 24, 2008, at 7:21 AM, Alex M wrote:

> My post got too much defragmenter. :(
> The AR5005VL is not 100% N but it output is the same as MIMO

No, it isn't.  The 5005VL is a beamforming chipset.  From the Atheros  
description:

	Multiple Radio Smart Antenna support with Transmit Beamforming and  
Receive Combining

The fundamental characteristic of a MIMO system is that multiple  
streams (of different or even identical) of data are in-flight  
simultaneously.

In (single-layer) beamforming, the same signal is emitted from each of  
the transmit antennas with appropriate phase (and sometimes gain)  
weighting such that the signal power is maximized at the receiver  
input. The fundamental benefit of beamforming is to increase the gain  
of a signal via constructive combining and to, in-turn, reduce the  
effects of multipath fading. In the absence of scattering, beamforming  
results in a well defined directional pattern, often called a 'beam'.

When the receiver has multiple antennas, the transmit beamforming  
cannot simultaneously maximize the signal level at all of the receive  
antenna and precoding is used.  Precoding requires knowledge of the  
channel state information (CSI) at the transmitter.

MIMO systems also perform spatial multiplexing or diversity coding.   
In spatial multiplexing, a high rate signal is split into multiple  
lower rate streams and each of these streams is transmitted from a  
different transmit antenna in the same frequency channel. If these  
signals arrive at the receiver antenna array with sufficiently  
different spatial signatures, the receiver can separate these streams,  
creating parallel channels for free. Spatial multiplexing is a very  
powerful technique for increasing channel capacity at higher SNR.

In diversity coding a single stream (unlike multiple streams in  
spatial multiplexing) is transmitted, but the signal is coded using a  
set of techniques known as space-time coding. The signal is emitted  
from each of the transmit antennas using certain principles of full or  
nearly-full orthogonal coding. Diversity coding exploits the  
independent fading in the multiple antenna links to enhance signal  
diversity.

When I was at Vivato, we built beamforming 802.11 systems.   Siavash  
Alamouti (google for him) worked with me there, and Vahid Tarokh  
consulted with us.  Given that these two developed the single *most- 
efficient* MIMO system known (and were recognized by the IEEE for  
such), I think I had good teachers.

> As to the calculations of the signal deviance, it has to be done in  
> the chip
> of the equipment, otherwise the PC cpu will be killes by overload of
> constant recalculations at each beacon, we don't want to do this,  
> current
> cards do the same majority of calculations are done inside the card,  
> and the
> output goes to driver, so we can see things like SNR, becone interval.

Your Intel CPU doesn't have enough floating point power to process the  
signals in a timely manner.   (And yes, I know that the 802.11 chips  
perform fixed-point arithmetic.)  Hell, your *nix system can't even  
generate the MAC-layer 802.11 ACKs with enough precision to comply to  
IEEE 802.11 specs, which is why they're commonly still done on the  
chipset via a finite state machine.

And, btw, the SNR you get from the card is almost always an  
approximation.

This thread has very little to do with m0n0wall.

Jim

> -----Original Message-----
> From: Lee Sharp [mailto:leesharp at hal dash pc dot org]
> Sent: Thursday, January 24, 2008 11:10 AM
> To: Monowall Support List
> Subject: Re: [m0n0wall] 802.11 draft N miniPCI cards for ALIX
>
> Jim Thompson wrote:
>
>> Note that the 5005VL isn't a MIMO chipset, its a beamforming chipset,
>> and Airgo != 802.11n.
>
> Seeing as how the standard still is not final, and they took out d 
> \some
> patent encumbered things that were in it, all preN stuff is !=  
> 802.11n.
> :)  What is all this fascination with N when everything falls bak to G
> from incompatibility?  It is very handy on the client side, but less  
> so
> on the AP side.
>
> 				Lee
>
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