Hi,
 I have written the minstrel rat control algorithm, which reports the 
percentage of lost packets at each of the available data rates.

It shows similar results to you.

On incredibly strong links, (receiver/transmitter adjacent) all rates work 
perfect.

As the link distance increases, the ofdm rates drop off, and fail. All 
that works is the 11b rates.

Admittedly, the ofdm rates are a different encoding technology, and will 
be more adversely affected by noise/interference. Maybe. But there 
are many 11g cards that do heaps better than atheros cards, as your 
report  below illustrates.

=============================================================

quoted text
> Any idea how we can get better receive sensitivity ?
Yes.

In a recent letter to this list from  Nick Kossifidis, the issue of 
calibration was raised. Currently, the ath_calibrate() method is called by 
a timer (every 30 seconds if the last calibration works, or every second 
if the last calibration fails). Nick was suggesting the need to drive the 
ath_calibrate method from the rate control algorithm. 

On rereading your letter, and Nick's letter, and some offline 
correspondence with Nick, I am wondering if there is a need for the 
ath_calibrate method to be run on changing to an ofdm rate, or run more 
frequently, or something. 

Nick was also suggesting removing the noise floor calculation code from 
the ath_calibrate method, so the two functions are executed at quite 
different times. 

To resolve these questions, I would really appreciate some input from 
those more knowledgeable in the area of inner workings of the 
hal,  OR
 suggested experiments..

Derek.



On Fri, 21 Sep 2007, Roman Skrobotz wrote:

quoted text
> On a Wistron CM9-GP Mini PCI card I did some Sensitivity measurements using a calibrated Rohde & Schwarz RF-test system.
> 
> I tested three different configurations:
> 
> a) Host: IBM R40 ThinkPad, Windows XP, Driver 4.12.38 (1/04/2005)
> 
> b) madwifi 0.9.3 svn r2100 with Linux 2.4.20
> 
> c) madwifi 0.9.3.2
> 
> Results:
> a) On the windows xp system the card shows an excellent receiver sensitivity which is far better than -90dBm on the low rates in both, 11b and 11g modes.
> 
> b) Running svn r2100 the sensitivity in 11b-Mode is also excellent. I have measured down to -98dBm at PER 8% using 1Mbit/s.
> But the behaviour in all 11g OFDM-rates is really strange: 
> It seems that the receiver refuses operation at signal levels below -77dBm.
> Characteristical for all WLAN-receivers is that the Packet Error Rate rises when the RF-level is about 3dB to 5 dB higher then the sensitivity limit.
> The typical limit is better than -90dBm for the lowest 11g OFDM-Rate 6Mbit/s.
> Interesting thing is that in this configuration the receiver suddenly looses about 70% of all packets at -77dBm RF-Level without having CRC-errors in RX-packets, so while really having a PER =0%, on all 11g-rates.
> (54Mbit/s excepted from this observation, cause it reaches its sensitivity limit typical at about -75dBm +- 2dB anyway)
> And there is a Hysteresis in the dependance on the RF-Level:
> Once the receiver is in the mode of loosing >70% of all packets, it won't stop loosing packets until the level is increased for at least 10dB (to -67dBm) for at least one second.
> The receiver starts to work properly again.
> Then the level can be slowly decreased again to -76dBm and nothing changes, all packets are received.
> When -77dBm are reached, the receiver looses >70% of all packets again - this procedure can be repeated as often you want.
> That means that this limit is not reached because of physical reception problems, the receiver has not reached his limit by far!
> It seems to be a configuration problem of the driver initialising the card.
> And it is obviously only dependant on the RSSI-Level and not on the quality of the radio reception...
> 
> c) We tried the 0.9.3.2 and the result was the same as seen in b) with one significant difference:
> The hysteresis behaviour was gone. There was one hard threshold: below -77dBm RF-level still >70% of packets were lost at 0% PER.
> But increasing the RF-Level of about 1..2dB, all packets were received again.
> For low 11g-Rates, the user will see a reduction of the expected coverage range down to about 1/10 of what is known as usual with this technology.
> 
> Any idea how we can get better receive sensitivity ?
> 
> Regards,
>   Roman 
> 
>  
> 
> 

-- 
Derek Smithies Ph.D.
IndraNet Technologies Ltd.
Email: derek@indranet.co.nz
ph +64 3 365 6485
Web: http://www.indranet-technologies.com/

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