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      05-04-2008, 01:23 PM   #1
kfiducia
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Technical Documents on Comfort Access

These are the FCC filings on both the comfort access unit and the key that goes with it.

A rough description of how it works is that an inductive 125 kHz signal is sent to the user's key through antennas in the interior and rear bumper, the response is sent using the 315 MHz transmitter in the key fob. The position of the antennas allows for location of the key based on signal strengths.




Below are links to the respective tech documents and pics. What inspired this research is an apparent defect in the functionality of the comfort access unit. Some users, including myself have been having problems, so I figured some more research would help people understand what goes on in these systems. What I can glean from this is that it would be possible to boost the transmitter power on the 125 kHz system to improve the responsiveness of the system. If the problem is within the response portion of the system, a higher gain antenna or more sensitive receiver could be implemented without affecting the battery life in the fob. Failing that, the transmitter power in the fob could be increased with obvious consequences. But at least you wouldn't be standing in the rain tugging on your door handle like a moron for 15 seconds before it opens.

Kyle

Comfort Access Block Diagram
Comfort Access Technical Description
Generic 125 kHz LF Unit Info

Comfort Access Fob Technical Doc
Fob Internals
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      05-04-2008, 04:12 PM   #2
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Interesting find (well, for the engineer in me at least).

From reading that it seems that if you're stuck outside tugging on the door handle it's most likely due to the door's 125kHz LF signal being a bit too weak.

The car sends a request to the CA fob using an LF signal when you touch the door handle, trunk lid or start button but the fob responds on the same RF transmitter that it uses for standard lock/unlock functionality at range. If pulling the door handle does nothing but pressing the fob unlock does then it's due to the fob not seeing the car's LF request.

If it was a case of the fob-to-car signal being too weak then the fob buttons wouldn't work and it would act like a dead fob battery.
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      05-05-2008, 03:21 PM   #3
kfiducia
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I disagree with that assertion, I don't think you can quite pinpoint the problem that easily. You have to account for system calculating the key position based on various signal strengths. If for some reason one of your antennas is not getting a strong signal from the fob in response to the ASK, it may be confused as to key position, or might disregard the response as noise. Unfortunately, we do not know the specifics of how it works, just the basics. But the system might be junking the response based on what it interprets as an "unknown position."

Which makes me wonder why the LF would not transmit a bit more information, such as position, rather than letting a more complicated system determine the position, like a series of antennas.
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      05-05-2008, 11:01 PM   #4
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From those pdfs there are 5 LF transmitters that can be turned on and off individually and a single RF transceiver. There may be additional RF receivers placed around the car but there is no mention of it in any of the documents.

Everything posted here points to the CA system sending requests to the FOB using those 5 LF antennas (not necessarily all at the same time) and receiving it's response from the fob using the same higher powered (relatively) RF receiver that receives the fob's button press indications from 20'-30' away. The determination of fob location appears to be made by fob either responding, not responding or responding with a signal strength to the LF signals (if the LF signal has a short-enough range the signal strength may not be needed).

I'm surprised there's no good info floating around on this. If you find a good breakdown where someone tore the system apart please post it here - I'm happy to be proven wrong.

This also may be of interest. The data-sheet for the controller:
http://pdf1.alldatasheet.com/datashe.../datasheet.pdf

Anyway, to anyone else not interested in the dirty details of CA... Just start ignoring my rants
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      05-06-2008, 12:32 PM   #5
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So how do we go about getting a stronger signal to help not look like an idiot? Is there anyone who has had success getting the strength of the signal up?
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      05-06-2008, 02:25 PM   #6
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awesome read
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      05-06-2008, 03:11 PM   #7
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Interesting.. I'm pretty sure that the 125KHz band is also used by RFID prox card readers to activate transmission on prox cards.

That's a very cool solution that prevents the fob from having to actively transmit except when prompted by the car... if it works like proximity cards, the LF signal induces a current in the fob which then tells it to wake up and transmit. In terms of hacking, perhaps it'd be possible to amplify that voltage on the fob side so it would be more sensitive to being activated by the car -- although that'd probably come at the expense of battery life.
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      05-06-2008, 04:29 PM   #8
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Has anybody else had issues with their CA after getting their windows tinted? Mine's intermittent now, sometimes not even recognizing the fob is in the car with me when I go to start it up.
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      05-06-2008, 05:08 PM   #9
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125kHz is a common freq for RFID, i originally thought the CA was RFID, thats why i went digging, i saw in the documentation that the system tries to determine the location of the key, lib, you are absolutely right, there is no indication of how, thats why i disagreed with the ease of the diagnostics you summarized. we are still in the same place, curious to know how. there are many possible methods but none clearly defined.

regarding the tint issue, i also have had my windows tinted, but using ceramic tint which should not interfere with the CA system, it would be rather frustrating if BMW did not plan for people to tint their windows. that being said, yes i do have a problem with my CA system working, i can't imagine what a fiasco that will be after having an outstanding issue with my lumbar since i bought the car with no hope in sight.
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      05-06-2008, 05:19 PM   #10
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Yes, I have Huper Optik ceramic tint as well...sometimes it works, sometimes it doesn't. Which really stinks, because, it's a really nice system and it worked perfectly for the first few days I had it without tint. The guys that installed the tint said the same thing, that ceramic films wouldn't interfere with the CA system. Hope somebody finds out a fix.
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      05-18-2009, 02:06 PM   #11
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So anybody figured out what causes the CA to stop working?
I happened to me after i had my windows tinted but I am sure its not about them blocking the signal as the same happens with the windows down.
What about the water used in the process of putting the window tint (It drips through the inside of the doors)
What kind of CA parts are inside the doors that could be affected?
Maybe the door handles don't know that you put your hand around them so no signal is sent through the antennas to the fob?
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