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RiffMaster
Joined: 14 Feb 2005
Posts: 19
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 01 Apr 2006 16:36 DECT distance limit |
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I know that in GSM the maximum distance range between mobile and BS is 35km (delay reason).
Does anybody know what is the distance limit in DECT? (assuming that is enough power and antenna gain to maintain the link budget).
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zolam
Joined: 03 Oct 2005
Posts: 104
Helped: 9
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| 01 Apr 2006 17:11 Re: DECT distance limit |
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Hi,
The comparison is nt quite rellevant because we are talking about two different aproaches:
The GSM is a cells-based network, with roaming, hopping and covering area - all nationalwide calculated by the service provider.
The DECT equipment is mainly a SOHO (home) based one. It is quite individually destinated.
The ETSI and other general standards in countries where regulation and safety policy exists, limit the power to 100mW, so generally, the maximum cover radius declared by the firms is 50 meters inside and 300 meters outside.
It takes into account different obstacles as armed beton walls or other "Faraday" nets, reflecting surfaces and so on.
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RiffMaster
Joined: 14 Feb 2005
Posts: 19
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| 01 Apr 2006 21:39 Re: DECT distance limit |
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For what I am looking, the comparison between GSM and DECT is relevant.
I have to explain a little bit more from where is coming the 35km limit in GSM. There is a procedure named “timing advance” used to correct the delay between mobile and BS. This TA has states from 0 to 63. The bit duration is 3.69us. The maximum delay can be corrected is 232.5us (63 x 3.69us). This maximum double delay corresponds to distance of approximately 34.9 km. [(300 x 232.5)/2] = 34875 meters
So the limit for distance between mobile and BS doesn’t have anything with RF performances, or ETSI specification.
What I am asking about DECT is not related to any standard or RF performances (RF power, sensitivity or antenna gain). I am looking to find the distance limit related to the TDMA approach used by DECT system (delay or something like this).
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Buriedcode
Joined: 06 May 2004
Posts: 316
Helped: 34
Location: London
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| 01 Apr 2006 22:34 Re: DECT distance limit |
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Hi RiffMaster (nice name btw)
You question is indeed relavent, even though, the actual range is indeed limited by Tx power (FCC regulations) and channel quality (multipath, noise etc..).
As you said, you're assuming that these are not of concern...you simply wish to know the maximmum range of DECT based on propagation delay ie: data being transmitted too far out of time-slot, beyond the correction capabilities of timing advance. Correct?
Well, from what I know of GSM...you just need to know the time-slot period, the 'guard period'. If the information is sent with a delay greater than the guard period, it will over-lap with the data in the next time-slot - ergo - goes south.
I know literally nothing about DECT, but a few seconds on google, I got the frame structure. Also, I read somewhere that its 'flexible', so I guess this can be set by the network.
Frame period = 5ms.
12 slots in each frame = 5/12 = 417us per slot.
Guard space, 56 bits = 49us.
Because the phone receives data delayed (because of distance)....and then sends some data back (with the same distance/delay) the maximum propagation delay is approx half the guard period = 49us/2 = 24.5us. Assuming a straight RF path, and that the signal travels at C (which it doesn't, very close though  )
Distance = speed *time = 3*10^8 * 24.5*10^-6 = 7350m = 7.35Km. Max distance, before timing advance. Couldn't find what the maximum timing advance is, but I think its 63 bits for Gsm, if its similar for DECT, it would probably double that range.
Anyway, I'm probably way off base....this is like being back in University
BuriedCode.
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RiffMaster
Joined: 14 Feb 2005
Posts: 19
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| 02 Apr 2006 9:47 Re: DECT distance limit |
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Hi BuriedCode,
My nickname is just a tribute to the real RiffMaster that was only one.
You, as an Englishman, have to know whom I am talking about
I found this DECT spec:
http://www.openhardware.ru/files/standartes/DECT/en_30017508v010700o.pdf
7.9 Delay
7.9.1 DECT network
The sum of the delays from the MRP to the digital line interface and from the digital line interface to the ERP (round-trip delay) shall not exceed 28,5 ms. If an analogue line interface is provided, the delay shall not exceed 29 ms including the A/D and D/A converters at the interface to the external network.
7.9.2 PP
The sum of the delays from the MRP to the air interface and from the air interface to the ERP (round-trip delay) shall not exceed 19,5 ms. This value includes the 5 ms delay of the reference FP looping back the ADPCM digital signal towards the PP.
Comparing 19.5ms (DECT) with 232.5us (GSM) means that the distance in DECT could be much higher than you said.
From my previous post you can find that in GSM the limitation is dictated by the TA limit of 63. I don’t know if in DECT exists something similar with this. According to the spec they are looking for a different kind of delay.
Cheers
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