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Data problems on fast moving mobiles

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  • #65652 Reply
    Jason
    Guest

    What will keep a fast moving mobile from having high RLC layer retransmissions and give it low effective throughput?
    This seems to be effecting 1900 tremendously but also a bit in 850. This is on our Nortel Network.

    #65653 Reply
    pix
    Guest

    hi jason,

    are you sure that the problem comes from the speed of the MS, and not from the fact that the frequencies are interfered ?
    – try to stop the car when you see this problem, and check whether it is still happening.

    how fast is your going ?

    Are we talking about EGPRS (MCS1-9) or GPRS (CS1-4) ? Which coding scheme is used during the problem ?
    I’ve read about the doppler effect, but it seems that it doesn’t have any impact on GSM. The example was at 100km/h, it generated a frequency offset of 90Hz… 90Hz compared to 200kHz… it’s neglictible.
    So I would really focus on an interference problem rather than a speed problem… but maybe I’m wrong.

    Regards,
    pix

    #65654 Reply
    Jason
    Guest

    Hi Pix,
    Thanks for looking at this. We have tested this by using a hand held Tems test phone. While driving above 35 mph (56 kph) our perceived throughput drops significantly and above 50 or 60 mph (100 kph) throughput is virtually zero. Once we drop below 35 mph it returns to an acceptable level. We have tested this up to 80 mph. When watching the TEMS phone we have observed that RLC retransmissions are for the most part 100% but the RLC BER stays fairly close to 0%. When stopped RLC retransmissions are around 50% average. LLC layer retransmissions and BER are almost always 0% with very rare exceptions, in any speed conditions.
    It seems to effect both EGPRS and GPRS, but from looking at the drive test, when stopped we are using EGPRS MCS-9, when moving fast we are on GPRS CS-1 and once dropping below about 35 mph we see several coding schemes in between the two extremes. On occasion the coding scheme will jump from CS-1 to MCS-5 while driving 75 mph, but it lasts for only a second or so.
    Currently we have no reason to believe that this would be a interference issue, the area has been scanned as well as the site itself for interference and the spectrum looks very clean. As a side note the same area where it works well when stopped, it does not work when driving through, we have tried this in both directions. We did find a parm for Nortel to help fast moving mobiles but I think this was for ‘bullet trains’ it had no effect on our tests.

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