From: Thomas T. <tt...@vt...> - 2011-08-08 23:18:57
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On Sun, Aug 7, 2011 at 9:56 AM, Nghia Phan <ngh...@gm...> wrote: > At the radio burst level, what is the bandwith requirement? 270.8kbps > (156.25bits every 577us) for the burst itself + ?? for the timestamp? On how > many bits is the timestamp (time of transmission) coded? Is there anything > else being transmitted to the USRP? Beyond the sample rate requirement, total transport bandwidth to the device is difficult to measure and not particularly useful in most cases. OpenBTS treats the timestamp as 64-bit sample count. The over-the-wire format depends on implementation - UHD uses a VITA-49 format of 32-bit seconds and a second field for ticks. Anything else sent to the device is, again, implementation dependent but might include things like UDP headers, streaming commands, and other control information. > On the receive side, beside the soft symbols (156.25 bits every 577us also I > guess?) on how many bits are the RSSI, receive timing error and timestamp > coded? Is there anything else being received from the USRP? Of the above, only the timestamp is received from the device. Other variables are calculated and stored as 32 or 64-bit floating point values. > Also can someone explain the sync mechanism between the software clock > (within GSM core within OpenBTS SW) and the “hardware clock” on the USRP? The GSM clock doesn't care about samples; it's based on slots and frames. To represent slots and frames, OpenBTS needs to track samples and maintain a constant offset between transmit and receive streams. Sample timestamps are used for this purpose. > What are the differences between the different clocks mentioned in the doc: > GSM frame clock, GSM master clock, USRP tags, GSM symbol clock (=USRP sample > clock),..? The frame clock counts frames. The master clock counts frames and timeslots. The symbol clock counts symbols. The 52MHz transceiver uses 1 sample-per-symbol at the USRP interface. The non-52MHz transceiver resamples the signal so symbol and sample counts will differ. Thomas |