I am thinking of developing a TDMA MAC + PHY using GNURadio for low speed communication (upto 200KBits per second) for prototyping. since OpenBTS has successively implemented a GSM BS using GNURadio, this seems to be a reasonable choice. but unlike OpenBTS, I want to avoid HW (clock changes) or FPGA programming and develop only on the GNURadio Platform.
recently I have seen the pre-cog (or EasyMAC) project presentation using the USRP N210: www.youtube.com/watch?v=pF_4dFQHAZE wiki: https://github.com/jmalsbury/pre-cog/wiki as I understood (and you are welcomed to correct me if I am wrong), UHD provides a timestamp (and tags) functionality for sending IQs and for receiving IQs, so all the latency and jitter problems will likely come from the host side (Drivers and GNU Radio application) since I do not need high speed data, the USB speed is more than enough. I do not know what limitations I will experience using this platform. I don't have any USRP devices on my hands yet, due to that fact I have a few questions regarding the use of USRP B100 (and not USRP N210): 1. What will be the expected latency using USB what will be the maximum jitter (unexpected latency)?, I have read the FAQ, but it is too theoretical, and the paper from 2007 could be out of date ( http://gnuradio.org/redmine/projects/gnuradio/wiki/UsrpFAQLatency ) 2. The USRP1 had accuracy problems with the clock when using OpenBTS, was this problem solved with USRP B100? in other words, could two or more USRPs work on a TDMA network without an external common clock? 3. what will be the minimum acceptable time slot duration on a system that does not require ACK mechanism? 4. on a system that requires ACK response what would be a minimum considerable time from a packet being sent until the ACK is received? 5. Are the timestamps for Rx and Tx on the same USRP correlated? 6. Are there any other considerations which can prevent EasyMAC or a similar implementation from working? 7. I know GNU Radio is not a true Real-time platform when running on linux General Purpose CPUs, how will this effect the TDMA? Thank you very much, and sorry for the long email :) Regards, Guy
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