Thursday, December 19, 2013

openBTS : use USRP-1 or not ?

Please note that using USRP-1 for use with OpenBTS can be done but there
are a number of disadvantages. A USRP-1 needs a number of modifications to be used with an external clock. This means moving and replacing (soldering) a number of small SMD components, creating a powersupply for the clocktamer and adding a clock input connector. Note that the USRP1 FAN output cannot supply enough power for a clocktamer without modifications. This modifications can be done by an experienced electronics engineer. You whould not have these
performed by someone who does not have experience in SMD soldering. Also note that a USRP-1 cannot use the standard UHD driver when working
with OpenBTS. It needs a gnuradio based driver. Support for this driver has stopped a long time ago, so you must patch and compile an old gnuradio version which you have to modify yourself for 52 Mhz use. That means that getting OpenBTS to run with a USRP-1 is quite complicated and a non standard procedure.

To get OpenBTS to run with a modified USRP-1 with clocktamer: Start with a linux system. You would then have to get the sourcecode of a very old gnuradio release (3.4.2 or older), patch/modify this source for default 52 Mhz use. Compile and install it  (You cannot use binary packages) and then compile and install all other required packages, finishing with compiling and installing OpenBTS using the right configure/compile options. After that it should work similar to working with modern N210 based systems.

Note that USRP-1 based OpenBTS systems are less reliable. The FPGA inside a USRP-1 is not fast enough to always reach the timing required for OpenBTS. Most of the time it works, but you may get occasional synchronisation /timing errors. Especially when the temperature is high.

With USRP-2 or USRP N210 bases systems you don't have these issues since
the FPGA inside them is much faster. They use gBit ethernet for communication with the host-PC which has much higher speed and lower latency compared to USB (used with USRP1).

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