Add readme.txt for USRP

git-svn-id: http://svn.eurecom.fr/openair4G/trunk@4192 818b1a75-f10b-46b9-bf7c-635c3b92a50f

targets/ARCH/USRP/USERSPACE/LIB/README.txt

+Now we are using USRP N200/210 and working with OAI v4164.
+
+** Get source code of UHD
+git clone git://github.com/EttusResearch/uhd.git
+
+** Install prerequisites
+sudo apt-get install libboost-all-dev libusb-1.0-0-dev python-cheetah doxygen python-docutils
+
+** Install UHD
+cd <uhd-repo-path>/host
+mkdir build
+cd build
+cmake ../
+make
+make test
+sudo make install
+sudo ldconfig
+
+** Check the speed of USRP
+The results should have almost no overflows and underflows, otherwise the OAI cannot work in realtime. 
+./benchmark_rate --tx_rate 6.25e6 --rx_rate 6.25e6 --duration 30
+
+** Build lte-softmodem-usrp
+cd targets/RTAI/USER
+./make_for_usrp.sh
+
+** Run lte-softmodem-usrp
+./lte-softmodem-usrp