openofdm/docs/source/usrp.rst

Integration with USRP

was originally developed on Ettus Research USRP N210 platform. This short guide explains how to modify the USRP N210's FPGA code base to accommodate .

USRP N2x0 FPGA Overview

The top level model of USRP N2x0 (N200 and N210) can be found in top/N2x0/u2plus.v. It instantiates the u2plus_core module, which contains the core modules such as the receiver and transmit chain. In particular, the receive chain includes rx_frontend, ddc_chain and vita_rx_chain. Similarly, the transmit chain includes vita_tx_chain, duc_chain and tx_frontend.

The code base contains placeholder modules (dsp_rx_glue and dsp_tx_glue) for extension. These modules are controlled by Verilog compilation flags and by default they are simply pass-through and have no effect on the signal processing at all.

Enable Custom Modules

Take the receive chain as an example, inside dsp_rx_glue module, it checks the RX_DSP0_MODULE macro and instantiates it if found. The macro can be defined in a customized Makefile. Make a copy of the top/N2x0/Makefile.N210R4, name it to top/N2x0/Makefile.N210R4.custom. And then make these changes.

• Change BUILD_DIR to $(abspath build$(ISE)-N210R4-custom). This will create a new build directory for our custom build.
• Comment out CUSTOM_SRCS and CUSTOM_DEFS. We will define them in a separate Makefile.
• Find Verilog Macros and change it to "LVDS=1|RX_DSP0_MODULE=custom_dsp_rx|RX_DSP1_MODULE=custom_dsp_rx|TX_DSP0_MODULE=custom_dsp_tx|TX_DSP1_MODULE=custom_dsp_tx|FIFO_CTRL_NO_TIME=1". This defines the macros to so that the custom modules are instantiated by the glue modules mentioned earlier.

After these changes, the two modules in custom/custom_dsp_rx.v and custom/custom_dsp_tx.v will be instantiated. By default they are simply pass-through. For instance, the output of RF frontend are directly connnected to the input of DDC, and the output of DDC are directly connected to the VITA RX module.

To integrate , we only need to insert it after the DDC but before VITA RX module. That is, the sample_in/sample_in_strobe of the dot11 module should be connected to the ddc_out/ddc_out_strobe signal.

Also note that two receive chains are defined in u2plus_core module, so that the two antenna ports can be configured in TX/RX or RX/RX mode. To save FPGA resource, you may want to comment out one of the RX chains to make more room for .