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rfnoc_atsc_rx.lwr

README.md

RFNoC-HLS-ATSC-RX

Andrew Valenzuela Lanez | andrew.lanez@navy.mil
Sachin Bharadwaj Sundramurthy | sbharad@eng.ucsd.edu
Alireza Khodamoradi | alirezak@eng.ucsd.edu

About

This repo contains the RFNoC atsc_rx OOT module for GNU Radio. Its blocks are to be implemented onto the FPGA of an RFNoC-suppported USRP (e.g., E3xx series or X3xx series) using RFNoC and Xilinx Vivado Design Suite 2015.4. They can plug-and-play into the GNU Radio gr-dtv ATSC receiver example and process an ATSC signal into a playable video file. Real time playback of a live ATSC signal processed through the gr-dtv ATSC receiver is possible on high-performance computers but not on most commodity computers. This makes ATSC receiver blocks ideal candidates for porting into RFNoC. Computation intensive tasks can be offloaded to FPGA logic while applying high-level synthesis optimization techniques to improve receiver throughput. This can bring GNU Radio ever closer to achieving real time ATSC playback on a typical commodity computer.

Example flowgraph with atsc_rx RFNoC blocks:

ex1

Screenshot of video from a live ATSC signal processed through atsc_rx RFNoC blocks:

dog

The atsc_rx module includes these blocks:

  • RFNoC: ATSC RX Filter
  • RFNoC: ATSC Receiver FPLL
  • RFNoC: DC Blocker
  • RFNoC: AGC
  • RFNoC: ATSC Viterbi Decoder
  • RFNoC: ATSC Deinterleaver
  • RFNoC: ATSC Reed-Solomon Decoder
  • RFNoC: ATSC Depad
  • RFNoC: ATSC RX Filter-FPLL
  • RFNoC: DC Blocker-AGC

All blocks above were built with these integrated features:

  • Vivado HLS Source & Testbench
  • HDL Testbench
  • FPGA Integration
  • UHD Integration
  • GNU Radio Integration

Directories

Here are descriptions of the top-level directories:

Folder Contents
rfnoc RFNoC main source tree files needed to build all blocks that have been verified operational on hardware and in GNU Radio. The src/rfnoc-atsc_rx subfolder contains files needed for UHD and GNU Radio integration. The src/uhd-fpga/usrp3 subfolder contains files needed for FPGA integration.
hls_patch Patch provided by the RFNoC dev team for image builder to detect various source files needed. Patch is needed for the current RFNoC release as of 7 July 2017.
blocks_in_progress Nearly completed blocks either more optimized for throughput or have more features than their counterparts in the main source tree but need timing, interface, or hardware performance issues resolved.

Noteworthy subdirectories:

Subfolder Contents
hls C++ source and testbench files for Vivado HLS.
testbenches SystemVerilog HDL testbench directories.
examples USRP X310 HG .bit FPGA image file of some blocks (not all blocks fit in a single image due to CE count, not area utilization) and associated GRC flowgraph.

Build

Assuming RFNoC has been setup (Getting Started with RFNoC Development covers all steps needed to build anything RFNoC in general), here are some extra build pointers:

Setup OOT Module and Blocks

Initialize the module:

$ cd {USER_PREFIX}/src
$ rfnocmodtool newmod atsc_rx
$ cd rfnoc-atsc_rx

Initialize any or all of the following blocks. Press Enter four times between each entry to accept default values:

$ rfnocmodtool add rxfilt
$ rfnocmodtool add fpll
$ rfnocmodtool add dcr
$ rfnocmodtool add agc
$ rfnocmodtool add viterbi
$ rfnocmodtool add deint
$ rfnocmodtool add rsdec
$ rfnocmodtool add depad
$ rfnocmodtool add fltpll
$ rfnocmodtool add dcragc

Copy/paste/replace/merge all contents of rfnoc into the local {USER_PREFIX} RFNoC development directory to populate files needed throughout the tree for GNU Radio, UHD, and FPGA integration.

GNU Radio and UHD integration

Install the files that were just copied:

$ mkdir build
$ cd build
$ cmake ..
$ make
$ sudo make install

RFNoC: FIFO blocks can be updated with more data type options to connect with atsc_rx blocks in GNU Radio Companion. Replace uhd_rfnoc_fifo.xml into {USER_PREFIX}/src/gr-ettus/grc then:

$ cd {USER_PREFIX}/src/gr-ettus/build
$ make
$ sudo make install
HDL Testbench

The RTL Export feature in Vivado HLS 2015.4 (user guide) can be used to first generate Verilog and Xilinx IP source files (*.v, *.dat and/or XCI folders) of a block. Paths to those files need to be assigned to the SIM_SRCS variable in the respective {USER_PREFIX}/src/rfnoc-atsc_rx/rfnoc/testbenches/noc_block_{NAME}_tb/Makefile of a block. Then run the testbench from the build directory:

$ cd {USER_PREFIX}/src/rfnoc-atsc_rx/build
$ make test_tb
$ make noc_block_[NAME]_tb

Note: make test_tb only needs to be run once for the atsc_rx OOT module, not per RFNoC block.

FPGA Integration

Note: Running Vivado HLS > RTL Export > Evaluate > Verilog can save time by predicting whether the design will meet timing rather than committing mutiple hours to build the image and then finding out. The following files specify which blocks are to be built into the FPGA. They are already populated with specified blocks for reference and can be modified to build a different combination of blocks as desired. Specified blocks must match among all files.

{USER_PREFIX}/src/rfnoc-atsc_rx/rfnoc/Makefile.srcs
{USER_PREFIX}/src/uhd-fpga/usrp3/lib/hls/Makefile.inc
{USER_PREFIX}/src/uhd-fpga/usrp3/top/x300/Makefile.srcs (or Makefile.OOT.inc in newer RFNoC releases)
{USER_PREFIX}/src/uhd-fpga/usrp3/top/x300/rfnoc_ce_auto_inst_x310.v

The last file assumes the image is for a USRP X310. Modify the appropriate file respective the USRP being used. Then build the image:

$ cd {USER_PREFIX}/src/uhd-fpga/usrp3/top/x300
$ make X310_RFNOC_HLS_HG

The last command assumes a USRP X310 and 1GigE Port0/10GigE Port1. Use a make command appropriate to the USRP and Ethernet port type being used. Use make cleanall if IP needs to be cleaned before a new build.

Design Considerations

The decision on which blocks to port into RFNoC hinged on two factors:

Frontend proximity: Porting frontend blocks from software into hardware increases deterministic processing before the datastream falls under the whim of an operating system scheduler. RX Filter, FPLL, DC Blocker, and AGC were selected. RX Filter minimizes unwanted ISI (intersymbol interference) then oversamples and interpolates the signal, FPLL (frequency and phase locked loop) is used for carrier acquisition, DC Blocker removes unwanted DC components, and AGC (automatic gain control) adjusts amplitudes to a reference value within a desired range.

Bottlenecks: Blocks that have higher consumption of runtime resources or cause buffers to fill are ideal candidates for porting. ControlPort Performance Monitor shows runtime consumption is primarily from Viterbi Decoder and secondarily from Reed-Solomon Decoder and top buffer consumption is from RX Filter:

gr-runtime

gr-buffers

Deinterleaver was selected to close the link between the Viterbi (or trellis) and Reed-Solomon decoders. Those blocks undo forward error correction encoded by the transmitter. Depad was targeted for its simplicity and to be tried as the first block to complete the RFNoC workflow. It strips extraneous bytes leaving an MPEG video file as the final output. All targeted blocks from the gr-dtv library are boxed out in red:

gr-dtv

The following flowgraph shows 6 of 10 blocks implemented in RFNoC. RX Filter was combined with FPLL and DC Blocker was combined with AGC at the HLS source level so that more CEs (Computation Engines) could fit in one FPGA image (combined area utilization of all blocks was not an issue). Throttle compensates for a rate mismatch (an action item for future iterations and discussed later).

grc

Two key parameters in the gr-dtv implementation of the receiver that drive sample rate requirements for all blocks are the 6.25 MHz sample rate coming out of the DDC (digital down coverter) at the receiver frontend and the oversampling ratio of the first block, RX Filter. It was decided that the 6.25 MHz input rate should not be modified to pass in a 6 MHz bandwidth ATSC channel:

spectrum

Reasonable oversampling ratios were found to range between 1.1 to 2 for the gr-dtv ATSC receiver software implementation to accumulate enough data to output video for playback. For an initial pass at implementing on hardware, it was decided to define target sampling rates based on the oversampling ratio of 1.1 to make implementation less constrained. Then, time permitting, iterate from there by increasing target rates and fine tune sample rates to match between blocks.

Implementation Notes

Workflow in the RFNoC framework is already well documented at Getting Started with RFNoC Development. The following focuses on implementation using Vivado HLS 2015.4 shown in the top part of this high-level workflow followed by results:

workflow

Developing HLS Source

Signal processing source files were written in C++ to be synthesizable. Vivado HLS optimization directives such as #pragma HLS PIPELINE, #pragma HLS UNROLL, #pragma HLS RESOURCE, and #pragma HLS ARRAY_PARTITION were used, among others. The tradeoff between optimizing to increase throughput versus optimizing to reduce utilization were kept in consideration. Synthesizability was checked using csynth_design (C Synthesis) and timing could be checked using export_design (RTL Export) with the Evaluate Verilog option enabled (though this last step can be time consuming and better left as a final step before FPGA integration in RFNoC).

Developing HLS Testbench

A testbench was written in C++ to send input data to the DUT and compare the output against a golden output using csim (C Simulation) in the C++ domain. The golden output was captured as a binary file using File Sink on the output of the counterpart or reference block in GNU Radio. Input was captured in a similar fashion with the original input source being a live ATSC signal fed from UHD: USRP Source as shown below. In the testbench, multiple function calls to the DUT per test run is encouraged to check the boundaries between returned output data sets and accumulate initiation interval statistics.

inout

If csim showed passing results and csynth_design showed the block to be synthesizable, then cosim (C/RTL Cosimulation) was run to translate the C++ code into RTL (Verilog, VHDL, and/or SystemC) and apply the testbench input stimuli and output checking in the RTL domain. To synthesize the input port of the RX Filter block, for example, into the AXI Stream interface used in RFNoC, the #pragma HLS INTERFACE axis depth=64 port=in was used on the top level function of the block. The depth parameter has no bearing on synthesis. Instead, it is a control parameter for the cosim testbench to know how to size its input FIFO so it matches the RX Filter input array size which does have bearing on synthesis.

Iterating Between HLS and RFNoC

After a block passed cosim and met timing in export_design with the Evaluate Verilog option enabled, it was ready to test against the RFNoC HDL testbench. The C++ source files fed into export_design got converted into Verilog and Xilinx XCI IP source files. Those output files were used as the DUT in the RFNoC HDL testbench. The binary input and golden output files used in HLS were converted to ASCII representations using MATLAB (Python would have worked as well) which were then used as input and golden output array variables in the SystemVerilog HDL testbench. If the DUT had bugs revealed by the HDL testbench or by running its FPGA implementation in GNU Radio, it was debugged in HLS or HDL testbench, re-packaged with export_design, then retested. This process was repeated until the RFNoC block implementation functioned as desired in GNU Radio.

Results

The following table shows current block output rates ("v2" blocks discussed later):

rates

RFNoC block utilization area reported by Vivado HLS C Synthesis in terms of units and percentage of block versions verified to function on hardware (none of these are "v2" blocks):

area

percentarea

A small oddity was found while testing the RFNoC: ATSC Receiver FPLL block in GNU Radio. All samples were being output with a gain of 3,2767.00480546441 (almost looks like a 15-bit shift left operation) applied to their expected values. This gain did not manifest when running the HLS testbench nor the HDL testbench. The source of this mysterious gain has not yet been found. To resolve this, the FPLL HLS source file was modified to simply divide all outputs by that gain. Of course, the HLS and HDL testbenches had to be updated to multiply that gain back onto the outputs before checking them.

In Progress

Settings Bus

Settings bus implementation is required to make parameters such as the oversampling ratio in RX Filter or delay line length in DC Blocker programmable. Provisions for the settings register bus were implemented in HLS source, HLS testbench, NoC block, HDL testbench, and GNU Radio and UHD integration. It was questionable whether the settings bus ports were getting synthesized correctly from HLS source files. Documentation for synthesizing the settings bus signals from Vivado HLS could not be found. The #pragma HLS INTERFACE ap_stable and #pragma HLS INTERFACE ap_none directives were experimented with to attempt synthesis of the set_addr, set_data, and set_stb settings bus ports from HLS source files. They are data port interface directives, however, they have no associated I/O protocol. Regardless, an FPGA image was made and tested in GNU Radio and the settings bus did not function as desired. Values were not observed to be propagating to the settings registers in Vivado Simulator:

xsim

Higher Throughput

As mentioned earlier, target sample rates were not met so there is room for improvement. A "v2" of the RX Filter, DC Blocker, and AGC blocks being worked on are more highly optimized but fail timing so they are not in a state to run on hardware. Reed-Solomon v2 passed all HLS timing and utilization checks but functioned erratically on hardware even when no critical warnings were reported from the build.

Current implementations and more details on the blocks discussed above are in blocks_in_progress.

More Blocks

The X310 10 CE limit was reached. Though there are tricks around this (such as what was accomplished by combining blocks at the HLS level) it would be nice if future iterations of RFNoC can support more CEs in a single X310 FPGA image. Then the entire ATSC receiver can be ported into RFNoC! With less hardware-to-software FIFOs, real time playback (the ultimate goal of this concept) would be in closer reach.

Conclusion

A shotgun approach to this project was used by selecting many blocks to port into RFNoC with hopes of hitting the bullseye. With Viterbi being the largest resource consumer and bottleneck and its RFNoC implementation working and meeting the target throughput, it can be considered that a bullseye was hit. Multiple other blocks struck right around the mark. Simpler blocks such as Deinterleaver and Depad far surpassed target sample rates and only came about to be developed during times when little to no progress could be made on more challenging blocks such as RX Filter, DC Blocker, RS Decoder, and Viterbi. This approach may have been inefficient at times with efforts being spread thin across many blocks. It is uncertain whether a more focused, sniper approach on fewer blocks would have yielded better results.

It was realized partway through the project that real time playback was an ambitious stretch goal. Although real time playback was not achieved in this iteration of development, HLS optimizations made it possible for several blocks to meet their respective targets and for all blocks to process data into playable video.

Tip: Instantiating many CEs increases the chance of critical warnings on timing or erratic behavior. When testing all blocks, it would be better to evenly distribute CE instances among 2 or 3 images (3 or 4 CEs per image) instead of maximizing the number of CE instances per image.