This document discusses implementing a low probability of intercept (LPI) radio transmitter using GNU Radio and a USRP radio hardware platform. It provides an overview of the system architecture, describing how GNU Radio software interfaces with the USRP hardware. It then outlines the DSP design flow, including writing custom signal processing blocks in C++. Finally, it details the implementation of the LPI radio transmitter, describing blocks for Manchester encoding, amplitude modulation using oscillators, and configuration of the USRP for transmission.

1. GNU Radio Exploring An implementation of LPI radioYuan Wangyuwang@ucsd.edu09/17/2009

2. AgendaA shortcut to GNU Radio

3. Hardware Platform - USRP

4. DSP Design Flow of GNU radio

5. Write your own blocks

6. Implementation of LPI radio transmitter2

7. System ArchitectureHardware FrontendUSRPHost ComputerRF Frontend(Daugtherboard)ADC/DAC andDigital Frontend(Mothermoard)GNU RadioSoftwareUSB2.0Your code goes here !http://mobiledevices.kom.aau.dk/fileadmin/mobiledevices/teaching/software_testing/Gnu_radio_lecture.pdf

8. System Architecture (Cont.)Software CoreKeep in mind: GNU radio has provided some useful APIs for DSP purposeWhat we are interested in is how to use these existing modules that have been provided in GNU radio to communicate between two end systemsHost ComputerDSPUSBHardware Frontend USRPRX/TXDaughterboardADC/DACFPGAUSBInterfacehttp://mobiledevices.kom.aau.dk/fileadmin/mobiledevices/teaching/software_testing/Gnu_radio_lecture.pdf

9. A shortcut to GNU RadioUSRP (Universal Software Radio Peripheral) Motherboard Up/Down converting, AD/DA converting, USB 2.0 interface FIR filter Four 64 MS/s 12-bit ADC

10. Four 128 MS/s 14-bit DAC

11. Four DDC with programmable decimation rates

12. Two DUC with programmable interpolation rates

13. High-speed USB 2.0 interface (480 Mb/s)

14. Modular architecture supports wide variety of RFdaughterboards Auxiliary analog and digital I/O support complex radio controls such as RSSI and AGCFully coherent multi-channel systemsPicture from www.ettus.com

15. A shortcut to GNU Radio (Cont.)USRP (Universal Software Radio Peripheral) daughterboard Power Amplifier, Antenna, etc. DC to 30 MHz receiver/transmitter1 MHz to 250 MHz receiver/transmitter50 to 860 MHz receiver800 MHz to 2.4 GHz receiver750-1050 MHz transceiver1150-1450 MHz transceiver1.5-2.1 GHz transceiver2.3-2.9 GHz transceiver (RFX2400 Using now)50 MHz to 1 GHz transceiver800 MHz to 2.2 GHztransceiver2.4 GHz and 5 GHz dualbandtransceiverPicture from www.ettus.com

16. USRP Block DiagramPicture from gnuradio.org

17. AD9862 with DUC (Tx.)Picture from gnuradio.org

18. DDC in FPGA (Rx.)Picture from gnuradio.org

19. GNU Radio Software CoreWrite signal Processing Block in C++

20. Build signal Flow graph with Python

21. Object Oriented Programming10

22. Base Class of GNU radio blockThe prototype of GNU radio block1) gr_sync_block()37101679

23. Base Class of GNU radio blockThe prototype of GNU radio block2) gr_interpolator_block() 371610101679

24. Basics: Data StreamsThe prototype of GNU radio block1) gr_decimator_block() 447710166996

25. GNU Radio Companion (GUI)

26. Build the DSP flow graph V2#!/usr/bin/env pythonfrom gnuradio import grfrom gnuradio import audiodef build_graph ():sampling_freq = 48000ampl = 0.1fg = gr.flow_graph () src0 = gr.sig_source_f (sampling_freq, gr.GR_SIN_WAVE, 350, ampl) src1 = gr.sig_source_f (sampling_freq, gr.GR_SIN_WAVE, 440, ampl)dst = audio.sink (sampling_freq)fg.connect ((src0, 0), (dst, 0))fg.connect ((src1, 0), (dst, 1)) return fgif __name__ == '__main__':fg = build_graph ()fg.start ()raw_input ('Press Enter to quit: ')fg.stop ()C++C++C++V1My APIAPIsV2C++C++C++V1My APIPython Flow graph

27. Advanced Topic: Write your own blocksEvery block(everything) is a C++ class typically derived from gr_block or its derived class.

28. Three components1. calit2_manchester_ff.h: Block statement2. calit2_manchester_ff.cc: Block implementation3. calit2.i: SWIG interface4. Other stuffs: Makefile.am, Makefile.swig.gen, testbenchPythonApplication developmentFlow graph constructionC++Signal processing blocksSchedulerControl flow graph

29. GNU Radio DevelopingOpensource software

30. Existing Projects: 802.11b, UCLA Zigbee, ATSC (HDTV), OFDM, DBPSK, DQPSK

31. CGRAN (Comprehensive GNU Radio Archive Network)

32. Features

33. Extensive library of signal processing blocks(C++)

34. Python environment for composing blocks (i.e. DSP flow graph)Implementaion of LPI radio Block DiagramAudio SourcePacked to Unpacked*32767Float to short0.32756short0x3E011,0,0,1,1,0,1Chunks to symbolsAmplitude Modulation on differentsub-carriers.1,-1,-1,1,1,-1,1USRPInterleave to N streamsSUMManchester Encoding…To hardware USRP board

35. Implementation of LPI radio (cont.) Manchester Encoder+1-11001if(in[i/16] > 0.0) {out1 = 1.0;out2 = 0.0;}else {out1 = 0.0;out2 = 1.0;}//create manchester output and upsample by 8for(int j = 0; j<8; j++){memcpy(&out[i+j], &out1, sizeof(float));}for (int j = 8; j<16; j++){memcpy(&out[i+j], &out2, sizeof(float));}

36. Implementation of LPI radio (cont.) Second order oscillator and AMA = 1; B = -Ω2 * dtd(x+t) = dx+dv*dt;d(v+t) = A*dv + B*dx;dvdx

37. Implementation of LPI radio (cont.) Sample source code // for different items on the streamsfor (inti = 0; i < noutput_items; i++){float *out = (float *) output_items[0]; float temp_sum = 0.0; //clean temp_sum for next item processingfor (unsigned int m=0; m < d_ncutoff; m++) // processing on different streams{const float *in = (float *) input_items[m]; d_sine[m].d_X = d_sine[m].d_X + d_sine[m].d_V*d_t;d_sine[m].d_V = d_sine[m].d_A*d_sine[m].d_V + d_sine[m].d_B*d_sine[m].d_X;temp_sum += d_sine[m].d_X*in[i]; // Amplitude Modulation Here} memcpy(&out[i], &temp_sum, sizeof(float)); // end of per item processing}

38. Implementation of LPI radio (cont.) USRP sink configuration#settings of USRPself.dac_rate = self.u.dac_rate() ## 128MS/sself.u.set_interp_rate(usrp_interp) ## Set interpolation ratetx_subdev_spec = usrp.pick_tx_subdevice(self.u) ## Locate daughter board(s)m = usrp.determine_tx_mux_value(self.u, tx_subdev_spec) ## Auto MUX setupself.u.set_mux(m)self.subdev = usrp.selected_subdev(self.u, tx_subdev_spec) ## Instantiate the daughter board## Tune to RF band: 2.45GHz support by RFX2400. import from command line optionself.u.tune(self.subdev.which(), self.subdev, target_freq) self.subdev.set_enable(True) ## Enable transmit

39. Implementation of LPI radio (cont.) Postmodulation at baseband

40. Implementation of LPI radio (cont.) Signals in the real world

41. Useful LinksHomepage (download, more links, etc)

42. http://gnuradio.org/trac/

43. A tutorial for GNU radio Python programming

44. http://gnuradio.org/trac/wiki/Tutorials/WritePythonApplications

45. Available Signal Processing Blocks

46. http://gnuradio.org/doc/doxygen/hierarchy.html

47. GNU Radio Mailing List Archives

48. http://www.gnu.org/software/gnuradio/mailinglists.html

49. CGRAN: 3rd Party GNU Radio Apps

50. https://www.cgran.org/

51. OFDM Implementation Presentation

52. http://gnuradio.org/trac/wiki/Wireless QAQuestions & Answers

53. Thank You!