Ultra-Wideband Imaging Radar Based on OFDM: Exploration of Its Potential

1. Ultra-Wideband Imaging Radar Based on OFDM: Exploration of Its Potential Presenter: Dr. Dmitriy Garmatyuk, Department of Electrical & Computer Engineering, Miami University _________________________________________________________ Presented on June 20, 2007 at Naval Research Lab, Washington D.C.

2. Miami University • In Ohio, not Florida • Established in 1809 • Named after Indian tribe • … the T-shirt says it all: _________________________________________________________

3. Talk Overview • History of OFDM • OFDM waveform design • UWB-OFDM in Communications and Radar • UWB-OFDM SAR: First steps • Range and cross-range imaging examples • Bigger picture: General scenario of interest • AFOSR project • Summary, Q/A _________________________________________________________

4. Brief History of OFDM • Originated by Bell Labs researcher R. W. Chang in 1966-68* • Next 20 years: System architecture prototype design, adaptation for digital broadcasting, mostly, by Thomson-CSF (currently, part of Thales Group), French electronics/communications company – best achievement was 70 Mbit/s HDTV link at 8 MHz bandwidth • The 90’s: OFDM was adapted for wireless LAN applications (20 MHz bandwidth, max of 54 Mbit/s link capacity) • February 14, 2002: FCC opens up 3.1 – 10.6 GHz for commercial use at –41.3 dBm/MHz, triggering R&D efforts in UWB communications among industrial companies**; OFDM is a primary candidate for system architecture • Now: MB-OFDM is still a #1 choice for WPAN (short-range PC-to-peripherals high data rate communications technology) and is being tapped for 4G (next-generation cellular) * “A Theoretical Study of Performance of an Orthogonal Multiplexing Data Transmission Scheme,” R. Chang and R. Gibby, IEEE Trans. on Communications, vol. 16, no. 4, April 1968. ** “Design of Multiband OFDM System for Realistic UWB Channel Environments,” A. Batra, J. Balakrishnan, G. R. Aiello, et al., IEEE Trans. on Microwave Theory and Tech., vol. 52, no. 9, Sept. 2004. _________________________________________________________

5. Fundamental Benefits and Reasons for Survivability • Dynamic spectrum allocation: User decides which sub-bands are to be occupied for each outgoing pulse • Digital-friendly architecture – as digital technology becomes cheaper, so do OFDM systems • Expandability: Bandwidth is determined solely by sampling speed • Robust against narrowband interference: Turn on/off sub-bands adaptively • Time synchronization is not a big issue: All processing is done in frequency domain • Very good spectral efficiency: One pulse can contain many bits of information _________________________________________________________

6. Simplest OFDM Transmitter Step 1: Decide how many sub-bands we want Step 3: Feed this vector to IFFT processor Example: 32 sub-bands (usually – 128 or 256) Positive frequency half-axis DC point Step 2: Create signal by populating the frequency vector Negative frequency half-axis (flipped)* _________________________________________________________ * MATLAB-specific notation

7. Quick calculation 2: The signal is an RF pulse at 31.25 MHz carrier frequency and 64 ns duration  theoretical spectrum is a sinc-function centered at 31.25 MHz and 31.25 MHz main-lobe bandwidth 2/Tpulse fc Simplest OFDM Transmitter – Cont’d Step 4: Feed the time-domain vector to DAC Quick calculation: If we assume 1 Gs/s speed of D/A conversion and 65 data points in the data vector, then there will be (65-1) samples at DAC’s output, each with 1 ns of duration  output signal will be 64 ns long _________________________________________________________

8. Orthogonality Illustration Step 5: Compose the frequency vector anew and place ‘1’ in adjacent positions Each sub-band has exactly zero interference from other sub-bands precisely at its carrier frequency (sub-carrier) _________________________________________________________

9. How to Make OFDM Ultra-Wideband? Quick calculation: If all sub-bands are ON, then the entire occupied spectrum is 0.5 GHz – or half the sampling rate. This holds for any number of sub-bands, or other system parameters – total potential bandwidth of an OFDM signal is always half the DAC speed, hence the non-existence of UWB-OFDM systems in the past. _________________________________________________________

10. UWB-OFDM in Communications • Can apply QPSK before feeding the frequency-domain vector to the DAC: Each sub-band will then represent a 4-bit symbol • Make use of fast integrated FFT/IFFT processors and D/A and A/D converters • With 128 sub-bands we can squeeze 128x4=512 bits into 128 ns pulse (theoretically)  translates to 4 Gb/s! • Practically, of course, some bits in the sequence will be needed for synchronization, etc, plus low power requirement will result in losses at the receiver and the necessity to re-transmit data several times, thus realistically 100-500 Mb/s are currently achievable • Pros: Fading/multi-path resistance, excellent spectral efficiency, good potential for interference mitigation, relatively cheap implementation in integrated CMOS technology, good scalability/spectrum flexibility potential • Cons: Doppler sensitivity, issue of high peak-to-average power ratio _________________________________________________________

11. UWB-OFDM Benefits for Radar • High waveform diversity potential • Dual-use architecture (radar/communications) • Noise-like waveforms for increased LPI/LPD • Ease of narrowband jamming and interference mitigation • High potential for coexistence with other services/radars • High resolution and multi-path potential • Modern technology allows for inexpensive implementation _________________________________________________________

12. First Step: UWB-OFDM SAR • Stripmap SAR topology was assumed • Backprojection algorithm in fast- and slow-time domains was chosen as a basis for image formation† • Standard SAR setup and analysis _________________________________________________________ † - REFERENCE: M. Soumekh, “Synthetic aperture radar signal processing with MATLAB algorithms,” John Wiley & Sons, 1999

13. Single range profile response First UWB-OFDM Radar Simulation Test-Bench _________________________________________________________

14. Range Profile Recovery: Standard SAR Focusing via matched filtering _________________________________________________________ D. Garmatyuk, “Simulated imaging performance of UWB radar based on OFDM,” Proceedings of The 2006 IEEE International Conference on Ultra-Wideband, pp. 237-242, Waltham, MA, September 2006.

15. Cross-Range Profile Recovery: OFDM Benefits from Easy Sub-Carrier Extraction In cross-range signals are represented in phase domain before computing their cross-correlation In OFDM single-frequency components in frequency domain are already available after FFT in the receiver*! where sRX(w0,u) represents radar signal at frequency w0 received when the radar platform was at the cross-range coordinate u; TFnis a reflectivity constant of nth target within the radar beamwidth; xn and yn are range and cross-range coordinates of the nth point target; and s0(w0,u) is defined as an ideal return from a unit reflector located at the centre of the radar-scanned target area – i.e. (xn, yn) = (Xc, 0), where Xc is the range distance to the centre of target’s area. _________________________________________________________ * Receiver:

16. Ref Phase Function Generation: An Illustration - Beamwidth Coverage Target yo = 0 _________________________________________________________

17. Cross-Range Imaging Result • Span of 16 meters was assumed and various PRFs were simulated _________________________________________________________

18. Full Image Successful target recovery for SNRs down to –20dB with resolution 0.1…1 meter _________________________________________________________

19. General Scenario of Interest _________________________________________________________ Scenario feasibility study will be presented at EuRAD’07 (October 11, Munich) and published in the proceedings (“Feasibility study of a multi-carrier dual-use imaging radar and communication system”, Dmitriy Garmatyuk, Jon Schuerger, Jade Morton, Kyle Binns, Michael Durbin, John Kimani; all – Miami University)

20. Senior Design Project (Spring’07): UWB-OFDM Image Communication System Simulator in MATLAB _________________________________________________________ To be presented in October at EuRAD’07

21. AFOSR-Sponsored Project • Objectives: • Design workable UWB-OFDM transceiver • Test imaging performance of UWB-OFDM radar • Test data communication performance of UWB-OFDM • Lay foundation for subsequent research of UWB-OFDM imaging radar networks • Plans and personnel: • 1st year: System component acquisition and theoretical analysis of realistic 256 sub-band (0.5 GHz BW) SAR • 2nd year: Imaging radar assembly and test • 3rd year: Image communication test and imaging radar network analysis (theory) • 1 faculty member (me) and 1 M.S. student (who is much interested in working for NRL or AFRL after graduation) _________________________________________________________

22. UWB-OFDM System Prototype Plan • Summer’07: FPGA-based digital transceiver design and assembly; • Fall-Winter’07: Digital testing and antenna system acquisition and test; • Spring’08: RF assembly and test • Summer’08: Complete system test and implementation _________________________________________________________

23. Topics NOT Covered So Far • RF detriments in the transceiver and how they will affect system performance • Doppler effect* • Clutter effects on various frequencies in UWB-OFDM bands • Custom design (e.g. high transmit power, integrated ASIC-based digital part) • Weight/power/complexity trade-offs for practical usage models • Intelligent signal design (e.g. to reduce PAPR) • Actual effects of jamming on performance _________________________________________________________ * But TU-Delft (The Netherlands) researchers have concluded that it is possible to perform Doppler estimation using OFDM: G. E. A. Franken, H. Nikookar and P. van Genderen, “Doppler tolerance of OFDM-coded radar signals,” in Proc. 3rd European Radar Conf., 2006, pp. 108-111.

24. Summary: UWB-OFDM system at Miami U High-resolution airborne radar imaging (SAR, 0.3…1 meter resolution theoretical bounds) Broadband image data communication between airborne platforms SNR -10.07dB Potential for image-based navigation in GPS-denied environments (future topic) Complete simulation-based feasibility study is ~80% done and hardware assembly plan commenced in April’07 Questions, discussion… _________________________________________________________ • D. Garmatyuk, “Ultrawideband imaging radar based on OFDM: System simulation analysis,” Proceedings of SPIE, Radar Technology X, Vol. 6210, pp. 66-76, Orlando, FL, May 2006. • D. Garmatyuk, “Simulated imaging performance of UWB radar based on OFDM,” Proceedings of The 2006 IEEE International Conference on Ultra-Wideband, pp. 237-242, Waltham, MA, September 2006. • 3. D. Garmatyuk, Y. Jade Morton, “On co-existence of in-band UWB-OFDM and GPS signals,” Proceedings of The 2007 Institute of Navigation National Technical Meeting, San Diego, CA, January 2007.