Ultra-Wideband Imaging Radar Based on OFDM: Exploration of Its Potential. Presenter: Dr. Dmitriy Garmatyuk, Department of Electrical & Computer Engineering, Miami University. _________________________________________________________.
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Presenter: Dr. Dmitriy Garmatyuk, Department of Electrical & Computer Engineering, Miami University
Presented on June 20, 2007 at Naval Research Lab, Washington D.C.
* “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.
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
Step 2: Create signal by populating
the frequency vector
Negative frequency half-axis (flipped)*
* MATLAB-specific notation
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
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
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)
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.
† - REFERENCE: M. Soumekh, “Synthetic aperture radar signal processing with MATLAB algorithms,” John Wiley & Sons, 1999
First UWB-OFDM Radar Simulation Test-Bench
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.
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.
Ref Phase Function Generation: An Illustration Sub-Carrier Extraction
- Beamwidth Coverage
yo = 0
Cross-Range Imaging Result Sub-Carrier Extraction
Full Image Sub-Carrier Extraction
Successful target recovery
for SNRs down to –20dB
with resolution 0.1…1 meter
General Scenario of Interest Sub-Carrier Extraction
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)
Senior Design Project (Spring’07): UWB-OFDM Image Communication System Simulator in MATLAB
To be presented in October at EuRAD’07
AFOSR-Sponsored Project Communication System Simulator in MATLAB
UWB-OFDM System Prototype Plan Communication System Simulator in MATLAB
Topics NOT Covered So Far Communication System Simulator in MATLAB
* 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.
Summary: UWB-OFDM system at Miami U Communication System Simulator in MATLAB
High-resolution airborne radar imaging (SAR, 0.3…1 meter resolution theoretical bounds)
Broadband image data communication between airborne platforms
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