OFDM SAR Signal Analysis And Imaging Methods

Orthogonal Frequency Division Multiplexing(OFDM)technology has been widely used in high speed wireless communications with its good anti-frequency selective fade ability.OFDM signals have excellent waveform diversity,anti-fade ability,communication potential and can be easily processed by digital methods.Due to the similarity of radar and communication systems,in recent years,OFDM radar has caught much attention.This paper focuses on OFDM synthetic aperture radar(SAR)and investigates OFDM SAR signal quality and processing methods.In Section 1,the background and significance of this dissertation is detailed.OFDM's current research,especially the research on OFDM SAR,is summarized.The main research areas and problems are also analyzed.The main contents of this dissertation are also introduced.In Section 2,based on SAR system geometry model and OFDM signal model,the general format of OFDM signal's generalized ambiguity function(GAF)is derived.The GAFs of general chirp-like coded OFDM signal,i.e.P3-OFDM and P4-OFDM,multi-carrier phase coded OFDM(MCPC-OFDM),chaotic phase coded OFDM(PC-OFDM),binary phase shift keying OFDM(BPSK-OFDM)and quadrature amplitude modulation OFDM(QAM-OFDM)are simulated.Results show that the code type of OFDM signals can greatly influence the OFDM SAR's GAF quality.In Section 3,OFDM signal with sufficient cyclic prefix is analyzed.The echo model of CP-OFDM SAR signal is established.Using parameter estimation theory,the inter-range-cell-interference-free(IRCI-free)SAR imaging algorithm is proposed.The Cramer-Rao bound of the scatter point value is derived.Our algorithm has lower computation burden and better estimation precision.In Section 4,systematic radar signal design which can suppress range ambiguity to the largest extent is investigated as high azimuth resolution and wide swath width cannot be achieved simultaneously in traditional SAR systems.Our signal design method is based on matched filtering.With OFDM signal's waveform diversity and the range ambiguity suppression principles,a novel OFDM radar signal with random subcarrier coefficients is proposed.Compared with classical up-/down-chirps used to solve range ambiguity problems,this signal has similar SAR processing algorithm but can be digitally processed more easily.Also,using this signal can achieve wider unambiguous swath width and better range ambiguity suppression ability,and can reconstruct high quality SAR images on ambiguous extended targets.Section 5 concludes the research results of this paper and points out the problems needed further investigation.