Type of Document Dissertation Author Benghuzzi, Mohsin M. URN etd-09042003-145334 Title Passive Detection Suppression of Cyclostationary Phase Coded Waveforms Degree Doctor of Philosophy Department Electrical and Computer Engineering, Department of Advisory Committee
Advisor Name Title Frank Gross Committee Chair James Simpson Committee Member Rodney Roberts Committee Member Simon Foo Committee Member Keywords
- Signal Detection
- Coded Waveforms
Date of Defense 2003-06-01 Availability unrestricted AbstractCommon military radar systems transmit sophisticated pulse compression waveforms for range information. Although much progress has been made in suppressing the platform radar cross section to avoid detection by enemy active radar, little has been done to protect the platform from being detected due to its own on board emissions. An aircraft can have great stealth properties but have a “loud” pulse compression waveform and is therefore liable to detection by enemy passive receivers.
A technique that reduces pulse peak power levels and maintains high range-resolution is known as pulse compression. Pulse compression is a coding technique in which a radar pulse of duration T is subdivided into N sub-pulses. This technique reduces transmitted power, and therefore, reduces detectability by distributing power over time. A matched filter is used at the receiver to reduce (compress) the effective width of the reflected coded pulse.
A disadvantage of using coded waveforms is that the transmitted signal has detectable features. These features can be easily detected by enemy passive detection equipment. Pulse coding generates features such as carrier frequency, chip rate, and frequency of hopping. In particular, presence of carrier frequency in coded waveforms gives rise to cyclostationary properties. Cyclostationary properties in a phase coded waveform are exhibited in the form of spectral lines in the transformed signal generated
using a nonlinear transformation. This study focuses on suppression of cyclostationary properties present in a transformed Welti coded cw waveform with the nonlinear transformation being that of the quadratic type.
In this dissertation, energy suppression of the cyclostationary spectral lines generated in the power spectral density of the square of the Welti coded cw signal is achieved by filtering the Welti code itself. The filter utilized is of the notch type that is centered around the dc frequency. Three various bandwidths of the energy suppression Welti code notch filter are employed. The bandwidths are chosen in relation to the first null of the Welti code sinc spectrum and are set at: [special characters in settings not supported in ASCII format -- see PDF file].
Spectral line energy suppression is determined by comparing average power within a frequency bandwidth after the square of the filtered and unfiltered Welti coded cw waveforms. The squared cw waveform reference bandwidth within which average power is compared is chosen to be equal to the bandwidth of the main–lobe in the Welti code spectrum, that is [special characters not supported in ASCII format], or 6 MHz, and is centered around twice the carrier frequency, or around 24 MHz. Average power within the reference bandwidth of the unfiltered Welti coded cw waveform is measured to be 10.9 dBm.
Filtering using the first Welti code notch filter bandwidth of [special characters not supported in ASCII format], or 60 kHz results in no code distortion. Filtering using this bandwidth also causes no degradation in the Welti code’s desirable complementary characteristics as the composite autocorrelation’s narrow central peak is retained and the time side-lobes still cancel. The average power within the cw waveform reference bandwidth, for this notch filter bandwidth case, is measured to be 8.98 dBm, which is a reduction of approximately 36.6% from the average power for the unsuppressed cw waveform.
Filtering using the second energy suppression Welti code notch filter bandwidth of [special characters not supported in ASCII format], or 120 kHz results in slight code distortion; nonetheless, the overall shape of the Welti code is maintained. For this bandwidth case also, filtering causes no degradation in the Welti code composite autocorrelation’s narrow central peak characteristic, and the time side-lobes still virtually cancel. The average power within the cw waveform reference bandwidth is measured to be 7.32 dBm, which is a reduction of approximately 56.5 % from the average power for the unsuppressed cw waveform.
Filtering using the third energy suppression Welti code notch filter bandwidth of [special characters not supported in ASCII format], or 240 kHz resulted in highly noticeable code distortion. For this bandwidth case, the narrow central peak characteristic also remained unchanged, however, no complete side-lobe cancellation resulted. The average power within the cw waveform reference bandwidth was measured to be 4.47 dBm, which is a reduction of approximately 77.4 % from the average power for the unsuppressed cw waveform.
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