CPM-Based Radar Waveforms for Efficiently Bandlimiting a Transmitted Spectrum

Abstract

In this thesis it shall be demonstrated how a polyphase-coded radar waveform can be implemented using a continuous phase modulation (CPM) framework so as to achieve spectral containment while maintaining a constant envelope to maximize energy-on-target. Current implementations of waveforms such as derivative phase shift keying (DPSK) and minimum shift keying (MSK) are subject to spurious spectral components referred to as "spectral regrowth". To design a waveform that removes these unwanted frequency components, the solution must not disturb the characteristics of the waveform to a point where it is no longer desirable to be used in the radar scenario, namely the power efficiency, range resolution, and target detectability. Power efficiency can be achieved by limiting choices to waveforms of constant modulus, or amplitude. The choice of a continuous phase waveform introduces a decrease in dynamic range. However, signal processing techniques will be presented as a means to increase the sensitivity. A version of the Least-Squares mismatch filtering will be implemented in a fashion that accommodates the continuous nature of the CPM structure. The DPSK and MSK techniques are applicable only to binary-coded waveforms. The CPM implementation will be formulated in a manner that gives it the added advantage of being applicable to polyphase-coded waveforms as well. The ability to utilize polyphase codes greatly increases the number of codes available, which is a direct benefit due to more diverse codes and longer code lengths. This trait can be exploited to use new longer polyphase codes increasing the pulse compression gain, hence target detectability. Results indicate that spectral spreading can be greatly decreased with the CPM implementation. The limiting factor on complete spectral containment for the CPM framework is the rise/fall-time as the waveform transitions on and off respectively. It will be shown that some tapering of the amplitude during these transition periods can be very beneficial in limiting spectral regrowth.