I. Introduction
Coded excitation that uses an elongated burst has long been known to be capable of enhancing the penetration depth of ultrasound while maintaining its spatial resolution. O'Donnell et al. [1] demonstrated an improvement of SNR by 15 dB with coded excitation over conventional pulse-echo ultrasound in the clinical ultrasound frequency range. There are two types of coded excitation approaches: phase modulation and frequency modulation. Chiao et al. [2] compared these two methods and discussed the advantages and the disadvantages of each approach. Although systems for phase modulation transmissions are simpler to implement utilizing switching circuits [3], the levels of range side lobes are affected by phase distortion during acoustic propagation in a nonlinear media. In contrast, frequency modulations, i.e., chirp-coded excitation, result in smaller range side lobes (<40 dB) and need only single transmissions to form one scan line.