A closed-loop Doppler measurement for velocity estimation in mobile, multipath environments

Due to the complexity of the multipath environment, velocity estimators generally derive a speed estimate based on the statistics of the received signal, which depend on the scattering distribution for a specific location. One significant, but often overlooked, challenge in mobile-user velocity estimation is that of the frequency reference at the receiver. Velocity estimators require a precise frequency reference to detect small changes in frequency or phase shift that are caused by the motion of the user. The oscillators that are used in communication equipment typically have tolerances in the same range as the expected Doppler shifts. Without a precise frequency reference at the receiver, the carrier recovery loop tracks the received carrier frequency which may have been subjected to a frequency shift. In the most general case, the scattering environment is also unknown. To resolve these problems of precision and ambiguity in the Doppler spectrum, a closed-loop measurement between transmitter and receiver must be performed. The paper introduces a closed-loop Doppler measurement using the frame rates of existing communication standards and having application to velocity estimation in mobile wireless systems.