A Comparison of Techniques for Non-Data-Aided Carrier Tracking of Phase-Modulated Signals

Communications between ground stations and nanosats in low earth orbit (LEO) require acquisition and tracking of large Doppler frequency offsets due to the relative velocity the between the transmitter and the receiver. The Doppler frequency shift varies with time, reaching its fastest rate of change as the small satellite reaches its closest approach to the ground station. Non data-aided techniques for acquiring and tracking the carrier frequency offset without requiring the processing of symbols have been developed to address this problem. One technique is the use of a frequency-locked loop (FLL) comprised of band edge filters that convert the energy in the modulated signal’s excess bandwidth into a control signal proportional to the frequency offset. Alternatively, the Doppler frequency offset, in addition to the modulation rate of a phase modulated signal, can be obtained by simply multiplying the incoming signal by itself or by a time-delayed version of itself. The frequency offset component of this processed signal can be extracted with a phase-locked loop (PLL) which filters the excess noise and removes the tones associated with the original signal’s symbol modulation rate. This PLL filtered signal can then be used in a FLL to correct the observed time-varying Doppler frequency. This paper presents a comparison between these two techniques for BPSK signals in both high and low SNR environments, highlighting the advantages and disadvantages of each approach. MATLAB results for each tracker are shown with varying SNR, static Doppler, and dynamic Doppler frequency offset