An access system for a vehicle includes a receiver and an access module. The receiver is configured to receive a signal transmitted from a portable access device to the vehicle. The access module is configured to: generate a differentiated signal based on the received signal; up-sample the differentiated signal to generate a first up-sampled signal; obtain or generate an expected signal; up-sample the expected signal to generate a second up-sampled signal; cross-correlate the first up-sampled signal and the second up-sampled signal to generate a cross-correlation signal; based on the cross-correlation signal, determine a phase difference between the first up-sampled signal and the second up-sampled signal; determine a round trip time of the signal received by the receiver; and permit access to the vehicle based on the round trip time.