  This paper presents tighter lower bounds on the maximum aperiodic ambiguity
function (AF) magnitude of unimodular sequences under certain delay-Doppler low
ambiguity zones (LAZ). These bounds are derived by exploiting the upper and
lower bounds on the Frobenius norm of the weighted auto- and cross-AF matrices,
with the introduction of two weight vectors associated with the delay and
Doppler shifts, respectively. As a second major contribution, we demonstrate
that our derived lower bounds are asymptotically achievable with selected Chu
sequence sets by analyzing their maximum auto- and cross- AF magnitudes within
certain LAZ.
