  This paper introduces a novel class of code sets, called "symmetrical
Z-complementary code sets (SZCCSs)" , whose aperiodic auto- and cross-
correlation sums exhibit zero-correlation zones (ZCZs) at both the front-end
and tail-end of the entire correlation window. Three constructions of (optimal)
SZCCSs based on general Boolean functions are presented. As a second major
contribution, we apply SZCCSs to design optimal training sequences for
broadband generalized spatial modulation (GSM) systems over frequency-selective
channels.
  Key words: Complementary code set, channel estimation, training sequence
design, generalized spatial modulation, frequency-selective channels.
