  In secure precise directional modulation (DM) networks, two practical
random-subcarrier-selection (RSS) methods are proposed to transmit confidential
message to the desired user per orthogonal frequency division multiplexing
(OFDM) symbol with only single receive power peak formed by constructing random
subcarrier set and performing a randomization procedure. This scheme completely
addresses the crucial problem facing secure precise wireless transmission
(SPWT), how to achieve the SPWT per OFDM symbol while the traditional SPWT
holds only in statistically average sense. Several necessary conditions for
SPWT per OFDM is derived and proposed: randomly distributed, sparse, and
distinct subcarrier distance between two pair of adjacent antennas. Two random
subcarrier sets (RSSs), quadratic subcarrier set (QSS) and prime subcarrier set
(PSS), are constructed, where the former means the subcarrier index associated
with any antennas is the square of antenna index, and the latter implies that
the subcarrier indices over all antennas are prime numbers. Subsequently,
following those conditions for SPWT per OFDM, a random factor is defined, and a
randomization procedure (RP) is proposed. Its detailed process includes the
following steps: integer mod, ordering, blocking, and block interleaving where
BI is repeated until the random factor is greater than the predefined
threshold. This yields a single main receive energy peak (SMREP) at the desired
position with other positions, outside the SMREP, harvesting only weak receive
energy seriously corrupted by AN.
