Time-hopping (TH) is a communications signal technique which can
be used to achieve anti-jamming (AJ) or low probability of
intercept (LPI). It can also refer to pulse-position modulation,
which in its simplest form employs 2k discrete pulses (referring
to the unique positions of the pulse within the transmission
window) to transmit k bit(s) per pulse. To achieve LPI, the
transmission time is changed randomly by varying the period and
duty cycle of the pulse (carrier) using a pseudo-random
sequence. The transmitted signal will then have intermittent
start and stop times. Although often used to form hybrid spread-
spectrum (SS) systems, TH is strictly speaking a non-SS
technique. Spreading of the spectrum is caused by other factors
associated with TH, such as using pulses with low duty cycle
having a wide frequency response. An example of hybrid SS is TH-
FHSS or hybrid TDMA (time division multiple access).