In this letter, the power spectral density (PSD) of a direct-detected
optical signal with overlapping pulse-position modulation (OPPM) schem
e is derived. It is demonstrated that this spectrum contains a strong
periodic component at the signal symbol frequency, thereby enabling a
simple PLL device to provide joint symbol and slot synchronization. Va
riance of timing error for a linearized PLL synchronizer is shown to b
e mainly dominated by, and is inversely proportional to, the product o
f PLL closed-loop bandwidth and symbol duration of the OPPM signal. Pe
rformance of a shot-noise limited PLL-based optical OPPM communication
system is then assessed in terms of the overall system symbol error r
ate.