Pulse position modulation (PPM) fiber optic architectures

Pulse position modulation (PPM) is a form of signaling wherein each transmitted symbol represents more than one bit. Each symbol (a pulse) is transmitted in one of M slots in a frame. Each symbol represents k bits, where k=log₂M. Among intensity modulated/direct detection (IM/DD) communication systems it is favored if the system is average power limited. This is because, for the same average power, it transmits log₂M more bits than non-return-to-zero (NRZ) modulation schemes. However, this advantage comes at the expense of spectral efficiency, which is (log₂M)/M bits/s/Hz for M-ary PPM. In this paper we will describe some rules for architecting M-ary PPM transmitters and receivers, especially for the fiber-optic and Si-photonics design regimes. These regimes can be defined in terms of the time slot (Ts) vs data rate (R) and M relationship because [20*Ts/ns]cm defines the delay line quantization required in the transmitter and receiver architectures. This relationship is shown in Figure 1. It suggests electronic, fiber-optic, and Si-photonics implementation regimes.