In this paper, differential pulse-position modulation (DPPM) over an indoor optical wireless link is represented as a Markov
source transmitted over a finite-state machine channel. Then, the MAP, Max-Log-MAP and SOVA algorithms are employed
for soft-decision decoding of DPPM. The error performance with intersymbol interference is evaluated and compared to
hard-decision decoding and MLSD. It is shown that the performance of Max-Log-MAP and SOVA is closed to that of
MLSD and MAP with lower complexity.
We investigate the performance of a differential amplitude
pulse-position modulation with return-to-zero coding (DAPPM-RZ) over an indoor optical wireless channel. We compare the performance of DAPPM-RZ(A=2) with DAPPM(A=2), DPPM and DH-PIM. The result shows that, over a non-dispersive channel, DAPPM-RZ yields better power efficiency than DAPPM. It requires about 1.5 dB less transmit power.
However, the bandwidth of DAPPM-RZ is double that of DAPPM. Compared to DPPM, the bandwidth of DAPPM-RZ is about the same as that of DPPM but DAPPM-RZ yields less power efficiency. When the number of bits/symbol(M) is above 3, the DAPPM-RZ is superior to DH-PIM_2 in terms of power efficiency but has less bandwidth efficiency. Over a dispersive channel, given the same value of M, DAPPM-RZ outperforms DPPM, DAPPM (without RZ) and DH-PIM<sub>2</sub> when the normalized rms delay spread is high.