Effect of unequal transmission power allocation in turbo-coded optical wireless communication system

In this paper, the turbo coded on-off keying with unequal transmission power allocation (UTPA) in optical wireless communications is evaluated. The turbo coded with UTPA allocates the different power on the message bit and parity bits at the turbo encoder outputs with total transmission power fixed. In the additive white Gaussian noise (AWGN) channel, the turbo coded with UTPA can improve the frame error rate (FER) performance. However, in the optical wireless communications, the effect of UTPA is not investigated. In the paper, the FER performance of the turbo coded on-off keying with UTPA is analyzed in the presence an avalanche photodiode (APD) noise, thermal noise, scintillation, and signal dependence noise. Consequently, the FER performance of the turbo coded on-off keying with UTPA is better than that of the traditional turbo coded on-off keying. It is found that there is an optimum power ratio. When the average received laser power is -47.9[dBm] and there is not scintillation, the optimum power ratio, denoted R, is 1.7. The turbo coded on-off keying with UTPA system with R=1.7 is superior to the conventional system with R=1.0 significantly. It is found that the performance improvement of the turbo coded on-off keying with UTPA becomes small when increasing the variance of scintillation.