4179.
A non-equal-probability quadrature amplitude modulation (QAM) scheme was proposed based on constellation point fission for visible light communication systems, in which the even-order non-equal-probability 68QAM (NEP-68QAM) and odd-order NEP-69QAM constellations with source entropy of 6 were studied. Similar to the NEP-72QAM constellation based on nonlinear encoding, both the NEP-68QAM and NEP-69QAM achieved a larger minimum Euclidean distance than the equal-probability 64QAM constellation. However, the peak-to-average power ratio of NEP-68QAM and NEP-69QAM was lower than that of NEP-72QAM, and they were easier to implement. Furthermore, an experimental demonstration was built to evaluate the performance of the two non-equal-probability constellations in the practical visible light channel. When the nonlinear effect of the light emitting diode (LED) was weak, NEP-68QAM achieved the lowest bit error rate due to the complete Gray coding. When severe nonlinear distortion of the LED appeared, NEP-69QAM had the strongest anti-nonlinear capability and thus offered the best performance. Considering a bit error rate threshold of 3.8×10
-3, when the DC bias current was 150 mA, compared with the NEP-72QAM and 64QAM, the peak-to-peak voltage (Vpp) operating range of the NEP-68QAM scheme increased by 7.8% and 31.7% respectively, while the Vpp operating range of the NEP-69QAM scheme increased by 16.9% and 42.9% respectively.… …
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