Amplitude probability distribution of intermodulation distortion in multichannel digital optical cable transmission

The statistical behavior of the amplitude probability distribution of intermodulation distortion interference in multichannel optical-cable TV systems was experimentally investigated. In multichannel transmission, the non-linearity of a laser diode (LD) or an electrical amplifier can cause intermodulation distortion (composite-second-order beat; CSO, composite-triple-beat; CTB, etc.). Even though it has been discussed as laser-clipping distortion, intermodulation distortion is usually distortion from AM-VSB carriers. The statistical analysis and evaluation of the distortion in transmitted channel is in controversial. We evaluated the distortion in 20 frequency-division-multiplexed 16-QAM channels, with each carrier carrying 80 Mbps for an optical cable TV system. We first enumerated the distortion components causing interference in each transmission channel so as to identify the intermodulation products. Then, in selected channels, we precisely measured the power of each kind of distortion and the amplitude distributions of the intermodulation distortion from sinusoidal and digital-modulated carriers on cable TV as a function of optical modulation depth (OMD) of LD. And we clarified how the probability distribution function (PDF) changed as the OMD increased. Also, the BER performance of a 16-QAM signal was measured and compare to the intermodulation behavior of the different distortion sources. We found evidence that the amplitude distribution of intermodulation distortion from digital carriers differs from that of thermal noise. Experimental results showed that the PDF of the intermodulation distortion changed when the ratio of intermodulation distortion among all undesired signals varied with the Ohio. The BER performance varied with intermodulation of both analogue and digital carriers even when the carrier to interference noise power ratio (CIR) is the same.