have permitted an accurate fit of the experimental results. It is worth noting that this model permits us to calculate closed expressions that describe the effect on SCM system performance due to important noise sources, such as intermodulaw x tion distortion due to external modulator nonlinearities 4 , distortion effects due to dispersive transmission of chirped w x signals 12 , or to Kerr effect.
On the contrary, a model in which the modulating signal is the sum of sinusoidal carriers does not permit us to derive general closed expressions able to explain the experimental results in a reliable way. Moreover, the instrumentation required in this case for a system test is particularly expensive, and it could be difficult to obtain the reliable and repeatable values of the system parameters that are required by standardization and homologation procedures.
This suggests the use, for standard definition, of Gaussian random signals as test loads, for which closed analytical expressions are available, that permit an accurate and exhaustive interpretation of the experimental results. Moreover, this approach would lead to reliable and repeatable tests requiring low-cost instrumentation. From a spectral point of view, the proposed approach would lead to test signals exclusively referred to the actual channeling characteristics: for example, two random signals with constant power spectral density but with different levels in relation to the analog and the digital channels.
In conclusion, this approach is equivalent to the methodology that was successfully adopted for testing and homologation of the FDM systems, similar to the SCM systems. After that several attempts based on the use of discrete carriers failed.
CONCLUSIONS
ลฝ
. Hybrid analogrdigital AMrQAM SCM optical transmission systems permits us to realize broadband network platforms with a high degree of flexibility for services management. An important problem for these systems is related to the degradation of QAM channel performance due to a clipping effect in semiconductor lasers. In this article a theoretical model has been presented in order to calculate the BER of QAM channels considering the effect of clipping impulsive noise. This model has been used to evaluate experimental results referred to a multichannel SCM hybrid AMrQAM system, working at a 1.3-m wavelength. Moreover, a proposal for a standard definition and system homologation procedure that is based on the analytical model, and permits an accurate and exhaustive evaluation of important distortion effects occurring in SCM optical transmission systems has been presented.