The paper discusses block coded multilevel QAM radio systems having nonlinear distortions. The noise induced by the nonlinearity is characterized by histograms calculated from the simulated sampler of the received signal. The overall noise distribution is given both for independent and dependent block symbols. Using this distribution the remaining bit error probability after decoding can be calculated in closed form analysis. The calculation results show that the vulnerability of multilevel QAM radio systems to nonlinear distortions can significantly be reduced by the application of even simple block coded modulation schemes.>