A Fourier-Bessel series based analysis that allows real-time symbol-by symbol error performance evaluation for mobile radio direct sequence code division multiple access (DS-CDMA) systems, is described. This technique analyses radio communication aeronautic links, and generates average bit error rate (BER) vs. E/sub b//N/sub 0/ estimation. Using the proposed theoretical analysis, a double differential QPSK (DDQPSK) modulation scheme, operating in a realistic flat fading channel, accompanied by the presence of additive white Gaussian and impulsive E-mixture noise, Doppler shift and multi-access-interference (MAI), is studied.