This paper shows an analytical approach for the evaluation of the tight upper bound of the bit error rate (BER) with CDMA-DS and applying the soft decision of Viterbi decoder technique as a contribution for future satellites applied to the earth observation. In this first part, the model is not stressed by some random signals as a jammer or some interference signal. BER expression has been evaluated as a function of the throughput efficiency (Te), the soft decision of levels (Q), the code rate (R), constraint length (L) and the processing gain, (G). The numerical results are showing that a good chance to reach a high efficiency of the system could be employing a code such as, R = 1/2 with L=7, in order to improve the BER and to increase the throughput efficiency. Also, using the soft decision decoder levels such as Q=8, a higher performance is achieved for this model. Finally, the coding gain is depicted as a function of the bit energy by a noise spectral density (Eb/Ni) and it is compared to case with no coding (w/c).