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In this study the authors present an in-depth study, analysis and discussion on maximum likelihood (ML)-based receiver for a typical spectrally encoded spread-time CDMA in the presence of multiple narrowband interference (NBI) signals in an additive white Gaussian noise channel. Furthermore, the authors demonstrate that by combining useful properties of ML-based receiver and three-level codes, that is, codes with values of -1, 0, +1, the authors can introduce a new strategy in which superior performance with respect to previous receiver structure based on two-level codes, that is, codes with values of -1, +1, can be attained. With the help of an example, the authors drive, first, the required signal-to-noise-interference power ratio (SNR) in a multiple access communication environment with multiple NBIs and, second, SNR attenuating parameter owing to multiple NBIs for two- and three-level codes. Finally, the authors relate the above parameters to a number of simple descriptive factors such as bit error rate. The results are validated by an extensive simulation on the aforementioned system.