An efficient coding and modulation system for transmission of digital data over plastic optical fibers is disclosed. The digital signal is coded by a three-level coset coding. The spectral efficiency of the system is configurable by selecting the number of bits to be processed in each of the levels. The first level applies to the digital data a binary BCH coding and performs coset partitioning by constellation mapping and lattice transformations. Similarly, second level applies another binary BCH coding, which may be performed selectably in accordance with the desired configuration by two BCH codes with substantially the same coding rate, operating on codewords of different sizes. The third level is uncoded. The second and third levels undergo mapping and lattice transformation. After an addition of the levels, a second-stage lattice transformation is performed to obtain a zero-mean constellation. The symbols output from such three-level coset coder are then further modulated.