Nonbinary LDPC-coded modulation for high-speed optical fiber communication without bandwidth expansion

We propose a scheme that can attain the same transmission bit rate as the corresponding conventional polarization-division-multiplexed (PDM) quadrature amplitude modulation (QAM) scheme while occupying lower bandwidth and, hence, achieving a higher spectral efficiency. In contrast to the conventional approach, which increases the symbol rate and thus the occupied bandwidth to transmit the redundant symbols due to forward error correction (FEC), the proposed approach expands the underlying signal constellation in size and reduces the FEC code rate accordingly to form a mechanism that can achieve coded transmission without bandwidth expansion. Such a scheme can find applications in scenarios where there exist stringent bandwidth restrictions and bandwidth expansion is not considered as a viable option. Although the idea of constellation expansion in lieu of bandwidth expansion is mainly associated with Ungerboeck's trellis-coded modulation (TCM), our proposed nonbinary low-density parity-check (LDPC)-coded modulation scheme shows that block-coded modulation schemes can also be used with expanded constellations to achieve transmission without bandwidth expansion and without resorting to TCM. Our results reveal that for small to medium constellation sizes, the proposed scheme can preserve bandwidth while not experiencing significant increase in required optical signal-to-noise ratio (OSNR). For large constellation sizes, however, to keep the increase in required OSNR at manageable levels, we propose using controlled bandwidth expansion where constellation expansion and bandwidth expansion are used simultaneously to obtain a balance between the two critical system parameters of bandwidth and required OSNR.