SE is defined as the information capacity of a single channel (in bit/s) divided by the frequency spacing Δf (in Hz) between the carriers of the WDM comb:
SE = Rs log2(M) /Δf (1+r)
where Rs is the symbol rate, M is the number of constellation points of the modulation format, and r is the redundancy of the forward error correction (FEC) code, for example, r = 0.07 for an FEC with overhead (OH) equal to 7%
More the SE, more data can be transmitted in a fiber. The total system capacity (defined as the maximum information in bit/s that can be transmitted by the WDM comb) is obtained as the product between the SE and the available bandwidth. The maximisation of the SE thus plays an important role in the maximisation of the overall system capacity.
The total system capacity (defined as the maximum information in bit/s that can be transmitted by the WDM comb) is obtained as the product between the SE and the available bandwidth. The maximisation of the SE thus plays an important role in the maximisation of the overall system capacity.
In the past years, the SE of optical systems has significantly increased, mainly due to the advent of coherent-detection technologies, which enabled the use of high-order modulation formats based on polarization-division multiplexing (PDM) [2], such as PDM-QPSK (quadrature phase-shift keying), with M = 4, PDM-16QAM (quadrature-amplitude modulation), with M = 8, and PDM-64QAM, with M = 12. However, the use of high-order modulation formats requires a higher optical signal-to-noise ratio (OSNR), which may result in a significantly reduced achievable transmission distance.
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