Last Updated: August 16, 2025
3 min read
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.
To increase the SE, and consequently, the overall system capacity, involves reducing the frequency spacing Δf between the WDM sub-carriers. Here normalized frequency spacing f and symbol Rate Rs defined as
f =Δf /Rs
For ultimate spectral efficiency, WDM channel spacings are reduced until the optical spectra of neighboring channels start to overlap noticeably. But this imposes linear crosstalk between adjacent WDM channels and becomes a main source of degradation. An efficient countermeasure to limit the crosstalk is based on an accurate spectral shaping of each sub-channel of the WDM comb is known as “Nyquist-WDM,”
Where the transmission of PDM-QPSK WDM signals with channel spacing equal to the symbol rate
The technique has also been successfully applied to the generation and transmission of higher-order modulation formats, such as PDM-8QAM, PDM-16QAM, PDM-32QAM, and PDM-64QAM with frequency spacing values equal or very close to the symbol rate.
Depending on the normalized frequency spacing f among the WDM channels, three different categories of Nyquist-WDM signaling, which are
f = 1 (i.e., Δf = Rs): Ideal Nyquist-WDM
f > 1 (i.e., Δf > Rs): Quasi-Nyquist-WDM
f < 1 (i.e., Δf < Rs): Super-Nyquist-WDM
