Abstract
Direct detection transceivers offer advantages, including low cost and complexity, in short- and medium-haul links. We carried out studies seeking to identify the signal formats which offer the highest information spectral densities and maximum transmission distances for direct detection links. The performance of two spectrally efficient optical signal formats, single-sideband (SSB) Nyquist pulse-shaped subcarrier modulation (SCM) and SSB orthogonal frequency-division multiplexing (OFDM), are compared by means of simulations. The comparison is performed for a range of wavelength-division multiplexing (WDM) net information spectral densities up to 2.0 b/s/Hz by varying the signal bandwidth, modulation cardinality, and WDM channel spacing. The signal formats' tolerance to signal-signal beating interference, resulting from square-law detection, is investigated, and the Nyquist-SCM format is found to suffer lower penalties from this nonlinearity at high information spectral densities. In 7 × 28 Gb/s WDM transmission at 2.0 b/s/Hz (with electronic predistortion and EDFA-only amplification), Nyquist-SCM signals can be transmitted over distances of up to 720 km of standard SMF in comparison to a maximum of 320 km with the OFDM signal format.
Highlights
T HE demand for high bit-rate transmission using costeffective solutions is continuously increasing, especially in short and medium reach optical links
The roll-off factor of the pulse shaping filter and RF-subcarrier frequency were optimised for the Nyquist-subcarrier modulation (SCM) signal whereas bit loading was carried out with the orthogonal frequency-division multiplexing (OFDM) signal to optimise their tolerance to the signal-signal beating interference (SSBI)
The Nyquist-SCM quaternary phase-shift keying (QPSK)/16-quadrature amplitude modulation (QAM) signal outperformed the equivalent adaptively modulated OFDM signal when the total bandwidth was lower than 8.75/10.5 GHz, leading to, respectively, ∼75%/50% overlap between the signal-signal beating products and sideband
Summary
T HE demand for high bit-rate transmission using costeffective solutions is continuously increasing, especially in short and medium reach optical links. ODB transmission has been demonstrated at ISDs of 0.8 b/s/Hz [4] and 1 b/s/Hz [5], but it is fundamentally limited to 1 b/s/Hz. we focus on single sideband (SSB) SCM formats (Nyquist-SCM and OFDM) with quadrature amplitude modulation (QAM) of the RF-subcarrier(s), which are detected by beating with the optical carrier. Their tolerance to SSBI is directly compared by varying the spectral guard band between the optical carrier and sideband in both single channel and wavelength division multiplexing (WDM) system architecture. ERKILINCet al.: PERFORMANCE COMPARISON OF SINGLE-SIDEBAND DIRECT-DETECTION NYQUIST-SUBCARRIER MODULATION
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