$5\times240$ Gb/s WDM DD Transmission Over 80 km With Spectral Efficiency of 5.25 bit/s/Hz

With the exponential growth of the data traffic, dense wavelength-division-multiplexing (DWDM) is required for future data center interconnect (DCI) applications with increasing demand for spectral efficiency (SE). In this letter, we propose an improved Kramers-Kronig algorithm including DC-offset adjustment and phase clipping after Hilbert transformation. Using this scheme, we experimentally demonstrate a single-polarization WDM direct detection (DD) transmission over 80 km with a record SE of 5.25 bit/s/Hz by transmitting 5 single-side-band (SSB) 256-QAM modulated channels at 30 Gbaud on 35-GHz grid in the C-band.