  We experimentally investigate transmitting high-order quadrature amplitude
modulation (QAM) signals with carrierless and intensity-only measurements with
phase retrieval (PR) receiving techniques. The intensity errors during
measurement, including noise and distortions, are found to be a limiting factor
for the precise convergence of the PR algorithm. To improve the PR
reconstruction accuracy, we propose a distortion-aware PR scheme comprising
both training and reconstruction stages. By estimating and emulating the
distortion caused by various channel impairments, the proposed scheme enables
enhanced agreement between the estimated and measured amplitudes throughout the
PR iteration, thus resulting in improved reconstruction performance to support
high-order QAM transmission. With the aid of proposed techniques, we
experimentally demonstrate 50-GBaud 16QAM and 32QAM signals transmitting
through a standard single-mode optical fiber (SSMF) span of 40 and 80 km, and
achieve bit error rates (BERs) below the 6.25% hard decision (HD)-forward error
correction (FEC) and 25% soft decision (SD)-FEC thresholds for the two
modulation formats, respectively. By tuning the pilot symbol ratio and applying
concatenated coding, we also demonstrate that a post-FEC data rate of up to 140
Gb/s can be achieved for both distances at an optimal pilot symbol ratio of
20%.
