We design a 2-bit high-pass delta-sigma modulator based on 4 th -order cascaded resonator feedback structure, and experimentally demonstrate IFoF transmission of 1.88GHz bandwidth 16384-QAM in 32Gb/s DML-DD link. 0.58% EVM is achieved after 20km SSMF transmission.
In this work, we systematically analyze the impact of three kinds of Mach-Zehnder modulator (MZM) imbalances, including bias deviation, amplitude mismatch, and differential time skew in intensity-modulation direct-detection (IM-DD) links. It is shown that, for power fading limited transmission, the imbalances can be utilized as advantages rather than impairments. Specifically, the bias deviation with single-arm driven mode and amplitude mismatch with differential driven mode can increase the available bandwidth by shifting the frequency of fading notches. Meanwhile, time skew provides another way to avoid fading by shaping the double sideband (DSB) signal into a vestigial sideband (VSB) with an asymmetrical transfer function. In the transmission experiment, 34 Gbaud Nyquist 6/8-ary pulse amplitude modulation (PAM-6/8) signals are used for investigation in a 20 km dispersion-uncompensated standard single-mode fiber (SSMF) link. With the help of a Volterra nonlinear equalizer, all three kinds of imbalances can achieve bit-error rates (BERs) below the 7% and 20% hard-decision forward error correction (HD-FEC) thresholds for PAM-6 and PAM-8 signals, respectively. The received power sensitivity is also compared at the back-to-back (BTB) case and after fiber transmission. Both numerical simulation and experimental demonstration confirm that the dispersion-induced power fading can be effectively suppressed with bias, amplitude, or skew imbalance, providing a feasible solution for transmission distance extension of C-band DD links.
We experimentally and numerically investigate the minimum electrical bandwidth for 50GBaud PAM4 detection with polybinary spectral shaping. With only 19GHz electrical brick-wall bandwidth, 100Gb/s PAM-4 transmission is demonstrated with 24% bandwidth compression at BTB and 1km SSMF scenarios.
With digital brick-wall filter-based coefficients estimation and Tomlinson-Harashima precoding, we experimentally demonstrate faster-than-Nyquist subcarrier modulation using a 20GHz O-band DML. At the 20% SD-FEC threshold, up to 105%, 45.5%, and 18.2% FTN rates are achieved for QPSK, 16-QAM and 32-QAM formats, respectively.
Frequency division multiplexing (FDM) passive optical network (PON) is considered as a competitive solution for 5G-advanced and 6G mobile fronthaul (MFH) to support multiple radio units (RUs) with shared trunk fiber and distributed unit (DU). However, it is likely to cause uplink signal overlapping by laser wavelength drift, and guard bands should be reserved between FDM channels, which limits the spectral efficiency (SE). In this work, we propose to aggregate multiple uplink FDM channels using power division multiplexing and jointly adopt successive interference cancellation (SIC) at the receiver to improve the uplink SE. We first theoretically derive the SE of such power- and frequency-division multiplexing (PFDM) PON with both guard band and SIC together, and reveal that PFDM PON can achieve higher SE than the pure FDM PON with guard band only. Focusing on the case of two overlapped uplink signals, we propose a pilot-tone assisted SIC to improve the estimation accuracy of frequency offset and phase noise in heavily overlapped uplink signals. In the proof-of-concept experiments, compared to the orthogonal case without considering the guard band, 0.29 bit/s/Hz SE penalty due to the residual interference after SIC is observed when two uplink signals are completely overlapped. The results showed that, in the presence of laser drift, PFDM PON becomes more spectrally-efficient than FDM PON when the relative wavelength drift exceeds 20% of the signal bandwidth.
We experimentally demonstrate 1/2-bit delta-sigma modulation and transmission in a 40Gbaud QPSK/16-QAM coherent system. After 24km SSMF transmission, up to 2.5Gbaud 4194304-QAM Nyquist signal is successfully transmitted at 71.5dB SNR.
We propose and experimentally demonstrate a digital coherent fronthaul system exploiting sample bits interleaving and uneven 16-QAM. The results indicate that 6dB EVM sensitivity enhancement can be achieved compared with standard 16-QAM.
We propose a pilot-tone assisted successive interference cancellation scheme to mitigate the laser drift-induced inter-subcarrier interference. 50% and 15% bandwidth reduction are experimentally demonstrated with QPSK and 16QAM formats, fitting well with theoretical derivation.
We propose and experimentally demonstrate fading-free direct detection transmission of PAM signal enabled by Alamouti coding and dual-drive MZM. After 80km SSMF transmission, 32Gbaud PAM-4/6/8 signal can achieve KP4, 7% and 20% FEC thresholds, respectively.
A customized algorithm is proposed to adaptively mitigate ISI and SSBI considering O/E front-end response simultaneously based on a MISO equalizer in SSB direct detection system, and validated in 30GBaud SSB-32QAM 80km SSMF transmission experiment.
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