RESUMEN
In this paper, for the first time, a probability-aided maximum-likelihood sequence detector (PMLSD) is experimentally investigated through a 64-GBaud probabilistic shaped 16-ary quadrature amplitude modulation (PS-16QAM) transmission experiment. In order to relax the impacts of PS technology on the decision module, a PMLSD decision scheme is investigated by modifying the decision criterion of maximum-likelihood sequence detector (MLSD) correctly. Meanwhile, a symbol-wise probability-aided maximum a posteriori probability (PMAP) scheme is also demonstrated for comparison. The results show that the PMLSD scheme outperforms the direct decision scheme about 1.0-dB optical signal to noise ratio (OSNR) sensitivity. Compared with symbol-wise PMAP scheme, PMLSD scheme can effectively relax the impacts of PS technology on the decision module and a more than 0.8-dB improvement in terms of OSNR sensitivity in back-to-back (B2B) case is obtained. Finally, we successfully transmit the PS-16QAM signals over a 2400-km fiber link with a bit error ratio (BER) lower than 1.00×10-3 by adopting the PMLSD scheme.
RESUMEN
In this paper, a polar coded probabilistic shaping (PS) 8-ary pulse amplitude modulation (PAM8) based on many-to-one (MTO) mapping is investigated for short-reach optical interconnection. By ingeniously assigning parity bits to ambiguities positions, no extra PS redundancy and no complex distribution matcher are required in the scheme comparing to traditional probabilistic amplitude shaping (PAS). The noise distributions after different transmission distances are studied and an optimal clock recovery method for PS signal is proposed to degrade the impact of severe eye skew effect on BER performance. The experimental results show that up to 1.2 dB and 0.8 dB shaping gains are respectively achieved over back-to-back (BTB) and 2-km standard single mode fiber (SSMF) transmission. With the help of the proposed optimal clock recovery method in the PS scheme, the shaping gain is improved from 0.15 dB to 0.4 dB after 10-km transmission. Moreover, compared to low-density parity-check (LDPC) code, the polar coded PS-PAM8 can provide an additional coding gain of 2.2 dB with code length of 256, which proves the performance superiority of polar code in short code length. Therefore, the proposed polar coded PS-PAM8 with low complexity and satisfactory BER performance is believed to be an alternative solution for the cost-sensitive short-reach optical interconnection.
RESUMEN
We investigate the parallelized performance of the conventional Kramers-Kronig (KK) and without the digital up-sampling KK (WDU-KK) receivers in a 112-Gbit/s 16-ary quadrature amplitude modulation (16-QAM) system over a 1440-km standard single-mode fiber (SSMF). A joint overlap approach and bandwidth compensation filter (OLA-BC) architecture is presented to mitigate the edge effect caused by the Hilbert transform and the Gibbs phenomenon induced by the FIR filter, respectively. Moreover, the computational complexity of the OLA-BC based parallelized KK/WDU-KK receiver is also discussed. Parallelized KK/WDU-KK receivers based on the presented OLA-BC architecture can effectively mitigate the edge effect and the Gibbs phenomenon together with more than two orders of magnitude improvement in terms of bit-error-ratio (BER) compared with parallelized KK/WDU-KK receivers without OLA-BC receivers in back-to-back (B2B) case. Finally, we successfully transmit the 16-QAM signals over 960-km SSMF with a BER lower than 7% hard-decision forward error correction (HD-FEC) threshold (3.8 × 10-3) and 1440-km SSMF with a BER lower than 20% soft-decision FEC (SD-FEC) threshold (2 × 10-2).
RESUMEN
In this paper, an improved polar decoder based on non-identical Gaussian distributions is proposed and experimentally demonstrated for optical pulse amplitude modulation (PAM) interconnection. The principle of the polar coded PAM system is illustrated theoretically and the non-identical Gaussian distributions based log-likelihood ratio (LLR) estimation is introduced in the polar decoder to mitigate nonlinearity. Transmission systems of 28-Gbaud 4-level pulse amplitude modulation (PAM-4) and 8-level pulse amplitude modulation (PAM-8) based on commercial 10-GHz directly modulated laser (DML) are both demonstrated over 10-km standard single-mode fiber (SSMF) in C-band without dispersion compensation. Experimental results show that, aided by the improved polar decoder, the channel nonlinearity can be taken into consideration and additional sensitivity gains of 0.7 dB and 1 dB are respectively achieved compared with traditional polar decoder for PAM-4 and PAM-8 systems.
RESUMEN
With the rapid development of cloud services, data-center applications and the Internet of Things, short-reach communications have attracted much more attention in recent years. 4-level pulse amplitude modulation (PAM4) is a promising modulation format to provide both high data rate and relatively low cost for short-reach optical interconnects. In this paper, a joint clock recovery and feed-forward equalization algorithm (CR-FFE) is proposed to simultaneously eliminate the inter-symbol interference (ISI) and track large sampling clock offset (SCO) in PAM4 transmission. The algorithm estimates timing error according to the difference between two tap coefficients of fractionally spaced equalizers, thus solving the problem of incompatible prerequisites between clock recovery and channel equalization. A 10GHz directly modulated laser (DML) based 50-Gbit/s PAM4 transmission experiment is implemented to investigate the performance of the proposed algorithm. Experimental results show that the proposed CR-FFE algorithm can resist SCO up to 1000 ppm after 40 km standard single-mode fiber (SSMF) transmission under the 2x10-2 SD-FEC BER threshold, which is dramatically improved comparing with that of 20 ppm in traditional CR cascaded by FFE algorithm.
RESUMEN
This paper numerically and experimentally investigates the performance of free-space optical receiver using modes diversity coherent receipt under moderate-to-strong turbulence. By utilizing a three-mode photonic lantern with digital maximum ratio combining, a 40 Gbps QPSK optical signal is received. The turbulence strength is measured by the ratio of beam diameter to atmospheric coherence length, D/r0. The larger the D/r0, the stronger the turbulence is, and vice versa. Compared with conventional single mode fiber based receipt, the required transmitted power can reduce by 4.6 dB and 4 dB at 10% interruption probability under turbulence strength D/r0 = 8 and 16. The required transmitted power at bit error ratio of 2.2 × 10-2 can relax by 4.2 dB and 5 dB under turbulence strength D/r0 = 8 and 16.
RESUMEN
This paper proposes and experimentally demonstrates an all-fiber conversion method that transfers the polarization-division multiplexing (PDM) signals to the mode-division multiplexing (MDM) signals. The conversion scheme is based on a mode converter and a polarization-mode controller. The input X-polarized/Y-polarized fundamental modes are converted to the first-order linear-polarized LP11A/LP11B modes with crosstalk of -10 dB/-18 dB and insertion losses of 3.04 dB/3.1 dB at 1550 nm, respectively. Using the proposed converter, 11.2 GBaud/s polarization-division-multiplexed quadrature-phase-shift keying (PDM-QPSK) and 16 quadrature-amplitude-modulation (PDM-16QAM) signals are successfully converted to 11.2 GBaud/s MDM-QPSK and MDM-16QAM signals.