As data traffic in optical fiber network increases, there has been a lot of research that explores how to increase transmission capacity. Recent works suggest the multi-dimensional optical transmission as a novel solution. Multi-dimensional optical transmission is a signal transmission technique using orthogonal dimension sources such as intensity and phase of optical signal. However, previously proposed scheme has nonlinearity issue due to high frequency components in the modulation sequence and interference issue due to the transceiver structure. These issues limit the performance enhancement of multi-dimensional optical transmission. To overcome this limitation, geometric and probabilistic constellation shaping technique is aided to optical intensity-phase-polarization multidimensional modulation in direct detection system. We used quadrature amplitude modulation – differential phase shift keying – polarized intensity rotational frequency shift keying (QAM-DPSK-PIRFSK) modulation scheme to ensure orthogonality among optical intensity, phase and polarization. However, inter-dimensional interference (IDI) still occurs during direct phase detection using Mach-Zehnder delay interferometer (MZDI). To mitigate the IDI effect, RF-QAM signal modulated in optical intensity dimension is shaped geometrically and probabilistically. We used Hexagonal-QAM(HQAM) as a scheme of geometric constellation shaping to increase density of symbol and decrease peak symbol power. Also, probabilistic shaping is aided to HQAM model. With the mathematical analysis on IDI, symbol appearance probability is adjusted according to the intensity of the RF-QAM symbol. We verified the validity of the proposed technique with optical fiber transmission simulation.
Data traffic is increasing explosively because of the development of various mobile applications and services for 5G and B5G communication. Optical intensity modulation has been used in optical access networks. However, due to the modulation bandwidth limitation of the optical modulator and the fiber dispersion, it is difficult to support the increasing data traffic only using optical intensity modulation. Therefore, multidimensional optical transmission using various optical resources has been researched. Modulation using optical polarization is widely studied as one of the multidimensional modulation resources because the system is simpler than other techniques. However, due to the squarelaw detection, which is an intensity-based signal reception, the signal needs to be received for each dimension respectively at the receivers, so the complexity of the optical receiver is increased as multiple resources are used. In this paper, we propose a single PD based signal receiving technique for the multidimensional signals modulated on optical intensity and optical polarization. The multidimensional optical signal is distorted by interdimensional interference. The proposed technique minimizes interference when receiving optical intensity and polarization modulated signals using a single PD. The proposed technique can separate each modulated signal for each modulation dimension of the multidimensional optical signals. We experimented the multidimensional signal detection using a single PD. We experimentally verified the BER performance of the QAM-PIRFSK signal transmission.
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