Current satellite communication systems based on RF technology are often limited by the available spectrum. Free-Space Optical Communication Links are a promising solution to overcome this bottleneck. The available spectrum enables Terabit-per-Second data rates, and the point-to-point nature of FSO links makes spectrum regulation unnecessary. Therefore, the use of optical links is investigated in a number of scenarios, as e.g. for GEO feeder links serving multimedia applications. Transmitter Diversity might be a solution to overcome atmospheric impacts on the ground-to-satellite transmission in an optical GEO feeder link application. Transmitter Diversity can e.g. be implemented by using various spectral channels. Despite the virtually unlimited bandwidth available in atmospheric transmission windows, practical implementations of an optical GEO feeder link need to use available hardware, namely fiber amplifiers in the C- and L-band – Therefore, a more efficient use of the available spectrum will enable in a higher system throughput. In this paper, we present a bandwidth efficient transmitter diversity scheme, called "phase-division in bit-time". It is based on Intensity Modulation with Direction Detection, and makes use of phase modulation for the transmitters to mitigate the impact of atmospheric phase distortions on the quality of the received signal.
KEYWORDS: Digital signal processing, Receivers, Phase shift keying, Modulation, Signal detection, Homodyne detection, Astronomical imaging, Personal digital assistants, Optical design, Data communications
Due to higher data rates, better data security and unlicensed spectral usage optical inter-satellite links (OISL) offer an attractive alternative to conventional RF-communication. However, the very high transmission distances necessitate an optical receiver design enabling high receiver sensitivity which requires careful carrier synchronization and a quasi-coherent detection scheme.
KEYWORDS: Digital signal processing, Receivers, Filtering (signal processing), Homodyne detection, Signal to noise ratio, Signal detection, Optical filters, Phase shift keying, Modulation, Satellites
We present simulative and experimental investigations of different coherent receiver designs for high-speed optical intersatellite links. We focus on frequency offset (FO) compensation in homodyne and intradyne detection systems. The considered laser communication terminal uses an optical phase-locked loop (OPLL), which ensures stable homodyne detection. However, the hardware complexity increases with the modulation order. Therefore, we show that software-based intradyne detection is an attractive alternative for OPLL-based homodyne systems. Our approach is based on digital FO and phase noise compensation, in order to achieve a more flexible coherent detection scheme. Analytic results will further show the theoretical impact of the different detection schemes on the receiver sensitivity. Finally, we compare the schemes in terms of bit error ratio measurements and optimal receiver design.
A simple analytical method to estimate the OSNR sensitivity of multi-level amplitude, phase and combined modulation
formats is shown. The results are compared to numerical simulations with coherent and direct detection technique using
RZ and NRZ pulse shape.
Nonlinear equalizers based on nonlinear Volterra theory and maximum-likelihood sequence estimators (MLSE) are
investigated to mitigate the nonlinear distortions resulting from the incomplete sideband suppression in optical single
side band modulation (OSSB) systems. Through theoretical analysis and simulations, we demonstrate that electrical
dispersion compensation (EDC) by using these nonlinear equalizers can achieve better performance compared to
conventional analog equalizers including feedforward equalizers (FFE) and decision feedback equalizers (DFE).
Multilevel intensity modulation such as 4-ASK signalling can reduce the bandwidth requirement in multimode fiber (MMF) links. However, 4-ASK signalling is susceptible to the laser nonlinearity. In addition, even with 4-ASK signalling, the maximum transmission distance over installed MMF is still limited by differential mode delay (DMD) to less than 300m for 10GE. We propose and demonstrate that directly modulated laser nonlinearity and modal dispersion resulting from DMD can be eliminated simultaneously by using a nonlinear electrical equalizer for 4-ASK signalling in high-speed MMF short links.
We present both optical and electrical compensation of Differential Mode Delay (DMD) in Multimode Fiber (MMF) links. Based on the rigorous analysis of optical compensation of DMD, a new kind of Dispersion Compensating Fiber (DCF) is proposed. We show that 10Gb/s Ethernet reach for conventional MMF can be extended to 300m by splicing 10~35m of this DCF. We prove that ISI resulting from DMD can be mitigated with electrical compensation by using linear or nonlinear equalizers. Moreover, we demonstrate that combination of optical and electrical compensation can dramatically enhance the bandwidth of installed MMF. Simulations carried out on six types of typical installed MMF for Overfilled Launch (OFL) as well as Restricted Mode Launch (RML) condition exhibit that transmission distance for conventional MMF with bandwidth-distance product 500MHz-km can be extended to 1000m or beyond with 2dB Eye-Opening Penalty (EOP).
In this paper, an overview of promising solutions for multi-level modulation formats, primarily based on phase shift keying (PSK), in optical WDM-transmission is presented and some recent results are summarized. With PSK modulation the option of multi-level signalling is offered which means that more than one bit is mapped on one transmitted symbol and thus offering bandwidth reduction for increased spectral efficiency. We investigate the performance with respect to receiver sensitivity, non-linear fibre impairments, chromatic and polarization mode dispersion. Last but not least the implementation-effort at high speed must comply with economic constraints.
In this paper, an overview of promising solutions for new modulation formats in optical WDM-transmission is presented and some recent results are summarized. We compare these formats with the standard conventional intensity modulation scheme. For real world applications it is necessary that these formats not only offer bandwidth reduction but also prove sufficient performance with respect to interference and intermodulation from adjacent channels, non-linear fibre properties, chromatic and polarization mode dispersion and noise resistance. Last but not least the implementation-effort at high speed must comply with economic constraints.
In order to increase the transmittable data rate and to enlarge the transmission distance, the spacing between WDM channels has to be decreased while the optical transparent length must be increased both giving rise to interchannel crosstalk induced by fiber nonlinearities like cross-phase modulation (XPM). Thus, the development of modulation techniques being robust towards these effects is necessary. Recently, phase shift keying (PSK) techniques have attracted remarkable interest. For PSK-techniques, the optical power as a function of time is approximately constant (for nonreturn- to-zero (NRZ) signaling) or periodic (for return-to-zero (RZ) signaling). This is an advantageous property for the reduction of nonlinear phase modulation (PM) induced by the effect of XPM. On the other hand, since for PSKtechniques the information is carried by the phase of the optical carrier, the sensitivity to the nonlinear PM is high. In our contribution, we present an analytical model for the XPM-induced PM. With the help of this model and the visualization of XPM in the complex plane, we prove that the differential self-homodyne implementation of PSK is robust towards the nonlinear PM while PSK-techniques using a local oscillator in the receiver are extremely sensitive.
In this paper we investigate the maximum unrepeatered and uncompensated transmission on standard single mode fiber (SSMF) with single-sideband (SSB) modulation. To increase the optical signal to noise ratio (OSNR) we use a backward pumping Raman source. With the combination of both the bandwidth reduced modulation scheme and the OSNR improvement with Raman amplification we can enlarge the error-free uncompensated and unrepeatered transmission length for SSB modulation up to 200km.
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