Ngoc T. Dang

Bit error rate analysis of rectangular QAM/FSO systems using an APD receiver over atmospheric turbulence channels

We theoretically analyze the performance of free-space optical (FSO) systems using rectangular quadrature-amplitude modulation (QAM) and an avalanche photodiode (APD) receiver over atmospheric turbulence channels. Both log-normal and gamma-gamma channel models are used in the analysis for the cases of weak/moderate and strong atmospheric turbulence. The system bit error rate, when Gray code mapping is employed, is theoretically derived taking into account various link conditions and system parameters, including the APD shot noise, thermal noise, channel attenuation and geometrical loss, atmospheric turbulence strengths, and link distances. The numerical results show that using APD with a proper selection of the average gain could greatly benefit the performance of the system; as a matter of fact, in the case of optimal gain, the system using an APD receiver could provide 7 dB gain in comparison with the one with a positive-instrinsic-negative receiver. We also quantitatively discuss the impact of link conditions and system parameters on the selection of optimal APD gain.

All-Optical Relaying FSO Systems Using EDFA Combined With Optical Hard-Limiter Over Atmospheric Turbulence Channels

In this paper, a novel relaying technique is proposed to improve the bit-error rate (BER) performance and distance coverage of high-speed all-optical free-space optical (FSO) communication systems. Particularly, an optical amplify-and-forward (OAF) relaying technique using erbium-doped fiber amplifier (EDFA) combined with optical hard-limiter (OHL) is introduced. The use of OHL enables EDFA-based OAF relaying FSO systems to prevent the accumulation of amplified background noise, which significantly degrades the system performance, when deploying multiple relays. For performance evaluation, we theoretically analyze the proposed system over atmospheric turbulence channels modeled by Gamma-Gamma distribution. A closed-form expression for the end-to-end BER bounds is, therefore, analytically formulated, taking into account other impacts of atmospheric channels, including atmospheric attenuation and geometric spreading of the optical beam, as well as noises caused by the background light and receiver. The numerical results, which are validated by Monte-Carlo simulations, confirm the superiority of the proposed system in comparison with the conventional ones.

Performance improvement of FSO/CDMA systems over dispersive turbulence channel using multi-wavelength PPM signaling.

Previous studies show that, compared to on-off keying (OOK) signaling, pulse-position modulation (PPM) is favorable in FSO/CDMA systems thanks to its energy efficiency and simple detection. Nevertheless, when the system bit rate increases and the transmission distance is far, the FSO/CDMA systems using PPM signaling critically suffer from the impact of pulse broadening caused by dispersion, especially when the modulation level is high. In this paper, we therefore propose to use multi-wavelength PPM (MWPPM) signaling to overcome the limitation of PPM. To further improve the system performance, avalanche photodiode (APD) is also used. The performance of the proposed system is theoretically analyzed using a realistic model of Gaussian pulse propagation. To model the impact of intensity fluctuation caused by the atmospheric turbulence, the log-normal channel is used. We find that, by using MWPPM, the effects of both intensity fluctuation and pulse broadening are mitigated, the BER is therefore significantly improved. Additionally, we quantitatively show that the system performance is further improved by using APD, especially when the average APD gain is chosen properly.

Reducing the Dispersion Effects in Multiwavelength Optical CDMA Systems by Using MCM Signaling

We propose a novel multiwavelength optical code-division multiple-access (OCDMA) system using multicode modulation (MCM) signaling. For the multicode demodulator, we employ a maximum-likelihood detection and optical hard limiters. The work in this paper theoretically analyzes the performance of the proposed system over linear dispersive channels, while taking into account the impacts of multiple-access interference (MAI), optical-beating interference (OBI), and receiver noise. It is seen that the use of MCM can significantly relax the effects of chromatic dispersion. In addition, MAI and OBI can be effectively mitigated in the proposed system. As a result, in comparison with the conventional systems using on-off keying (OOK) modulation and pulse-position modulation (PPM), the proposed system can support a higher user bit rate and a larger number of users. Also, a lower transmitted power is required and a longer transmission length can be achieved.

Effects of atmospheric turbulence and misalignment fading on performance of serial-relaying M-ary pulse-position modulation free-space optical systems with partially coherent Gaussian beam

A novel method is presented to analyse the effects of atmospheric turbulence and misalignment fading (or pointing error) on performance of serial-relaying M-ary pulse-position modulation (PPM) free-space optical (FSO) systems. This study is more comprehensive than previous ones, since the effect of beam size variation due to turbulence by using the partially coherent Gaussian beam model is taken into account. In addition, a closed-form expression is formulated for bit error rate of serial-relaying M-ary PPM FSO systems over Gamma–Gamma atmospheric turbulence channel, taking into account the effects of atmospheric attenuation, extinction ratio and signal-dependent noise. The authors find that the laser sources coherent parameter, which governs the beam size at the receiver, plays an important role in the system design. If this parameter is not chosen properly, the system impairment will be either dominated by pointing error or geometric spreading loss. Thanks to the use of serial-relaying and M-ary PPM, the effects of atmospheric turbulence and misalignment fading is mitigated; hence the ability of combating atmospheric turbulence and the transmission distance of FSO systems are significantly improved. In addition, useful information for system design, such as the required number of relays for a specific turbulence strength and transmission distance, could be obtained from the numerical results.

BER analysis of all-optical AF dual-hop FSO systems over Gamma-Gamma channels

In this paper, we analyze the performance of all-optical dual-hop free-space optical communication (FSO) systems over strong atmospheric turbulence channels. In our analysis, turbulence channels are modeled by Gamma-Gamma distribution and dual-hop transmission is based on optical amplify-and-forward (OAF) technique. We formulate both exact-form and closed-form expressions for bit-error rate (BER) of M -ary pulse-position modulation (PPM) dual-hop FSO systems taking into account the effects of various noises as well as path loss and geometric spreading of optical beam. The numerical results show that strong turbulence has a severe effect on the performance of FSO systems. In addition, background noise and thermal noise at the destination are dominant compared to other terms of noise from the source and the relay. Thanks to OAF technique, the required transmitted power is reduced accordingly to the amplifier gain thus the transmission distance can be extended.

All-optical AF relaying FSO systems using EDFA combined with OHL over Gamma-Gamma channels

In this paper, we newly propose and theoretically analyze the performance of all-optical amplify-and-forward (OAF) relaying free-space optical (FSO) systems using erbium-doped fiber amplifier (EDFA) combined with optical hard-limiter (OHL) over atmospheric turbulence channels. The use of OHL enables OAF relaying FSO systems to remove accumulated background noise, which is one of the main factors that limit the system performance. The performance of proposed systems is analytically studied by which closed-form expression for bit-error rate (BER) is formulated, taking into account the effects of atmospheric turbulence channels and noises caused by background radiation and receiver. The numerical results, which are validated by Monte-Carlo (M-C) simulations, confirm the superiority of the proposed systems in comparison with conventional ones.

Hybrid free-space optics/millimeter-wave architecture for 5G cellular backhaul networks

This paper proposes a cost-effective, scalable network architecture, which combines millimeter-wave (MMW) and free space optics (FSO) technologies that can be employed as a complementary solution for pre-deployed optical fiber based 5G cellular backhaul networks.

A survey of performance improvement methods for free-space optical communication systems

Free-space optical communication (FSO) is a line-of-sight (LOS) technology that enables data transmission based on the propagation of light in free space. Compared to radio frequency (RF) communication systems, FSO ones provide high data rates, unregulated bandwidth, high security, and low power. Therefore, FSO systems are rapidly gaining interest as an effective means of transferring data at high rates over short distances. However, FSO still faces many challenges, including weather-dependent atmospheric attenuation, atmospheric turbulence, and misalignment fading (or pointing error). In this paper, we present some of the exciting research approaches that have been suggested to evaluate and improve the performance of FSO systems. The principle as well as the advantages and disadvantages of each improvement method will be analyzed. In addition, we outline some future research challenges and opportunities.

Performance analysis of incoherent multi-wavelength OCDMA systems under the impact of four-wave mixing

In this paper, we comprehensively analyze the impact of four wave mixing (FWM) on the performance of incoherent multi-wavelength optical code-division multiple-access (MW-OCDMA) systems. We also consider many other interferences and noises, including multiple access interference, optical beating interference, and receiver noise, in the analysis. From the numerical results, we can find the power ranges of different MW-OCDMA systems, in which the impact of FWM is dominant and consequently results in an increase in the bit-error rate of the systems. We also find that the impact of FWM becomes more severe when the frequency spacing is small and/or dispersion-shifted fiber is used. In addition, we quantitatively discuss the impact of FWM on the number of supportable users and power penalty in the MW-OCDMA systems.