Hien T. T. Pham

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.

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.

Novel multi-code pulse position modulation for performance improvement of 2-D OCDMA systems

In OCDMA systems, pulse position modulation (PPM) and multi-code modulation (MCM) have been proven as efficient methods for respectively mitigating multiple access interference (MAI) and dispersion. In this paper, we propose to combine PPM and MCM to create a novel modulation scheme, multi-code pulse position modulation (MCPPM). We then theoretically analyze the performance of the 2-D OCDMA system using MCPPM over a linear dispersive channel, while taking into account the negative impact of MAI, optical-beating interference (OBI), and receiver noise. It is seen that the use of MCPPM can simultaneously relax the effects of MAI, OBI, and dispersion. As a result, in comparison to the conventional systems using PPM and MCM, the one using MCPPM can support a higher user bit rate and a larger number of users. Also, a lower transmitted power can be achieved.

All-optical two-way relaying dual-hop FSO systems using network coding over atmospheric turbulence channel

All-optical two-way relaying is an advanced techniques that helps to improve the data-rate and throughput of free-space optical (FSO) communication systems. In this paper, we propose to implement all-optical two-way relaying in dual-hop FSO systems using network coding. The relay node that is based on optical regenerate-and-forward relaying is equipped with an all-optical XOR gate and optical hard-limiters. Performance of the proposed system is analyzed under the effect of atmospheric turbulence and noise induced by the background light. Based on the closed-form expression for bit-error-rate (BER) derived in this paper, the numerical results demonstrate the feasibility of the proposed systems. In addition, the numerical results provide the useful information for system design such as optimum range of threshold power, required transmitted power, and supportable transmission distance.

Performance of relay-assisted free-space optical CDMA systems over log-normal atmospheric turbulence channels

In this paper, we propose to use relay transmission as a powerful technique for performance improvement of free-space optical code-division multiple-access (FSO/CDMA) systems. Relay nodes are based on chip detect-and-forward to avoid decoding process that makes the relay nodes more complex. We formulate a closed-form expression for bit-error rate (BER) of proposed FSO/CDMA systems over log-normal atmospheric turbulence channels taking into account channel loss due to attenuation and beam divergence. Multiple-access interference and background noise are also considered in our analysis. The numerical results show that the relay transmission is an efficient solution to improve the system performance. Thanks to this solution, FSO systems can achieve low BER, long distance, and high data rate. In addition, the advantages of using two-dimensional (2-D) prime code over 1-D one are also investigated in this paper.

A comprehensive performance analysis of PPM-based FSO systems with APD receiver in atmospheric turbulence

In this paper, a model of Gaussian pulse propagation over atmospheric turbulence channel is used to comprehensively analyze the performance of PPM-based free-space optical communication (FSO) systems with APD receiver. This model simultaneously takes into account all effects of atmospheric turbulence including intensity fluctuation and pulse broadening, which has not been considered in previous works. In addition, the impacts of APD shot noise, background noise, and thermal noise are included in our analysis. The numerical results show that, when M ≤ 128, the main factor that limits the system performance is intensity fluctuation. However, when M >; 128, optical pulse is so short that the effect of pulse broadening becomes dominant. Moreover, we found that APD gain of 60 is the optimum value that helps to achieve the lowest bit-error rate. Finally, based on channel capacity, we are able to determine the maximum transmission length of the system.

Novel 2-D OCDMA systems using multi-code modulation and optical hard-limiters

A novel 2-D OCDMA system using multi-code modulation (MCM) and optical hard-limiters (OHLs) is proposed in this paper. The proposed system has abilities of simultaneously mitigating dispersion, multiple access interference (MAI), and optical beating interference (OBI). The numerical results show that the proposed system allows for low bit error rate, large number of supportable users, low required power, and high users bit rate. For example, the system using 8-MCM can support 38 users at the users bit rate 2.5 Gbit/s when the transmitted power per bit is −5 dBm and the transmission length is 20 km.

Effects of noise and distortion on performance of OFDM millimeter-wave RoF systems

In this paper, we theoretically investigate the effects of noise and nonlinear distortion on the performance of orthogonal frequency-division multiplexing (OFDM) signal carried by millimeter-wave radio over fiber (RoF) systems. Various noise components are considered in this paper including shot noise, thermal noise and relative intensity noise. In addition, we take into account the effect of nonlinear distortion caused by two non-linearity devices that are Mach-Zehnder modulator (MZM) and photodetector. Numerical results show that the effect of noise can be effectively mitigated by increasing the optical transmitted power or the modulation index of MZM. However, when modulation index is larger than a specific value, the nonlinear distortion is likely dominant over the noise, which results in degradation of the system performance. Optimum values of modulation index for some special cases are shown in this paper.

Performance of multi-hop M-ary PPM FSO systems with SIMO links over strong atmospheric turbulence channel and pointing errors

Strong atmospheric turbulence and pointing errors are major challenges in free-space optical communication (FSO) systems. In this paper, in order to overcome these challenges, we propose to employ multi-hop transmission with single-input multiple-output (SIMO) links (i.e., receiver diversity) in FSO systems. In addition, M-ary pulse-position modulation (PPM) and symbol detect-and-forward relaying scheme will be used to further improve the system performance. We formulate the closed-form expressions for bit error rate (BER) of multi-hop M-ary PPM FSO systems with equal gain combining (EGC) and maximum ratio combining (MRC) diversity techniques taking into account the effects of background noise and thermal noise. Numerical results indicate that the combination of multi-hop transmission and receiver diversity is a great solution for reducing effects of strong atmospheric turbulence and pointing errors.