Mojtaba Mansour Abadi

FSO Detection Using Differential Signaling in Outdoor Correlated-Channels Condition

In this letter, we introduce a detection technique based on differential signaling scheme for outdoor free space optical communications. This method requires no channel state information (CSI) and does not suffer from the computational load compared with the conventional receivers, where adjusting dynamically the detection threshold level (either based on CSI knowledge, or by using pilots) leads to increased computational time and reduced link throughput. This letter shows that the performance of the proposed technique only depends on the correlation of propagating optical beams. Especially under highly correlated-channels condition the fluctuation of the detection threshold level in the receiver is significantly small. We also show experimentally that under weak turbulence regime the variance of detection threshold level reduces for the correlated channel.

Quantized Feedback-Based Differential Signaling for Free-Space Optical Communication System

We propose a quantized feedback-based differential signaling scheme for 2 × 1 free space optical (FSO) multiple-input single-output communication systems. The analytical bit error rate (BER) for the proposed differential signaling scheme is derived and compared with the existing differential signaling scheme for FSO systems. The asymptotic BER of the proposed scheme is also determined and it is shown by using the asymptotic BER expression that the proposed scheme achieves full spatial diversity of two. Moreover, the effect of the feedback error on the performance of the proposed scheme is studied and it is demonstrated by simulation-based and analytical results that the proposed scheme significantly outperforms a previously proposed differential signaling scheme in terms of turbulence fades reduction for tolerable feedback errors.

Experimental verification of long-term evolution radio transmissions over dual-polarization combined fiber and free-space optics optical infrastructures.

This paper describes the experimental verification of the utilization of long-term evolution radio over fiber (RoF) and radio over free space optics (RoFSO) systems using dual-polarization signals for cloud radio access network applications determining the specific utilization limits. A number of free space optics configurations are proposed and investigated under different atmospheric turbulence regimes in order to recommend the best setup configuration. We show that the performance of the proposed link, based on the combination of RoF and RoFSO for 64 QAM at 2.6 GHz, is more affected by the turbulence based on the measured difference error vector magnitude value of 5.5%. It is further demonstrated the proposed systems can offer higher noise immunity under particular scenarios with the signal-to-noise ratio reliability limit of 5 dB in the radio frequency domain for RoF and 19.3 dB in the optical domain for a combination of RoF and RoFSO links.

Dual Purpose Antenna for Hybrid Free Space Optics/RF Communication Systems

The concept of hybrid free space optical/radio frequency (FSO/RF) communications has been considered for the last mile access network applications. The existing schemes mostly use two separate transceiver antennas for outdoor terrestrial links of few kilometres long. In this paper, we propose and investigate a single hybrid FSO/RF antenna that could be used in high-speed access networks. We outline the design procedures based on the concept of Cassegrain antenna, and show how the FSO transceiver is incorporated into the original design. The proposed antenna is fabricated and extensive measurements are carried out for the radiation pattern, return loss, RF signal-to-noise ratio and the FSO received power. By performing link budget analysis and the results obtained from experiments, we demonstrate the operation of proposed hybrid antenna with 1 × 3 single-input multiple-output configuration and an equal gain combining at the receiver under moderate regime turbulence regime.

Investigation of moderate-to-strong turbulence effects on free space optics — A laboratory demonstration

The increasing demand for higher bandwidth is the motivation behind research studies in free space optical (FSO) communications. In particular, the impact of the atmospheric channel on the FSO link performance has been investigated for decades. Among known channel effects, turbulence is the one, which is thoroughly studied from theoretical and experimental point of views. However, due to limitation of laboratory equipment and channel length, only experimental works in the weak turbulence regime are reported. Therefore, in this paper a dedicated indoor controlled laboratory chamber is used to simulate the outdoor FSO communications for moderate-to-strong turbulence regimes. Instead of increasing the link distance, we generate high temperature gradients along the propagation path of 7 m by blowing cold air in a parallel direction to the laser beam. The performances of the on off keying intensity modulated FSO link is assessed by means of the Q-factor and the optical intensity distribution plots of received signals, thus giving an insight into the level of scintillation achieved in moderate to strong turbulence regimes.

Comparison of different combining methods for space-diversity FSO systems

This paper gives an overview of different signal combining schemes for free space optical (FSO) communication systems employing spatial diversity (SD) for the case of lognormal weak turbulence atmospheric channels. The receiver configurations are explained and categorized into two domains, namely optical and electrical. Moreover, a new combining scheme in the electrical domain, called “logical” domain combining, is introduced. We present closed-form expressions of bit-error-rate for the combining schemes, considering the typical case of a single-input multiple-output (SIMO) system. Through some numerical analyses, we show that depending on the number of receivers and the turbulence strength, the proposed logical combining method outperforms the other ones.

Hard switching in hybrid FSO/RF link: Investigating data rate and link availability

This paper investigates the performance dependence of a hybrid free space optical (FSO)/radio frequency (RF) communication system on the data rate, link distance and link availability. In existing hybrid FSO/RF systems with hard switching, an algorithm is used to switch between FSO and RF and vice versa based on the feedback of the channel state information. In particular, two methods based on time hysteresis (TH) and power hysteresis (PH) are widely used in hard switching protocols. Although switching parameters including the time and power thresholds have been studied, the effect of link parameters has not been investigated in the literature. In this paper, we analyse the performance of a hybrid FSO/RF system under realistic channel conditions for both TH and PH switching schemes. By means of analysis and simulation, we show that increasing the data rate of individual FSO or RF link does not necessarily result in higher data rate and improved link availability in a hybrid FSO/RF system.

Using differential signalling to mitigate pointing errors effect in FSO communication link

In this paper, we investigate a differential signalling (DS) scheme to mitigate pointing errors effect in intensity-modulation/direct-detection (IM/DD) free space optical (FSO) communication systems with the non-return-to-zero on-off keying (NRZ-OOK) modulation format. The most common approach to detect a digital signal is based on comparing the received signal with an optimal threshold level. Normally, this optimal threshold level is set at the mean value of the signal in a clear channel. However, this detection threshold method is inefficient in a channel with fading effects. DS has been reported to mitigate the fluctuation of optimal detection threshold in NRZ-OOK IM/DD FSO communication systems only to combat atmospheric fading effects (e.g., fog and turbulence) and to reduce the background noise of the received signal. In this paper, we adopt the DS scheme to mitigate the optimal detection threshold level fluctuations induced by pointing errors. We also discuss the conditions in which DS cancels out the fluctuation of threshold level. The proposed technique is supported by carrying out experimental investigation of the FSO link with pointing errors. Measured results confirm that the effect of PE on the received signal was highly correlated (i.e., ρ = 0.92, where ρ is the correlation coefficient between channels) thus resulting in reduced variance value of the combined signal threshold level. We also derive an expression for bit-error-rate for the system which is supported by simulation.

Channel characterization of a last-mile access radio over combined fibre and free-space optics system

In this paper, we characterize a radio over fibre and radio over free space optic system for the last mile access network in urban areas and as a solution to the cellular backhaul networks. Special focus is put on the performance of such systems under the atmospheric turbulence. We show that the proposed system performance is almost the same for the entire frequency range of 0.4-4 GHz under the weak or no turbulence regimes. Finally we show that for proposed system with BPSK and QPSK at different RF carrier frequencies the turbulence effect has marginal impact.

A report on H-FSO/RF antenna measurement for outdoor applications

This paper presents a report on experimental set up and results for a hybrid free space optics (FSO)/radio frequency (RF) dual purpose antenna for outdoor applications. The basic design of the antenna is discussed and we present two sets of early experimental results for the signal to noise ratio (SNR) and the Q-factor for the RF and FSO simplex link at the far-field distance of RF antenna, respectively. Over a link span of 15 m the measured SNR and the Q-factor are 45.68 dB and 15.69, respectively, demonstrating the functioning transmission capability of the hybrid FSO/RF link employing the proposed antenna.