Abstract-In this paper, the impairments of FSO communication systems caused by angular fluctuations including beam misalignment and angle-of-arrival (AOA) fluctuations are modelled in the presence of both atmospheric turbulence and transceiver vibrations. In particular, assuming FSO receivers with a limited field-of-view (FOV), the fading caused by AOA fluctuations is studied. The outage probability expressions for both coherent and direct detections are derived in both shot-noise-limited and thermal-noise-limited regimes. For direct detection, the optimal receiver FOV that achieves the minimum outage probability is considered. Furthermore, the issue of imperfect phasefront tracking in practical coherent receivers is investigated.

This work investigates spatial-mode multiplexing (SMM) for practical free-space optical communication (FSO) systems using direct detection. Unlike several works in the literature where mutually incoherent channels are assumed, we consider mutually coherent channels that accurately describe SMM FSO systems employing a single laser source at the transmitter with a narrow linewidth. We develop an analytical model for such mutually coherent channels and derive expressions for aggregate achievable rate (AAR). Through numerical simulations, it was shown that there exist optimal transmit mode sets which result in the maximal asymptotic AAR at high transmitted power. Moreover, in order to resolve the reliability issues of such SMM FSO systems in the presence of turbulence, a so-called mode diversity scheme is proposed that can be easily implemented along with SMM FSO systems. It is demonstrated that mode diversity can significantly improve the outage probability and the ǫ-outage achievable rate performance of the multiplexed channels in SMM FSO systems degraded by turbulence.

Free-space optical (FSO) communication provides wireless optical connectivity with high data rates and low-cost implementation; however, its performance is strongly influenced by the power attenuation due to the infrequent adverse weather conditions. This paper proposes a novel highly sensitive dual-mode receiver comprising an array of single-photon avalanche diodes (SPADs) and a PIN photodiode (PD) to enhance the availability of FSO links. In adverse weather conditions, the receiver operates in the SPADmode; whereas, in the clear weather conditions, the receiver works in the PD-mode. A hybrid receiver controller is employed in the proposed receiver to adaptively control the switching process based on the received light levels. The adaptive controller also adjusts the incident photon rate of the SPAD array using a variable optical attenuator (VOA) to optimize the performance of the SPAD unit. Our extensive performance analysis illustrates the superior achievable data rates of the proposed receiver under various weather conditions compared to the traditional FSO receivers with either PD or SPAD array.

In many practical scenarios, single-photon avalanche diodes (SPADs) are good solutions to improve the performance of optical communication systems due to their high sensitivity to photon arrival. SPAD receivers can be implemented in large arrays to achieve higher data rates and additional protection against background light; however, they suffer from a significant intersymbol interference (ISI) if the SPAD dead time is comparable or larger than the symbol duration, i.e., sub-dead-time signal transmission. This work proposes a novel detection scheme designed for high-speed SPAD-based systems to effectively mitigate the degradation induced by ISI. Different from traditional receivers, in the proposed scheme, the information extracted from both the counts and arrival times of photons are utilised for the optimal symbol detection in the presence of the non-linear and random ISI effect due to dead time. Our extensive numerical and experimental results demonstrate the superiority of the proposed photon time information based detection (PTID) scheme in terms of both BER performance and background light tolerance of the communication link. In addition, a linear approximation of the SPAD-based channel is investigated, which illustrates that the traditional equalization methods are effective under some specific circumstances.

Photon counting detectors such as single-photon avalanche diode (SPAD) arrays can be utilized to improve the sensitivity of optical wireless communication (OWC) systems. However, the achievable data rate of SPAD-based OWC systems is strongly limited by the nonlinearity induced by SPAD dead time. In this work, the performance of SPAD-based OWC system with orthogonal frequency division multiplexing (OFDM) is investigated and compared with that of on-off keying (OOK). We employ nonlinear equalization, peak-to-average power ratio optimization by adjusting the OFDM clipping level, and adaptive bit and energy loading to achieve a record experimental data rate of 5 Gbps. The contrasting optimal regimes of operation of the two modulation schemes are also demonstrated.