Free-space optical (FSO) communication has recently gained a lot of interest for last-mile terrestrial applications. Some of its advantages include high data rates, ease of deployment, license-free operation, and high security. However, the weather-dependent optical wireless channel introduces attenuation and intensity variations known as scintillation which impose severe challenges for reliable data transmission. The distance dependence of both attenuation and scintillation motivates the use of relays as a means of improving the system performance and extending the range of communication. In this paper, we advocate the use of all-optical relays equipped with erbium-doped fiber amplifiers (EDFAs), which, in contrast to conventional FSO relays with electrical amplification, avoid optical-to-electrical and electrical-to-optical conversions. We develop accurate signal and noise models for fixed and variable gain all-optical and electrical relaying which include the effects of all relevant system parameters and types of noise. For performance evaluation, we analyze the outage probability of all-optical relaying in lognormal fading for dual-hop and multi-hop transmission. Our results show that all-optical relays, while simpler from an implementation point of view, outperform electrical relays unless the number of relays is very large. Moreover, for a fixed source-destination distance, performance improves as the number of hops (relays) increases up to a certain point beyond which adding more hops deteriorates performance.

Free-space optical (FSO) communications has recently received considerable attention for last-mile terrestrial applications and wireless backhaul in mobile communication systems. The performance of these systems is mainly impaired by weather-dependent attenuation and the intensity variations of the channel known as scintillation. The distance-dependence of both attenuation and scintillation motivates the use of relays as a means of improving system performance and extending the communication link. In this paper, we propose an exact and practical model for all-optical relays employing erbium-doped fiber amplifiers (EDFAs), which avoid the optical-to-electrical and electrical-to-optical conversions required in conventional FSO relays employing electrical amplification. The proposed model is shown to be more accurate than existing simplified models and an outage probability analysis for all-optical EDFA-based relaying in the presence of lognormal fading is presented. Our results show that FSO systems with simple all-optical relays achieve a better performance compared to systems with relays employing electrical amplification.

Atmospheric turbulence-induced fading is one of the main impairments affecting free-space optics (FSO) communications. In this paper, we consider FSO systems with intensity modulation and direct detection (IM/DD) and derive a closed- form expression for the asymptotic pairwise error probability of general FSO space-time codes (STCs) for two lasers and an arbitrary number of photodetectors for channels suffering from Gamma-Gamma fading. Furthermore, we provide a simple design criterion for FSO STCs which is used to establish the quasi-optimality of previously proposed FSO repetition codes. We also show that STCs optimized for RF systems achieve full diversity in FSO systems but are suboptimal as far as the coding gain is concerned. Simulation results confirm the analytical findings of this paper.

Atmospheric turbulence induced fading is one of the main impairments affecting free-space optics (FSO) communications. In recent years, Gamma-Gamma fading has become the dominant fading model for FSO links because of its excellent agreement with measurement data for a wide range of turbulence conditions. However, in contrast to RF communications, the analysis techniques for FSO are not well developed and prior work has mostly resorted to simulations and numerical integration for performance evaluation in Gamma-Gamma fading. In this paper, we express the pairwise error probabilities of single-input single- output (SISO) and multiple-input multiple-output (MIMO) FSO systems with intensity modulation and direct detection (IM/DD) as generalized infinite power series with respect to the signal- to-noise ratio. For numerical evaluation these power series are truncated to a finite number of terms and an upper bound for the associated approximation error is provided. The resulting finite power series enables fast and accurate numerical evaluation of the bit error rate of IM/DD FSO with on-off keying and pulse position modulation in SISO and MIMO Gamma-Gamma fading channels. Furthermore, we extend the well-known RF concepts of diversity and combining gain to FSO and Gamma-Gamma fading. In particular, we provide simple closed-form expressions for the diversity gain and the combining gain of MIMO FSO with repetition coding across lasers at the transmitter and equal gain combining or maximal ratio combining at the receiver.

Atmospheric turbulence induced fading is one of the main impairments affecting free-space optics (FSO). Gamma-gamma fading has become the dominant fading model for FSO links because of its excellent agreement with measurement data for a wide range of turbulence conditions. In this paper, we express the bit error rate of intensity modulated FSO with direct detection in single-input single-output and multiple-input multiple-output gamma-gamma fading channels as generalized infinite power series with respect to the signal-to- noise ratio. For fast numerical evaluation these power series are truncated to a finite number of terms and an upper bound for the associated approximation error is provided. Another contribution of this paper is the extension of the well-known RF concepts of diversity and combining gain to FSO and gamma-gamma fading.

Two new Bluetooth receiver designs, the so-called modified limiter-discriminator detector with integrate and dump filtering (LDI) and noncoherent sequence detection (NSD), have recently been proposed in the literature. These receivers have been shown to improve the Bluetooth system performance in terms of physical-layer measures like bit-error rate (BER) and packet-error rate (PER) compared to the conventional LDI receiver. In this paper, we present a more comprehensive performance comparison for these receivers, which includes consideration of the spatial distribution of Bluetooth devices, channel propagation, data traffic, scheduling, automatic repeat request (ARQ), and baseband packet selection. The performance is measured in terms of practically relevant metrics such as end-to-end delay and throughput. Our numerical and simulation results verify that incorporating the new receivers into Bluetooth devices can considerably improve data-rate and quality-of-service performance compared to employing state-of-the-art LDI receivers.