Ing. Jan Bohata, Ph.D.

We demonstrate a fifth generation (5G) new radio (NR) signal transmitted by an analog optical and seamless antenna wireless connection at the frequency band of 60 GHz to exploit a high-frequency unlicensed frequency range. An optical frequency doubling technique, using two Mach–Zehnder modulators operating in a carrier suppressed and linear regime, respectively, was adopted to obtain the desired millimeter wave frequency at the photodetector’s output. The proposed system was tested with the 5G NR signals with a maximum bandwidth of 200 MHz and 64 quadrature amplitude modulation format. It was shown that the signal transmitted through the optical fiber and free space optical link with 1 m long seamless antenna transmission at 62 GHz was capable of meeting the signal quality requirements in terms of error vector magnitude. Moreover, the system phase noise performance showed an almost negligible difference between the various system configurations.

We propose a stable full-duplex transmission of millimeter-wave signals over a hybrid single-mode fiber (SMF) and free-space optics (FSO) link for the fifth-generation (5G) radio access networks to accelerate the Industry 4.0 transformation. For the downlink (DL), we transmit 39 GHz subcarrier multiplexing (SCM) signals using variable quadrature amplitude modulation (QAM) allocations for multi-user services. As a proof of operation, we experimentally demonstrate the transmission of 3 Gb/s SCM signals (1 Gb/s per user) over a hybrid system consisting of a 10 km SMF and 1.2 m FSO link. For the uplink (UL), satisfactory performance for the transmission of 2.4 Gb/s 5G new radio (NR) signal at 37 GHz over the hybrid system is experimentally confirmed for the first time, to the best of our knowledge. The measured error vector magnitudes for both DL and UL signals using 4/16/64-QAM formats are well below the third generation partnership project (3GPP) requirements. We also further evaluate by simulation the full-duplex transmission over the system in terms of received optical and RF powers and bit error rate performance. A wireless radio distance of approximately 200 m, which is sufficient for 5G small-cell networks, is estimated for both DL and UL direction under the heavy rain condition, based on the available data from Spain. Furthermore, simulation for the DL direction is conducted to verify the superior performance of the system using variable QAM allocation over uniform QAM allocation. Using a variable modulation allocation, up to five users (2 Gb/s per user) can be transmitted over a hybrid 10 km SMF and 150 m FSO link.

In this paper, we demonstrate the experimental transmission of radio signal in the n79 new radio (NR) 5G frequency band over a fronthaul optical infrastructure consisting of a 10 km standard single-mode fiber, 100 m long free-space optics channel and a seamless 2 m long radio frequency link for downlink and uplink characterization. The predefined signal using 64-quadrature amplitude modulation and 20 MHz bandwidth has been used for testing while the error vector magnitude (EVM) parameter was measured. The results are also presented in terms of received optical and electrical power as well as the signal-to-noise ratio. The successful transmission over the whole proposed network with the EVM below the required limit of 9% is demonstrated for the received optical power of 0 dBm and 2.3 dBm for the downlink and uplink, respectively.

This paper describes the experimental demonstration of the hybrid optical/millimeter wave signal generation and transmission over combined optical fiber and free space optics fronthaul network with a seamless antenna link. An electrical bandpass filter is used to filter out the spectrum after photodetection in order to realize the seamless antenna transmission. The successful transmission of 64/256-quadrature amplitude modulation (QAM) 5G signal with up to 200 MHz bandwidth is presented by using two different setups: one is based on two Mach-Zehnder modulators (MZM) and the other employs a directly modulated laser (DML) to provide more cost efficient fronthaul solution. The DML based approach reveals mildly better performance in comparison to the MZMs in terms of higher achieved signal-to-noise ratio and lower error vector magnitude (EVM). More specifically, the best signal-to-noise ratio and EVM achieved with the DML based setup has been 31.5 dB and 3. 3%, respectively, compared to 30.3 dB and 3.8% with the MZMs based setup while transmitting 256-QAM signal with 100 MHz bandwidth. However, both setups kept the EVM well below the given 9% and 4.5% limit for 64- and 256-QAM, respectively.

This letter presents results of an experimental measurement campaign involving the deployment of combined radio over the fiber and radio over free space optics (FSO) technology in the cloud-based fifth generation (5G) fronthaul network operating in the millimeter wave (mmW) area. For this purpose, we have used 10 km of optical fiber, 50 m long outdoor FSO link and 1 m long antenna seamless radio frequency transmission at 39 GHz. The results show excellent performance in terms of the phase noise and the signal-to-noise (SNR) ratio. The error vector magnitude performance depends on the modulation format and are below the standard limits for the 5G new radio signals, with 400 MHz QPSK and 64-QAM showing almost identical results for SNR of up to ~19 dB whereas 256-QAM signal offering the best spectral efficiency. Moreover, we investigate the received mmW signal deterioration due to the atmospheric conditions in the FSO channel.

The telecommunication world is experiencing the 5th generation (5G) networks deployment including the use of millimeter wave (mmW) frequency bands to satisfy capacity demands. This leads to the extensive use of optical communications, especially the optical fiber connectivity at the last mile access and the edge networks. In this paper we outline fiber and free space optics (FSO) technologies for use as part of the 5G optical fronthaul network. We investigate two different mmW transmission schemes based on (i) the conventional analog radio over fiber transmission using one Mach-Zehnder modulator (MZM) with double sideband (DSB) optical modulation, and (ii) an optical-based frequency doubling with one MZM biased at the null point to introduce carrier suppression DSB (CS DSB) transmission and second MZM used for data modulation. Both systems are assessed in terms of the error vector magnitude, signal-to-noise ratio, dynamic range and phase noise. We consider a configuration for the fronthaul network in the frequency range 2 (FR2) at 27 and 39 GHz with the scale of bandwidth up to 400 MHz with M-quadrature amplitude modulation and quadrature phase shift keying. Results are also shown for FR1 at 3.5 GHz. Moreover, we investigate for the first time the 5G new radio signal transmission under strong turbulence conditions and show the turbulence-induced FSO link impairment. We finally demonstrate the CS DSB scheme performs well under chromatic dispersion-induced fading for the frequency up to 40 GHz and single mode fiber length of 30 km, whereas the DSB format seems more appropriate for an antenna seamless transmission.

We present a hybrid radiofrequency and microwave photonic link at 25 GHz using the chromatic dispersion of an optical fiber to steer the beam of a three-element planar dipole-based phased antenna array (PAA). Our team has designed and developed an in-house built PAA, experimentally verified its parameters, and successfully demonstrated optically controlled beam steering as measured in an anechoic chamber.Moreover, a detailed analysis of the optically based beam steering in the proposed microwave photonics system has been carried out, with data transmission achieving an error vector magnitude as low as 5.6% for the frequency of 25 GHz and 20 MHz bandwidth.

Flexible multiband signal transmission is proposed and demonstrated over photonically generated millimeter wave signal at 40 GHz. The proposed scheme is based on a low cost directly modulated laser (DML) for data transmission and Mach-Zehnder modulator (MZM) for carrier suppressed double sideband modulation as the optical frequency multiplication scheme employed for millimeter wave signal generation. Multiple bands transmission with 50 MHz 16-quadrature amplitude modulation (QAM) have been experimentally demonstrated along 10 km standard single mode fiber (SSMF) link with a total bitrate of 1.2 Gb/s under a minimum RoP of -1.1 dBm. Further experiments with each band employing different modulation formats and bandwidths show successful performance up to 200 Mb/s using quadrature phase shift keying (QPSK), 16-QAM, 64-QAM bands, as a proof of concept towards flexible deployment of future networks, i.e. employing centralized radio access network (C-RAN) architecture.

Multiband data signal transmission using intermediate frequency over fiber (IFoF) is proposed and demonstrated with local photonic millimetre wave (mmW) generation. The proposed scheme is based on a low cost directly-modulated laser (DML) and Mach-Zehnder modulator (MZM) biased at a null point for optical carrier suppression to realize optical frequency multiplication (OFM). Transmission of 4- and 64-quadrature amplitude modulation (QAM) signals providing data rates of 3 and 1.5 Gbps, respectively, over 25 km standard single mode fiber (SSMF) are demonstrated and signals are delivered at 40 GHz to be radiated by the remote radio heads (RRHs). As a proof of concept, also multiband signal of various modulation formats and bandwidths have been transmitted successfully along 25 km fiber link and delivered in the mm W band. Quality signal assessment by error vector magnitude (EVM) measurement at mmW bands is demonstrated and an estimation of the optical sensitivity of the system is provided.

Local and remote photonic millimeter wave (mmW) signal generation schemes are theoretically and experimentally evaluated in order to compare both approaches for practical deployment in a cloud radio access network (C-RAN) fronthaul network. The paper presents a full comprehensive formulation of the frequency response of a system based on a directly modulated laser transmitting data over 40 GHz signal which is generated by external carrier suppressed modulation and optical frequency multiplication. Theoretical and experimental characterization of the system response at baseband and mmW band for local and remote generation setups show very good agreement. The remote configuration leads to a higher electrical output power (i.e., 15 dB higher in 25 km fiber links) than the local generation setup in the mmW band due to the combined effect of chirp and fiber dispersion, although intermodulation distortion is higher in the former case. Transmission experiments using quadrature phase-shift keying (QPSK) signals with 250 MHz bandwidth centered at 0.5 GHz over 10 and 25 km fiber links also confirm the superior performance of the remote setup, whereas the local setup leads to similar results to optical back-to-back (OB2B) measurements, which is also validated with data signals centered at different frequencies within the laser bandwidth frequency range. Finally, experimental results show the quality of the recovered signals in terms of error vector magnitude (EVM) as a function of the received electrical power and demonstrate that no further penalties are introduced by photonic mmW signal generation with respect to electrical back-to-back (EB2B) levels.

The fifth generation of mobile network (5G) places increased demands on the communication infrastructure, which should be formed in particular by optical networks. Moreover, the use of millimeter-wave (mmW) frequency bands contributes to higher degree of the radio frequency and optical systems convergence. Therefore, we show in this paper the comparison between two wireless seamless connections as part of the radio over fiber (RoF) system, namely free space optical link and mmW wireless link operating at the 5G mmW frequency bands of 27 and 39 GHz. We also demonstrate the impact of including a 10 km of optical fiber to a RoF system with a seamless antenna connection at 27 GHz.

This contribution describes the impact of non uniform thermal-induced turbulence on 64-QAM LTE signal transmission at 25 GHz millimeter wave (mmW) signal along hybrid RoF/RoFSO links. Microwave photonic signal generation has been employed by using the optical frequency doubling with intensity modulator biased at the minimum transmission point. The optical and electrical power budgets of different scenarios have been defined and error vector magnitude (EVM) performance has been measured. It is shown that the uniform scenario with high turbulence in the middle of the link is the one with greater impact on the quality of the received data.

With the emergence of the fifth generation and beyond mobile networks, both visible light communications (VLC) and radio frequency (RF) or millimetre wave (mmW) systems are expected to maintain the connectivity in various environments. In outdoor environments the link (VLC or RF) availability is paramount, which is affected by channel conditions. In particular, in vehicular communications other vehicles, harsh environment, and road geometry and structure will have the impact on the link connectivity and availability. In such cases, a front-end antenna solution, which benefits both optical and RF communication links, can be seen as an attractive option that can be fitted in future vehicles. In this paper, we present the design and practical implementation of a planar hybrid VLC/mmW antenna operating at 20.8 GHz and show measured results for characterization of RF and VLC links as well as communications performance. We have used the widely adopted on-off keying and quadrature amplitude modulation schemes with different orders todemonstrate data rates of 5 Mb/s and up to 100 Mb/s for the VLC and mmW links, respectively. By measuringthe bit error rate and the error vector magnitude for VLC and RF links, respectively for each modulation we have shown that the proposed hybrid planar antenna is suitable for example in a typical vehicle-to-ehicle communications.

Multi-element antenna beam steering in freelicence higher frequency bands is one of the crucial features of 5G networks enabling better tracking of the users. In this paper, we present an experimental microwave photonics transmission system operating at 26 GHz where beamforming is fully realized in the optical domain. The system is designed to be deployed as a part of the mobile fronthaul network with an optical fiber span of 15 km. As a proof of concept, a planar 3-element antenna array has been developed and radiation patterns were measured in an anechoic chamber with high agreement between experimental and simulation results.

A directly modulated laser usage is proposed for 16-QAM signal transmission over hybrid optical links using two optical frequency multiplication schemes based on external modulation for 40 GHz millimeter-wave (mmW) signal generation. We provide an experimental comparison of two possible approaches: quadrature bias point external modulation system using additional optical filter and null transmission point with suppressed carrier. The systems are discussed in terms of cost and complexity of mmW photonic transmission. High error vector magnitude performance of mmW transmission over fiber and free space optics links is demonstrated and an estimation of the sensitivity of the systems is provided.

We present a simple but highly accurate modeling technique for real photonic crystal fibers (PCFs) characterization. We determine the influence caused by idealized model parameters. Our technique can be applied to arbitrary PCF air-hole structures, as it takes into account all structural distortions. It requires only an image of the PCF cross-section to create an accurate PCF model. Model outputs are presented in comparison with the measurement of chromatic dispersion curve and the effective mode area. We provide a study on the impact of imprecise determination of glass refractive index on the PCF model accuracy. We demonstrate how the simplification of the air-hole deformations can influence the chromatic dispersion curve. Finally, we show the effect of precise PCF modeling on example of supercontinuum generation.

This paper presents theoretical and experimental bit error rate (BER) results for a free-space optical (FSO) multiple-input-multiple-output system over an arbitrarily correlated turbulence channel. We employ an erbium-doped fiber amplifier at the receiver (Rx), which results in an improved Rx’s sensitivity at the cost of an additional non-Gaussian amplified spontaneous emission noise. Repetition coding is used to combat turbulence and to improve the BER performance of the FSO links. A mathematical framework is provided for the considered FSO system over a correlated non-identically distributed Gamma-Gamma channel; and analytical BER results are derived with and without the pre-amplifier for a comparative study. Moreover, novel closed-form expressions for the asymptotic BER are derived; a comprehensive discussion about the diversity order and coding gain is presented by performing asymptotic analysis at high signal-to-noise ratio (SNR). To verify the analytical results, an experimental set-up of a 2×1 FSO-multiple-input-single-output (MISO) system with pre-amplifier at the Rx is developed. It is shown analytically that, both correlation and pre-amplification do not affect the diversity order of the system, however, both factors have contrasting behaviour with respect to coding gain. Further, to achieve the target forward error correction BER limit of 3.8×10−3 , a 2×1 FSO-MISO system with a pre-amplifier requires 6.5 dB lower SNR compared with the system with no pre-amplifier. Moreover, an SNR penalty of 2.5 dB is incurred at a higher correlation level for the developed 2×1 experimental FSO set-up, which is in agreement with the analytical findings.

Radio over fiber (RoF) transmission systems have been developing rapidly, especially for applications in 5G networks. In scenarios unsuitable for fiber-optics, radio over free-space optics (RoFSO) presents a suitable solution. Nevertheless, free-space optics (FSO) suffers from atmospheric conditions. The use of the 2000 nm band offers several advantages over the commonly used 1550 nm region. We focus on proof-of-concept evaluation of such a 2000 nm RoFSO transmission system. Measured characteristics are compared with a similar 1550 nm RoFSO system. We demonstrate both systems for QPSK and 64-QAM LTE formats, at 5 GHz and 10 GHz with a 20 MHz bandwidth.

In this paper, we study the impact of correlation on the bit error rate (BER) and the channel capacity of a free-space optical (FSO) multiple-input-multiple-output (MIMO) system employing optical space shift keying (OSSK) over a fading channel. In order to study a practical correlated channel, we consider the effect of channel correlation due to both small-and large-scale eddies and show that the use of OSSK over correlated FSO channel can lead to an improved system performance with increasing correlation level of upto 0.9. In this work, we first develop an analytical framework for different performance metrics of the OSSK multiple-input single-output system with correlation and then extend our investigation by proposing an asymptotically accurate mathematical framework for MIMO. We also validate all the analytical results using MATLAB simulations. Finally, we develop an experimental setup of FSO with two correlated links to study the throughput and latency of the links at different turbulence levels.

Microwave photonics is a promising solution to transmit millimeter wave (mmW) signals for the 5th generation (5G) mobile communications as part of a centralized radio access network (C-RAN). In this paper, we experimentally evaluate the impact of turbulent free space optics links on photonically generated mmW signals in the frequency range of 26−40 GHz . We analyze the remote generation of mmW signals over hybrid links based on free-space optics (FSO) and standard single mode optical fiber (SSMF) with −39.97dBm received electrical power and phase noise level at 100kHz as low as −95.92dBcHz at 26GHz . Different thermal distributions along the FSO link have been implemented and Gamma-Gamma model has been employed to estimate the thermally induced turbulence. The results show high electrical power decrease and fluctuation of the generated mmW signal according to the particular level of the turbulence in terms of refractive index structure parameter and thermal distribution along the FSO link. 8Gb/s 16-quadrature amplitude modulation (QAM) data transmission at 42GHz has been demonstrated over the hybrid link with minimal error vector magnitude (EVM) value of 5% whereas turbulent FSO link introduced up to 5dB power penalty.

We present a theoretical and experimental study on the impact of different thermal-induced free-space turbulence distributions on the M–quadrature amplitude modulation (M-QAM) signal transmission in radio frequency K-band over hybrid optical links of standard single mode fiber (SSMF) and free-space optics (FSO). Frequency multiplication using an external intensity modulator biased at the null transmission point has been employed to photonically generate radio signals at a frequency of 25 GHz , included for the frequency bands for fifth-generation (5G) mobile networks. Moreover, extensive simulations have been performed for 10Gb/s with 4-, 16-, and 64-QAM over 5 km of SSMF and 500 m long FSO channels under scenarios with different turbulence levels and distributions. Proof-of-concept experiments have been conducted for 20 MHz with 4- and 64-QAM over 5 km of SSMF and 2 m long FSO channels under turbulence conditions. Both theoretical and experimental systems have been analyzed in terms of error vector magnitude (EVM) performance showing feasible transmission over the hybrid links in the received optical power range. Non-uniform turbulence distributions are shown to have a different impact on M-QAM modulation formats, i.e., turbulence distributions with higher strength in the middle of the FSO link reveal a 1.9 dB penalty when using 64-QAM signals compared to a 1.3 dB penalty using 4-QAM signals, whereas higher penalties have been measured when 4-QAM format is transmitted over turbulence distributions with larger magnitude in the second half of the FSO link. The results have been validated by theoretical predictions and lead to practical consequences on future networks’ deployment.

This paper investigates an optical transmission architecture of a passive optical network (PON), which is compatible with the millimeter-wave radio-over-fiber and free-space optics systems (RoF-FSO) under weak-to-strong atmospheric turbulence (AT) regimes to enable seamless connectivity as part of next-generation broadband wireless access networks. We first analyze and evaluate in simulation the transmission performance of the integrated system at 40 GHz for 10 Gb/s N-pulse amplitude modulation (N-PAM) with N = 2, 4. Link performance shows that, 4-PAM outperforms 2-PAM in terms of tolerance to the combined impairment of fiber chromatic dispersion and AT. We then experimentally demonstrate the proof-of-concept integrated RoF-FSO-PON at 25 GHz using 20 MHz M-quadrature amplitude modulation (M-QAM) signals with M = 4, 16, 64. We show that, for QAM with a higher-order M, the link performance is being more affected by the combined impairments.

Microwave photonics provides attractive solutions for millimeter wave (mmW) signal generation. In this paper, we demonstrate photonically generated multiple mmW signals transmission over a wavelength division multiplexed (WDM) hybrid optical network based on optical fiber and free-space optics (FSO) links. The experimental results demonstrate the generation and reconfigurable signal distribution from a central office to base stations in the frequency range 14 40 GHz with phase noise levels below -87 dBc/Hz. Moreover, 10 Gb/s data transmission has been demonstrated over photonically generated 40 GHz mmW signal. We show that FSO technology provides a possible solution for mmW fronthaul in 5th generation networks to extend the optical access network providing increased wireless accessibility and maintaining transmission capacity.

A new hybrid microwave photonic link based on a polarization division multiplexing Mach-Zehnder modulator (PDM-MZM) is proposed. The link enables co-transmission of millimeter-wave (mmW) and sub-6 GHz wireless signals over a seamless single-mode fiber (SMF) and free-space optics (FSO) channels. Optimization of the chromatic dispersion (CD)-induced power fading regardless of the power fading due to the non-deterministic atmospheric turbulence (AT) is simultaneously demonstrated. Extensive simulation analysis is first presented to examine (i) the impact of CD on mmW (25 GHz) and sub-6 GHz (2.6 GHz) signals, envisioned for the 5th generation networks, and (ii) optimization of CD-induced power fading by changing the phase relations between the optical carrier and optical sidebands in each polarization channel using single tunable polarization controller. A proof-of-concept experiment is finally performed to simultaneously deliver 25 GHz and 2.6 GHz signals with 4/16/64-quadrature amplitude modulation over (i) 20 km SMF and 2 m radio wireless link and (ii) 20 km SMF, 4.2 m FSO (with AT) and 2 m radio wireless links. The optimization of the CD-induced power fading is experimentally verified and link performance shows high tolerance to CD with no power penalties and the measured error vector magnitudes well below the required limits. The predicted bit error rates are also below the forward error correction threshold of 2×10−4

A 5 Gb/s 32-QAM data transmission over a hybrid standard single-mode fibre (SSMF) and free-space optics (FSO) link has been experimentally demonstrated in the 42 − 90 GHz range for 5G networks deployment. Four-wave mixing (FWM) nonlinear effect in a semiconductor optical amplifier (SOA) experienced by a carrier suppressed double sideband modulated signal has been employed to photonically generate millimetre wave (mmW) signals with reduced electronics bandwidth. Both simulation and experimental results have been shown to describe the error vector magnitude (EVM) performance of the system. Large operation bandwidth is shown in our experimental setup, where measured penalties for 42 and 66 GHz are below 1 dB whereas 90 GHz leads to 3 dB penalty at 12 % EVM threshold. Sensitivity has also been estimated for optical back-to-back (OB2B), after SSMF and hybrid link transmissions for different mmW frequencies.

This paper investigates integrated radio-over-fiber and radio-over-free-space optic links in a passive optical network architecture for ubiquitous wireless coverage. Two optical millimeter-wave generation techniques, namely optical heterodyning and optical modulator-based up-conversion are used. In the first configuration, simulation results are achieved for transmission of 20-40 Gb/s 4-PAM over the 60 GHz heterodyned hybrid link. In the second configuration, the transmission of 20 MHz 4-QAM over the up-converted 25 GHz hybrid system is examined. System performance evaluation and practical feasibility are carried out in terms of the received optical powers, bit error rates, eye diagrams, error vector magnitude and constellation diagrams.

In the modern wireless networks, millimeter-wave radio-frequency (RF) bands are becoming more attractive as they provide larger bandwidth and higher data rates than the today-used systems operating at frequencies below 6 GHz. In addition, according to the fact that coaxial cables exhibit extremely high attenuation for millimeter-wave RF signals, analog radio over fiber techniques (RoF) form a promising technology for delivering unaltered radio waveform to a remote antenna. This paper experimentally analyzes three types of RoF modulations, namely a directly modulated laser, an electro-absorption modulator, and a Mach-Zehnder Modulator. The primary focus is on the implementation of each RoF transmitter in an RoF system, such as those in 5G networks. The experimental study includes a detailed characterization of an RoF system with a 50-m long outdoor free-space RF channel operating in the frequency band of 25 GHz. Frequency response (S-parameters) and third-order nonlinear distortion are investigated in detail. Tests of EVM performance were conducted using an orthogonal frequency division multiplexing signal modulated with 16-quadrature amplitude modulation (16-QAM) with a long-term evolution signal. It is demonstrated that the transmitters studied can operate under a 13.5% EVM limit given for 16-QAM. Apart from the detailed system performance, the considerable power fluctuations in the 25 GHz free-space RF outdoor channel are reported.

The authors propose an integrated Mach–Zehnder interferometer and diversity combining receiver to mitigate the atmospheric turbulence-induced fading in a millimetre-wave (mmW) radio-over-free-space optics (RoFSO) link. They use a carrier frequency of 28 GHz as recommended for the fifth-generation wireless access networks and consider two optical mmW signal generation schemes, namely double-sideband (DSB) and single-sideband (SSB). In direct detection (DD)-based RoFSO, the link performance is limited by atmospheric turbulence. They show that the proposed Rx can overcome this detrimental effect, which is verified by investigation of a 10 Gb/s 16-quadrature amplitude modulation orthogonal frequency-division-multiplexing signal at 28 GHz over a 1 km free-space optics link under weak and strong turbulence regimes. For the DSB scenario, the proposed Rx offers improved error vector magnitudes of about 0.8 and 5.7%, and modulation error ratios of 1.3 and 4.9 dB under weak and strong turbulence regimes, respectively, compared with the DD receiver (DD Rx). For the SSB scenario under weak turbulence, the proposed Rx achieves a 4 dB improvement in the receiver sensitivity and four orders of magnitude enhancement in the bit error rate over the DD Rx. The proposed Rx can be integrated on a single chip for further cost reduction.

In this paper, we numerically and experimentally present the bandwidth constraints of a cost-effective 5G mobile fronthaul based on a directly-modulated laser for data modulation and a Mach-Zehnder modulator-based optical double sideband with carrier suppression scheme for optical millimeter wave (mmW) signal generation. The effect of chirp, fiber dispersion and a combination of both on different bandwidth M-Quadrature Amplitude Modulation (M-QAM) signals, i.e. M = 4, 16 and 64, at 40 GHz has also been investigated. Simulation results are first carried out to evaluate the impact of higher chirp of the directly-modulated laser on the link performance as a function of modulation format and signal bandwidth. We then experimentally demonstrate the same scheme transmitting M-QAM signals with bandwidths ranging from 50 to 1000 MHz over a 10 km long single mode fiber. Both experimental and simulation results show that larger signal bandwidths lead to higher optical power penalties due to the combined effect with the error vector magnitudes (EVMs), however still satisfying the required limits of 3GPP standard for all QAM signals. Experimental measurements also show the feasibility of including free space optics links in the optical distribution network with no further significant penalties. Finally, a multiband signal (three-band) transmission is demonstrated leading to an increase of the total bitrate with the measured EVMs are well below the EVM requirement.

We propose and investigate for the first time a seamless millimeter-wave (mmW) radio-over-fiber (RoF) and radio-over-free-space optics (FSO)-based downlink for use in a passive optical network architecture using 4-, 16- and 64-quadrature amplitude modulation (QAM) for broadband wireless access (BWA) networks. The proposed system is implemented in both experiment and simulation to realize continuous and ubiquitous coverage in urban and rural areas. We outline, a proof-of-concept demonstration of 4-, 16- and 64-QAMs at data rates of 34, 67 and 100 Mb/s, respectively transmitted over a 15 km standard single-mode fiber (SMF), which is then optically up-converted to 25 GHz for transmission over a 10 km of SMF and a 2 m of FSO channel under a non-uniform turbulent condition. We show the measured error vector magnitude (EVM) values of 13, 9.2 and 7.3% for 4-, 16- and 64-QAM, respectively, which are below the corresponding standard EVM requirements and therefore confirm the practicality of the proposed hybrid system. Depending on the data rates, each modulation can be adaptively configured. We report a simulation of a 10 Gb/s 4- and 64-QAM hybrid RoF-FSO downlink under an extended non-uniform turbulence regime to verify the feasibility of the proposed scheme for use in practical applications. By implementing the decision-directed carrier phase recovery and linear electrical equalization, EVMs can be efficiently reduced below the required limits. We further evaluate the proposed system performance in terms of the bit error rates, constellation diagrams, received optical and mmW powers. Using state-of-the-art K-band power amplifiers and conical horn antennas, the maximum wireless transmission distance is estimated to be about 135 m for use in the last-mile BWA networks.

In this chapter, an all-optical FSO relay-assisted system is proposed to mitigate the destructive effects of the distance-dependent AT-induced fading. Relays are inserted directly in the link in order to reduce the AT-induced link loss, thus extending the link span and ensuring higher link availability as well as improving the overall system performance. Two all-optical relaying schemes are proposed and investigated, namely all-optical amplify-and-forward (AOAF) and all-optical regenerate-and-forward (AORF) FSO relay-assisted approaches. For the AOAF approach, the performance analysis of triple-hop AOAF FSO communications is done under the impact of nonhomogeneous atmospheric turbulence. The AORF relaying approach is then proposed to overcome the limitation imposed by AOAF system, where the signal and noise are accumulated at each relay, thus limiting the number of relay nodes that can be used.

This paper presents experimental results for an all-optical free-space optical (FSO) relay-assisted system by employing an all-optical regenerate and forward (AORF) scheme in order to increase the transmission link span. The ultra-short pulse (i.e., 2 ps) regeneration technique based on Mamyshev method is adopted. We have developed a dedicated experimental test-bed composed of optical fiber components and FSO links to demonstrate the proposed scheme and evaluate its performance in terms of the Q-factor and bit error rate (BER) under turbulence regimes for both single and dual-hop network architectures. We show that, using the AORF a hundred times improvement in the BER performance is achieved compared to the amplify-and-forward scheme for a fixed signal-to-noise ratio under turbulence conditions.

This paper outlines experimental investigation of the performance of the wireless optical network based on an all-optical triple-hop free space optical (FSO) communications employing amplify-and-forward relaying under the influence of atmospheric turbulence. We present new results for the bit error rate (BER) performance for seven possible turbulence network scenarios for relay-assisted FSO link and validate them with numerical simulations based on Gamma-Gamma turbulence model showing a good agreement between them. We also show results, which elucidate the impact of non-homogeneous turbulence along the entire transmission link span for the multiple-hop relay-assisted FSO system. More specifically we show that the BER performance considerably deteriorates for the case where turbulence is near to the receiver end. We outline that for a target BER of 10−4 the signal-to-noise ratio penalty can be as high as 9 dB compared to the case with no turbulence.

We experimentally demonstrate photonic millimetre wave signal generation and distribution over hybrid networks based on wired and wireless links. Optical frequency multiplication based on carrier-suppressed modulation has been employed for generating millimetre wave signals in the 40 - 90 GHz frequency range and 10 Gb/s 32-QAM signals have been successfully transmitted over fiber/FSO networks showing the potentiality of such architectures for future 5G networks deployment in terms of flexibility and coverage.

Two experimental configurations of a hybrid K-band (25 GHz) microwave photonic link (MPL) are investigated for seamless broadband wireless access networks. Experimental configurations consist of optical fiber, free-space optics (FSO) and radio frequency (RF) wireless channels. We analyze in detail the effects of channel impairments, namely fiber chromatic dispersion, atmospheric turbulence and multipath-induced fading on the transmission performance. In the first configuration, transmission of the 64-quadrature amplitude modulation (QAM) signal with 5, 20 and 50 MHz bandwidths over 5 km standard single-mode fiber (SSMF), 2 m turbulent FSO and 3 m RF wireless channels is investigated. We show that, for QAM with a high bandwidth, the link performance is being affected more by atmospheric turbulence. In the second configuration, the 20 MHz 4/16/64-QAM signals over a 50 km SSMF and 40 m FSO/RF wireless links are successfully transmitted with the measured error vector magnitude (EVM) values of 12, 9 and 7.9%, respectively. It is shown that, for all transmitted microwave vector signals, the bit error rate is lower than the hard-decision forward-error-correction limit of 3.8×10−3. Moreover, an extended FSO link span of 500 m for 25 GHz hybrid MPL with 16-QAM at 10 Gb/s under the weak and strong turbulence regimes is evaluated via simulation analysis to mimic a practical outdoor system.

In this paper, various link characteristics and bit error rate (BER) performance of a free-space optical (FSO) multiple-input-single-output (MISO) system with repetition cod- ing (RC) is investigated both experimentally and theoretically. The study is performed in the presence of atmospheric turbulence (AT) over a correlated channel with an erbium-doped fiber am- plifier employed at the receiver (Rx). It is shown experimentally that the measured AT levels of two parallel links are different and the links experience non-identical fading influence. The analytical BER expression for an optically pre-amplified correlated FSO- MISO system with non-identical AT channel is derived. The BER results of the considered system with pre-amplifier is compared with the BER results of an FSO system without pre-amplifier, and it is shown analytically that a 2×1 FSO system with pre-amplifier offers better BER performance in terms of coding gain of 6 dB over the system without pre-amplifier. Moreover, it is also shown that the FSO system with RC gives better performance with identical links as compared to non-identical links. Further, it is also revealed that the diversity order of the considered system is independent of correlation and use of pre-amplifier. All the analytical results are validated by comparing them with Matlab simulations.

We experimentally demonstrate a seamless hybrid transmission of radio-over-fiber (RoF), radio-over-free-space optics (RoFSO) and millimeter wave wireless link using long-term evolution (LTE) 4/16/64-QAM signals for broadband wireless access applications. The signals are transmitted and evaluated over the hybrid channel consisting of 5 km of standard single-mode fiber (SMF), 2 m FSO under the turbulence level up to 3.2×10-11 m-2/3 and 3.3 m 25 GHz wireless channel. The performance shows that, the error vector magnitudes (EVM) are below the 3GPP standard for 4/16/64-QAM signals. We show that, the bit error rate (BER) for all signals is below the forward error correction limit of 10-3. We also perform simulation analysis at higher bit rates of 10 and 15 Gb/s for the hybrid link of SMF (5 km) and FSO (extended up to 500) m under the turbulence level of 1.3×10-15 m-2/3 for the practical outdoor scenario. We achieved acceptable performance for 4- and 16-QAM at 10 and 15 Gb/s.

In this paper we present experimental results from precise measurements of mode field distribution in various commercial multimode (MM) fiber types. We characterize the mode field distribution by the encircled flux (EF) parameter. EF determines optimal launch conditions in the core of the MM fiber, which is crucial especially for accurate characterizing of insertion losses at fiber connections, e.g. in data centers. For the purpose of EF measurement, a universal testbed enabling mode field characterization, integral optical power estimation or fiber connector cleanness evaluation has been proposed and experimentally verified. The uniqueness of the proposed testbed lies in backlighting of the tested MM fiber. This backlighting enables accurate positioning of the MM fiber end-face with the collimating lens to measure in the near-field area. The testbed allows the deployment of variable MM fibers and is also wavelength independent. Therefore it has the high potential within growing usage of plastic or large core fibers for short-range communications, e.g. in automotive or avionics. The obtained results are discussed within a selected real application. Moreover, since the mode field distribution is evaluated even for fibers, which do not have available any standards for EF characterization, we present new recommendations for optimal launch conditions in such fibers.

We present the experimental demonstration of photonic doubling to achieve 25 GHz millimeter wave signal generation, following the 5th generation network frequency band recommendations. A hybrid optical network combining up to 50 km long radio over fiber and 40 m long radio over free space optics systems plus 40 m long radio frequency (RF) 25 GHz wireless channel is demonstrated. The proposed system employs double sideband suppressed carrier modulation with 27 dB optical carrier suppression ratio and RF carrier phase noise below - 107 dBc/Hz with 10 kHz offset. Furthermore, the proposed system is optimized in terms of RF input power and the system quality is tested in terms of error vector magnitude for different modulation formats and variable fiber section lengths.

We propose and experimentally demonstrate a photonics-assisted converged radio-over-fiber (RoF), radio-over-free-space optics (RoFSO) and millimeter-wave (MMW) wireless transmission system for use in broadband wireless access (BWA) networks. The focus is at the emerging frequency band of 25 GHz, as recommended for fifth-generation networks. As a proof-of-concept demonstration, all-optical up-converted long-term evolution test models with 4-, 16- and 64-quadrature amplitude modulation (QAM) are transmitted and evaluated over the proposed hybrid link under weak-to-strong atmospheric turbulence regimes. Link performance shows that, the error vector magnitudes are below the 3GPP standard for 4-, 16- and 64-QAM. We also show that, for all QAM signals under turbulence conditions, the bit error rate performance is below the forward error correction limit of ${10^{ - 3}}$ . Simulation analysis is also performed for the 10 Gb/s hybrid systems under turbulence for an extended FSO link up to 500 m to emulate a practical outdoor environment. Furthermore, we analytically estimate the attainable MMW wireless range for different rain rates in Prague, Czech Republic. The obtained experimental and simulation results confirm the feasibility and potential of the proposed hybrid system for next-generation last mile BWA networks.

This Letter outlines radio-over-fiber combined with radio-over-free-space optics (RoFSO) and radio frequency free-space transmission, which is of particular relevance for fifth-generation networks. Here, the frequency band of 24–26 GHz is adopted to demonstrate a low-cost, compact, and high-energy-efficient solution based on the direct intensity modulation and direct detection scheme. For our proof-of-concept demonstration, we use 64 quadrature amplitude modulation with a 100 MHz bandwidth. We assess the link performance by exposing the RoFSO section to atmospheric turbulence conditions. Further, we show that the measured minimum error vector magnitude (EVM) is 4.7% and also verify that the proposed system with the free-space-optics link span of 100 m under strong turbulence can deliver an acceptable EVM of <9% with signal to noise ratio levels of 22 dB and 10 dB with and without turbulence, respectively.

We present an experimental hybrid radio over fiber (RoF) and radio over free space optics (RoFSO) system employing a directly modulated laser (DML) for a radio frequency band of 24 – 26 GHz. Moreover, a 3.6 m long radio free space link is combined with a pair of wideband antennas. We outline the specific settings of the proposed high-frequency system for deployment in the centralized fronthaul networks as part of the 5th generation wireless technologies. We show a potential extension in terms of the added optical loss when using a 100 MHz signal bandwidth. In order to determine the optimum system performance, the setting of directly-modulated laser bias current is investigated. Measured results illustrate that the performance of the proposed system is less susceptible for up to medium turbulence conditions by maintaining an error vector magnitude (EVM) value below the limit of 9 %, given for 64- quadrature amplitude modulation (64-QAM). The lowest recorded EVM of the whole system for 64-QAM and a 100 MHz bandwidth at a frequency 24 GHz is 4.1 % while using a band pass filter to reduce amplified spontaneous emission noise in the system.

Optical networks are commonly deployed in harsh environmenst. Based on measurement we provide new statistics of optical link performance in high-variant temperature environment with the impact of polarization mode dispersion to the system reliability.

Multi-mode fiber (MMF) networks are under higher pressure with the wide deployment of high data-rate transmission systems. Reachable distance and system reliability in particular is essential for MM fiber usage in strategic areas like avionics or big data centers. Therefore, knowledge of the mode-field distribution within MMFs is crucial for proper design and optimization of such MMF systems. An optimally excited mode-field in MMFs leads to lower insertion losses (IL) at MMF connections and also contributes to achieving a higher effective modal bandwidth (EMB). To properly measure IL in a MM network and characterize the network for high data rates, the encircled flux (EF) parameter determines an ideal mode distribution in a MMF. This paper studies all above-mentioned aspects, to fully describe mode behavior in MM fiber connections. Moreover, potential accidents in terms of MM fiber offset in adaptors are tested and completed with simulations. Behavior of different optical sources on EF is presented.

This paper demonstrates a hybrid radio over multi-mode fibre and free space optics (RoMMF-FSO) system that can be used to extend the transmission range of the 4th generation long-term evolution (4G-LTE) signal in access networks. A single mode filtering technique (SMFT) is used to enhance 4G-LTE performance. The proposed scheme is evaluated in terms of the system transfer function, laser beam profile, and error vector magnitude (EVM). We show that using SMFT increases the RoMMF-FSO system bandwidth by 2 GHz and improves the received optical power by 13.6 dB. Moreover, the proposed system enhances the EVM by 4%. The measured results show that using a 1 km MMF instead of a 1 km SMF will marginally increase the measured EVM from ~6.6% to ~7% with a 0.2 dB power penalty with respect to the LTE EVM limit of 12.5% as is specified for 16-quadrature amplitude modulation. The proposed system is validated practically under atmospheric turbulence conditions to mimic the outdoor environment. Measured EVM results are verified theoretically through transmitting LTE signals with turbulent using log-normal model. We also show that for a FSO link span of 500 m to meet the EVM target of 12.5% the SNR power penalties are ~2 dB and ~11 dB for Rytov variance of 1.2×10sup>-4 and 0.1, respectively compared with no turbulence.

Over the last two decades, a large amount of optical fiber (OF) cables has been deployed as part of the global communication networks. Both the aging of OFs as well as the need to increase transmission data rates, particularly in the backbone, have become hot topics. We present the study of the aged OF deployment in various optical networks including free space optics (FSO) link as a part of modern optical communication networks. Here, we show extended results obtained using a dedicated OF testbed focusing on the long-term monitoring of polarization mode dispersion (PMD) because of its time-varying nature. The adaptation of polarization multiplexed radio over fiber (RoF) and radio over FSO (RoFSO) systems as well as 10 Gbps on-off-keying (OOK) non-return-to-zero (NRZ) intensity modulation with the direct detection system, which is common cost-effective transmission system in passive networks, are demonstrated. Moreover, simulation of 100 and 200 Gbps return-to-zero (RZ) differential quadrature phase shift keying (DQPSK) with direct detection is outlined to verify the impact of aged OF network connected with FSO under turbulence conditions. Results reveal more than 6 dB of power penalty with the aged OF route for 100 Gbps systems. In addition, there is a 0.8 dB power penalty due to the strong seasonal induced PMD fluctuations. The influence of scintillations in terms of Rytov variance for the FSO link is also investigated for weak to moderate turbulence. Finally, we derive an expression for the long-term mean PMD value determined over one-month measured frequency response.

Due to the high demands for the reliability of data centers, the determination of properly excited optical field inside the multimode (MM) fiber core belongs to the key parameters while designing such a MM optical system architecture for data centers. Appropriately excited mode field of the MM fiber provides optimal power budget in connections, leads to the decrease of insertion losses (IL) and achieves effective modal bandwidth (EMB), which is essential for high data rates. Crucial is especially the encircled flux (EF) parameter, which should be properly defined for combinations of variable optical sources and MM fiber infrastructure to provide proper mode-field distribution. In this paper, we present detailed investigation and measurements of the mode field distribution for short MM optical data center links with the emphasis on their reliability. Such characterization is essential for the design of large MM networks. Various scenarios were tested in terms of IL and mode-field distribution to reveal potential problematic scenarios. Furthermore, we focused via simulations and experiments on the estimation of particular defects and errors, which can realistically occur like eccentricity or connector misalignment. Their dependence on EF statistics and functionality of data center infrastructure was evaluated. Finally, we provide recommendations for data center systems and networks, using OM3 MM fiber connections

This paper presents novel experimental results for a 10 Gbps triple-hop relay-based all-optical free space optical (FSO) system by employing the amplify-and-forward (AF) relaying scheme. We provide a mathematical framework for the end-end signal-to-noise ratio (SNR) and the bit-error rate (BER) performance and confirm that the derived analytical results reasonably match experimental results especially at relatively high SNR. The evaluated BER performances under different atmospheric turbulence regimes (modeled by the Gamma-Gamma distribution) show that the considered relay-assisted FSO system offers a significant performance improvement for weak to strong turbulence regimes, even without knowledge of the channel state information. More precisely, at a target BER of 10-5 the proposed scheme offers ~ 5 dB and ~4 dB of SNR gain compared to the direct transmission for turbulence strengths C_n^2 of 3.8×10^(-10) m-2/3 and 5.4 ×10^(-12) m-2/3, respectively.

Today's optical networks are composed of thousands of kilometers of aging optical cables. Many of these cables are located in harsh environments which contribute to induced birefringence of the fibers and a corresponding increase of polarization mode dispersion (PMD). This paper introduces derived statistics from the longest-known running evaluation of a PMD measuring campaign and an investigation into how higher optical power affects these aging systems. Results indicate strong seasonal dependence of PMD on temperature for an optical cable testbed exposed to atmospheric changes, leading to a 16 % increase of mean PMD value in summer. This fluctuation causes bit error rate (BER) limits to be exceeded for 10 Gbps and 40 Gbps non-return-to-zero (NRZ) signals which is a critical issue for applications where high reliability is required. Moreover, due to the high optical power load within old optical infrastructures, a more than 0.15 dB increase of relative loss per year in tested routes, compared to reference routes, has been observed.

We present a detailed procedure of precise chromatic dispersion and mode field characterization of unknown photonics crystal fibers for their further utilization in nonlinear applications. To verify our methods an Endlessly single-mode photonic crystal fiber (ESM-PCF) was used. A finite-element method simulation software was utilized for mode field distribution and chromatic dispersion simulation based on the particular fiber structure. Subsequently, a measurement setup for dispersion characterization was proposed, based on the Mach-Zehnder interferometer. Experiment results include as well the influence of various optical elements in the configuration on the measured fiber parameters. Second experimental setup was realized with the possibility of parallel mode-field diameter measurement and visualization of the fiber core structure. Effective area and dispersion measurements results were compared to the simulation outputs. We achieved up to 5 nm accuracy of zero-dispersion wavelength for a 28 cm long ESM-PCF and maximum mismatch of 2 μm2 with effective mode-field area determination. Our procedures and characterization outputs could be applied to specialty optical fiber design and characterization mainly for nonlinear applications such as supercontinuum generation.

In this paper, the transmission schemes for Centralized Radio Access Network Architecture (C-RAN) based on combination of two technologies - Radio over Fiber (RoF) and Radio over FSO (RoFSO) are simulated and experimentally verified. The proposed setups are optimized for Long Term Evolution (LTE) with 20 MHz bandwidth using 64 Quadrature amplitude modulation in terms of Error Vector Magnitude (EVM). At the first, the measurements of Polarization Division Multiplexing using combination of RoF a RoFSO (PDM-RoF/FSO) is compared with simulation models. This is further extended by combination of PDM-Wavelength Division Multiplexing (WDM)-RoF/FSO and PDM-Subcarrier Multiplexing (SCM)-RoF/FSO, respectively. Results indicate better performance of PDM-SCM-RoF/FSO than WDM RoF/RoFSO in terms of launch power to reach EVM limit.

In this paper the results from extended measurement of dual-polarization (DP) analogue radio over fiber (RoF) in a long term evolution (LTE) cloud radio access network (C-RAN) architecture are presented. This technique is proposed for fiber connections between central offices and remote base stations. Investigation of various optical fiber length is carried out to determine the best system performance in terms of error vector magnitude (EVM) and bit error rate. Maximal achieved distance for the case of LTE bandwidth of 20 MHz is 50 km displaying permissible EVM value of 8.5 % at the radio frequency of 2.6 GHz when using 64 QAM modulation scheme.

This paper proposes an optimum hybrid radio over multi-mode fiber and free space optics (RoMMF–FSO) system to enhance the last mile bottleneck issues of wireless communication systems. Single mode fiber (SMF) and gradient index (GRIN) lenses are used as mode filtering techniques to optimize the system performance by mitigating the modal effects produced by multimode optical fiber networks. We characterize the system based on the beam profile, transfer function, and error vector magnitude (EVM) measurements. The results reveal that both techniques can improve the EVM performance significantly by ~19% and ~30% using a SMF filter and SMF-GRIN lenses, respectively. Furthermore, the analysis of the obtained measurements show the SMF-GRIN lenses is more effective than SMF only with lower EVM values and negligible difference in the optical launch power.

This paper describes the experimental verification of the utilization of long-term evolution (LTE) radio over fibre (RoF) and radio over free space optics (RoFSO) systems using dual-polarization signals for cloud radio access network (C-RAN) applications determining the specific utilization limits. A number of FSO configurations is 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 combination of RoF and RoFSO links.

In this paper we present experimental results from extended measurement campaign for an analog dual polarization (DP) radio over fiber and radio over free space optics (FSO) link as part of the cloud mobile network architecture. System utilization with long-term evolution (LTE) test models is described and different link types are evaluated in terms of the error vector magnitude (EVM). Results imply that combined radio over fiber and radio over FSO schemes in combination with Erbium-doped fiber amplifier (EDFA) can provide almost the same performance for a fiber subpart length of between 10 km to 50 km. Moreover, both sub-systems can operate with a high dynamic range and meet the required EVM limit of 9% for 64- quadrature amplitude modulation (QAM) with the minimum optical signal to noise ratio (OSNR) of 24 dB for a 50 km fiber link and a LTE bandwidth 20 MHz. In addition, the combination of 10 km fiber and FSO link is tested under turbulence FSO channel condition showing EVM values below the threshold of 9 % even under a strong turbulence regime.

In this paper, we propose a hybrid radio over multi-mode/single-mode fibre and free space optics (RoMMF/SMF-FSO) system to enhance the 4th generation-long term evolution (4G-LTE) signal performance when used in local area access networks. A single mode fibre pigtailed gradient index lens is used to minimise the modal effect of the optical link at different turbulence strengths. The proposed system is characterised in terms of the system transfer function, error vector magnitude (EVM), and optical launch power (OLP). Results demonstrate the robustness of the proposed scheme in terms of increased available passband bandwidth by 2 GHz and improving the transmission shape by reducing the system transfer function ripple by more than 5 dB. Successful transmission of the 4G-LTE signal is achieved over a MMF/SMFFSO channel with a 1.5 dB power penalty under the higher turbulence level (i.e., Rytov variance of 0.1).

Relay-assisted free space optics (FSO) communications becomes one of the promising solutions to improve the FSO link capabilities by implicitly reducing the transmission distance and exploiting distance-dependent fading variance of turbulence. Highly motivated by the capabilities of the system, this paper presents bit error rate (BER) performance of an all-optical 10-Gbps FSO relay based system using amplify-andforward signaling is investigated through numerical simulations and experimental implementation. Results show that BER improves up to several orders of magnitudes when using relay based links over the same turbulent distance.

In this paper we characterize a radio over fiber 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 dual polarization (DP) radio over a free-space optical (FSO) communication link using a long-term evolution (LTE) radio signal is proposed and analyzed under different turbulence channel conditions. Radio signal transmission over the DP FSO channel is experimentally verified via error vector magnitude statistics. Based on the results we show that transmitting of the LTE signal over the FSO channel is a potential solution for the last-mile access or backbone networks, when using multiple-input multiple-output based DP signals.

A dual polarization (DP) radio over a free-space optical (FSO) communication link using a long-term evolution (LTE) radio signal is proposed and analyzed under different turbulence channel conditions. Radio signal transmission over the DP FSO channel is experimentally verified by means of error vector magnitude (EVM) statistics. Based on the results we show that transmitting the LTE signal over the FSO channel is a potential solution for the last-mile access or backbone networks, when using multiple-input multiple-output based DP signals.

We report on a novel mode-field adapter that is proposed to be incorporated inside tapered fused-fiber-bundle pump and signal combiners for high-power double-clad fiber lasers. Such an adapter allows optimization of signal-mode-field matching on the input and output fibers. Correspondingly, losses of the combiner signal branch are significantly reduced. The mode-field adapter optimization procedure is demonstrated on a combiner based on commercially available fibers. Signal wavelengths of 1.55 and 2 μm are considered. The losses can be further improved by using specially designed intermediate fiber and by dopant diffusion during splicing as confirmed by preliminary experimental results.

One major drawback of coherent optical orthogonal frequency-division multiplexing (CO-OFDM) that hitherto remains unsolved is its vulnerability to nonlinear fiber effects due to its high peak-to-average power ratio. Several digital signal processing techniques have been investigated for the compensation of fiber nonlinearities, e.g. digital back-propagation, nonlinear pre- and post-compensation and nonlinear equalizers (NLEs) based on the inverse Volterra-series transfer function (IVSTF). Alternatively, nonlinearities can be mitigated using nonlinear decision classifiers such as artificial neural networks (ANNs) based on a multilayer perceptron. In this paper, ANN-NLE is presented for a 16 QAM CO-OFDM system. The capability of the proposed approach to compensate the fiber nonlinearities is numerically demonstrated for up to 100-Gb/s over 1000 km and compared to the benchmark IVSTF-NLE. Results show that in terms of Q-factor, for 100-Gb/s at 1000 km of transmission, ANN-NLE outperforms linear equalization and IVSTF-NLE by 3.2 dB and 1 dB, respectively.

In our contribution we report novel mode field adapter incorporated inside bundled tapered pump and signal combiner. Pump and signal combiners are crucial component of contemporary double clad high power fiber lasers. Proposed combiner allows simultaneous matching to single mode core on input and output. We used advanced optimization techniques to match the combiner to a single mode core simultaneously on input and output and to minimalize losses of the combiner signal branch. We designed two arrangements of combiners’ mode field adapters. Our numerical simulations estimates losses in signal branches of optimized combiners of 0.23 dB for the first design and 0.16 dB for the second design for SMF-28 input fiber and SMF-28 matched output double clad fiber for the wavelength of 2000nm. The splice losses of the actual combiner are expected to be even lower thanks to dopant diffusion during the splicing process.

Oscillation of phenomena birefringence under atmospheric conditions is reported. Paper contains measured values of polarization mode dispersion from long term monitoring scenario and gives illustration about measuring of long time installed cables. Several commonly utilizing measuring techniques were used to determine birefringent properties of the fibers. Results are correlated with temperature changes during different terms to achieve proper comprehensive conception of progress.

Results from tests and analyses of multimode optical fibers for an avionic optical network under a variety of stress conditions are presented. Experiments revealed vibrational and temperature changes of distinct multimode fibers. Results lead to the discussion of influenced insertion losses and especially reduced bandwidth corresponding to modal distribution changes. It was determined that these crucial parameters could affect system reliability when an airplane network intersects thermal and vibrational variable environments.

Wireless optical links can bring new added technologic approach atop of today’s wireless networks forming their backbones and subparts. However several measurements have to be performed in order to get close insight on their availability within infrastructures. The paper reports recent tests at the Czech Technical University in Prague in cooperation with Northumbria University. New testbeds have been developed for outdoor and indoor measurements. Together with reports from packet and BER measurements, paper introduces complementary results on spatial coherence measurements for single link influenced by turbulence.

Optical fibers provide many benefits to the telecommunication systems and their usage has more and more current even in the avionics industry [1]. It offers an attractive solution for airplanes, such a replacing copper conductors to reduce weight of the plane, electro-magnetic interference, bring high-speed communications connection, implementation of optical sensors, etc. Nevertheless for extremely high requirements on safety it is also necessary to precede all inflight influences. For summary of potential threats see e.g. [2]. One can easily found a several conditions of a harsh environment or an unfriendly fiber surrounding which could have a fatal impact to the transmission characteristic as attenuation statistics leading to link drops, time jitters or pulse degradations in time domain. There are different temperature gradients on the deck through which the infrastructure can pass while experiencing differentiated degradation and thus changed conditions within a physical layer. This paper is focused on an analysis of temperature-depended influences on insertion losses and other parameters of optical connectors, combining two types of multimode fibers, purposed for installation within an aircraft optical network.