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Free-space optical (FSO) communication system provides several advantages over radio frequency (RF) system offering high bandwidth, low cost, small space requirements and more secure transmission which is free from Electromagnetic Interference (EMI). However, when the transmitted light signal passes through the atmosphere it experiences attenuation and fluctuations due to atmospheric turbulence. This paper analyzes the bit error rate (BER) performance of FSO communication systems under strong atmospheric turbulence for on-off keying (OOK), binary phase-shift keying (BPSK), differential phase shift keying (DPSK), quadrature phase shift keying (QPSK) and 8-phase shift keying (8-PSK) for link distances of 500, 1,000, 1,500 and 2,000 m. The probability density function (pdf) of the received irradiance is modelled using the gamma-gamma distribution model. It is found that the system exhibits the best BER performance and compensates the lowest power penalty at BER of 10 for BPSK modulation compared with other modulation techniques which makes BPSK more appropriate to be used with FSO turbulent system.

Graphene-based photodetectors have attracted significant attention for high-speed optical communication due to their large bandwidth, compact footprint, and compatibility with silicon-based photonics platform. Large-bandwidth silicon-based optical coherent receivers are crucial elements for large-capacity optical communication networks with advanced modulation formats. Here, we propose and experimentally demonstrate an integrated optical coherent receiver based on a 90-degree optical hybrid and graphene-on-plasmonic slot waveguide photodetectors, featuring a compact footprint and a large bandwidth far exceeding 67 GHz. Combined with the balanced detection, 90 Gbit/s binary phase-shift keying signal is received with a promoted signal-to-noise ratio. Moreover, receptions of 200 Gbit/s quadrature phase-shift keying and 240 Gbit/s 16 quadrature amplitude modulation signals on a single-polarization carrier are realized with a low additional power consumption below 14 fJ/bit. This graphene-based optical coherent receiver will promise potential applications in 400-Gigabit Ethernet and 800-Gigabit Ethernet technology, paving another route for future high-speed coherent optical communication networks. Graphene-based photodetectors have many advantages for applications. Here, the authors demonstrate a high-speed optical coherent receiver for optical communications based on graphene-on-plasmonic slot waveguide photodetectors.

Radio-frequency technologies are widely applied in many fields such as mobile systems, healthcare systems, television and radio broadcasting, and satellite communications. However, one major problem in wireless communication based on radio frequencies is its impact on human health. High frequencies adversely impact human health more than low frequencies if the signal power transgresses the permissible threshold. Therefore, researchers are investigating the use of visible light waves (instead of the radio-frequency band) for data transmission in three major areas: visible light communication, light fidelity, and optical camera communication. In this paper, we propose a scheme that upgrades the camera on–off keying (COOK) scheme by using it with the multiple-input multiple-output (MIMO) scheme; COOK has been recommended by the IEEE 802.15.7-2018 standard. By applying technologies, such as matched filter, region of interest, and MIMO, our proposed scheme promises to improve the performance of the conventional scheme by improving the data rate, communication distance, and bit error rate. By controlling the exposure time, the focal length in a single camera and using channel coding, our proposed scheme can achieve the communication distance of up to 20 m, with a low error rate.

This paper simulates the high speed optical modulation fiber systems for ultra high spectral efficiency improvement through the digital shift keying techniques employment. The electrooptic modulators are used with compensated fiber communication system. The optical fiber system is composed of three hybrid fibers that are namely single mode/highly nonlinear/dispersion shifted fibers. The Raman amplification system is demonstrated with 10 km dispersion compensated fiber (DCF) for ultra high speed long distance transmission can be achieved up to 450 km. The digital shift keying techniques are used through this work that are namely ASK (amplitude shift keying), PSK (phase shift keying) and FSK (frequency shift keying). The signal per noise ratio/BER and optical/electrical power are measured to detect the high system spectral efficiency. The distributed bidirectional Raman amplification is used for the efficient power transferred through the optical fiber system. The hybrid fiber system with distributed Raman amplification have achieved the optimum optical modulated fiber systems.

Free space optical (FSO) communication is a wireless alternative to fiber-based communication as it may provide a backbone emergency communication link in disaster-hit areas. This paper proposes a full duplex millimeter wave (mm-wave) enabled FSO system to provide a high data rate communication link for search and rescue operations. The proposed system consists of a central unit which is connected to multiple unmanned aerial vehicles (UAVs) through an optical-amplify and forward relay over an FSO channel represented by Log-Normal channel model. An optical comb is generated and Laguerre-Gaussian modes of each wavelength of the optical comb are exploited to carry 10 Gbps differential phase shift keying modulated signals. 60 GHz mm-wave signals are generated at the remote UAVs by employing optical heterodyne detection for downlink transmission towards the user equipment. For the uplink transmission, a dedicated wavelength is used to carry 10 Gbps on-off keying baseband data for live streaming of the disaster-hit area. Furthermore, a single hop scheme is employed to counter the non-line of sight issue of the FSO link. The performance of the proposed model is evaluated under different weather conditions and refractive index structure parameter values. A maximum distance of 3.3 km is achieved for clear sky under low turbulence conditions. Moreover, the effect of misalignment due to hovering of the UAVs on the system performance is also investigated and tolerance in the x (horizontal) and y (vertical) directions is observed at the forward error correction BER limit of 10 - 9 . The simulation results reveal that the proposed system has the potential to provide reliable and quick emergency communication services in disaster-struck areas.

Since the last two decades free space optical (FSO) communication transcends the RF communication and microwave systems because of its capability of carrying high bandwidth, license-free, long-range, small size, high bandwidth, low cost and ease of deployment. FSO communication has few limitations as well like beam dispersion, scintillation, weather effects etc. This paper demonstrates the implementation of FSO link and analysis of transmission performance of FSO channel for broadband access networks. The proposed FSO link is simulated in opti-system to analyze the physical characteristics of transmitter and receiver. The transmission performances of FSO link are validated using on/off keying (OOK) and differential phase shift keying (DPSK) at various atmospheric conditions with different data rates and transmission ranges. The transmission performance of the FSO link is analyzed under the influence of key parameters like launch power, beam divergence, atmospheric attenuation, receiver sensitivity and receiver aperture. The proposed model of the FSO link is investigated using key parameters like optical signal to noise ratio (OSNR), noise figure and bit error rate (BER).

This paper throws light on chaotic shift keying-based speech encryption and decryption method. In this method, the input speech signals are sampled and its values are segmented into four levels, namely L 0, L 1, L 2, and L 3. Each level of sampled values is permuted using four chaotic generators such as logistic map, tent map, quadratic map, and Bernoulli's map. A chaotic shift keying mechanism assigns logistic map for L 0, tent map for L 1, quadratic map for L 2, and Bernoulli's map for L 3 for shuffling the speech samples at every level. Further, the sampled values are permuted using Chen map which uncovers the chaotic behavior. Various testing methods are applied to analyze the efficiency of the system. The results prove that the proposed system is highly secured against the attackers and possesses a powerful diffusion and confusion mechanism for better speech communication in the field of telecommunication.

In this article, a multifunctional optical processing unit for differential eight phase‐shift‐keying (D8PSK) and differential quadrature phase‐shift‐keying (DQPSK) based on four‐wave mixing (FWM) effects in semiconductor optical amplifier (SOA) is proposed. All‐optical wavelength conversion, D8PSK‐to‐DQPSK format conversion and signal encryption of 10‐Gbaud DQPSK and D8PSK optical signals are achieved simultaneously by simulation. Detailed theoretical analysis and simulation results are conducted to verify the feasibility of the scheme. The Q factor of the electrical signals and the bit error rate recovered from wavelength and format converted components are approximated to be around 6, and 10−9, which indicates the feasibility of the scheme. For the encrypted signal, the FWM effect of another SOA is utilized to decrypt the encrypted signal, and the encryption function of 10‐Gbaud DQPSK and D8PSK optical signals is also verified. All‐optical wavelength conversion, D8PSK to DQPSK format conversion and signal encryption of 10‐Gbaud DQPSK and D8PSK optical signals are achieved simultaneously by using a SOA‐FWM‐based multifunctional optical switching unit. Detailed theoretical analysis and simulation results are conducted to verify the feasibility of the scheme.

A new multi−carrier orthogonal double bit rate differential chaotic shift keying (MC−ODBR−DCSK) communication system is presented in this paper. With two composite signals generated by an orthogonal chaotic signal generator as reference signals, 2M bits of information data are transmitted on M−channel subcarriers, improving transmission speed and energy efficiency. In addition, the receiver does not require a radio frequency (RF) delay circuit to demodulate the received data, which makes the system easier to implement. This paper analyzes Data−energy−to−Bit−energy Ratio (DBR) of the system. The bit error rate performance of the system is simulated to verify the impact of parameters such as chaotic maps, semi-spread spectrum factor, and sub-carrier number. At the same time, the bit error rate performance of the MC−ODBR−DCSK system is compared with traditional DCSK systems in Rician fading and additive Gaussian white noise (AWGN) channels.

Free-space optical communication is a promising means to establish versatile, secure and high-bandwidth communication between mobile nodes for many critical applications. While the spatial modes of light offer a degree of freedom to increase the information capacity of an optical link, atmospheric turbulence can introduce severe distortion to the spatial modes and lead to data degradation. Here, we demonstrate experimentally a vector-beam-based, turbulence-resilient communication protocol, namely spatial polarization differential phase shift keying (SPDPSK), that can reliably transmit high-dimensional information through a turbulent channel without the need of any adaptive optics for beam compensation. In a proof-of-principle experiment with a controllable turbulence cell, we measure a channel capacity of 4.84 bits per pulse using 34 vector modes through a turbulent channel with a scintillation index of 1.09, and 4.02 bits per pulse using 18 vector modes through even stronger turbulence corresponding to a scintillation index of 1.54. Resistance to turbulence is an ongoing challenge for point-to-point freespace communications. Here the authors present a protocol for encoding a large amount of information in vector beams that are transmittable through a moderately strong turbulent channel without adaptive beam compensation.