Realization of free space optics with OFDM under atmospheric turbulence
Introduction
The growth of internet traffic in conjunction with an increase in the number and range of new services has placed pressure on radio networks operating on low-speed infrastructure. Free space optics (FSO) has the combined features of prevalent telecommunication technologies, i.e. wireless and fiber optics. FSO communication has attracted significant attention recently in high data rate wireless links and provided the essential combination of qualities required to bring traffic to the optical fiber backbone. FSO technology is also a promising solution for the “last mile” problem. FSO networks can be used as information bridges between nodes in local area networks and wireless local loops [1]. In addition, FSO provides secure transmission because of negligible interception using point-to-point laser signals. High capacity, low power consumption, license-free and low deployment costs are some of the merits of FSO [2], [3]. FSO has similar working principles as fiber optic communication, the main difference being the use of the atmosphere instead of optical fiber as the channel. An FSO network can be implemented as a point-to-point, mesh or point to multipoint architecture [4]. Low power infrared beams, not harmful to the human eye, can transmit data through the air between links comprising distance of few meters to several kilometers [5]. FSO provides good solutions for broadband networks, especially in geographical areas where optical fiber deployment is not feasible but there are some performance limitations. The most dominant limiting factors are atmospheric conditions such as fog, dust, snow or rain that debilitates the transmission path and may close down the network. Thus, in the design of FSO systems, these atmospheric parameters must be considered [6]. The link equation for free space optics [7] iswhere dR defines the receiver aperture diameter, dT is the transmitter aperture diameter, θ is the beam divergence, R is the range and α is the atmospheric attenuation. In FSO links, multipath fading is another limiting factor.
Significant effort is imperative for reducing multipath fading due to atmospheric turbulence. OFDM has immense potential for mitigating multipath fading due to atmospheric turbulences in FSO as data is distributed over a large no of orthogonal carriers that are sufficiently spaced at narrow frequencies with overlapping bands. The use of fast Fourier transform (FFT) provides orthogonality to the subcarriers, preventing the demodulators from seeing frequencies other than their own. The application of OFDM offers higher data capacity, secure transmission, high speed and smooth upgrade [8]. The rest of the paper is organized into following sections: Section 2 describes the simulation setup for OFDM–SOA system and Section 3 describes the result and discussion. The paper is concluded in Section 4.
Section snippets
System description
The proposed OFDM–FSO system is modeled using OptiSystem™ from Optiwave Corp. 10 Gbps data is generated through 4-level QAM sequence generator using 2 bits per symbol. The QAM data signal is modulated by an OFDM modulator using 512 subcarriers, 1024 FFT and 32 cyclic prefix code before being modulated at 7.5 GHz using a QAM modulator as shown in Fig. 1. This QAM signal is transmitted over free space by means of a continuous wave (CW) laser having a wavelength of 193.1 THz and power of 0 dBm.
The FSO
Results and discussion
The results are presented in this section. Fig. 3 depicts the measurement of SNR and received power for ODSB and OSSB modulation formats under clear weather condition by considering attenuation of 0.11 dB/Km. It has been observed that after 20 km, an improvement of 3.2 dB is measured in the case of ODSB. The OSSB technique suffers more severely from fading. After 60 km, both the OSSB and ODSB perform equally well.
The total received power after 20 km is measured as −51.46 dBm and −54.64 dBm for ODSB
Conclusion
In this work, a novel system for 10 Gbps OFDM based on FSO is designed with the integration of the semiconductor optical amplifier (SOA). From our results, it is concluded that with the use of pre- and post-amplification technique, the FSO system will prolong to 185 km under clear weather conditions with acceptable SNR and received power. When the atmospheric attenuation is increased and reaches to heavy fog conditions, then the achievable distance is extended to 2.5 km with acceptable SNR and BER.
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