In mobile communication system, due to the limitations of mobile device such as low power supply as well as small size, most of the processing should be done at the Base Station. Using multi-receive structure at the Base Station really helps better recovery of the original signal by combining different received signals. In this paper, for the first time, receive diversity is used in single-hop hybrid Free Space Optical / Radio Frequency (FSO / RF) communication system. Also it is the first time that a single-hop hybrid FSO / RF system is investigated at saturate atmospheric turbulence regime. For the first time, closed-form expression is derived for Outage Probability of the proposed system and verified through MATLAB simulation. Results indicate a significant improvement in the performance of the proposed structure compared with common FSO and RF systems with receive diversity. Therefore it can be concluded that although the proposed structure requires a complex receiver, but addition of this complexity could significantly reduce processing or power consumption required for performance maintenance of the system.
Deep Dive into A novel hybrid FSO / RF communication system with receive diversity.
In mobile communication system, due to the limitations of mobile device such as low power supply as well as small size, most of the processing should be done at the Base Station. Using multi-receive structure at the Base Station really helps better recovery of the original signal by combining different received signals. In this paper, for the first time, receive diversity is used in single-hop hybrid Free Space Optical / Radio Frequency (FSO / RF) communication system. Also it is the first time that a single-hop hybrid FSO / RF system is investigated at saturate atmospheric turbulence regime. For the first time, closed-form expression is derived for Outage Probability of the proposed system and verified through MATLAB simulation. Results indicate a significant improvement in the performance of the proposed structure compared with common FSO and RF systems with receive diversity. Therefore it can be concluded that although the proposed structure requires a complex receiver, but addition
Over the last decades, high data rate demands led to more attention to Free Space Optical (FSO) communication system and made it as the main competitor of traditional communication system. This is due to the higher bandwidth and capacity of FSO system compared with Radio Frequency (RF) system. RF system is appropriate in term of cost, but provides lower data rate compared with FSO system. In contrast, FSO system provides both low cost and high data rate [1].
Diversity is a widely used efficient technique for improving performance of communication systems. In mobile communication systems, receive diversity is more practical, because the transmitter (mobile phone) has many limitations and cannot deserve much power consumption or processing complexity. At the receiver side (Base Station), receive diversity technique gathers different copies of the transmitted signal, which are encountered with different fading channels. Combination of these copies could really help better recovery of the transmitted signal [2]. Maximum Ratio Combiner (MRC), Equal Gain Combiner (EGC) and Selection Combiner (SC) are some of the well-known combiners.
Millimetre-wave RF system can achieve data rates equal with FSO system. Weather condition and atmospheric turbulence affect performance of FSO system, and make it un-reliable. However, their impact on FSO and RF links is not the same [3], i.e. in FSO link performance degradation is mostly because of fog and atmospheric turbulence, and heavy rain doesn’t affect it. In contrast, RF link is sensitive to heavy rain, and does not care about fog and atmospheric turbulence. Combination of FSO and RF systems is an efficient solution for improving performance of both FSO and RF systems [4].
The so called hybrid FSO / RF systems are available in singehop [3,[5][6][7][8][9][10][11][12][13][14], dual-hop [15][16][17], and multi-hop [18][19][20] structures. Single-hop structure is more preferable for short range communications. In this structure, data is transmitted through two parallel FSO and RF links simultaneously [7,8,13] or by use of a switch [5,6]. Switch technique suffers from continues switches in situations with many changes in weather conditions. In simultaneous transmission this problem no more exists, also it does not require feedback.
In this paper a novel single-hop hybrid FSO / RF system with parallel simultaneous transmission is presents. To the best of author’s knowledge it is the first time in a hybrid single-hop FSO / RF system, receive diversity is used. Also it is the first time that a single-hop hybrid FSO / RF system is investigated at saturate atmospheric turbulence regime. Considering RF link in Rayleigh fading and FSO link in Negative Exponential atmospheric turbulence, for the first time, closed form expressions are derived for Outage Probability of the proposed structure. MATLAB simulations verified derived expressions.
In FSO and RF links, ECG and MRC schemes are used, respectively. The main motivation of this kind of selection is that in FSO link, no closed form expression can be derived for probability density function (pdf) and Cumulative Distribution Function (CDF) of MRC scheme in an FSO link with receive diversity in Negative Exponential distribution; also according to discussion in chapter 7 of [21], no closed form expression can be derived for the pdf and CDF of ECG scheme in a RF link with receive diversity in Rayleigh distribution. That is why the combiner of FSO and RF links are not selected the same.
In FSO system, Intensity Modulation and Direct Detection (IM/DD) is used. Combination of FSO and RF links brings advantages of both FSO and RF systems such as reliability, high data rate, security, etc.. On the other hand, Compared with other singlehop FSO / RF systems, the multi-receive structure of the proposed system could really help the receiver to make better decision by combining different received copies of the original signal, which are Rest of the paper is organized as follows: section II describes the system model. Section III derives pdf of the presented structure, section IV derives Outage Probability of the presented system. Section V compares analytical and simulation results. Section VI is conclusion of this study.
As shown in Fig. The SNR at the input of each of MRC and EGC combiners, and thus SNR at the input of SC combiner, is calculated in this section.
The pdf of Negative Exponential atmospheric turbulence of is as follows [22]:
where ๐ผ ๐ , atmospheric turbulence intensity of ๐ -๐กโ ; ๐ = 1,2, โฆ , ๐ FSO path, has Negative Exponential distribution with 1/๐ mean and 1/๐ 2 variance. Assuming ๐ฅ as the transmitted signal, the received signal at the ๐ -๐กโ FSO receiver, becomes ๐ฆ ๐ = ๐๐ผ ๐ ๐ฅ + ๐ ๐ ; where ๐ is the optical to electrical conversion efficiency and ๐ ๐ , the input noise of ๐ -๐กโ FSO receiver, is Additive White Gaussian Noise (AWGN) with zero mean and ๐ 2 variance. Output of EGC combiner becomes as follows:
Therefore, SNR at
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