A 60 GHz RoF generation system based on optical beat of narrowband Bragg filters
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Radio over fiber is considered as a very promising technology that will indisputably compete as a viable solution for the distribution of current and future broadband wireless communication systems. In this paper, we propose a new millimeter-wave generation system: the 60 GHz system which is based on two narrowband Bragg filters forming with the coupler a Y system. The millimeter wave is obtained by the optical beat technique detection of two optical waves. The frequency interval between the two waves generated by the Bragg filters gives the 60 GHz millimeter wave. The simulation results demonstrate that an optimal BER less than 10−9 for different values of flow rate and a good quality factor Q can be reached by using Bragg filters with a very narrow band. In addition, we simulate the 60 GHz system for various lengths of optical fiber. The eye diagram remains clear and open for fiber lengths that reach 50 km.
KeywordsNarrowband Bragg filter Radio over fiber Beat BER
To preempt the saturation of the 2.4 or the 5 GHz unlicensed bands used by Wi-Fi technologies and to increase the connection speed, the industrials are heading toward the unlicensed millimeter-wave band (mmW) that is limited in the interval from 57 to 66 GHz. The unlicensed millimeter-wave band offers large channels of 2.16 GHz bandwidth with high authorized emitted powers allowing advanced integration into radio terminals. However, the 60 GHz technology poses the problem of propagation losses, which limits the typical signal transmission distance to 60 GHz that varies from a few meters to several tens of meters.
To extend the coverage area, fiber-over-the-air (RoF) technology has been the subject of much scientific research over the last decade for the generation and optical transmission of RF signals. The RoF consists to transport the information over optical fiber by modulating the light source with the radio signal. Two types of modulation can be used: a direct modulation with the radio signal or an external modulation with intermediate frequency. Radio-over-fiber technology thus combines the advantages of fiber communication (very wide bandwidth, immunity to electromagnetic interference and low losses) [1, 2] along with radio communication.
RoF technology is used in several applications similar to fiber to the home (FTTH) [3, 4, 5], mobile communication [6, 7, 8] and wireless broadband systems [9, 10, 11, 12, 13]. Multiple studies with various methods have also recently been proposed [14, 15, 16, 17]. Among the most recent studies for the generation of frequencies close to 60 GHz [18, 19, 20, 21] is the use of frequency-division orthogonal multiplexing (OFDM, multi-carrier modulation) proved by a RoF tunnel certificate operating two wireless radio links . This millimeter wave is generated by the gain-switching laser that is a result of the beating of two uncorrelated optical tones  on the one hand or by using Mach–Zehnder modulators (MZM) [24, 25] on the other.
The contribution provided by this paper is based on a miniaturized optical system that generates millimeter waves without electrical-to-optical (EO) conversion or optical-to-electrical (OE) conversion. The proposed system is a Y-shaped coupler with two inputs, each having a very high-bandwidth Bragg filter (the two bands are slightly different). Both inputs of the couplers will lead to a single output .
In this paper, we propose a new all-optical system to generate the 60 GHz millimeter-wave band, based on the beat of the wavelengths reflected by a narrowband Bragg filters.
The optical beat achieved by Bragg filters shows performance by comparing with other recently used techniques for the generation of millimeter carriers such as laser beats . Narrowband Bragg filters can give well-defined frequencies, the thing that gives good results in terms of quality factor and BER. On the other hand, the proposed system is an all-optical system; the optical devices replace the electronic components especially the E/O or O/E converter used in other 60 GHz millimeter systems which use the optoelectronic oscillator .
The frequency interval of the coupled spectra generates a 60 GHz signal, with flow rates of 5 Gbit/s and 10 Gbit/s, for a BER less than 10−9 and distances up to 50 km.
2 Generation of radio frequency signals on optical fiber by optical beat detection
The 60 GHz millimeter-wave system proposed in this paper is based on the optical beat technique that replaces the lasers with a narrowband Bragg filters which will be slave filters with a master laser. The Bragg filters each hang on one of the sidebands of the master laser.
3 Narrowband Bragg filter
Bragg reflectors have been the subject of development of many applications in the field of optical communication systems. Bragg gratings (BGs) are used to provide narrow spectrum filters, couplers, mirrors, sockets and sensing elements that define the required basic functions. Another area for the use of the Bragg filters is that of wavelength multiplexers and demultiplexers that make in-line optical filters, particularly band-pass filters and wavelength-specific reflectors.
4 Model description
5 Simulation results and discussion
5.1 Effect of reflected band of the Bragg filters on quality factor (Q) and BER for 5 Gbit/s
5.2 Eye diagram for 5 Gbit/s and 10 Gbit/s for different SSMF lengths
In this paper, we have attempted to demonstrate the generation and transmission of a 60 GHz mm-wave signal based on narrowband Bragg filters. The millimeter wave is obtained by optical beat of the coupled narrow bands generated by the Bragg filters. The simulation results show the improvement in the BER and the quality factor Q for 5 Gbit/s and 10 Gbit/s flow rates also for a length of fiber link up to 50 km.
The system proposed in this paper can be used in several RoF applications: ADSL, cellular communications, wireless systems, etc.
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no competing interests
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