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Abstract - The paper presents experimental results of a optical incoherent MIMO transmission with two channels over a gradient, multimode 4.4 km fiber (GI MMF) with a 50 μm core. Simultaneously, two channels was transmitted with rate of 200 Mb/s for everyone. A QPSK modulation has been used. A quality of the realized transmission has been nearly perfect.
Index Terms - incoherent optical MIMO transmission, multimode fiber, QPSK modulation,
I. INTRODUCTION
PPLYING multiple antennas at both the transmitter and receiver side can, especially when then environment provides rich scattering, greatly improve the capacity/ throughput of a wireless communication link in flat fading [1]. The multiple delay spread, which is a one of major limitations in conventional radio transmission systems, is used here as a way to obtain this spectacular feature. The MIMO technology offers significant increases in data throughput and link range without additional bandwidth or transmit power. It achieves this by higher spectral efficiency (more bits per second per hertz of bandwidth) [2].
However, the possibility of using this technique is not limited only to the wireless transmission. This technique can be successfully used in the optical lightwave systems for transmission over multimode fiber (MMF) [3-6]. Obviously, all spectacular properties of this technique are also available in the optical domain. The MIMO technique can improve the very limited transmission capabilities of this medium.
At short-reach optical networks MMF, primarily graded-index (GI), has been the medium of choice. Its large core diameter makes MMF splicing and launching light process easier comparatively to the single-mode fiber (SMF). Lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs) which operate at the 850 nm and 1300 nm wavelength can be used. On the other hand the bandwidth of the MMF is significantly lower to compare with the SMF for the same reason. Greater diameter of the core translate into bigger number of modes
Manuscript received November 07, 2001. This work was supported in part by the Dean Faculty of Electronic and Information Engineering of Warsaw University of Technology under Grant 504/M/1036/0246/2011.
M. Kowalczyk is with the Telecommunications Institute of Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warsaw, Poland (e-mail: mkowalczyk@tele.pw.edu.pl).
which are carried simultaneously through the waveguide with different propagation constants β. The lower bandwidth is due to the differential mode delay (DMD) among the propagating modes [7,8]. A value of the bandwidth- distance product parameter, which very well characterized of MMF possibilities transmission, typically not exceed 500 MHz× km. It means that satisfy of growing needs in relation to bandwidth, offered by networks based on this type of fiber, in the context of new services and standards within conventional transmission form can be very problematic.
To solve this problem in recent years new approaches are proposed for increase the possibilities of transmission of MM fibers [7,9-11]. One of them is use of the MIMO techniques.
The current article presents some experimental results for this type transmission.
II. THEORY
Generally optical MIMO fiber transmission systems can be divided in two groups coherent [3] and incoherent [4-6]. The former type uses coherent optical detection, which makes practical realization difficult. Described at this paper system is instance of the second type. To better understand its working mechanism some theoretical background has been given below. Broad analysis of the performance of such systems has been presented in [12].
The incoherent MIMO fiber system was described for the first time in [4,5]. M identical radio frequency (RF) carriers are modulated by M separate digital data streams, which modulated M laser sources. Obtained in this way optical signals are combined in a optical coupler/splitter and as one are launched into a MM fiber. At the receiver side, the light is split into M optical paths (by a second splitter), detected, and then processed electronically. What is important, each of the transmitting lasers excites in the MM fiber slightly different modes. Furthermore, both the input optical coupler and output optical splitter have mode selective properties (i.e. to some extent they filter modes/mode groups). Therefore, each connection between i-th receiver and j-th transmitter is realized by different modes.
In order to write the overall relation describing the mechanism of operation of this type of system we will use the Fig.1.
Experimental incoherent optical MIMO
transmission 2x2
over GI 50μm MM Fiber
M. Kowalczyk
A
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Fig. 1. Optical MIMO fiber transmission system model
Based on the model shown in Fig. 1, we can write the relation between M input signals [s1...sM]T and corresponding M output signals [r1...rM]T as
[
r
i]
[
[
h
ij]
[
s
j]
(1)Here, H is the transmission matrix containing the transfer functions between all receiver/ transmitter pairs. The matrix elements are complex numbers as they take into account both the attenuation and the phase shift of the RF carrier between i-th receiver and j-th transmitter. By inverting the matrix H we may recover the original signals using the formula:
[
s
j]
[
[
h
ji]
11[
r
i]
(2)Obviously, the matrix H may not be singular and its numerical conditioning directly determines the values of the channel SNR [7,12]. The matrix H-1 may be found directly by some optimization algorithm or calculated at the system initialization stage.
The detailed analysis presented in [12] shows that, although it is possible to implement incoherent optical MIMO transmission in baseband of MMF (solution takes form a MGDM system [7]), its full benefits may be reach if transmission is realized beyond of the baseband (passbands area). This is a big advantage, because the transmission can be realized as a extend of conventional transmission which using baseband as a scope of frequencies, Fig.2.
Fig. 1. The frequency characteristic of a GI MMF with marked areas of baseband and passbands along with the type of transmission that can be conducted in these areas.
III. EXPERIMENTALSETUP
In order to investigate the feasibility of incoherent optical MIMO transmission over MM fiber was built two-channel system whose block diagram is given in Fig. 3.
Fig. 3. Experimental setup of optical incoherent MIMO 2x2 transmission, AWG - arbitrary waveform generator, Amp - amplifier, LDR – laser driver, PD – photodiode, DigOsc - digital oscilloscope
At the realized experimental setup as a source of transmitted signals was used an arbitrary waveform generator (AWG). Two pattern sequences simulated two independent modulated RF carriers with QPSK modulation format were generated. RF carriers were modulated by two independent 200 Mb/s information signals. For simplicity only two of four QPSK states were actively used, that is the switching was performed between orthogonal (0o, 90o) components only. This had an additional advantage that the received signals might be presented in the well known form of eye diagrams, which had used as a transmission quality measure. Additionally was remember original unmodulated RF carrier signal, which was used in a demodulation process. The demodulation and recovering original signals was realized off-line beyond experimental system by using personal computer (PC) and MATLAB software. Two DFB lasers (Finisar 1310-4L-LC) operating at 1310 nm were modulated by those RF carriers signals (a direct intensity modulation was used). Obtained in this way optical signals were coupled into the MM fiber via commercial broadband 2x2 coupler. The GI silica MM fiber (Corning InfiniCor) with 50 μm core diameter and 4.4 km length has been used. After the propagation along the fiber, at the destination node, the received optical signal was divided between receiving channels by means of another 2x2 coupler/splitter. Each electrical channel started with
> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 a photodiode (JTSU, ETX-100 1310) and a transimpedance
amplifier. Then the signals were simultaneously sampled by a digital oscilloscope with sampling rate 10 Gsample/s. The recorded sequences were 100 μs long. They were saved and stored for further off-line processing.
IV. RESULTS
The off-line processing involved finding a form of inverted matrix H-1 for which quality of recovered transmitted signals was the best. A special program implemented at a MATLAB environment sought components of H-1 matrix for which Q parameter for each channel achieved a maximum value. Obtained in this way values of the parameter Q for each channel for the ‘best case’ exceed the value of 7.6, which means in practice an error-free transmission. Corresponding to this ‘best case’ obtained eye diagrams are presented in Fig. 4.
Fig. 4. Eye diagrams obtained for two-channels optcical incoherent MIMO transmission
V. CONCLUSION
Obtained results confirmed the effectiveness of MIMO transmission on the basis of optical transmission using MM fibers. Error-free transmission of the total bit rate at close to 0.5 Gb/s was obtained. The MIMO technique offers new possibilities, which are not available for conventional form of optical transmission over MM fibers.
ACKNOWLEDGMENT
The author acknowledge financial support granted by the Dean Faculty of the Electronic and Information Engineering of Warsaw Technical University under the program of grants for young scientists.
REFERENCES
[1] ] E. Biglieri, R. Calderbank, A. Constantinidies, A. Goldsmith, A. Paulraj, H. V. Poor, MIMO Wireless Communications, Cambridge University Press, 2007
[2] A. Zelst, Physical Interpretation of MIMO Transmissions, Proceedings Symposium IEEE Benelux Chapter on Communications and Vehicular Technology, Eindhoven, 2003
[3] A. Tarighat et al., “Fundamentals and Challenges of Optical Multiple-Input Multiple-Output Multimode Fiber Links”, IEEE Communications
Mag., vol. 45, pp. 57-63, 2007
[4] H.R. Stuart, “Dispersive multiplexing in multimode fiber”, Proc. OFC 2000, ThV2, pp. 305-307, 2000
[5] H.R Stuart, “Dispersive Multiplexing in Multimode Optical Fiber”,
Science, vol. 289, pp. 281-283, 2000.
[6] M. Kowalczyk, J. Siuzdak, “Four Channel Incoherent MIMO Transmission over 4.4 km MM Fiber”, Microwave & Optical
Technology Letters, vol. 53, no. 3, pp.502-508, January 2011
[7] C.P. Tsekrekos et al., “Design Considerations for a Transparent Mode Group Diversity Multiplexing Link”, IEEE Photonics Techn. Letters, vol. 18, pp. 2359-2361, 2006
[8] P. Pepeljugorski, S. Golowich, A. Ritger, P. Kolesar, A. Risteski, “Modeling and simulation of next-generation multimode fiber links”,
IEEE Journal of Lightwave Technology, vol. 21, no. 5, pp. 1242-1255,
May 2003
[9] J.P. Weem, P. Kirkpatric, J. M. Verdiell, Electronic dispersion compensation for 10 Gigabit communication links over FDDI multimode fiber, Optical Fiber Communication Conference 2005 [10] M.Kowalczyk, J.Siuzdak, SCM transmission in MM fiber with
automatic selection of the subcarrier frequency, Microwave & Optical Technology Letters, vol. 51, no 5, pp. 1212-1214, May 2009
[11] I. Gasulla, J. Copmany, 1 Tb/s_km WDM transmission over multimode fibre link, Proc. of ECOC, Brussels, 2008
[12] J.Siuzdak, “RF carrier frequency selection for incoherent MIMO transmission over MM fibers”, J. Lightwave Techn., vol. 27, pp. 4960-4963, 2009
Marcin Kowalczyk received the M.Sc. degree in telecommunications from Kielce University of Technology in 2007. Starting from 2007 till 2010 was a PhD student at the Telecommunications Institute of Warsaw University of Technology (IT-WUT). He received his PhD degree in December 2010. s PhD student he worked on transmission beyond a baseband in multimode fibers to increase their transmission capacity by using Subcarrier Multiplexing and MIMO techniques. Currently, he continues research work at this area. His interests include light propagation in optical fibers, microwave electronics and some aspects of digital signal processing. He is member of Optical Communications Group (OCG). The group is a part of the IT-WUT and is led by Professor Jerzy Siuzdak. The mission of the OCG is to conduct world class research and education in the field of optical communication system technology He authored and co-authored over 30 publications.
