THE UNIVERSITY OF TULSA THE GRADUATE SCHOOL ENHANCING FSO LINK PERFORMANCE IN ADVERSE CONDITIONS USING A FIBER-BUNDLE BASED RECEIVER DESIGN by Nathan F. Hutchins A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science in the Discipline of Electrical Engineering The Graduate School The University of Tulsa 2015 COPYRIGHT STATEMENT Copyright © 2015 by Nathan F. Hutchins All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of the author. iii ABSTRACT Nathan F. Hutchins (Master of Science in Electrical Engineering), Enhancing FSO Link Performance in Adverse Conditions using a Fiber-Bundle Based Receiver Design Directed by Dr. Peter LoPresti (100 words) Redesigning the optical receiver is a necessary step in creating more reliable freespace optical mobile communication. The receiver design investigated in this research consists of a fiber bundle based receiver that allows for better optical communication in turbulent conditions by using an array of small powerful lenses to capture light. These lenses allow signals to be collected that would be lost to a standard receiver. Through experimentation, it was concluded that the bundle based receiver design collected at least ten times more signal power in all levels of turbulence for 850nm, 1310nm, and 1550nm wavelengths compared to the standard receiver. iv ACKNOWLEDGEMENTS I would like to thank Dr. LoPresti for all the help he has given me over these years and guiding my research till completion. I would also like to thank Dr. Ashenayi and Dr. Holmstrom for serving on my committee. v TABLE OF CONTENTS COPYRIGHT STATEMENT .................................................................................... iii ABSTRACT............................................................................................................... iv ACKNOWLEDGEMENTS ....................................................................................... v TABLE OF CONTENTS ........................................................................................... vi LIST OF FIGURES ................................................................................................... vii LIST OF TABLES ..................................................................................................... ix CHAPTER 1 INTRODUCTION......................................................................... 1.1 Project Overview....................................................................................... 1.2 Summary of the Problem ......................................................................... 1.3 Prior Relative Work ................................................................................. 1.4 Thesis Objective ........................................................................................ 1 1 2 2 3 CHAPTER 2 BACKGROUND ........................................................................... 2.1 Basic Theory .............................................................................................. 2.2 The Problem with Turbulence ................................................................. 2.3 Bundle-Based Receiver Design ................................................................ 2.4 Kolmogorov, Rytov, and Cn2 .................................................................... 4 4 6 9 12 CHAPTER 3 EXPERIMENTATION ................................................................ 3.1 Experimental Setup .................................................................................. 3.2 Data Analysis ............................................................................................ 15 15 22 CHAPTER 4 CONCLUSIONS AND FUTURE WORK .................................. 4.1 Conclusions ................................................................................................ 4.2 Future Work .............................................................................................. 35 35 36 REFERENCES .......................................................................................................... 37 APPENDIX A MATLAB CODE .......................................................................... 38 vi LIST OF FIGURES 2.1 Standard Optical Receiver ................................................................................... 4 2.2 Standard Optical Receiver ................................................................................... 5 2.3 Standard Optical Receiver with Beam Alignment Device .................................. 5 2.4 Beam Misalignment on Standard Receiver.......................................................... 7 2.5 Graph of Reference Laser Signal ......................................................................... 8 2.6 Beam Scattering Due to Turbulence on Standard Receiver................................. 9 2.7 Fiber-Bundle Based Receiver Design .................................................................. 10 2.8 Calumniating Transmitter Design with Linear Fiber Array ................................ 10 2.9 Short Focal Length Lens used to tie the Signal to the Fiber Bundle ................... 11 2.10 Ray Diagram of Bundle Receiver ...................................................................... 12 3.1 Flow Chart and Diagram of Experimentation ...................................................... 15 3.2 Function Generator and Pseudo-Random Bit Generator ..................................... 16 3.3 Electrical-Optical Converters and Amplifiers ..................................................... 17 3.4 Linear Fiber Array Transmitter. Alignment laser in background. ....................... 17 3.5 Turbulence Box with Glass Sides and Hot Plate Inside ...................................... 18 3.6 Fiber Based Receiver, Front View ....................................................................... 19 3.7 Fiber Based Receiver, Side View ........................................................................ 20 3.8 Oscilloscope Display, Channel 1 is the Reference laser, Channel 2 is the Transmitted Signal, and Channel 4 is the Received signal .................................. 21 3.9 National Instruments Machine used to Record and Save Transmitted/Received data and Values for Reference Laser ............................... 21 vii 3.10 Screenshot of National Intermentβs Signal Acquisition Software after Recording .......................................................................................................... 22 3.11 Oscilloscope Graph of Bundle Receiver Low Turbulence ................................ 25 3.12 Oscilloscope Graph of Bundle Receiver High Turbulence................................ 25 3.13 Oscilloscope Graph of Standard Receiver Low Turbulence.............................. 26 3.14 Oscilloscope Graph of Standard Receiver High Turbulence ............................. 26 3.15 MATLAB Plot 850nm Received Signal on Standard Receiver Low Turbulence ........................................................................................................ 27 3.16 MATLAB Plot 850nm Received Signal on Standard Receiver High Turbulence ........................................................................................................ 27 3.17 MATLAB Plot 1550nm Received Signal on Bundle Receiver Low Turbulence .......................................................................................................... 28 3.18 MATLAB Plot 1550nm Received Signal on Bundle Receiver High Turbulence ........................................................................................................ 28 3.19 Graph of Cn2 (written as Cn^2 in the plotting program) for the Zeros for the 1310nm Laser, Cn^2_0 is the Bundle Receiver, Cn^2_0_STD is the Standard Receiver............................................................................................... 29 3.20 Graph of Cn2 for the Ones for the 1310nm Laser, Cn^2_1 is the Bundle Receiver, Cn^2_1_STD is the Standard Receiver ...................................... 30 3.21 Graph of Cn2 for the Zeros for the 850nm Laser, Cn^2_0 is the Bundle Receiver, Cn^2_0_STD is the Standard Receiver ............................................ 31 3.22 Graph of Cn2 for the Ones for the 850nm Laser, Cn^2_1 is the Bundle Receiver, Cn^2_1_STD is the Standard Receiver ............................................ 31 3.23 Graph of Cn2 for the Zeros for the 1550nm Laser, Cn^2_0 is the Bundle Receiver, Cn^2_0_STD is the Standard Receiver ............................................ 32 3.24 Graph of Cn2 for the Ones for the 1550nm Laser, Cn^2_1 is the Bundle Receiver, Cn^2_1_STD is the Standard Receiver ............................................ 33 3.25 One Scale Graph of Cn2...................................................................................... 34 viii LIST OF TABLES 3.1 Example Output Data Form MATLAB in Excel ................................................. ix 24 CHAPTER 1 INTRODUCTION 1.1 Project Overview Free space optics is the use of light propagation to communicate without a fiber or transfer media other than open (free) space. The term free space refers to air, outer space, atmosphere, or vacuum (i.e. not optical fiber or other waveguide). The technology is capable of extremely high data rates compared to other wireless forms of communication but is typically costly and very susceptible to weather, temperature, and atmospheric changes. With the increase in the number of mobile platforms and the need for high speed data communications between them, it is becoming necessary to improve on the current free space optical system design to make it compatible with the limitations imposed by a mobile operating environment. In particular, the receiver requires a larger field of view to reduce the effects of alignment errors; both transmitter and receiver require designs that use less power, and optical tracking methods are needed to keep the transmitter and receiver pointed at each other. These modifications would allow mobile high speed communication through free-space optics to support the high data rates required by current and future mobile applications. 1 1.2 Summary of the Problem Air turbulence is a significant issue for free space optical transmission. The scattering effects of turbulence on the optical beam are irregular and cause deflection of the beam away from the receiver and changes in the angle of arrival at the receiver, which cause hotspots and dead spots on receiverβs detector. Hot spots can cause receiver saturation or blinding, while dead spots cause data to be lost or misinterpreted. The irregular nature of turbulence, which affects every free-space optical communications system regardless of the transmission distance, makes the effects difficult to predict and to counteract. However, the amount of signal degradation due to turbulence can be reduced through the proposed redesign of a traditional receiver. 1.3 Prior Relative Work In earlier research, the effects of using different wavelengths for signal transmission on the operation of the bundle-based receiver under study herein were investigated. In this research, it was concluded there was minimal effect on receiver performance as a function of wavelength for 1550 nm and 1310 nm wavelengths. According to simulations that modeled the communication link, there was also minimal effect on the performance when using an 850nm wavelength. 2 1.4 Thesis Objective The goal of this research project is to investigate how a bundle-based receiver design helps mitigate the effects of turbulence , and by how much, while keeping the size, weight, and power of the optical receiver as low as possible. 3 CHAPTER 2 BACKGROUND 2.1 Basic Theory The basic free space optical communication system consists of a transmitter and receiver. The standard optical transmitter is a telescope that is used to direct the propagation of light after it exits the optical fiber feeding the transmitter. This arrangement can be as simple as one lens (Figure 2.1) or as complicated as a large series of lenses and mirrors. Figure 2.1 Standard Optical Receiver The basic optical receiver is usually a series of lenses or mirrors used to gather as much of the propagated light as possible and funnel it into an optical fiber or an optical detector (Figure 2.2). 4 Figure 2.2 Standard Optical Receiver The traditional receiver design can be augmented with a small alignment device, at the cost of additional weight and power consumption, to slightly increase the field of view. If the light exits the lens at an angle that would normally cause the light to miss the detector, the beam steering device can be rotated to redirect the light back onto the detector surface. This is demonstrated in Figure 2.3. Figure 2.3 Standard Optical Receiver with Beam Alignment Device 5 This communication pair works well in stationary applications and good weather conditions with low turbulence. Stationary installations can draw power from the electrical grid and can typically support large mechanical systems to reduce vibration. Low turbulence limits the change in angle of arrival to small angles that can still be captured by the receiver. Difficultly occurs when either one of the pair must be mobile or when the turbulence becomes large. 2.2 The Problem with Turbulence One major problem with the traditional receiver in free space optical systems is misalignment. There are several types of misalignment, which include physical misalignment and signal misalignment. Physical misalignment is where the transmitter and receiver are not aligned correctly, so that the transmitted beam does not enter the center of the receiver and/or enters at a larger angle than the receiver is designed for. Receivers with and without alignment-aiding devices have a maximum amount of misalignment that can be corrected. It is possible, as shown in Figure 2.4 below, that the misalignment too great to allow for correction. 6 Figure 2.4 Beam Misalignment on Standard Receiver Signal misalignment can be caused by the introduction of turbulence. Deflection and diffraction by the turbulent air makes the incoming power distribution of the signal very irregular in time and in space. This can cause hot spots and dead spots on the detector or can cause the power to miss the detector entirely. Traditional receivers have a limited range of misalignment that can be corrected, and so the collected power is very strongly modulated by the turbulence effects. Turbulence can be defined as the change of refractive index of a material due to heat, movement, and particulate dispersion in the air. These factors cause pockets, or eddies, of different refractive index to form, and these eddies can vary in both size and density of distribution. The value that is typically used to describe the intensity of the turbulence, and used herein to compare the performance of different receivers, is the index-of-refraction structure constant, πΆπΆππ2 .This is the numerical value of the amount of energy contained in the turbulence , which will cause a corresponding negative effect on the optical signal transmission. 7 Figure 2.5 Graph of Reference Laser Signal Figure 2.5 shows the variation due to turbulence of a continuous wave reference laser that was used to measure the turbulence in the laboratory. The blue line shows the signal of the reference laser as seen by the detector in a cool, non-turbulent laboratory environment, with a Cn2 of 8.8x10-16 which is one of the lowest recordings observed in the lab. The red line in the Figure is the signal of the reference laser with a hot, high turbulence laboratory environment, with a Cn2 of 1.2x10-12, which is over one thousand times more turbulent than the low turbulence case. This is the real effect on a transmitted optical signal, and a representation of the effect is shown in Figure 2.6. The turbulence changes the power distribution and angle of arrival of the signal and causes hot-spots, which saturate the detector, as well as dead spots and signal misalignment. Since there is no way to predict how turbulence will affect a signal these changes in the signal collected by the receiver can lead to fatal errors in a communication system. 8 Turbulence Figure 2.6 Beam Scattering Due to Turbulence on Standard Receiver 2.3 Bundle-Based Receiver Design This thesis investigates a fiber bundle based receiver design which allows for a larger viewing angle, also called the field of view. The creation of a larger viewing angle will not only help in cases of misalignment but will also help mitigate the effects of scattering due to turbulence and other airborne interference. The transmitter design consists of a series of lenses used to collect and recollimate the beam for best transfer of optical power to the electrical receiver. The bundle-based receiver, through the use of several small lenses, has many points on which the maximum signal can be incident and thus collected. After the signal is captured by one of the small lenses the signal is coupled to a corresponding fiber in the fiber bundle and then collimated using an array of graded index lenses. The collimated signal is then focused by an aspheric or parabolic lens which is used to focus the signal onto a fiber or 9 photodetector, as shown by Figure 2.7. The parabolic lens reduces aberrations so that the focal spot is small enough to be entirely collected by the small area of very fast detectors. Figure 2.7 Fiber-Bundle Based Receiver Design The use of a linear fiber array in the transmitter design is for some future work related to tracking. In this research only the middle pair of fibers was used for transmission (Figure 2.8) to keep light directed at a stationary receiver. Figure 2.8 Calumniating Transmitter Design with Linear Fiber Array The design of the fiber bundle-based receiver produces a larger viewing angle by using several small, very short focal length lenses to channel the input signal into the receiving fibers, even at large angles of arrival. These powerful lenses can bend or 10 redirect the signal light into the fiber bundle with more consistently for a larger range of input angles. As shown in Figure 2.9, even if the signal is coming in at a very wide angle the collecting lenses are strong enough to redirect, or bend, the signal back toward the central axis of the lens and then be channeled into the fiber bundle fiber. This increase in the range of arrival angles that can be collected creates the increased field of view. Another full ray diagram is shown in Figure 2.10. The lens in Figure 2.9 is one of the 3mm focal length lenses shown in Figure 2.10. Figure 2.9 Short Focal Length Lens used to tie the Signal to the Fiber Bundle 11 Figure 2.10 Ray Diagram of Bundle Receiver With this design the bundle-based receiver is able to capture more signal power. Another feature of the receiver is that it has more than one lens for capturing light. Each fiber in the bundle has its own capturing lens, so as the signal moves off one of the lenses it will move onto another lens which will collect it, thus maintaining signal strength and integrity. 2.4 Kolmogorov, Rytov, and Cn2 Atmospheric fluctuations in the refractive index, n, are almost purely caused by small changes in temperature. Since the variations in pressure and humidity are usually small enough to be neglected, the well-known Kolmogorov power-law spectrum, defined by the equation 1 below, can be used to approximate the turbulence [1-4]. π·π·ππ (π π ) = 0.033πΆπΆππ2 π π β11β3 , 1οΏ½πΏπΏ βͺ π π βͺ 1οΏ½ππ 0 0 12 (1) Ξ¦n describes the distribution of spatial variations in the refractive index in terms of the spatial frequency ΞΊ. L0 and l0 are defined as the values of the inertial subrange, and represent the largest and smallest scales on which the refractive index is expected to vary. For basic modeling, L0 and l0 are usually assumed to be infinity and zero respectively. In some cases, it may be necessary to write the Kolmogorov power-law spectrum to include the effects of the distance of propagation of the wave. This can be written as shown in equation 2, π·π·ππ (π π , π§π§) = 0.033πΆπΆππ2 (π§π§)π π β11β3 , (2) which shows that value of Cn2 is a function of the distance of propagation, and thus affects the Kolmogorov power-law spectrum as a whole. Since the Kolmogorov power-law spectrum and the fluctuations in refractive index are statistically homogeneous, the propagation of the wave can be represented by the stochastic wave equation β2 πΈπΈ + ππ 2 ππ2 (π π )πΈπΈ + 2β[πΈπΈ β β log ππ(π π )] = 0, (3) where R is the point in space, n(R) is the index of refraction at R and k is given by equation 4, and β is given by equation 5, ππ = β2 2ππ ππ , ππ2 (4) ππ2 β2 = βx2 + ππππ 2 + ππππ 2 . (5) For this set of conditions and equations, the covariance function can be used to predict and calculate the behavior of light propagating through the turbulent air, since the Kolmogorov power-law spectrum and the stochastic wave equation are both deltacorrelated, meaning they both are continuous and random with time [1-4]. 13 When using the Kolmogorov power-law spectrum to study plane waves, the value of Cn2 can be calculated by using the covariance function of the refractive index. The result of the calculation is the Rytov variance, which describes the received intensity, and is given by equation 6, πππ π 2 = 1.23πΆπΆππ2 ππ 7οΏ½ 11οΏ½ 6 πΏπΏ 6, (6) where L is the length of propagation and k is determined by equation 4 [1]. Other equations for the Rytov variance can be derived for spherical waves and Gaussian beams. For the experiments conducted as part of this work, the plane wave equation is most appropriate. This set of equations are considered to be accurate as long as the experiment can satisfy five simple, but very realistic assumptions 1. Wave backscattering can be neglected 2. Depolarization effects can be neglected 3. The refractive index is delta correlated in the direction of propagation 4. The paraxial approximation will hold 5. The experiments are limited to the use of weak turbulence cases Since the approximations do hold for the experiments conducted in the laboratory, the Rytov variance, equation 6, is used in the calculations of the effective turbulence observed during the experiments that were conducted and are described in chapter 3. 14 CHAPTER 3 EXPERIMENTATION 3.1 Experimental Setup Figure 3.1 Flow Chart and Diagram of Experimentation To test the performance of the bundle-based receiver under turbulent conditions, it is necessary to not only implement successful free space optical communication but also to recreate turbulence in a repeatable fashion in the laboratory. The overall experimental setup that accomplishes this is diagramed in the flowchart in Figure 3.1. For generating the signal, a function generator is used to drive a pseudo-random bit sequence generator that then drives the optical source. The function generator was operated at 100 kHz with a 4.8 Vpp square wave and an offset of 2.6 V DC. The square wave is used as the clock signal for the pseudo-random bit sequence generator which produces a pseudorandom bit stream that repeats every 248-1 bits. (Figure 3.2). 15 Figure 3.2 Function Generator and Pseudo-Random Bit Generator The output of the pseudorandom bit sequence generator is connected to an Electrical-to-Optical (EO) converter, where the electrical signal amplitude modulates an internal laser source. Three separate EO converters were used (Figure 3.3), which converted the electrical signal to one of three optical wavelengths 1550nm, 1310nm, or 850nm. The optical signal from the EO converter is coupled by optical fiber to an optical amplifier to increase the optical signal power. The output of the amplifier is coupled by another optical fiber to the middle fiber of the transmitter array (Figure 3.4), with the exception of the 850 nm case for which no amplifier was available. 16 Figure 3.3 Electrical-Optical Converters and Amplifiers Figure 3.4 Linear Fiber Array Transmitter. Alignment laser is in the background. After the optical signal exits the transmitter, it travels through a turbulence simulation box that contains a hot plate and a series of fans (Figure 3.5). This box is used to create different strengths of turbulence in the laboratory. Increasing the temperature of 17 the hot plate in the box increases the turbulence, which in turn increases the effect of turbulence on the optical signal that will be detected by the receiver. A 633 nm alignment/reference laser is used to measure the baseline turbulence in the box for analysis purposes and to provide a point of comparison for evaluating how the receiver performs as a function of turbulence. The transmitted signal is then captured by the receiver and coupled to a detector so the signal can be analyzed. In the experiments that were conducted, all tests are run on two receivers, one traditional receiver and one bundle-based receiver. Figure 3.5 Turbulence Box with Glass Sides and Hot Plate Inside . The fiber bundle receiver is constructed using a hexagonal array of small, 3mm focal length lenses that couple the signal to an array of fibers with a 400 micro meter 18 core. Each fiber in the bundle collects optical power from one lens in the array. The fibers then transmit the signal to an array of graded index lenses which collimate the signal. The collimated light is incident on a 25.4 mm diameter aspheric lens with a focal length of 20mm that focuses the signal onto the detector. The aspheric lens is used to achieve a small enough focal spot to match the small area of the detector, so that the maximum amount of signal power is coupled to the detector. Figure 3.6 shows the 3mm focal length collecting lenses at the front of the receiver. Figure 3.6 Fiber Based Receiver, Front View The side view in Figure 3.7 shows the actual fiber bundle in the receiver that channels the signal from the collecting lenses to the parabolic (aspheric) lens and the detector. 19 Figure 3.7 Fiber Based Receiver, Side View The optical signal collected by the detector is converted into a voltage signal by the detector, and this voltage signal can be observed on an oscilloscope (Figure 3.8) and recorded by a National Instruments PIX-1042 (Figure 3.9) running LabVIEW Signal Express 2012 (Figure 3.10) both of which are connected to the output of the receiver by a coaxial cable. The PIX-1042 and the LabVIEW software samples the voltage signal at the rate of 5·106 samples per second (5 MS/s). Once the sampled data has been acquired from both the signal laser and the reference laser, the LabVIEW software saves the data to a text file that can be viewed and analyzed in MATLAB. 20 Figure 3.8 Oscilloscope Display, Channel 1 is the Reference laser, Channel 2 is the Transmitted Signal, and Channel 4 is the Received signal Figure 3.9 National Instruments Machine used to Record and Save Transmitted/Received Data and Values for Reference Laser 21 Figure 3.10 Screenshot of National Intermentβs Signal Acquisition Software after Recording Because of the memory limitations of MATLAB and the National Instruments machine only about 15 seconds of data can be recorded per run. Since the runs were limited in length, and a sufficiently large volume of data is needed for proper statistical analysis, several runs were performed at each turbulence setting for each transmitting wavelength and the results of the data analysis were averaged. 3.2 Data Analysis After all the data is collected it is analyzed using a MATLAB script which reads each bit and labels it as a 1 or 0 according to its signal strength relative to the average 22 value of the overall data set. After the data is sorted, the standard deviations of the values of the 1βs and 0βs were calculated. The standard deviation of the laboratory reference laser was also calculated and used as the baseline for comparing the quality of the signals captured by the receivers. The MATLAB analysis proceeded as follows. The first step was to determine the mean value of all of the data in the run. This provides a baseline for determining ones or zeros since the National Instruments machine provides data that has been normalized with the mean of the incoming signal. From the mean, the script can decide whether or not the bit was a one or zero. A bit is a one if the value of the bit is above the mean, while a bit is a zero if the value of the bit is below the mean. The standard deviation of the reference laser is calculated using the MATLAB function βstd(x)β, which returns a value for the standard deviation based on the data collected from the detector observing. The value of Cn2 can then be calculated using equation 7 πΆπΆππ2 = ππ2 7 11 1.23βππ οΏ½6 βππ οΏ½6 (7) which is based on the formula for the Rytov variance (equation 6), assuming weak turbulence. Note that, since the reference laser was always present, that is, it is always a one, there was no need to sort between zeros and ones as was needed with the signal laser. After the data was analyzed by MATLAB it was to Excel to perform averaging and to plot results. The wavelengths used in the lab were 633nm for the reference laser, and 850nm, 1310nm, and 1550nm for the signal lasers. The total length from transmitter to receiver was 1.68 meters, which is the effective length over which the turbulence occurred. Using 23 these values and the information for the standard deviation, Ο, the effective Cn2of the path (box and air) traversed by the signal laser was calculated. An example of the Excel sheet used for this calculation is shown in table 3.1. The sheet shows the results of several different experimental runs. The results for each turbulence case for the two different receivers were compared using the average of all the runs at a given temperature setting of the hotplate. Table 3.1 Example Output Data Form MATLAB in Excel sigma_0 sigma_1 Sigma^2_0 Sigma^2_1 Lambda L K Cn^2_0 Cn^2_1 1550 Box 1.48021E-03 1.87880E-03 2.19102E-06 3.5299E-06 1.55E-06 1.68 4.05E+06 1.34437E-14 2.16588E-14 1550 Box 2 1.51302E-03 1.76703E-03 2.28924E-06 3.12239E-06 1.55E-06 1.68 4.05E+06 1.40463E-14 1.91584E-14 1550 Box 3 1.56786E-03 1.64177E-03 2.45818E-06 2.69542E-06 1.55E-06 1.68 4.05E+06 1.50829E-14 1.65386E-14 1550 Box 4 1.57003E-03 1.76240E-03 2.465E-06 3.10604E-06 1.55E-06 1.68 4.05E+06 1.51247E-14 1.90581E-14 1550 Box 5 1.57394E-03 1.84693E-03 2.4773E-06 3.41113E-06 1.55E-06 1.68 4.05E+06 1.52002E-14 2.09301E-14 1550 Box 6 1.56806E-03 2.06636E-03 2.4588E-06 4.26986E-06 1.55E-06 1.68 4.05E+06 1.50867E-14 2.61991E-14 1550 Low 1.45155E-03 1.97636E-03 2.10701E-06 3.90601E-06 1.55E-06 1.68 4.05E+06 1.29282E-14 2.39665E-14 1550 Low 2 1.47475E-03 2.19843E-03 2.17488E-06 4.83312E-06 1.55E-06 1.68 4.05E+06 1.33446E-14 2.96551E-14 1550 Low 3 1.46706E-03 2.24288E-03 2.15225E-06 5.03053E-06 1.55E-06 1.68 4.05E+06 1.32058E-14 3.08664E-14 1550 Low 4 1.47316E-03 2.32184E-03 2.1702E-06 5.39093E-06 1.55E-06 1.68 4.05E+06 1.33159E-14 3.30777E-14 1550 Low 5 1.46010E-03 2.39198E-03 2.13189E-06 5.72156E-06 1.55E-06 1.68 4.05E+06 1.30808E-14 3.51064E-14 1550 Low 6 1.43883E-03 2.39492E-03 2.07022E-06 5.73564E-06 1.55E-06 1.68 4.05E+06 1.27025E-14 3.51928E-14 1550 med 1.39020E-03 2.30054E-03 1.93265E-06 5.2925E-06 1.55E-06 1.68 4.05E+06 1.18583E-14 3.24738E-14 1550 med 2 1.36573E-03 2.27473E-03 1.86522E-06 5.17438E-06 1.55E-06 1.68 4.05E+06 1.14446E-14 3.1749E-14 1550 med 3 1.35436E-03 2.34360E-03 1.83429E-06 5.49246E-06 1.55E-06 1.68 4.05E+06 1.12548E-14 3.37007E-14 1550 med 4 1.37463E-03 2.31476E-03 1.88961E-06 5.35813E-06 1.55E-06 1.68 4.05E+06 1.15943E-14 3.28764E-14 1550 med 5 1.37300E-03 2.30269E-03 1.88513E-06 5.30238E-06 1.55E-06 1.68 4.05E+06 1.15668E-14 3.25344E-14 1550 med 6 1.34907E-03 2.23500E-03 1.82E-06 4.99522E-06 1.55E-06 1.68 4.05E+06 1.11672E-14 3.06497E-14 1550 High 1.38441E-03 2.00649E-03 1.9166E-06 4.02599E-06 1.55E-06 1.68 4.05E+06 1.17599E-14 2.47027E-14 1550 high 2 1.40042E-03 1.97132E-03 1.96119E-06 3.8861E-06 1.55E-06 1.68 4.05E+06 1.20335E-14 2.38443E-14 1550 High 3 1.38221E-03 1.96113E-03 1.91051E-06 3.84601E-06 1.55E-06 1.68 4.05E+06 1.17225E-14 2.35984E-14 1550 high 4 1.35359E-03 1.93409E-03 1.83221E-06 3.74071E-06 1.55E-06 1.68 4.05E+06 1.12421E-14 2.29523E-14 1550 High 5 1.39842E-03 1.94269E-03 1.95559E-06 3.77405E-06 1.55E-06 1.68 4.05E+06 1.19991E-14 2.31568E-14 1550 high 6 1.36442E-03 2.01899E-03 1.86163E-06 4.07633E-06 1.55E-06 1.68 4.05E+06 1.14226E-14 2.50116E-14 Avg Avg 1.46641E-14 2.05905E-14 1.30963E-14 3.13108E-14 1.1481E-14 3.23307E-14 1.16966E-14 2.38777E-14 The first column in table 1 shows the six runs that were performed at each temperature setting and the data is averaged together in the last two columns. The four temperatures tested were with the hotplate off (Box only run) and the three different setting of the hotplate low, medium, and high. After taking six runs at every temperature, Cn2 was averaged for both the ones and zeros and then graphed. 24 Figure 3.11 Oscilloscope Graph of Bundle Receiver Low Turbulence Figure 3.12 Oscilloscope Graph of Bundle Receiver High Turbulence Figures 3.11 and 3.12 show captured oscilloscope traces. Channel two(green or bottom trace) is a sample of the data signal that was sent to the EO converter, and channel four (pink or middle trace) is a sample of the signal collected by the bundle 25 receiver for the low (3.11) and high (3.12) turbulence cases. Channel one (yellow or top trace) is output signal from the reference laser detector. Figure 3.13 Oscilloscope Graph of Standard Receiver Low Turbulence Figure 3.14 Oscilloscope Graph of Standard Receiver High Turbulence Figures 3.13 and 3.14 show the same information as Figures 3.11 and 3.12 for the case of the standard receiver. 26 Figure 3.15 MATLAB Plot 850nm Received Signal on Standard Receiver Low Turbulence Figure 3.16 MATLAB Plot 850nm Received Signal on Standard Receiver High Turbulence 27 Figure 3.17 MATLAB Plot 1550nm Received Signal on Bundle Receiver Low Turbulence Figure 3.18 MATLAB Plot 1550nm Received Signal on Bundle Receiver High Turbulence 28 Figures 3.15-3.18 show graphs of the data over a large range of samples produced using MATLAB. Figures 3.15 and 3.16 are for the standard receiver at 850 nm and Figures 3.17 and 3.18 are for the bundle-based receiver at 1550 nm. From these Figures it can be seen how the signal is deformed by the introduction of turbulence in the system. In particular, in Figures 3.17 and 3.18, it is observed that, at high turbulence, the average voltage signal for a one (ignoring the spike artifacts from the amplifier) is both smaller and more variable than the voltage signal for a one when lower turbulence is present. 1.28E-13 1.26E-13 1.24E-13 1.22E-13 1.2E-13 1.18E-13 1.16E-13 1.14E-13 1.12E-13 1.1E-13 3E-15 2.3E-14 4.3E-14 6.3E-14 Cn2 8.3E-14 1.03E-13 2.3E-13 2.2E-13 2.1E-13 2E-13 1.9E-13 1.8E-13 1.7E-13 1.6E-13 1.5E-13 1.4E-13 1.3E-13 1.2E-13 1.23E-13 Cn2 Traditional Rx Cn2 Bundle Rx 1310nm Cn2 Zeros Of The Lab Cn^2_0 Cn^2_0_STD Figure 3.19 Graph of Cn2 (written as Cn^2 in the plotting program) for the Zeros for the 1310nm Laser, Cn^2_0 is the Bundle Receiver, Cn^2_0_STD is the Standard Receiver 29 1E-14 3E-14 5E-14 Cn2 7E-14 9E-14 4.3E-13 3.9E-13 3.5E-13 3.1E-13 2.7E-13 2.3E-13 1.9E-13 1.5E-13 1.1E-13 1.3E-13 Cn2 Traditional Rx Cn2 Bundle Rx 1.8E-13 1.6E-13 1.4E-13 1.2E-13 1E-13 8E-14 6E-14 4E-14 2E-14 0 -1E-14 1310nm Cn2 Ones Of The Lab Cn^2_1 Cn^2_1_STD Figure 3.20 Graph of Cn2 for the Ones for the 1310nm Laser, Cn^2_1 is the Bundle Receiver, Cn^2_1_STD is the Standard Receiver Figures 3.19 and 3.20 show the analysis results for the 1310 nm signal laser. In the Figures, the Cn2 in the box, as measured by the reference laser, is plotted on the horizontal axis and the effective value of Cn2 for the two receivers (Bundle receiver, Blue, on the left axis, and Standard receiver, Orange, on the right axis) is plotted as a function of the box value. The four points in the plot correspond to no turbulence (left-most point) through high turbulence (right-most point). It can be observed that an increased Cn2 in the box is directly correlated with the increase of effective Cn2 observed by the standard receiver for both the sent zeros (Figure 3.19) and ones (Figure 3.20). The difference in the effective Cn2βs of the standard and bundle receivers is about a factor of 2 for the 1310 nm laser. That is, looking at average value per run, the Cn2 experienced by the standard receiver is about twice the value of Cn2 experienced by the bundle receiver. 30 850nm Cn2 Zeros 7E-16 7E-16 6.8E-16 6.9E-16 6.6E-16 6.85E-16 6.4E-16 6.8E-16 6.2E-16 6.75E-16 6E-16 6.7E-16 5.8E-16 6.65E-16 6.6E-16 Cn2 Standard Rx Cn2 Bundle Rx 6.95E-16 0 5.6E-16 1E-14 2E-14 3E-14 4E-14 5E-14 6E-14 7E-14 8E-14 Cn2 Of The Lab Cn^2_0 Cn^2_0_STD Figure 3.21 Graph of Cn2 for the Zeros for the 850nm Laser, Cn^2_0 is the Bundle Receiver, Cn^2_0_STD is the Standard Receiver 850nm Cn2 Ones 7.3E-16 7.3E-16 7.1E-16 7.25E-16 6.9E-16 7.2E-16 6.7E-16 7.15E-16 6.5E-16 7.1E-16 6.3E-16 7.05E-16 6.1E-16 7E-16 5.9E-16 6.95E-16 5.7E-16 6.9E-16 0 2E-14 4E-14 6E-14 Cn2 Standard Rx Cn2 Bundle Rx 7.35E-16 5.5E-16 8E-14 Cn2 Of The Lab Cn^2_1 Cn^2_1_STD Figure 3.22 Graph of Cn2 for the Ones for the 850nm Laser, Cn^2_1 is the Bundle Receiver, Cn^2_1_STD is the Standard Receiver 31 Figures 3.21 and 3.22 show the analysis results for the 850 nm signal laser, and are organized in the same manner as Figures 3.19 and 3.20. As before, it can be observed that the increased Cn2 in the box is directly correlated to the increase of the effective Cn2 observed by the standard receiver for both the sent zeros (Figure 3.21) and ones (Figure 3.22). The dependence of the effective value of Cn2 on turbulence for the 850 nm experiment is very similar to that of the 1310 nm experiments with the exception of the observed behavior at the highest turbulence level. At this point, it is expected that the signal is moving off one of the collecting lenses and moving onto another collecting lens in the lens array. This allows the receiver to collect the optical power, just from a different fiber. Since the standard receiver only has one collecting lens, similar to the case shown in Figure 2.4, the effective value of Cn2 will continue to rise as turbulence increases, since the beam from the lens will fail to be directed onto the detector. 1.5E-14 3.9E-14 1.45E-14 3.8E-14 1.4E-14 3.7E-14 1.35E-14 3.6E-14 1.3E-14 3.5E-14 1.25E-14 3.4E-14 1.2E-14 3.3E-14 1.15E-14 1.1E-14 0 5E-14 1E-13 1.5E-13 2E-13 2.5E-13 Cn2 Traditional Rx Cn2 Bundle Rx 1550nm Cn2 Zeros 3.2E-14 3E-13 Cn2 Of The Lab Cn^2_0 Cn^2_0_STD Figure 3.23 Graph of Cn2 for the Zeros for the 1550nm Laser, Cn^2_0 is the Bundle Receiver, Cn^2_0_STD is the Standard Receiver 32 3.4E-14 3.2E-14 Cn2 Bundle Rx 3E-14 2.8E-14 2.6E-14 2.4E-14 2.2E-14 2E-14 1.8E-14 0 5E-14 1E-13 1.5E-13 Cn2 Of The Lab Cn^2_1 2E-13 2.5E-13 1.88E-13 1.86E-13 1.84E-13 1.82E-13 1.8E-13 1.78E-13 1.76E-13 1.74E-13 1.72E-13 1.7E-13 3E-13 Cn2 Traditional Rx 1550nm Cn2 Ones Cn^2_1_STD Figure 3.24 Graph of Cn2 for the Ones for the 1550nm Laser, Cn^2_1 is the Bundle Receiver, Cn^2_1_STD is the Standard Receiver Figures 3.23 and 3.24 show the analysis results obtained for the 1550 nm signal laser. These plots show similar behavior to the previous plots, where the effective Cn2 is on the order of ten times lower for the bundle receiver compared to the standard receiver, as shown quite distinctly in Figure 3.25. In Figure 3.24 the same effect on the effective Cn2 is observed as in Figures 3.20 and 3.22, where at high turbulence the movement of the signal power onto other lenses of the bundle can again be observed. 33 Cn2 Rx 2.2E-13 2E-13 1.8E-13 1.6E-13 1.4E-13 1.2E-13 1E-13 8E-14 6E-14 4E-14 2E-14 0 1550nm Cn2 Ones 0 5E-14 1E-13 Cn2 1.5E-13 2E-13 2.5E-13 3E-13 Of The Lab Cn^2_1 Cn^2_1_STD Figure 3.25 One Scale Graph of Cn2 In summary, the analysis results shown in the graphs not only show that the bundle receiver has, on average, a lower value of effective value of Cn2 than a standard receiver, but it also has the effect of reducing the effective value of Cn2 by a small amount, as the bundle-based receiver possesses more lenses onto which the optical power can fall and be collected.. As the signal drifts off one lens due to turbulence it drifts onto another lens and is again collected, which reduces the effective Cn2 value since signal integrity is maintained. 34 CHAPTER 4 CONCLUSIONS AND FUTURE WORK 4.1 Conclusions In this thesis, two different free space optical receiver designs were compared, the standard design and the bundle-based design, to determine whether or not the bundlebased design provided any advantage over the standard design for operation under turbulent conditions. After experimental analysis was performed, it was found that the bundle-based design reduces the effective value of Cn2 in most of the cases tested, which demonstrates its ability to reduce the effect of turbulence on a free space optical signal. This is an important result, as the receiver design will help to improve and expand the use of free space optical communication in mobile and high turbulence applications. 4.2 Future Work Moving forward there are a few steps that need to be taken to improve the overall understanding and performance of the new system. First, increase the core diameter of the fibers used in the bundle from 400ΞΌm to 600ΞΌm. It is expected that the larger core will allow the receiver to collect light from a wider range of arrival angles and thus provide greater ability to collect significant optical power even when strong turbulence is present. 35 Second, simulations are currently being created that aim to predict the behavior of the receiver for the case of weak turbulence. These simulations require more work before they can be considered correct and complete. Successful simulations will not only increase understanding of the behaviors but also allow discovery and application of design rules for different cases. Third, it is necessary to investigate the use of the fiber bundle-based receiver in mobile applications; the addition of movement increases the signal degradation of free space optical communication and requires that optical tracking must be investigated and implemented in conjunction with the fiber bundle receiver design. The transmitter, using all of the fibers in the transmitting array, may be used to track a moving receiver by employing an optical switch to change between the fibers along the linear fiber array, which will deflect the beam from side-to-side and would allow the transmitter beam to track the receiverβs movements without complicated electronics or large and heavy mechanical systems such as gimbals. 36 REFERENCES 1. J. D. Schmidt, βNumerical Simulation of Optical Wave Propagation,β Bellingham, WA SPIE Press, 2010. 2. E. Hecht, βOptics,β 4th ed. San Francisco, CA Person, 2002. 3. L. C. Andrews and R. L. Phillips, βLaser Beam Propagation through Random Media,β 2nd ed. Bellingham, WA SPIE Press, 2005. 4. F. L. Pedrotti and L. S. Pedrotti, βIntroduction to Optics,β 2nd ed. Englewood Cliffs, NJ. Prentice Hall, 1993. 5. N. F. Hutchins et al. βWavelength Dependence of a Fiber-Bundle Based FSO Link,β in IEEE Globecom, 2014 © IEEE. IEEE catalog number CFP1400E-USB. ISBN 978-1-4799-7701-7. 6. L. C. Andrews et al. βLaser Beam Scintillation with Applications,β Bellingham, WA SPIE Press, 2001. 37 APPENDEX A MATLAB CODE 001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 clc p=9; x=dataset; for n=1500 %% determine the bit start if abs(x(n)-x(n+1))>.8*mean(x)&&abs(x(n+1)-x(n+2))<1*mean(x) p=n+1; break end end r=x(pend); %%%% p is the bit starting point thri=mean(r); % up_trans=0; count_level_1=0;% counting samples of level 1 % dwn_trans=0; count_level_0=0;% counting samples of level 0 m=0; h=0; q=0; cnt_1=0; l=0; cnt_0=0; for i=1length(r) if r(i)>thri cnt_0=0; count_level_1=count_level_1+1;% counting samples of level 1 m=m+1; data_level_1(m)=r(i); index_data_level_1(m)=i; cnt_1=cnt_1+1;% counter for counting the # of samples at every continues Nr of level_1 %sampling in the middle of eye opening if (mod((cnt_1-5),50)==0) h=h+1; data_1(h)=r(i); g(h)=i;% end %%%%%% else % % % data_1=Level_1(m-cnt_1+1m); % % cnt_1=0; count_level_0=count_level_0+1;% counting samples of level 0 l=l+1; data_level_0(l)=r(i); 38 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 index_data_level_0(l)=i; cnt_0=cnt_0+1; if (mod((cnt_0-5),50)==0) q=q+1; data_0(q)=r(i); t(q)=i; end end end Nr_1=length(data_level_1);% #of bits of level 1 Nr_0=length(data_level_0);% #of bits of level 0 max_level_1=max(data_level_1); min_level_1=min(data_level_1); max_level_0=max(data_level_0); min_level_0=min(data_level_0); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% I_1=mean(data_level_1); I_0=mean(data_level_0); sigma_1=std(data_level_1); sigma_0=std(data_level_0); %%% optimum threshold for all samples(10)included in 1 bit I_D = ((sigma_0 * I_1) + (sigma_1*I_0))/(sigma_0 + sigma_1); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% % %%%%remove the DC level % r=r-I_D; % data_level_1= data_level_1-I_D; % data_level_0= data_level_0-I_D; % % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%plots rrr=r(1500); figure(3) plot(r) xlim([1 500]) figure(4) subplot(1,2,1) hist(data_level_1,length(data_level_1)) set(get(gca,'child'),'FaceColor','r','EdgeColor','r'); ylabel('No. of occurrence') title('pdf of received samples of level 1 bit ') subplot(1,2,2) hist(data_level_0,length(data_level_0)) set(get(gca,'child'),'FaceColor','b','EdgeColor','b'); ylabel('No. of occurrence') title('pdf of received samples of level 0 bit ') %Normalising the pdf of 1 and 0 levels so the integration leads to 1 [n_1,X_out1]=hist(data_1,length(data_1)); %n_1=n_1./sum(n_1.*X_out1); n_1=n_1./sum(n_1); %mean_1=sum(X_out1.*n_1);%expected value of bit 1 mean_1=mean(data_1);%expected value of bit 1 39 102 max_data_1=max(data_1); 103 min_data_1=min(data_1); 104 [n_0,X_out0]=hist(data_0,length(data_0)); 105 %n_0=-n_0./sum(n_0.*X_out0); 106 n_0=n_0./sum(n_0); 107 %mean_0=sum(X_out0.*n_0);%expected value of bit 0 108 mean_0=mean(data_0);%expected value of bit 0 109 max_data_0=max(data_0); 110 min_data_0=min(data_0); 111 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 112 figure(5) 113 subplot(1,2,1) 114 hist(data_1,length(data_1)) 115 set(get(gca,'child'),'FaceColor','r','EdgeColor','r'); 116 ylabel('No. of occurrence') 117 title('pdf of received bits of level 1') 118 subplot(1,2,2) 119 hist(data_0,length(data_0)) 120 set(get(gca,'child'),'FaceColor','b','EdgeColor','b'); 121 ylabel('No. of occurrence') 122 title('pdf of received bits of level 0 ') 123 N_1=length(data_1); 124 N_0=length(data_0); 125 % optimum Threshold 126 I_D_data=((std(data_0)*mean_1)+(std(data_1)*mean_0))/(std(data_0)+std(data_1)); 127 rv=mean(data_1.^2)/mean(data_1)^2-1;% Rytov_var=mean(data_1.^2)/mean(data_1)^2-1 128 Q=(mean(data_1)-mean(data_0))/(std(data_1)+std(data_0));% Q-factor 129 Ber_op=(1/2)*erfc(Q/sqrt(2)); 130 % for sampling in the middle of eye opening using Q_factor 131 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 132 figure(6) 133 subplot(1,2,1) 134 bar(X_out1,n_1) 135 set(get(gca,'child'),'FaceColor','r','EdgeColor','r'); 136 ylabel('No. of occurrence') 137 title('pdf of received bits of level 1') 138 subplot(1,2,2) 139 bar(X_out0,n_0) 140 set(get(gca,'child'),'FaceColor','b','EdgeColor','b'); 141 ylabel('No. of occurrence') 142 title('pdf of received bits of level 0 ') 143 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 144 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 145 %%%%%%%Calculating BER from pdf of 1 level and 0 level for sampling in 146 %%%%%%%middle of opening eye 147 %th=0;%%%%modified it latter 148 th=I_D_data; 149 [i]=find(X_out1<th); 150 v_1=X_out1(i);%bit values that is small than threshold 151 nn_1=n_1(i);% Nr of occurrences these bits 152 [j]=find(X_out0>th); 153 v_0=X_out0(j); 154 nn_0=n_0(j); 155 %% Calculating BER according to the Pdf integrations. 156 ber=(N_1/(N_1+N_0))*abs((sum(nn_1)))+(N_0/(N_1+N_0))*(sum(nn_0)); 157 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 40 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 %%%%%%calculating Cn^2 lamda=1.55e-6; k=(2*pi/lamda)^(7/6); l=(3.05)^11/6; Cn_sq=rv/(1.23*k*l);% Cn_sq=rv/(1.23 k^7/6 Lp^11/6);k=2*pi/lamda ; Lp=3.05m %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%curve fitting. I=X_out1(1)X_out1(2)-X_out1(1)X_out1(end); I0=mean_1;%% the singal without turbulence a=1/(sqrt(2*pi*rv)); b=(log(I/I0)-rv/2).^2; y_wt=exp(-b/(2*rv))./(a*I); figure(7) %plot( I,y_wt,'c') plot( I,1e-3.*y_wt,'c')%1e-3 just for normalization %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%Normalizing the pdf for including 10 samps and plot the pdf [n_level_1,X_out_level_1]=hist(data_level_1,length(data_level_1)); %n_1=n_1./sum(n_level_1.*X_out_level_1); n_level_1=n_level_1./sum(n_level_1); %mean_level_1=sum(X_out_level_1.*n_level_1);%expected value of bit 1 including 10 samps mean_level_1=mean(data_level_1);%expected value of bit 1 including 10 samps [n_level_0,X_out_level_0]=hist(data_level_0,length(data_level_0)); %n_0=-n_0./sum(n_level_0.*X_out_level_0); n_level_0=n_level_0./sum(n_level_0); %mean_level_0=sum(X_out_level_0.*n_level_0);%expected value of bit 0 including 10 samps mean_level_0=mean(data_level_0);%expected value of bit 0 including 10 samps th=I_D; [i]=find(X_out_level_1<th); v_level_1=X_out_level_1(i);%bit values that is small than threshold nn_level_1=n_level_1(i);% Nr of occurrences these bits [j]=find(X_out_level_0>th); v_level_0=X_out_level_0(j); nn_level_0=n_level_0(j); %% Calculating BER according to the Pdf integrations. ber_level=(Nr_1/(Nr_1+Nr_0))*abs((sum(nn_level_1)))+(Nr_0/(Nr_1+Nr_0))*(sum(nn_level_0)); %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% figure(10)%%%%%%%%%%%after normalization subplot(1,2,1) bar(X_out_level_1,n_level_1) set(get(gca,'child'),'FaceColor','r','EdgeColor','r'); ylabel('No. of occurrence for including 10 samps in bit') title('pdf of received bits of level 1') subplot(1,2,2) bar(X_out_level_0,n_level_0) set(get(gca,'child'),'FaceColor','b','EdgeColor','b'); ylabel('No. of occurrence for including 10 samps in bit') title('pdf of received bits of level 0 ') %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 41
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