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  • 1. Final Report Free Space Optical Communication Link Using LEDs ECE 4007 Senior Design Project Section L01, FSO Group Adam Swett Clayton Huff Trang Thai Nguyen Trinh Submitted May 1, 2008 FSO Group (ECE 4007 L01) 1
  • 2. Table of Contents Executive Summary………………………………………….. iii 1. Introduction………………………………………….…….. 1 1.1 Objective…………………………………………. 1 1.2 Motivation..………………………………………. 2 1.3 Background………………………………………. 2 2. Project Description and Goals……………………………. 3 3. Technical Specifications……………………………………. 5 4. Design Approach and Details………………………………6 4.1 Design Approach…………………………………. 6 4.1.1 Design Revision…………………………... 6 4.1.2 Shunt Peaking Circuit..…………………… 7 4.1.3 LED Modulation…………………………. 8 4.1.4 Free Space Optical Receiver Design………. 8 4.1.5 Free Space Optical Channel Setup……….…12 Intensity Modulation……………...13 Optical Channel Design…………..14 4.1.6 PCB Considerations and Revisions…………22 4.2 Codes and Standards…………………………….... 25 4.3 Constraints, Alternatives, Tradeoffs…………….... 28 5. Schedule, Tasks, and Milestones………………………….. 28 6. Project Demonstration…………………………………….. 29 7. Marketing………………………………………………….. 30 7.1 Marketing Analysis......…………………………… 31 7.2 Cost Analysis…………………………………...… 31 8. Summary……………………………………………….…... 32 9. References………………………………………………….. 33 Appendix 1 ………………………………………………….….34 FSO Group (ECE 4007 L01) ii
  • 3. Appendix 2 ……………………………………………………..35 Appendix 3 ...........................................................................…...36 Executive Summary The Free Space Optical (FSO) LED link has the ability to connect two devices at high-speed while taking advantage of the high bandwidth of optical communication and the low cost and low power consumption of LEDs. This link will provide an alternative to traditional RF wireless communication that is currently nearing its bandwidth limitations. As the speed increases for data transmitted over a wire, it is necessary that wireless communication continues unbounded. The FSO link also outperforms USB 1.1, USB 2.0, and Bluetooth allowing for an additional market and perhaps a new standard for data transmission. This will become necessary as file sizes increase and multimedia dominates the business world. A key advantage to the LED system will be cost. The FSO link will consist of a transmitter and a receiver. The transmitter will have a target cost less than $50 and will operate on two AA batteries. The receiver will also cost $50. Overall the cost of this system is significantly less than a comparable LASER optical link and draws less power. The FSO link will be built using top-of-the-line components and cost will continue to decrease as the components become standard. LEDs are more directional than radio waves, which prevents eaves-dropping. The result of the project after testing, debugging, and data analysis was a working link at one meter with a speed of 160 Mbps and a bit error rate 10-10. The target market for this project is the average consumer who has a need for high-speed and low cost data transmission. FSO Group (ECE 4007 L01) iii
  • 4. Free Space Optical Communication Link 1. Introduction There are various forms of wireless communication available such as satellites/antennas, WiFi, and FSO communication with lasers or VCELs. Currently, there are no forms of FSO communication that use a LED as the transmitting light source. The FSO link using a LED will send data via free space between a transmitting unit and a receiving unit. 1.1 Objective The objective of the FSO link using a LED is to create a new form of optical communication that can be marketed with small size and low cost. The link consists of a transmitter and a receiver both of which are battery powered and portable. The transmitter circuitry takes in a data stream of "1s (+3.3V)" and "0s (0V)" and then modulates the current through an LED. The LED sends photons with intensities proportional to the input data bits across the free space link to a PIN photo-diode. The PIN diode is connected to the receiver which converts optical power into an output voltage. The size of each module is approximately the same and should accommodate all components needed to function. The LED, PIN diode, and other circuitry will be able to operate off of a portable battery due to low power consumption. Inexpensive parts and low power consumption present a lower marketing cost. The link was designed for two different applications. The first is a short range application, such as, sending data from a flash drive to a computer, and this application has a data transfer rate of 100 Mb/s at a FSO Group (ECE 4007 L01) 1
  • 5. range up to 10 cm. The second application sends data at 25 Mb/s at a range up to 1 m, and this application could apply to a central classroom module sending data to laptops or PDAs. 1.2 Motivation The current form of free space optical communication using light emission resides in the realm of lasers and VCELs. These devices produce optical intensities high above any LED, and they can also switch faster from high intensity to low intensity and vice versa. The drawback to using lasers and VCELs is the cost and power consumption. Lasers cost much more than LEDs and require more expensive driver circuitry. They also consume more power due to the stimulated emission process. LED are both less expensive and require less power to operate than lasers. This feature would allow for a low cost FSO link that operates at high-speeds. Another important advantage of the FSO based on LEDs is that security of transmission can be achieved. LEDs should be used in FSO communication so that the cost and size of the final product will have numerous applications and will potentially replace the existing indoor wireless networks.. 1.3 Background The current wireless technologies such as Bluetooth, WiFi, or WiMax have low data transmission rate compared to that of the wired LAN systems. These networks require licensing and pose security risks. Many efforts over the last decade have developed an alternative for wireless networks. FSO networks can be set up within a day with absolute security because the transmission is based on line-of-sight. The data rate for optical systems is in the Gbps range. However, this technology is very expensive FSO Group (ECE 4007 L01) 2
  • 6. preventing it from replacing the current wireless networks such WiFi. LEDs were proposed to replace the laser as the light sources in order to lower the cost while maintaining the high-speed data rate and the security feature that FSO offers. There have been experiments conducted using LEDs as the light source for a FSO link However; there are no commercially available products that use LED FSO technology. A company in Japan designed a LED driver that could modulate an LED at 400 MHz, but LEDs were incapable of modulating at those speeds at the time [1]. 2. Project Description and Goals This project has two stages: • To create a link with a minimum data rate of 100 Mbps at a range of 10 cm in a controlled environment. • To achieve a data rate of 25 Mbps at a range of 1 m under normal conditions using the same link. The controlled conditions are defined as the FSO link maintained within 10 cm and operated in a noise free environment (i.e., in an enclosed pipe with no light source other than the transmitting LED). The room conditions are defined as the FSO link at a distance of 1 m being exposed to ambient light sources in a closed space. The test will be conducted without excessive light such as sunlight. The LED link is designed to transmit data bits from a voltage generator to an oscilloscope through free space using light from an LED. The LED operates in the spectral range of infrared (above 700 nm in wavelength). A PIN photodiode is employed on the receiver circuit to collect the light sent from the transmitting LED. A system block diagram is shown in Figure 1. FSO Group (ECE 4007 L01) 3
  • 7. Figure 1. A block diagram of the FSO-LED system The project is subjected to the minimum performance as described in the first step of the approach. The second step is subjected to the minimum performance in which both data transmission rate and range shall exceed that achieved in the first step. An ideal target for the second step of the project within the time frame of the semester is to achieve a minimum transmission rate of 25 Mbps at the range of 1 m. Due to the theoretical limitation of the LED operation, the maximum data rate may not reach 1Gbps. Nevertheless, at the transmission speed of 100 Mbps, the low cost LED link is fully capable of replacing the wired LAN network and enables wireless communication between numerous portable devices such as Mp3 players, digital cameras, portable storages, and laptops. 3. Technical Specifications FSO Group (ECE 4007 L01) 4
  • 8. The transmitter/receiver design includes crucial specifications that must be met in order to achieve a working FSO link. These proposed and actual specifications are listed in Table 1. Table 1. Proposed Versus Actual FSO Link Specifications Specifications Proposed Actual Tx Power Consumption 200 mW 690 mW Tx Supply Voltage +5 V +3.3 V Rx Power Consumption 200 mW 98 mW Rx Supply Voltage +5 V -5 V, +5 V Link Range 10cm  1m 10 cm  1.01m Link Bandwidth 25  100 Mbps 25  160 Mbps Transmitter Size ~ 4 x 4 inches 2 x 1.5 inches Receiver Size ~ 4 x 4 inches 1.5 x 1.5 inches The LED and PIN diode perform the photon transmission and receiving, and they must be operated with the correct voltage and current levels to achieve the necessary optical intensity. The input voltage to the transmitter will range from 0 –> +3.3 V, and the power rail of the circuits will be at +3.3 V. The short range application for the FSO link should send data at 100 Mbps at a distance of 10 cm, and the long range application should have a data rate of 25 Mbps at a distance of 1 m. There should not be any rigorous alignment required for the link set up. At the distance of 50 cm, the data rate should remain constant within a 10o cone between the transmitter and receiver. FSO Group (ECE 4007 L01) 5
  • 9. 4. Design Approach and Details 4.1 Design Approach 4.1.1 Design Revision The Digital driver proved to be much more difficult to design than was originally planned. The proposed circuit was experimental in nature meaning that strict simulations could not accurately model the response of the circuit in real conditions. The spice models for the LED did not exist and many of the parameters necessary were not available. The alternative method to attaining the parameters was through a frequency analyzer; this also was not a viable option. As such the proposed digital driver circuit that was originally proposed could not be implemented in the final design. Instead the MAXIM 3967 Driver was implemented. The suggested circuit for the driver was given in the specification sheet. Figure 2. Suggested Maxim circuit for MAX3967A LED driver. FSO Group (ECE 4007 L01) 6
  • 10. In the design implemented a shunt peaking circuit was added, the modulation current resistor was set with a base of 700 ohms and a variable resistor following it in series, and all pre-bias pins were tied to VEE. 4.1.2 Shunt Peaking Circuit Figure 3. Schematic of shunt peaking circuit used in transmitter. In an effort to lower power consumption in more typical LED applications manufactures often will trade optical power and lower power consumption for speed. Often times it is not necessary for an LED to switch quickly, however, in the case of data transfer it becomes integral. Shunt peaking is a method that can be used to increase the switching time of an LED using circuit components. The basic shunt peaking circuit is shown in the figure. The two most important components are the inductor L p and resistor Rp. These provide the ratios that determine the new extinction ratio of the LED. The FSO Group (ECE 4007 L01) 7
  • 11. capacitors are used as a DC block to protect the LED. Finally the resistor Rs provides a return path for current that would otherwise pass through the LED when OUT+ is turned off. Two equations govern the use of the shunt peaking circuit. The first thing that must be determined is k which is the fraction of drive that will be used for shunt peaking. This is a function of the total power needed to drive the link and the amount of power that is currently being received. Once the value for the shunt peaking resistor is found the desired fall time of the LED is entered into the first equation and a suitable value for the inductor is found. The values found in the shunt peaking equations are only theoretical. With typical lab values of Inductors and resistors it is then necessary to experimentally determine what combination of components produces the best signal. 4.1.3 LED Modulation The LED used was the Optek OPF345a which has a rise and fall time of 4.5 ns and was chosen because this time is relatively fast. In order to achieve the highest data rate, the LED should not be completely turned off. This speeds the process in which minority carries can effectively be created in the junction. A high enough bias level must be chosen to turn on the LED while not turning on the LED all the way, this will avoid damaging the LED. 4.1.4 Free Space Optic Receiver Design A typical optical receiver consists of many stages as shown in Figure 4. Within the scope of our project, the receiver board design is limited to the front-end components. This includes a photodiode and a pre-amplifier (first block immediately following the photodiode in Figure 4). Therefore the data collected is raw data and is subjected to FSO Group (ECE 4007 L01) 8
  • 12. further refinement of filtering and error corrections. Our receiver schematic is shown in Figure 5 below where the p-i-n (PIN) photodiode OPF430 by Optek Inc. and the OPA657 .transimpedance amplifier (TIA) chip by Texas Instruments Inc. were used Figure 4. Different stages in a typical optical full receiver. Figure 5. Receiver board schematic. The photodiode collects photons from the light source and converted it into current. This current is extremely small and corresponds to the sensitivity of the photodiode. Typically the photodiode generates about 0.55 uA for 1 uW optical power received. Therefore it is followed by a pre-amplifier to amplify the electrical signal for further processing. A high-impedance technique is often used to develop a voltage FSO Group (ECE 4007 L01) 9
  • 13. proportional to the light detector current. The impedance serves to reduce the thermal noise and improve the receiver sensitivity. However, the leakage current could saturate the PIN diode preventing the modulated signal from being detected. An improvement can be obtained with the use of and LC feedback circuit. As the Q increases so does the impedance of the LC circuit [5]. This alternative method is referred to as transimpedance amplifier, which is the first stage of the receiver shown in Figure 4. Typical transimpedance amplifier is shown in Figure 6. The bandwidth of the commercially available transimpedance amplifier usually covers kHz to GHz range. Figure 6. Tuned transimpednace amplifier with high Q [5] In the receiver design, the TIA OPA657 was chosen because it has a wide modulation bandwidth product (1.6 GHz) with a rise/fall time of 1ns. It has a JFET input stage that allows low noise that is appropriate for low power optical system since the PIN generated current is low, thus the noise should be kept significantly lower. The PIN OPF430 was chosen because it has a small rise time of 2ns at low reverse bias voltage (5V) and a good responsitivity (0.55A/W). The PIN together with the TIA make up the front-end receiver with effective rise/fall time of about 3ns, which is sufficient to cover our targeted modulation bandwidth of 150MHz. FSO Group (ECE 4007 L01) 10
  • 14. In Figure 5 the receiver schematic shows a feedback loop that sets the bandwidth of the circuit. This feedback loop typically consists of a resistor and a capacitor that can be determined according to the following relationship. (Eq.1) RF and CF are the feedback loop resistance and capacitance, CD is the diode capacitor, and GBP is the gain bandwidth product [12]. The OPA430 PIN can be modeled in simulation as shown in Figure 7. The PIN is reversed biased at -5V however; voltage dropped across the PIN capacitor is about -3.5V to -4.5V. The decoupling circuit is to eliminate the offset voltage at the output and to minimize the bias current going into the TIA. The 100uA DC current included in the OPA430 equivalent circuit is to account for the ambient light in the room and is discussed further in the next section. Figure 7. The equivalent circuit model of the PIN OPA430. The circuit was simulated in TINA TI simulation software [13]. Our circuit theoretical response to a step input is a flat line, however a ringing is observed in real circuit response (Figure 8) this is due to the parasitic capacitance that was introduced into the circuit by defects in manufacturing such as intrinsic capacitors within discreet FSO Group (ECE 4007 L01) 11
  • 15. components and imperfect solder joints. The real circuit response can be simulated by adding capacitance values into the theoretical circuit as illustrated in Figure 5. The Bode plots of voltage gain magnitude and phase is plotted in Figure 9. The transfer function shows that receiver has a bandwidth about 150MHz and relatively constant phase. Figure 8. The receiver circuit step response. Figure 9. The Bode plots of the receiver circuit. 4.1.5 Free Space Optic Channel Set Up In order to construct a fast and reliable transmission link, the optical channel has to ensure that sufficient optical power is collected at the photodiode. In addition, the channel has to minimize transmission loss so that the power consumption may be kept FSO Group (ECE 4007 L01) 12
  • 16. low. The channel therefore can be characterized based on optical propagation, link power budget, and bit-error-rate (BER) factors. These considerations also allow the LED link to be evaluated with respect to its performance and operating conditions such as range and alignment sensitivity between the transmitter and receiver. Intensity Modulation and Direct Detection The information that is transmitted in an optical link requires some form of modulation in order to for the signal to be encoded into a physical parameter. This physical parameter is usually the intensity, the frequency, or the polarization of the emitted light. The modulating signal is then demodulated in the receiver to recover the transmitted information. The optical link is based on low cost LEDs which emit incoherent light and has a wide spectral width as well as a large angle spread. They are almost perfect Lambertian sources. Therefore it is extremely difficult to collect appreciable signal power in a single electromagnetic mode, thus the only practical modulation is intensity modulation (IM). As a result, the most feasible down-conversion technique is direct detection (DD) in which the current produced by the light detector is directly proportional to the square of the received electric field [2]. Visible LEDs are mostly manufactured for illumination instead of communication therefore their switching speed is relatively low. In order to achieve transmission rate above 100Mbps, commercially available infrared LED (850nm OPF345A by Optek Inc.) is employed in our optical LED communication link. Correspondingly, the photodiode is required to have comparable modulation bandwidth and an optical spectrum in the range of 800-900 nm of wavelength in order to collect the emitted light from LED. The PIN photodiode (OPF430 by Optek Inc.) can satisfy these requirements and is employed in our system. FSO Group (ECE 4007 L01) 13
  • 17. In a DD system, the optical incident power is detected by a photodiode and converted into an electrical current. In room condition of operation, there is always ambient light that enters the photodiode along with the signal light. The power of the ambient noise, Pamb is determined as Pamb = Namb∙Bopt [Eq.2] ,where Namb denotes the ambient radiation, and Bopt denotes the optical bandwidth of the photodiode. Therefore, the resulting electrical current has a DC component (IDC) of, IDC = R∙Pamb [Eq.3] and a signal current of, is = R∙Ps [Eq.4] where R denotes the responsivity of the photodiode (unit of A/W), and P s is the optical power of the signal [3]. The photodiode also produce a dark current typically on the order of nA and is negligible. Since a transimpedance is used in the receiver to convert the current generated by the PIN into a voltage signal, the responsivity that characterizes the receiver board can be calculated in terms of incident power and voltage generated as, Vpp = R∙Ptot [Eq. 5] where Vpp denotes the peak-to-peak voltage generated, and Ptot is the total power that is the sum of Pamb and Ps. Here the responsivity has unit of V/W. Optical Channel Design After investigations, it was recognized that radiated power emitted by the LED cannot be coupled sufficiently into the PIN directly even with a head-to-head arrangement (the PIN is placed immediately in front of the LED). This is due to a small detection area of the PIN (Optek OPF430) which is designed for optical fiber coupling and the diverse radiation pattern of the LED. Therefore the use of lenses is essential to enable a working FSO Group (ECE 4007 L01) 14
  • 18. and reliable data transmission. The lenses are set up and attached to specific locations relative to the LED and PIN as illustrated in Figure 10. Figure 10. Optical Link geometry The lenses are plano-convex having 1 in. diameter and focal length of 50 mm. The LED and PIN are placed at the focal point of each lens as shown in Figure 10 so that they can couple the light into a collimated beam. The radiation pattern of the LED can be considered as mostly confined in the cone of angle φ but only the portion of light within the cone of angle θ is coupled into the lens channel. Parallel light rays traveling from lens 1 to lens 2 is non-divergent, thus there is no lost of power across the optical channel of free space range R (Figure 10). The PIN is placed at the focal point of lens 2, therefore it receives almost all the power that is coupled at lens 1 and effectively converted that into electrical currents. In the final products, the lens can be packaged together with the LED and PIN to form a transmitter (Tx) and receiver (Rx) packages. The beam width may be adjusted into a divergent beam by shortening the distance f1 (Figure 10) if a wider angle FSO Group (ECE 4007 L01) 15
  • 19. of illumination is desired when power budget of the link allows. This feature addresses how easy to aim the receiver in order to establish the transmission. The received optical power P at the receiver is expressed as, [Eq. 6] where Pt is the transmitted power from an LED, ф is the angle of irradiance with respect to the transmitter axis, is the angle of incidence with respect to the receiver axis, and d is the distance between an LED and a detector’s surface [4]. T s(ψ) is the filter transmission g(ψ) is the concentrator gain. ΨC is the concentrator field of view. Semi-angle m is the order of Lambertian emission, and is given by the transmitter half angle (at half power) as [Eq. 7] Here, m= 1 from Ψ1/2= 60o (Lambertian transmitter) [4]. From the geometry shown in Fig. 2 below, we can set ф = ψ. The concentrator is referred to lenses employed to enhance the power at the receiver. Figure 11. Geometrical set up of the propagation light. [3] However, the FSO link within the scope of this project is designed to produce a collimated beam. Therefore, the free space distance d is theoretically free of constraint and unlimited in the system of Fig. 1. Since the focal length f1 (50 mm) and the lens FSO Group (ECE 4007 L01) 16
  • 20. diameter (1 in. or 25.4 mm) are fixed parameters, angle θ is constant and is about 28.5o. The optical power is measured at A (PLED) and at D (PPIN) and the coupling efficiency is calculated as the percent ratio of the two values as shown in Table I, where Vp-p denotes the voltage amplitude for LED square waves set at different frequencies. Data in Table 2 was taken with our designed driver and receiver boards using link configuration shown in Figure 10. The responsivity is calculated according to Eq. 5. Table 2. Power versus frequency measurements on our designed Tx and Rx boards Freq (MHz) 160 100 50 Vpp (mV) (1sf) 785 960 965 PLED (mW) 0.75 0.79 0.8 PPIN (mW) 0.49 0.53 0.55 Efficiency (%) 65 67 69 R (V/W) 1.6E+03 1.8E+03 1.8E+03 An evaluation of our optical channel is set up as illustrated in Figure 12. The Rx is moved sideway relative to the Tx, i.e. it is offset by a distance s (Figure 12) from the collimated beam. Thus the power collected at PIN is decreased accordingly, and is recorded with respect to the operating frequency and BER. Figure 12. Power budget evaluation using alignment offset. The responsitivity of our receiver board is evaluated in this procedure with respect to frequency as shown in Table. II. The evaluation procedure was performed at 160Mbps FSO Group (ECE 4007 L01) 17
  • 21. on two sets of boards: 1) the MAXIM evaluation board as the LED driver MAXIM 3967A and a 1GHz PIN with a separate post amplifier. 2) Our designed driver and receiver as described in earlier sections. Measured parameters include received power, the mean voltage of logic ‘1’ (I1) and logic ‘0’ (I0), and the RMS noise voltage for both logic levels (σ1 and σ0) (Appendix 1 and 2). Q values and therefore BER can be calculated according to Eq. 8-9 below. The received power is plotted against offset s and BER is plotted against the received power for both sets of boards in Figure 13-15. I1 − I 0 Q= [Eq. 8] σ1 + σ 0  −Q2    2   1  Q  e   [Eq. 9] BER = erfc  ≈ 2  2  Q 2π Table 3. Responsivity evaluated at 160MHz for our receiver board s (mm) PPIN (mW) Vpp (mV) R (V/W) 0 0.49 785 1.6E+03 2 0.48 740 1.5E+03 4 0.45 640 1.4E+03 6 0.41 535 1.3E+03 8 0.33 405 1.2E+03 10 0.24 290 1.2E+03 12 0.16 174 1.1E+03 14 0.08 71 8.9E+02 16 0.04 12 3.0E+02 FSO Group (ECE 4007 L01) 18
  • 22. 1.4 1.0E+00 1.2 1.0E-01 Received Power (mW) 1.0 0.8 1.0E-02 BER 0.6 1.0E-03 0.4 1.0E-04 0.2 0.0 1.0E-05 0 2 4 6 8 10 12 14 16 18 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Offset s (mm) Received Power (mW) (a) (b) Figure 13. Measurements at 160 Mbps on set 1 boards for a) the received power vs offset s and b) BER vs the received power. 0.6 0.5 Received Power (mW) 0.4 0.3 0.2 0.1 0.0 0 2 4 6 8 10 12 14 16 18 Offset s (mm) (a) FSO Group (ECE 4007 L01) 19
  • 23. 1.0E+00 1.0E-01 1.0E-02 1.0E-03 1.0E-04 BER 1.0E-05 1.0E-06 1.0E-07 1.0E-08 1.0E-09 1.0E-10 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Received Power(mW) (b) Figure 14. Measurements at 160Mbps on set 2 boards for a) the received power vs offset s and b) BER vs the received power. The power is decreased almost linearly as the offset distance is increased (Figure 13a-14a) since the beam is collimated and therefore the coupled power is proportional to the offset distance. BER is shown to increase as the received power is decreased in both Figure 13b and 14b. From Figure 5b, it is concluded that we have achieved a BER on the order of 10-10 at 160 Mbps significantly exceeding the data rate stated in the project proposal. At the offset of 14 mm with corresponding received power of 0.08 mW, we still achieve a BER of 10-8. The eye diagram for our FSO-LED link using our designed driver and receiver boards at 160 Mbps is shown in Figure 15. All measurements were independent of the range d. However, in the lab measurements were recorded at the range d of about 37 mm between lenses for set 1 boards, and at d of 1.011 m for set 2 boards. FSO Group (ECE 4007 L01) 20
  • 24. Note that this range of transmission is also exceeding the proposed value, and in theory it can be extended further without power penalty. However, the aiming angle between the receiver and the transmitter becomes narrower and thus more difficult to perform alignment. In final products, it is suggested that more than one LEDs and PINs are used so that the wider angle is covered over large distance of communication, which is highly achievable as the proof-of-concept has been shown from our measurements. The scope noise was recorded to be 829.5 uV, and the dark noise of receiver was recorded to be 1.34 mV both in RMS values. Figure 15. The eye diagram of FSO-LED link at 160 Mbps for 1ns division FSO Group (ECE 4007 L01) 21
  • 25. Figure 16. The eye diagram of FSO-LED link at 160 Mbps for 2ns division 4.1.6 PCB Considerations and Revisions The PCBs used in this project were two layered with the top layer containing signal traces and the bottom layer being a ground plane. The boards were designed using a software called "PCB Artist" and were fabricated by Advanced Circuits. The important considerations to the design of these boards are listed below. • The LED and PIN diode should be as close to the chips as possible to allow for quick switching and power efficiency. • The polygon outlined in blue is a copper pour area and should take up as much of the bottom layer as possible. Microstrip transmission lines are designed to run above an infinite ground plane; this design combats the effects of magnetic and electric fields. • The by-pass capacitors for the chip should be as close to the chip pin as possible. These capacitors filter out noise generated on the microstrip for DC traces. FSO Group (ECE 4007 L01) 22
  • 26. • The transmission lines for +IN and –IN need to be almost identical in size and length. It is recommended by the chip manufacturer to make the lines 50Ω, but this proved to be difficult and the board still worked without 50Ω lines (higher frequencies depend more on line impedance). • The transmission lines should never make 90º turns and should have proper widths depending on the current flowing through them. This will prevent the lines from bottle necking the current and acting like fuses. The PCB layouts for the transmitter and the receiver are shown in Figure 17b and the actual PCBs are shown in 17a. There are some necessary revisions for the next boards. Tx revisions: • The top of L2 (connected to the LED) should also connect to VCC. Connecting to the bypass capacitor C2 is a good choice since they are so close to each other. • Optional: the board size can easily be reduced by using 0402 or 0603 SMD components. Components this size must be manually created in PCB Artist. Rx revisions: • An AC coupling circuit must be implemented between the PIN diode and the op- amp. This takes the form of a shunt resistor connected in series with a capacitor. The resistor drains the leakage current coming from the diode and the capacitor blocks any DC component of the signal. The capacitor should be fairly large so that most lower frequencies are passed (if it is to high, the lower frequency content in a PRBS signal will be filtered out thus causing a substantial amount of bit error). FSO Group (ECE 4007 L01) 23
  • 27. • The –Vb connection to the PIN diode should be switched with the case ground connection (make the top thru hole of the diode footprint ground and make the bottom thru hole –Vb). • Optional: as mentioned with the Tx, the board size can be reduced by using smaller components. Figure 17a. Actual Prototype PCB, Receiver is on the left Transmitter is on the right. FSO Group (ECE 4007 L01) 24
  • 28. Figure 17b. PCB layouts for the transmitter (left) and the receiver (right). The blue parts of the boards indicate ground, the red indicates signal traces, the grey indicate thru holes not connected to ground, and the peach colored circles are holes used for mounting the boards. Values for each component on the Tx board are found in Figure A3 in Appendix 3 and for the Rx board in Figure 5. 4.2 Codes and Standards At this point, there are no standards for FSO communication. The Link that we will create will exceed the speeds of common wireless transmission which are included below. All of the following is referenced in [11]. Bluetooth Wireless Technology • Geared towards voice and data applications • Operates in the unlicensed 2.4 GHz spectrum • Can operate over a distance of 10 meters or 100 meters depending on the Bluetooth device class. The peak data rate with EDR is 3 Mbps • Able to penetrate solid objects • Is omni-directional and does not require line-of-sight positioning of connected devices Certified Wireless USB • Speed: Wireless USB is projected to be 480 Mbps up to 2 meters and 110 Mbps for up to 10 meters. Wireless USB hub can host up to 127 wireless USB devices • Wireless USB will be based on and run over the UWB radio promoted by the WiMedia Alliance. • Allows point-to-point connectivity between devices and the Wireless USB hub Wi-Fi (IEEE 802.11) • Bluetooth technology costs a third of Wi-Fi to implement • Bluetooth technology uses a fifth of the power of Wi-Fi • The Wi-Fi Alliance tests and certifies 802.11 based wireless equipment • 802.11a: This uses OFDM, operates in the 5 GHz range, and has a maximum data rate of 54 Mbps FSO Group (ECE 4007 L01) 25
  • 29. • 802.11b: Operates in the 2.4 GHz range, has a maximum data rate of 11 Mbps and uses DSSS. 802.11b is the original Wi-Fi standard • 802.11g: Operates in the 2.4 GHz range, uses OFDM and has a maximum data rate of 54 Mbps. This is backwards compatible with 802.11b • 802.11e: This standard will improve quality of service • 802.11h: This standard is a supplement to 802.11a in Europe and will provide spectrum and power control management. Under this standard, dynamic frequency selection (FS) and transmit power control (TPC) are added to the 802.11a specification • 802.11i: This standard is for enhanced security. It includes the advanced encryption standard (AES). This standard is not completely backwards compatible and some users will have to upgrade their hardware. The full 802.11i support is also referred to as WPA2 • 802.11k: Under development, this amendment to the standard should allow for increased radio resource management on 802.11 networks • 802.11n: This standard is expected to operate in the 5 GHz range and offer a maximum data rate of over 100 Mbps (though some proposals are seeking upwards of 500 Mbps). 802.11n will handle wireless multimedia applications better than the other 802.11 standards • 802.11p: This standard will operate in the automotive-allocated 5.9 GHz spectrum. It will be the basis for the dedicated short range communications (DSRC) in North America. The DSRC will allow vehicle to vehicle and vehicle to roadside infrastructure communication • 802.11r: This amendment to the standard will improve users’ ability to roam between access points or base stations. The task group developing this form in spring/summer 2004 • 802.11s: Under development, this amendment to the standard will allow for mesh networking on 802.11 networks. The task group developing this formed in spring/summer 2004 WiMAX (Worldwide Interoperability for Microwave Access and IEEE 802.16) • WiMax is a wireless metropolitan area network (MAN) technology • WiMax has a range of 50 km with data rates of 70 Mbps. Typical cell has a shorter range • The original 802.16 standard operated in the 10-66 GHz frequency bands with line of sight environments • The newly completed 802.16a standard operates between 2 and 11 GHz and does not need line of sight • Delays in regulatory approval in Europe due to issues regarding the use of the spectrums in the 2.8 GHz and 3.4 GHz range • Supports vehicle mobility for between 20 to 100+ km/hr. The 802.16e standard will allow nomadic portability • The IEEE 802.16a and the ETSI HIPERMAN (High Performance Radio Metropolitan Area Network) share the same PHY and MAC. 802.16 has been designed from the beginning to be compatible with the European standard FSO Group (ECE 4007 L01) 26
  • 30. • Created to compete with DSL and cable modem access, the technology is considered ideal for rural, hard to wire areas Infrared (IrDA) • IrDA is used to provide wireless connectivity for devices that would normally use cables to connect. IrDA is a point-to-point, narrow angle (30° cone), ad-hoc data transmission standard designed to operate over a distance of 0 to 1 meter and at speeds of 9600 bps to 16 Mbps • IrDA is not able to penetrate solid objects and has limited data exchange applications compared to other wireless technologies • IrDA is mainly used in payment systems, in remote control scenarios or when synchronizing two PDAs with each other Radio Frequency Identification (RFID) • There are over 140 different ISO standards for RFID for a broad range of applications • With RFID, a passive or unpowered tag can be powered at a distance by a reader device. The receiver, which must be within a few feet, pulls information off the ‘tag,’ and then looks up more information from a database. Alternatively, some tags are self-powered, ‘active’ tags that can be read from a greater distance • RFID can operate in low frequency (less than 100 MHz), high frequency (more than 100 MHz), and UHF (868 to 954 MHz) • Uses include tracking inventory both in shipment and on retail shelves 802.20 • Considered to be mobile wireless broadband wireless access. • Maximum data rate expected to be 1 Mbps, operating in licensed bands below 3.5 GHz • Supports vehicle mobility up to 250 km/hr 4.3 Constraints, Alternatives, Tradeoffs The main constraint for the system is the modulating frequency of the LED. LEDs operating at speeds over 250 MHz are expensive. In order for the FSO link to achieve speeds above 250 Mbps, additional logic must be employed. A major trade-off for the FSO link is speed vs. range. As the range of the link increases, the data rate decreases and vice versa. Due to this trade-off, FSO links must be FSO Group (ECE 4007 L01) 27
  • 31. designed for either short or long range applications. A primary goal of this project s is to design a suitable compromise between data rate and transmission range. 5. Schedule, Tasks, Milestones The project tasks are divided between the transmitting end and the receiving end of the FSO link. A. Swett and C. Huff are in charge of researching, designing, and building the FSO transmitter. Likewise, T. Thai and N. Trinh will be researching, designing, and building the FSO receiver. T. Thai is also responsible for setting up and evaluating the free space optical channel performance. Deliverables, demonstrations, and testing will be accomplished as a team. The Gantt chart is shown in Figure 18. Figure 18. Gantt chart for FSO project (Green = low risk, Yellow = medium risk, Red = high risk). FSO Group (ECE 4007 L01) 28
  • 32. The milestones of the project are shown as blue tick marks in Figure 18. Early milestones include finishing designs and assembly of the transmitter/receiver. Most of the team's milestones will be accomplished in the testing phase where link bandwidth and range should keep approaching the desired goals. 6. Project Demonstration Each part of the project will be extensively tested in the lab by two groups during the semester. A. Swett and C. Huff will test the transmitter end while N. Trinh and T. Thai will test the receiver end. In the end, the two groups will test the two ends and demonstrate that the connection works according to the requirement set forth by the project. The final demonstration of the project was held April 23rd. The link was demonstrated in the High Speed Optics Laboratory in the Tech Square Research Building. There were 2 demonstrations that exhibited connections between the transmitter and receiver with 1) a minimum speed of 150 Mbps and a distance of 30 cm 2) a speed of 160 Mbps at a distance of 1 m with normal room condition, i.e., normal ambient lighting. The group set up the two ends of the system with the function generator at the sending side and an oscilloscope at the receiver side as illustrated in Figure 1. The demonstration was considered successful with a BER of 10-10. 7. Marketing 7.1 Marketing Analysis The LED FSO Link will have to compete against Bluetooth and IrDA products that are currently on the market. Compared to Bluetooth and IrDA, LED FSO will have a much greater speed. Bluetooth adapters can cost from $20 to $120 depending on FSO Group (ECE 4007 L01) 29
  • 33. connection types (socket serial, USB, CompactFlash, or PC card). Their operating speed can achieve up to 3 Mbps but only with integrated system. The speed maxes out at 1 Mbps for adapters with the PC Card connection type. There are fewer products on the market for IrDA. The prices can range from $6 to $60, depending on the connection type. The speed of the IrDA connection ranges from 9.6 kbps to 16 Mbps. The speed of the LED link achieved up to 170 Mbps as tested. This speed can be increase in the future with better LED selection and improved receiver. The cost of the prototype for both receiver and transmitter is under $150. In production this cost will significantly decrease due to mass production and bulk sale of components. 7.2 Cost Analysis The total cost of building the prototype is estimated by assuming all four group members are receiving a typical entry-level engineering’s salary of $60,000 per year and working 15 hours per week on the project for 16 weeks. The $60,000 salary is for 48 weeks per year and 40 hours per week and therefore the salary is $31.25 hourly. The cost of labor is shown in Table 2. Table 2. Labor Cost Salary $60,000.00 Hourly wage for each member $31.25 Total hour per week of all members 60 hours Total hour of all members 960 hours Total labor cost $30,000.00 The cost of the prototype is shown in table 3. The amounts shown are what was proposed and what actually was spent. Also, there is a bill of materials included in Appendix 2. Table 3. Prototype Design Parts Cost FSO Group (ECE 4007 L01) 30
  • 34. Parts Type Targeted Cost Actual Cost EVKit for Tx $300 $300 Led and PIN-diode $25 $12 PCBs and Other Parts Used $100 $130 Total $425 $442 The total cost for the design and construction of the prototype is estimated to be $30,442. An estimated target price for the receiver based on parts ordered in large quantities is approximately $15. A target price for the final product should be approximately $50 each after factoring in production cost, marketing cost, and distribution cost. This price will allow the FSO link to compete with other products on the market. 8. Summary The project was successful according to the initial goals of the project. A link was achieved at 1 meter with a speed of 160 Mbps and a BER of 10-10. Initially the proposal included a link with a slower data rate and a longer range and a link with a faster data rate and a shorter range. In the final product it was discovered that the link range had little effect on the overall performance. It is possible to achieve greater speeds with future revisions of both the driver and receiver boards. Link alignment should be studied further to achieve a more reliable data transfer at wider angles. It is also possible to achieve better results by using more than one LED and PIN. Also, the pre-bias and modulation currents should be studied using the evaluation board and the custom receiver to determine if it is possible to further reduce thermal noise created by the LED. A package should be created in which an internal power supply (battery), casing, and lens are integrated. It will also be necessary to interface the transmitter and the receiver with a data source and memory such FSO Group (ECE 4007 L01) 31
  • 35. as USB and a hard drive. The receiver prototype should be developed into a commercial receiver with more filtering and error correction stages along with interfacing. 9. References [1] Suzuki, Tomihiro (July 1986). High-Speed 1.3-um LED Transmitter Using GaAs Driver IC. Journal of Lightwave Technology, vol. lt-4, no. 7. [2] M. D. Kotzin, “Short-range communications using diffusely scattered infrared radiation,” Ph.D. dissertation, Northwestern Univ., Evanston, IL, June 1981. [3] R. Otte, “Low-Power Wireless Optical Transmission,” Delft University Press, Delft, Netherlands, 1998. [4] C. G. Lee, C. S. Park, J. H. Kim, and D. H. Kim, "Experimental verification of optical wireless communication link using high-brightness illumination light-emitting diodes," Opt. Eng. 46, 125005 (2007). [5] D. Johnson “Handbook of Optical Through the Air Communications,” [Available Online]:, [Accessed Feb. 3, 2008]. [6] M. D. Kotzin, “Short-range communications using diffusely scattered infrared radiation,” Ph.D. dissertation, Northwestern Univ., Evanston, IL, June 1981. [7] J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE, vol. 85, no. 2, pp. 265–298, 1997. [8] C. G. Lee, C. S. Park, J. H. Kim, and D. H. Kim, "Experimental verification of optical wireless communication link using high-brightness illumination light-emitting diodes," Opt. Eng. 46, 125005 (2007). FSO Group (ECE 4007 L01) 32
  • 36. [9] B. Clarke, K. Hamilton, D. Hembree, T. Marsh, and C. Young, “Low-cost, High- speed FSO Communication Link,” Senior Design project, Georgia Institute of Technology, April, 2007. [10] D. Johnson “Handbook of Optical Through the Air Communications,” [Available Online]:, [Accessed Feb. 3, 2008]. [11] Bluetooth Corporation, [Online], [Available Online]:, [Accessed Feb. 2, 2008]. [12] Application Note OPA657, “1.6GHz, Low-Noise, FET-Input Operational Amplifier”, Texas Instruments Inc., Mar, 2006. [13] Available online. Accessed May 1, 2008. Appendix 1 Table 3. Received power and BER measurements with evaluation Maxim 3967A and the 1GHz-post Amp (using Tektronix CSA907R Error Detector, lowest capable BER reading is 10-8) s (mm) Po (mW) BER (10^-5) 2 1.2 1.6 4 1.1 1.2 6 0.95 1.8 8 0.8 2.8 10 0.63 3.4 12 0.45 11 14 0.26 110 16 0.08 N/A Table 4. Received power and BER measurements with our designed boards (using Tektronix 11801B Digital Sampling Oscilloscope) mean-V1 mean-V0 RMS RMS BER s (mm) (mV) (mV) Noise 1 Noise 0 Q (calc) (calc) 0 308 -302 52 48 6.1 5.4E-10 2 305.9 -298 52 47 6.1 5.4E-10 4 268.2 -261 47 40 6.1 4.9E-10 6 221.6 -218 38 35 6.0 7.6E-10 8 174.5 -172 30 28 6.0 7.6E-10 10 125.1 -126 21 21 6.0 1.2E-09 FSO Group (ECE 4007 L01) 33
  • 37. 12 74.3 -78.5 14 14 5.6 1.1E-08 14 29.3 -33.7 6 6 5.4 2.9E-08 16 2.4 -8.2 2 2 3.2 7.1E-04 Appendix 2 Table 5. Bill of Materials FSO Group (ECE 4007 L01) 34
  • 38. Appendix 3 Figure A3. Driver Schematic FSO Group (ECE 4007 L01) 35