Introduction Visible Light Communication Full Seminar Report and PPT
With the exponentially increasing data demand but limited available radio spectrum, alternatives will be necessary to accommodate the needs of wire-free communication systems. This chapter will illustrate the problems of current wireless communication systems and alternatives to these systems, as well as motivations and possible applications for visible light communications.
As societal dependence upon wireless systems continues to grow, wireless technology needs to expand to meet the demand. Phones, laptops, and global positioning systems are all devices that implement certain forms of wireless communication to send information to another location. However, the availability of current forms of wireless is very limited, and it is not necessarily safe to implement wireless radio, making it necessary to explore other alternatives to wireless communication to allow continued expansion upon communication systems and to ensure safe use.Figure 1 illustrates the frequency allocations of the radio spectrum in the United States. ct Visible light communication is a new way of wireless communication using visible light. Typical transmitters used for visible light communication are visible light LEDs and receivers are photodiodes and image sensors. We present new applications which will be made possible by visible light communication technology. Location-based services are considered to be especially suitable for visible light communication applications.
The Federal Communications Commission (FCC) regulates many wireless applications in the US, including radio, television, wire, satellite, and cable . Each application is given a frequency band in which it is allowed to operate to allow efficient use of the available frequency spectrum. From Figure 1, it is quite evident that this spectrum is very crowded. At the same time, there is a huge growth in demand in the limited radio frequency spectrum. From Figure 2, predictions estimated that as soon as even 2013, the US could potentially be in a spectrum deficit. Therefore, a more efficient way of utilizing radio frequency is necessary.
In addition to the crowding of the frequency spectrum, interference is also a concern for many existing wireless systems. Any simultaneous use of a frequency band will cause interference due to the electromagnetic nature of most wireless devices, which could result in incorrect or loss of information for those users involved. A prime example of this is the use of mobile devices on planes, which directly affects safety. Regardless of the reason, it is clear that it is not feasible to use wireless devices in certain environments in which safety, data integrity, and accuracy are highly important.
VLC systems have more flexibility and integrity than other communication systems in many regards. Since the medium for transmission in VLC systems is visible light and not RF waves that can penetrate walls, the issue of security is inherently solved because light cannot leave the room, containing data and information in one location. There is no way to retrieve and access the information unless a user is in a direct path of the light being used to transmit the data. In addition, LEDs are highly efficient and becoming more durable, adding to the integrity of these systems.
1.2 Alternatives in Progress
Currently, several alternatives to radio frequency communications exist. For example, there are cognitive radio, which utilizes radios programmed to adapt to surroundings by constantly analyzing the frequency spectrum to determine how the surrounding spectrum is currently being utilized, and laser communication systems, which transmits data through free space by shooting a laser with wavelengths close to the infrared spectrum to a receiver.
1.2.1 Cognitive Radio
Given that one major issue in wireless communication is the crowded frequency spectrum, many engineers spend their time and effort focusing on determining solutions for this issue. Since there is limited access to the frequency spectrum, these engineers are focusing on options that could optimize the spectrum. By optimizing the frequency spectrum’s usage, it would be possible to provide all end users a portion of the spectrum. As the current trend continues, devices that normally would not be able to wirelessly communicate, such as lamps or temperature sensors, will be connected to some type of wireless network. This will increase the number of end users and further crowd the frequency spectrum.
One area that engineers are focusing on to optimize the frequency spectrum involves cognitive radios. The difference between a cognitive radio and a typical radio system is that a cognitive radio is programmed to adapt to its surroundings. A cognitive radio is constantly analyzing the frequency spectrum to determine how the surrounding spectrum is being used. The system could potentially monitor the entire frequency spectrum, but that would require an antenna that has a large bandwidth. Since most antennas operate at a range of frequencies, cognitive radios will monitor that specific bandwidth and determine how it is occupied. Once the radio has determined how the spectrum is being occupied, it will choose non-occupied frequencies to transmit its information. While it is transmitting information, it continues to monitor the spectrum to determine whether other signals are attempting to access the same frequencies. If there are other signals, the radio will stop transmitting and switch to another unused frequency slot. This whole process is called Dynamic Spectrum Access and is a vital part of how a cognitive radio functions.
The idea of using cognitive radios for optimizing the use of the frequency spectrum will require the systems to focus on more than one frequency band. Since a majority of these bands have been dedicated to certain organizations, those organizations have priority or full control over the frequencies. Out of all the divided frequency bands, researchers are looking at the television bands. There are multiple television bands ranging between 54-72 MHz, 76-88 MHz, 174-216 MHz, 512-608 MHz, and 614-698 MHz which are used to provide certain television signals to the set top boxes in homes. Each band’s bandwidth is then further divided to allow all channels to have access to transmission. The reason the television band is the band of focus is how the spectrum is being used. At the Illinois Institute of Technology in Chicago, IL, a team of researchers monitored the frequency spectrum over a span of three years to determine how each frequency band was occupied. The occupancy was measured by monitoring the frequency band’s spectral density to a threshold. The following figure represents the occupancies of certain frequency bands .
Laser communication systems utilize wireless connections through the atmosphere, transmitting data through free space by shooting a laser. This form of wireless communication can be effective because it is not regulated by the government as it operates in a near infrared spectrum, hence avoiding any additional overcrowding of the spectrum with this form of communication. This allows for quick establishment of communication links, as it does not need to go through the various regulatory processes that would be necessary to set up an RF system. The system can work for a distance of up to 6 km with bitrates up to 1.25 Gbps. The system also uses relatively low power and has a low noise ratio. It is also secure, as any sort of eavesdropping on the data transmission will require viewing directly into the transmitter path, causing an interruption in transmission.
Unfortunately, the system requires a line-of-sight path from the transmitter to receiver. This renders the two functional blocks relatively immobile. If the path is not calibrated precisely, the laser could miss the receiver by a large distance, resulting in no data transmission. In addition, although invisible to the naked eye, the lasers used could result in damage to one’s eye if there is an extended exposure to the laser.
Failure, Hazard Analysis, Limitations, and Future
Several issues occurred along the way of our design and implementation, causing many of our initial goals to change and adjustments were made accordingly to meet deadlines and absolutely necessary functional requirements. These ranged from power issues on the analog transmission side of the system, to digital issues on the digital receiver side of the system.
Our final system met several, but not all, of our initial design goals. While the system is operational, it is able to transmit text at a transmission frequency of 500 Hz at a transmission distance of roughly 25 cm without the implementation of our power source. Certainly, these achieved goals leave much room for improvement and extensions.
4.1 Digital Issues
There have been many issues with the programming of the C2000 Launchpad evaluation kits. TI has several versions of example code that may not be the most up-to-date set of files, which caused compilation errors, initialization errors, etc. Often times to circumvent this issue, it was necessary to re- download sets of files on different computers to achieve for any sort of functionality.
Code Composer Studio, the integrated development environment (IDE) used to program our boards would frequently have issues reading files or would not compile due to errors, but not display what the error was. One major problem was an unresolvable error that occurred consistently on line 18 of our code no matter the fix we tried. The line was commented out and the code would not compile even though it had no significance on the functionality of the code. Many times, restarting the IDE solved the issue, but sometimes a complete reinstallation was necessary.
4.2 Analog Issues
4.2.1 LED Brightness
The original LEDs that were chosen and implemented in the prototype proved to be too dim to achieve a transmission distance of more than 20 cm. The initial goal of the design was be able to transmit data at a distance of at least one meter using solely visible light. In an attempt to achieve this goal, further research and value analysis on LEDs was conducted in hopes of finding brighter LEDs that fit the same specifications of the previous LEDs. The value analysis on LEDs can be found, and is explained more in-depth, in Section 126.96.36.199 LEDs.
Once the brighter LEDs were placed into the circuit the measurement was taken again to see if the transmission distance had improved as expected. The distance, however, did not improve by more than 10 cm. The LEDs were not receiving enough current from the MCU output to reach their brightest potential. In order to remedy this problem a Power MOSFET device was added with the purpose of supplying enough current to the LEDs, but only when the transmitted signal is ‘on’ or logic high. This ensures that the LEDs will only flash and activate the photodiodes when desired.
4.2.2 MOSFET Limitations
One version of our design involved using a MOSFET to increase the signal strength from the MCU that powered the LEDs on the transmitter end of the circuit. Our initial design is depicted below in Figure 26 but was quickly changed over to Figure 27 upon further investigation of how the MOSFET drain current works. In the first design the LEDs are connected in series with the drain of the MOSFET in an attempt to take the current from the MOSFET and power the LEDs when the device is on. However, being connected in series, the LEDs had no ground reference which made current flow impossible. After testing this in the lab, the design was quickly altered to that of the second picture to ensure the drain current flows through the LEDs as intended.
Another design fault was the lack of knowledge of using a MOSFET in a power application. Since the MOSFET is switching on and off at a high speed and has a high input capacitance, the logic output of the MCU does not supply enough current to charge the MOSFET gate fast enough. In order to bypass this problem, a Gate Driver was required to interface the two devices. This Gate Driver generates the current necessary to turn MOSFETs on and off from the input logic of a DSP or microcontroller. A lack of experience with MOSFETs in power applications was the cause of this problem and resulted in an inappropriate MOSFET for the design burning out during testing. When looking for a suitable Gate Driver it was found that the most of the devices available were surface mount which is not compatible with our design. Later in the design process, it was found the MOSFET had heating issues that caused failure in one of our boards so the MOSFET was excluded from the final design.
4.3 Future Improvements
4.3.1 Digital Improvements
Throughout the entire project, many of the issues that arose were from the digital components, the microprocessors. As mentioned earlier, it was needed to switch from the C2000 processor to the MSP430F5529 processor at the receiver because there were issues regarding sampling with the ADC on the C2000. The MSP430F5529 was a quick fix to the problem because it was familiar and available at the time of consideration. However, because its sampling rate is rather slow, it is not the best option for communication systems. In this section, we discuss other digital options such as FPGAs, and other digital signal processing chips.
A Field-Programmable Gate Array, or FPGA for short, is an integrated circuit that contains a large resource of logic gates and memory to implement digital computations. It is possible to customize the logic through a hardware description language such as Verilog. With an FPGA, it is possible to have parallel executions. This would allow the ADC to sample the incoming data without affecting any other process. Another process could take the data from the ADC and perform a spectral energy computation or even decoding the samples back into ASCII text.
FPGAs are also better suited for high frequency signals because the combinational logic inside the integrated chip typically can run as fast as the built in clock on the FPGA. In most cases, an FPGA’s internal clock can be as high as 100MHz or higher. With the high frequency operation, it would be possible to achieve a higher transmission rate as long as the ADC that is used can sample fast enough. It is possible to choose which ADC can be used because the ADC can be an external module that will be interfaced with the rest of the FPGA development board.
Unlike a FPGA, a microcontroller performs its functionality sequentially. Since the microcontroller’s ADC functions through interrupts, the time the interrupt takes to finish its process can have an effect on how fast data can be processed. In order to get a faster sampling rate, the number of computations in the interrupt needs to be done within as little processing cycles as possible or a faster processor may be needed.
188.8.131.52Better Processing Chips
When determining the initial microprocessors to use, the deciding factor to use the C2000 boards was their high sampling rate ADCs. Unfortunately, there were many issues that arose when using the board’s ADC which is why it was discarded on the receiver side. Due to the limiting time, the MSP430F5529 was chosen to replace the C2000. While the MSP430F5529 may have not been the best option for the Visible Communication System, it was a quick fix to produce a working prototype.
If a microprocessor is going to be considered again for future Visible Communication Systems, there are a few factors that should be considered before selecting the specific chip. The first factor is the ADCs sampling rate. Without a fast sampling rate, the entire systems transmission’s rate will be limited by the ADC. The second factor would be available sample code. One issue that arose with the C2000 was its lack of working sample code. By having sample code, it is much easier to design code for projects because there are models such as how to set an ADC to produce samples. The third factor is memory and processing speed. Without memory, it would be impossible to store the sampled data from the ADC. With a small amount of memory, the number of bits that can be transmitted at a time is limited because the memory has been filled. On top of having a sufficient amount of memory, it is ideal to have a fast processing processor. In order to decode the received message, the instructions to decode the received samples must run within a certain amount of samples that is not larger than the ADC’s sampling rate. If it takes too long to initiate instructions, it could affect how fast the ADC is actually sampling. Since most ADCs operate using interrupts, if the interrupt takes longer to process than the ADC’s sample period, the function is not operating in real time and can cause the receiver to not function properly.
184.108.40.206 Computer Interface
One of the main components of a Visible Communication System is its interface with other devices, such as computers or smartphones. A computer is an excellent source of interfacing the prototype system because the software that is used to program the digital side is a computer application. Since the processor used did not have enough memory to process the incoming data, the data had to be transferred to the computer to be processed in MATLAB. While it would have been better to process the data on the chip itself, there still needs to be a way to transfer the data to the computer. Due to the many issues that arose with transferring data to the computer, one short term fix was to export the collected ADC samples through CCS’s console. Once on the console, it would be possible to copy the information to a file to be later processed by MATLAB. To make the system function better, it would be better to have the data exported instantaneously to a file. While attempting to create an instantaneous process, one source that was looked at was the MSP430f5529’s sample code, emul Storage Keyboard.
The program was able to output data to a file depending on which button on the development board was pressed. We attempted to modify the code to run with the ADC interrupt to produce the data that would be outputted. Also the data would have been outputted to the file once all the information from the transmitter was received. Unfortunately, no progress was made with this approach. Because of this, other methods of transmitting data was looked into.
Another method that was looked into was the USB UART interface between the microcontroller and computer. With the UART interface, it was possible to send the data to a hyper terminal once all the information was received from the transmitter. A GUI interface could have then be used along with the hyper terminal to send the data to a file and initiate MATLAB to process the information. With the limited amount of time, it was decided to not pursue this option.
Visible light communication is a new way of wireless communication using visible light. Typical transmitters used for visible light communication are visible light LEDs and receivers are photodiodes and image sensors. We present new applications which will be made possible by visible light communication technology. Location-based services are considered to be especially suitable for visible light communication application. We showed advantages and disadvantages of
visible light communication and explained the effectiveness of location-based services for visible light communication by showing some examples. It is expected that visible light communication will be widely used as LED light market expands worldwide.