Nt1310 Unit 3 Assignment 1
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Implementation of Optical OFDM in a Wireless Environment
Aditya Matukumallia, Gupta Vivekananda. Na, Navneeth Kishore Reddy. Ka, Shanthi Prince*a a Department of Electronics and Communication …show more content…
Engineering,SRM University,
Kattankulathur - 603203, Tamil Nadu, India.
ABSTRACT
In this communication world, man has always been obsessed with optimization of high speed and secured communication systems. One of the major breakthrough in this process of evolution is the wireless communication, latest being the Wi-Fi and WIMAX systems. These systems are limited by security issues, expensive infrastructure, RF interference and the most cardinal being the bandwidth constraints. These limitations being sufficed by the basic properties of light and also the emergence of white LEDs as the leader in the lighting solutions globally provoked us to use them as a source of communication. The lower power consumption and the higher efficiency also make it a commercially viable communication system. To implement this system a proper modulating and multiplexing technique has to be chosen. Orthogonal Frequency Division Multiplexing (OFDM) along with Binary Phase Shift Keying (BPSK) serves this purpose. This paper primarily focuses on development of the proof of concept to establish the wireless optical communication link which can be used in intra satellite communication, public addressing systems, video streaming in airplanes. This can be further extended to high speed internet access in the future.
Keywords: Optical OFDM, Modulation, BPSK, digital signal processors, Optical wireless
1. INTRODUCTION
In the recent years the usage of white LEDs as commercial lighting solution has increased many fold due to their low power consumption, decrease in the cost of production and the size and form factor. So it is expected that by 2025, there would be a proper establishment of hardware network of LEDs thereby saving the world market $250 billion, reduce electricity demands from lighting by 62 %, eliminate 258 million metric tons of carbon emissions, avoid the building of 133 new power plants and save the US over $280 billion1. So we intend to utilize this established network to establish high speed communication links.
OFDM is a promising candidate for achieving high data rate transmission in mobile environment. The application of
OFDM to high data rate mobile communication system is being investigated by many researchers. Cimini2 proposed a cellular mobile radio system based on OFDM used with pilot based correction. It was shown to provide large improvements in BER performance in a Rayleigh Fading Environment. The application of OFDM to optical communications has only occurred very recently, but there are an increasing number of papers on the theoretical and practical performance of OFDM in many optical systems including optical wireless3. The concept of using parallel data transmission and frequency multiplexing was published in mid 1960s. After more than thirty years of research and development, OFDM has been widely implemented in high speed digital communications. Due to recent advances of digital signal Processing (DSP) and Very Large Scale Integrated circuit (VLSI) technologies, the initial obstacles of
OFDM implementation such as massive complex computation, and high speed memory do not exist anymore. The use of
Fast Fourier Transform (FFT) algorithms eliminates arrays of sinusoidal generators and coherent demodulation required in parallel data systems and makes the implementation of the technology cost effective4.
*shanthi.p@ktr.srmuniv.ac.in; phone 91-9444962179; fax 9144-27453903
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2. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
OFDM is a special case of frequency division multiplexing (FDM). OFDM is a combination of modulation and multiplexing. OFDM is a form of multi carrier modulation which splits the message to be transmitted into a number of parts. The OFDM concept is based on spreading the data to be transmitted over a large number of carriers, each being modulated at a low rate. The carriers are made orthogonal to each other by appropriately choosing the frequency spacing between them. The available spectrum is also split into a number of low rate carriers and the parts of the message are simultaneously transmitted over a large no of frequency channels. High spectral efficiency, inherent robustness against narrowband interference, sensitivity, synchronization, and it’s immunity towards Inter Channel Interference (ICI) and
Inter Symbol Interference (ISI) makes it a better choice over other modulation schemes.
To generate OFDM successfully, the relationship between the carriers must be understood and carefully controlled to maintain the orthogonality of the carriers. For this reason, OFDM is generated by firstly choosing the bandwidth required, depending upon the input data and the modulation scheme used. The required spectrum is then converted back to the time domain signal by performing an Inverse Fourier transform (IFFT). The IFFT performs the transformation very efficiently, and provides a simple way of ensuring the carrier signals produced are orthogonal. The IFFT converts a number of complex data points, whose size in the power of 2, into a time domain signal of the same number of points.
On the receiver side the Fast Fourier Transform (FFT), transforms the cyclic time domain signal into its equivalent frequency spectrum. This is done by finding the equivalent wave form, generated by a sum of orthogonal sinusoidal components. Frequency division multiplexing (FDM) is the process by which the total bandwidth available to the system is divided into a series of non overlapping frequency sub-bands that are then assigned to each communicating source and user pair.
While conventional FDM uses the frequency spacing of 2/T (T is the period over which the subcarriers are orthogonal) between neighboring subcarriers, OFDM uses the frequency spacing of 1/T, which is the minimum frequency spacing for orthogonality, between neighboring subcarriers by allowing the subcarrier spectra to overlap so that OFDM improves the spectral efficiency5.
2.1 OFDM System Block
Figure 1 shows the generic OFDM block. The transmitter section consists of an A/D (analog to digital) converter, which converts the analog input into a digital signal. The digital binary stream is given as an input to the S/P (serial to parallel) converter which divides the binary stream into required number of channels depending on the system design. The binary data on each individual channel is modulated using any of the phase shift keying techniques such as BPSK, QPSK, or
QAM. The baseband carrier frequencies of each channel should be harmonics in order to maintain orthogonality. In this paper we have considered BPSK modulation. Each BPSK signal is sampled equally in time. These samples must be given as an input to the IFFT block6. The samples on each BPSK signal are in time domain but the input of the IFFT block should be in frequency domain. A set consisting of nth sample of each of the BPSK signal can be considered as a signal in frequency domain. Suppose, each BPSK signal consists of N samples, the total number of sets of inputs given to the IFFT block are N and the corresponding N outputs are concatenated to give the OFDM signal, as shown in Figure
2.
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Figure 1. OFDM system block diagram
Figure 2. Inverse Fast Fourier Transform process
In figure 2 “l” corresponds to the samples of the BPSK signal F1 and “O” corresponds to the samples of the BPSK signal
F2.
All the signals in the wireless medium suffer from multipath fading effects. This is combated by the addition of cyclic prefix. The addition of cyclic prefix further prevents from the signal loss due to ISI (Inter Symbol Interference).
The first process in the receiver side includes the removal of the cyclic prefix just before the FFT block to get back the transmitted OFDM signal. Now when the OFDM stream consisting of N sets of samples is passed through the FFT module, it gives back the samples on which IFFT was performed. This when serial to parallel converted gives backs the
BPSK signals. This is illustrated in the Figure 3.
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Figure 3. Fast Fourier Transform process
These BPSK signals are converted back to the binary stream by demodulators. The binary streams are then combined back to get back the initial serial data. This data is passed through a DAC circuit to get back the original analog signal.
3. OPTICAL OFDM SYSTEM
As already discussed, due to the theoretically infinite bandwidth of light, zero RF interference, inherent robustness against narrow-band interference, high spectral efficiency, we have concatenated the OFDM system to an optical system.
3.1 Optical OFDM System Block Diagram
The Optical OFDM system is shown in Figure 4. The discrete blocks of the system are the DSP kit, optical transmitter and receiver and a computer with the terminal software.
Figure 4 Optical OFDM System block diagram
3.2 TMS320C6713 DSP kit
Since, the designed system includes performing IFFT, FFT and other signal processing blocks, the selected DSP processor is TMS320C6713 from Texas instruments which have an evaluation board developed by Digital spectrum which is commercially available as C6713 DSK.
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The DSK features the TMS320C6713 DSP, a 225 MHz device delivering up to 1800 million instructions per second
(MIPs) and 1350 MFLOPS. This DSP generation is designed for applications that require high precision accuracy.
Other hardware features of the TMS320C6713 DSK board include: Embedded JTAG support via USB, High-quality 24bit stereo codec, Four 3.5mm audio jacks for microphone, line in, speaker and line out, 512K words of Flash and 8 MB
SDRAM, Expansion port connector for plug-in modules, On-board standard IEEE JTAG interface, +5V universal power supply and an analog interface circuit for data conversion (AIC).
3.3 Code Composer Studio
The implementation on the TMS320C6713 DSK is done through a software interface called the Code Composer Studio.
Code Composer Studio extends the basic code generation tools with a set of debugging and real-time analysis capabilities though familiar environments like C/C++. Code Composer Studio supports all phases of the development cycle shown below in Figure 5.
Figure 5. Development cycle
The codes are developed for each sub module of the system using C language, and after the validation of the results using MATLAB simulations the codes are burnt into the DSP board.
3.4 Implementation of the concept of negative carriers through Conjugate Symmetric Matrix
The output of the IFFT module should drive the LED circuit. The LED circuit uses the OOK (On off Keying) modulation technique, which considers only the amplitude of the signal. The transmission of a complex carrier wave, using amplitude modulation leads to the shift in the frequency. Negative carriers are used to eliminate these disadvantages. This process involves sending the complex values along with their complex conjugate to the input of the
IFFT in such a manner that the output of the IFFT is real.
The above process can be implemented by using the conjugate symmetric matrix.
Suppose X is an array of N elements, then the conjugate symmetric matrix is an array of 2N elements. This matrix is given as an input to the IFFT so as to get only real values as the output. Although, the number of inputs to the IFFT has been doubled, the computational time is comparatively faster. This is because of the fact that this process does not involve the computation of complex numbers.
The output of the IFFT matrix is real irrespective of the input being real or imaginary. The reverse operation can be performed in the FFT in the receiver side to regenerate the complex values.
3.5 Optical Transmitter and Receiver
At the transmitting side, when any data or file is given as input to the computer it is transported to the input pin of
MAXIM 232 chip through the cable connected to the RS232 Port .The MAX 232 chip converts it as a TTL output.
This
TTL output is given as an input to the level shifter circuit, which provides an interface between the low voltage devices with high voltage devices. This output of the level shifter is given to a voltage controlled oscillator (NE566) for
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modulation whose output is required to drive the LED. The switching on and off of the LED is controlled by a transistor
BC547.
The output of voltage controlled oscillator is fed to the LED driver (IRF540) which requires a constant supply to
prevent any unwanted damage to the transmitting LED. Variation in voltage depreciates the intensity of LED output hence leading to loss of data. The modulated output is fed to the LED driver along with a constant current to drive the LED. An adjustable voltage regulator (LM317) is used to obtain a constant current. It is an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2V to 37V. It also implements internal current limiting thermal shutdown and safe area compensation making it essentially blow out proof. The LED switches
ON and OFF according to the incoming modulated data stream. The major advantage of using this LED driver is that it is very easy to drive and has very fast switching times making it ideal for high speed switching applications.
The receiver contains a photo detector which senses the light falling on it and converts into electrical signal. Then the electrical signal is amplified using operational amplifier because the signal might have incurred some losses during transmission. This amplified signal then demodulated using a phase locked loop (CD 4046) where the carrier is separated from the data. The available data is in the form of TTL which is then converted to RS 232 output and is fed to the computer. 3.6 Terminal software
Code vision AVR is a C cross compiler, integrated development environment and automatic program generator designed for the ATMELAVR family of microcontrollers. The C cross compiler implements all the elements of the ANSI C language as followed by the AVR architecture with some features for embedded systems need. For debugging embedded system, which employ serial communication the IDE has built in terminal.
4. CONCLUSION
The designed system discussed in the paper can be further optimized by implementing an RTOS (real time operating system) to support file transfer. The system can also be modeled supporting RF design to further enhance the system.
The DSP module, the RF modulators, high speed switches, flash RAM can be ported onto a PCB to meet the system requirements in form, size and data rates. The system can be further enhanced to develop a full duplex communication link. REFERENCES
[1] Nikki Rogan, “The future of LED Lighting is bright thanks to the CREE lighting the LED revolution tour,” 13
July 2012 www.creeledrevolution.com
[2] Cimini, L.J., “ Analysis and simulation of digital mobile channel using orthogonal frequency division multiplexing”, IEEE Transactions on Communications 33(7), 665-675 (1985)
[3] Jean Armstrong, “OFDM for Optical Communications,” Journal of Lightwave Technology, 27(3),189-204
(2009)
[4] Weinstein, S.B. and Ebert, P.M., “Data transmission by frequency division multiplexing using the discrete
Fourier transform," IEEE transactions on Communication Technology, 19(5), 628-634 (1997).
[5] Jan-Jaap van de,Ove Edfors, Magnus Sandel, Sarah Kate Wilsony, Per Ola “On Channel Estimation in OFDM
Systems,” Proceedings of Vehicular Technology Conference, 2, 815-819 (1995).
[6] Elgala, H., Mesleh, R., Haas, H. and Pricope, B., “OFDM Visible Light Wireless Communication Based On
White LED’s,” IEEE Proc. of Vehicular Technology, 2185-2189 (2007).
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Implementation of Optical OFDM in a Wireless Environment
Aditya Matukumallia, Gupta Vivekananda. Na, Navneeth Kishore Reddy. Ka, Shanthi Prince*a a Department of Electronics and Communication …show more content…
Engineering,SRM University,
Kattankulathur - 603203, Tamil Nadu, India.
ABSTRACT
In this communication world, man has always been obsessed with optimization of high speed and secured communication systems. One of the major breakthrough in this process of evolution is the wireless communication, latest being the Wi-Fi and WIMAX systems. These systems are limited by security issues, expensive infrastructure, RF interference and the most cardinal being the bandwidth constraints. These limitations being sufficed by the basic properties of light and also the emergence of white LEDs as the leader in the lighting solutions globally provoked us to use them as a source of communication. The lower power consumption and the higher efficiency also make it a commercially viable communication system. To implement this system a proper modulating and multiplexing technique has to be chosen. Orthogonal Frequency Division Multiplexing (OFDM) along with Binary Phase Shift Keying (BPSK) serves this purpose. This paper primarily focuses on development of the proof of concept to establish the wireless optical communication link which can be used in intra satellite communication, public addressing systems, video streaming in airplanes. This can be further extended to high speed internet access in the future.
Keywords: Optical OFDM, Modulation, BPSK, digital signal processors, Optical wireless
1. INTRODUCTION
In the recent years the usage of white LEDs as commercial lighting solution has increased many fold due to their low power consumption, decrease in the cost of production and the size and form factor. So it is expected that by 2025, there would be a proper establishment of hardware network of LEDs thereby saving the world market $250 billion, reduce electricity demands from lighting by 62 %, eliminate 258 million metric tons of carbon emissions, avoid the building of 133 new power plants and save the US over $280 billion1. So we intend to utilize this established network to establish high speed communication links.
OFDM is a promising candidate for achieving high data rate transmission in mobile environment. The application of
OFDM to high data rate mobile communication system is being investigated by many researchers. Cimini2 proposed a cellular mobile radio system based on OFDM used with pilot based correction. It was shown to provide large improvements in BER performance in a Rayleigh Fading Environment. The application of OFDM to optical communications has only occurred very recently, but there are an increasing number of papers on the theoretical and practical performance of OFDM in many optical systems including optical wireless3. The concept of using parallel data transmission and frequency multiplexing was published in mid 1960s. After more than thirty years of research and development, OFDM has been widely implemented in high speed digital communications. Due to recent advances of digital signal Processing (DSP) and Very Large Scale Integrated circuit (VLSI) technologies, the initial obstacles of
OFDM implementation such as massive complex computation, and high speed memory do not exist anymore. The use of
Fast Fourier Transform (FFT) algorithms eliminates arrays of sinusoidal generators and coherent demodulation required in parallel data systems and makes the implementation of the technology cost effective4.
*shanthi.p@ktr.srmuniv.ac.in; phone 91-9444962179; fax 9144-27453903
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2. ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING
OFDM is a special case of frequency division multiplexing (FDM). OFDM is a combination of modulation and multiplexing. OFDM is a form of multi carrier modulation which splits the message to be transmitted into a number of parts. The OFDM concept is based on spreading the data to be transmitted over a large number of carriers, each being modulated at a low rate. The carriers are made orthogonal to each other by appropriately choosing the frequency spacing between them. The available spectrum is also split into a number of low rate carriers and the parts of the message are simultaneously transmitted over a large no of frequency channels. High spectral efficiency, inherent robustness against narrowband interference, sensitivity, synchronization, and it’s immunity towards Inter Channel Interference (ICI) and
Inter Symbol Interference (ISI) makes it a better choice over other modulation schemes.
To generate OFDM successfully, the relationship between the carriers must be understood and carefully controlled to maintain the orthogonality of the carriers. For this reason, OFDM is generated by firstly choosing the bandwidth required, depending upon the input data and the modulation scheme used. The required spectrum is then converted back to the time domain signal by performing an Inverse Fourier transform (IFFT). The IFFT performs the transformation very efficiently, and provides a simple way of ensuring the carrier signals produced are orthogonal. The IFFT converts a number of complex data points, whose size in the power of 2, into a time domain signal of the same number of points.
On the receiver side the Fast Fourier Transform (FFT), transforms the cyclic time domain signal into its equivalent frequency spectrum. This is done by finding the equivalent wave form, generated by a sum of orthogonal sinusoidal components. Frequency division multiplexing (FDM) is the process by which the total bandwidth available to the system is divided into a series of non overlapping frequency sub-bands that are then assigned to each communicating source and user pair.
While conventional FDM uses the frequency spacing of 2/T (T is the period over which the subcarriers are orthogonal) between neighboring subcarriers, OFDM uses the frequency spacing of 1/T, which is the minimum frequency spacing for orthogonality, between neighboring subcarriers by allowing the subcarrier spectra to overlap so that OFDM improves the spectral efficiency5.
2.1 OFDM System Block
Figure 1 shows the generic OFDM block. The transmitter section consists of an A/D (analog to digital) converter, which converts the analog input into a digital signal. The digital binary stream is given as an input to the S/P (serial to parallel) converter which divides the binary stream into required number of channels depending on the system design. The binary data on each individual channel is modulated using any of the phase shift keying techniques such as BPSK, QPSK, or
QAM. The baseband carrier frequencies of each channel should be harmonics in order to maintain orthogonality. In this paper we have considered BPSK modulation. Each BPSK signal is sampled equally in time. These samples must be given as an input to the IFFT block6. The samples on each BPSK signal are in time domain but the input of the IFFT block should be in frequency domain. A set consisting of nth sample of each of the BPSK signal can be considered as a signal in frequency domain. Suppose, each BPSK signal consists of N samples, the total number of sets of inputs given to the IFFT block are N and the corresponding N outputs are concatenated to give the OFDM signal, as shown in Figure
2.
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Figure 1. OFDM system block diagram
Figure 2. Inverse Fast Fourier Transform process
In figure 2 “l” corresponds to the samples of the BPSK signal F1 and “O” corresponds to the samples of the BPSK signal
F2.
All the signals in the wireless medium suffer from multipath fading effects. This is combated by the addition of cyclic prefix. The addition of cyclic prefix further prevents from the signal loss due to ISI (Inter Symbol Interference).
The first process in the receiver side includes the removal of the cyclic prefix just before the FFT block to get back the transmitted OFDM signal. Now when the OFDM stream consisting of N sets of samples is passed through the FFT module, it gives back the samples on which IFFT was performed. This when serial to parallel converted gives backs the
BPSK signals. This is illustrated in the Figure 3.
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Figure 3. Fast Fourier Transform process
These BPSK signals are converted back to the binary stream by demodulators. The binary streams are then combined back to get back the initial serial data. This data is passed through a DAC circuit to get back the original analog signal.
3. OPTICAL OFDM SYSTEM
As already discussed, due to the theoretically infinite bandwidth of light, zero RF interference, inherent robustness against narrow-band interference, high spectral efficiency, we have concatenated the OFDM system to an optical system.
3.1 Optical OFDM System Block Diagram
The Optical OFDM system is shown in Figure 4. The discrete blocks of the system are the DSP kit, optical transmitter and receiver and a computer with the terminal software.
Figure 4 Optical OFDM System block diagram
3.2 TMS320C6713 DSP kit
Since, the designed system includes performing IFFT, FFT and other signal processing blocks, the selected DSP processor is TMS320C6713 from Texas instruments which have an evaluation board developed by Digital spectrum which is commercially available as C6713 DSK.
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The DSK features the TMS320C6713 DSP, a 225 MHz device delivering up to 1800 million instructions per second
(MIPs) and 1350 MFLOPS. This DSP generation is designed for applications that require high precision accuracy.
Other hardware features of the TMS320C6713 DSK board include: Embedded JTAG support via USB, High-quality 24bit stereo codec, Four 3.5mm audio jacks for microphone, line in, speaker and line out, 512K words of Flash and 8 MB
SDRAM, Expansion port connector for plug-in modules, On-board standard IEEE JTAG interface, +5V universal power supply and an analog interface circuit for data conversion (AIC).
3.3 Code Composer Studio
The implementation on the TMS320C6713 DSK is done through a software interface called the Code Composer Studio.
Code Composer Studio extends the basic code generation tools with a set of debugging and real-time analysis capabilities though familiar environments like C/C++. Code Composer Studio supports all phases of the development cycle shown below in Figure 5.
Figure 5. Development cycle
The codes are developed for each sub module of the system using C language, and after the validation of the results using MATLAB simulations the codes are burnt into the DSP board.
3.4 Implementation of the concept of negative carriers through Conjugate Symmetric Matrix
The output of the IFFT module should drive the LED circuit. The LED circuit uses the OOK (On off Keying) modulation technique, which considers only the amplitude of the signal. The transmission of a complex carrier wave, using amplitude modulation leads to the shift in the frequency. Negative carriers are used to eliminate these disadvantages. This process involves sending the complex values along with their complex conjugate to the input of the
IFFT in such a manner that the output of the IFFT is real.
The above process can be implemented by using the conjugate symmetric matrix.
Suppose X is an array of N elements, then the conjugate symmetric matrix is an array of 2N elements. This matrix is given as an input to the IFFT so as to get only real values as the output. Although, the number of inputs to the IFFT has been doubled, the computational time is comparatively faster. This is because of the fact that this process does not involve the computation of complex numbers.
The output of the IFFT matrix is real irrespective of the input being real or imaginary. The reverse operation can be performed in the FFT in the receiver side to regenerate the complex values.
3.5 Optical Transmitter and Receiver
At the transmitting side, when any data or file is given as input to the computer it is transported to the input pin of
MAXIM 232 chip through the cable connected to the RS232 Port .The MAX 232 chip converts it as a TTL output.
This
TTL output is given as an input to the level shifter circuit, which provides an interface between the low voltage devices with high voltage devices. This output of the level shifter is given to a voltage controlled oscillator (NE566) for
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modulation whose output is required to drive the LED. The switching on and off of the LED is controlled by a transistor
BC547.
The output of voltage controlled oscillator is fed to the LED driver (IRF540) which requires a constant supply to
prevent any unwanted damage to the transmitting LED. Variation in voltage depreciates the intensity of LED output hence leading to loss of data. The modulated output is fed to the LED driver along with a constant current to drive the LED. An adjustable voltage regulator (LM317) is used to obtain a constant current. It is an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2V to 37V. It also implements internal current limiting thermal shutdown and safe area compensation making it essentially blow out proof. The LED switches
ON and OFF according to the incoming modulated data stream. The major advantage of using this LED driver is that it is very easy to drive and has very fast switching times making it ideal for high speed switching applications.
The receiver contains a photo detector which senses the light falling on it and converts into electrical signal. Then the electrical signal is amplified using operational amplifier because the signal might have incurred some losses during transmission. This amplified signal then demodulated using a phase locked loop (CD 4046) where the carrier is separated from the data. The available data is in the form of TTL which is then converted to RS 232 output and is fed to the computer. 3.6 Terminal software
Code vision AVR is a C cross compiler, integrated development environment and automatic program generator designed for the ATMELAVR family of microcontrollers. The C cross compiler implements all the elements of the ANSI C language as followed by the AVR architecture with some features for embedded systems need. For debugging embedded system, which employ serial communication the IDE has built in terminal.
4. CONCLUSION
The designed system discussed in the paper can be further optimized by implementing an RTOS (real time operating system) to support file transfer. The system can also be modeled supporting RF design to further enhance the system.
The DSP module, the RF modulators, high speed switches, flash RAM can be ported onto a PCB to meet the system requirements in form, size and data rates. The system can be further enhanced to develop a full duplex communication link. REFERENCES
[1] Nikki Rogan, “The future of LED Lighting is bright thanks to the CREE lighting the LED revolution tour,” 13
July 2012 www.creeledrevolution.com
[2] Cimini, L.J., “ Analysis and simulation of digital mobile channel using orthogonal frequency division multiplexing”, IEEE Transactions on Communications 33(7), 665-675 (1985)
[3] Jean Armstrong, “OFDM for Optical Communications,” Journal of Lightwave Technology, 27(3),189-204
(2009)
[4] Weinstein, S.B. and Ebert, P.M., “Data transmission by frequency division multiplexing using the discrete
Fourier transform," IEEE transactions on Communication Technology, 19(5), 628-634 (1997).
[5] Jan-Jaap van de,Ove Edfors, Magnus Sandel, Sarah Kate Wilsony, Per Ola “On Channel Estimation in OFDM
Systems,” Proceedings of Vehicular Technology Conference, 2, 815-819 (1995).
[6] Elgala, H., Mesleh, R., Haas, H. and Pricope, B., “OFDM Visible Light Wireless Communication Based On
White LED’s,” IEEE Proc. of Vehicular Technology, 2185-2189 (2007).
IC100 - 154 V. 3 (p.6 of 6) / Color: No / Format: A4 / Date: 9/15/2012 12:20:04 PM
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