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OFDM Transceiver using HDL. Paul pethsomvong Zach asal Dr. in soo ahn Dr. yufeng lu May 6, 2014. Outline. Project Overview Project Goal Project Description Orthogonal Frequency Division Multiplexing (OFDM) Communication Channel Effect Specifications Results Conclusion - PowerPoint PPT Presentation
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OFDM Transceiver using HDLPAUL PETHSOMVONGZACH ASALDR. IN SOO AHNDR. YUFENG LUMAY 6, 2014
Outline• Project Overview
• Project Goal
• Project Description
• Orthogonal Frequency Division Multiplexing (OFDM)
• Communication Channel Effect
• Specifications
• Results
• Conclusion
• References2
3
Project Overview• Implements an Orthogonal Division Multiplexing
(OFDM) communication system.
Figure 1: Block Diagram
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Project Goal• To implement a complete OFDM system using
field programmable gate arrays (FPGA) and VHDL hardware description language. The OFDM system can be reconfigurable.
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Project Description• The project is divided into three stages.• Stage 1: MATLAB/Simulink
• Stage 2: FPGA board implementation with channel effects
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What is OFDM• Orthogonal frequency division multiplexing (OFDM) is a
technique for transmitting digital data by using a large number of modulated carriers with sufficient frequency spacing so that the carriers are to be orthogonal. [2]
• OFDM involves encoding data on multiple carrier frequencies, all of which are orthogonal to each other
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OFDM signal• g(t) is the complex envelope for OFDM signal.
• fn(t) are orthogonal carriers.
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Orthogonal
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Complex Envelope Representation
Figure 2: Complex Envelope Representaion
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Communication Channel Effects• Attenuation
• Multipath• Delay spread
• Doppler spread
• Multipath fading
• Inter-symbol interference
Attenuation• Reduction in power density
• Occurs as the distance from the source increases
• absorption, free-space loss, refraction , etc.
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Multipath• Delay Spread• delay spread is caused by multiple paths with
varying lengths.
• Doppler Spread• Caused by different movements of various
multipath reflectors
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Multipath Fading• Fading caused when different multipath
reflectors move around slowly and appear and disappear.
• Fading is due to the received signals cancelling each other out.
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Inter-symbol Interference (ISI)• One symbol interferes with subsequent symbols
• Each symbol spreads in time, the pulse for each symbol may be smeared into adjacent symbol slots
• Causes communication to be less reliable
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Applications & Advantages• Resistant to multipath fading• Low inter-symbol interference• Widely used in digital communication
systems• Used in 802.11(a, g, n, ac), 4G, and LTE• 4G - 4th generation wireless communication• LTE- Long Term Evolution
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FFT/IFFT• This is the most important block in the
OFDM communication system. • The FFT and IFFT are implemented using
the Xilinx core generator.
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Cyclic Prefix• Trim and align data
Figure 3: Cyclic Prefix
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Equipment• Genesys FPGA board
• Xilinx
• Diligent PmodDA2
• Diligent PmodAD1
Figure 4: Genesys FPGA board
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Specifications
Specification Chosen ValueFPGA System Clock 100 MHzCarrier Frequency 25 MHzSymbol Rate 12.5 MspsData Rate 50 MbpsFFT Size 32Symbol Time 8 * 10-8 secModulation Scheme 16-QAMFigure 5: Specifications
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• FPGA System Clock = 100 MHz• Symbol Rate Rs = 12.5 Msps• Data rate Rb= Rs · N = 50 Mbps• Bits per symbol N = 4
• Symbol time = 1/ Rs
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Stage 1 Simulink simulation• A single subcarrier carries data.
Figure 6: 1 Frequency
Stage 1 Simulink simulation• Two subcarriers carry data.
22Figure 7: 2 frequencies
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• 32 subcarriers
Figure 8: 32 frequencies
Simulink Model
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Simulink Model
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Simulink Model
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Simulink Model
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Simulink Model
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Simulink Model
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Simulink Model
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Simulink Model
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Simulink Model• Model contains transmitter, receiver, and a
Gaussian channel model.
• Channel model• Y(t) = x(t)*h(t)
• where x(t) is the transmitted signal, h(t) is the impulse response of the channel, and y(t) is the received signal.
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Simulink results
Figure 9: Input and Output from Transceiver
Stage 2: Implementation with channel effects• The transmitter and receiver are implemented on
a FPGA board using VHDL.
• Analog-to-digital converter (ADC) and digital-to-analog converter (DAC) modules are included.
• Channel effects degrade the overall system performance.
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Figure 10: 16-QAM Transmitted
36Figure 11: 16-QAM Received
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Figure 12: Itransmitted and Ireceived
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Figure 13: Qtransmitted and Qreceived
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Conclusion
• In this project, a complete OFDM transceiver has been designed,
simulated and tested.
• First the simulink model of OFDM transceiver was constructed and
simulated
• After verifying the simulink model, VHDL is used to describe the
transceiver
• The system has been implemented on a Virtex 5 FPGA board.
• 16 QAM is used for baseband modulation. Some modules from the
previous project was reconfigured and used for the project
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References• [1] Anthony Gaught, Alexander Norton, and
Christopher Brady., “FPGA-based 16 QAM communication system”, Spring 2013.
• [2]Leon Couch, Jr. “Analog and Digital
Communication Systems”, 8th edition, 2012,
Prentice-Hall.
Questions
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