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

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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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