Design and Implementation of Orthogonal Frequency Division Multiplexing (OFDM) Signaling

Study by: Alan C. Brooks – Stephen J. Hoelzer

Advisors: Dr. In Soo Ahn – Dr. Thomas L. Stewart

EE 451 – December 8, 2000

This project will concentrate on Orthogonal Frequency Division Multiplexing (OFDM) research, simulation, and implementation. OFDM is really a modulation technique especially appropriate for wireless communication because of its potential to deal with inter-symbol interference (ISI). After researching OFDM, simulation in MATLAB is going to be completed. The primary thing about this project is going to be while using simulation results as helpful tips for implement OFDM on the DSP board. With respect to the DSP hardware used, the MATLAB code will have to be transformed into either C or Simulink. This will make our code suitable for the program tool for your particular DSP board. To check the DSP code, we’ll verify the input and output vectors from the MATLAB simulation and DSP implementation correspond.

Using the rapid development of digital wireless communication recently, the requirement for high-speed mobile data transmission has elevated. New modulation techniques are now being implemented to maintain the need more communication capacity. Processing power has elevated to some extent where OFDM is becoming achievable and economical. Because so many wireless communication systems being developed use OFDM, it’s a useful research subject. A few examples of current applications using OFDM include DSL, DAB (Digital Audio Broadcasting), High definition tv broadcasting, IEEE 802.11 (wireless networking standard).

This project includes MATLAB simulation and DSP implementation. Utilizing a DSP design tool for example dSpace and Simulink or TMSC60 tools, a genuine time OFDM transmitter is going to be built.

Figure 1 shows a simplified flowchart from the MATLAB simulation code.

Figure 1: OFDM Simulation Flowchart

The transmitter first converts the input data from the serial stream to parallel sets. Each group of data contains one symbol, Si. for every subcarrier. For instance, some four data could be [S0 S1 S2 S3 ].

Before performing the Inverse Fast Fourier Transform (IFFT), this situation data set is arranged around the horizontal axis within the frequency domain as proven in Figure 2. This symmetrical arrangement concerning the vertical axis is essential for implementing the IFFT to control this data.

Figure 2: Frequency Domain Distribution of Symbols

An inverse Fourier transform converts the regularity domain data set into examples of the related time domain representation of the data. Particularly, the IFFT is helpful for OFDM since it generates examples of a waveform with orthogonal frequency components.

Then, the parallel to serial block produces the OFDM signal by sequentially outputting time domain samples.

The funnel simulation allows study of the results of noise, multipath, and clipping. With the addition of random data towards the transmitted signal, simple noise could be simulated. Multipath simulation involves adding attenuated and delayed copies from the transmitted signal towards the original. This simulates the issue in wireless communication once the signal propagates on the majority of pathways. For instance, a receiver could see an indication using a direct path in addition to a path that bounces off a structure.

Finally, clipping simulates the issue of amplifier saturation. This addresses an operating implementation condition in OFDM in which the peak to average power ratio is high.

The receiver performs the inverse from the transmitter. First, the OFDM data are split from the serial stream into parallel sets. The Short Fourier Transform (FFT) converts time domain samples back to a frequency domain representation. The magnitudes from the frequency components match the initial data. Finally, the parallel to serial block converts this parallel data right into a serial stream to recuperate the initial input data.

The present form of the MATLAB simulation accepts binary, text, or seem as input. After that it generates the related OFDM transmission based on variable setup parameters.

Figure 3: MATLAB Simulation with Seem Input

Figure 3 shows an example from the graphs generated through the current MATLAB code. Top of the left plot may be the input seem file. After modulation, the related OFDM transmission is proven around the upper right. Let’s imagine, an ideal funnel was assumed meaning the received signal (lower right) exactly matches the transmitted. Finally, the received OFDM signal is demodulated to breed the initial input data, proven within the lower left plot.

Figure 4: Spring 2001 Schedule

As proven in Figure 4, the primary goals to complete in Spring 2001 include finalizing MATLAB, generating the related DSP code, after which testing the machine. The overlap between tasks implies that inside a two engineer team, parallel work pathways is going to be transported out simultaneously.

At length, MATLAB code is going to be improved to create a hardware implementation achievable. This can involve adding QAM and guard periods, in addition to windowing, interleaving, and coding features. The next thing is to discover the DSP tools. By writing simple test simulations, the functionality from the DSP tools is going to be explored. Once these tool are understood, the MATLAB code is going to be converted in to the appropriate format for that DSP board. Simulink may be used using the dSpace board, however the TMSC60 board uses C. The testing phase calls for verifying the DSP board produces results in conjuction with the MATLAB code. Also, when the OFDM system works properly, the options from the OFDM transmission is going to be examined. This might include phase shift and synchronization techniques.

[1] Van Nee, Richard, and Prasad, Ramjee. OFDM for Wireless Multimedia Communications. Boston: Artech House, 2000.

[2] O’Leary, Seamus. Understanding Digital Terrestrial Broadcasting. Massachusetts: Artech House, 2000.

[3] Bahai, Ahmad R. S. and Saltzberg, Burton R. Multi-Carrier Digital Comunications:Theory and Applying OFDM. New You are able to: Kluwer Academic/Plenum Publishers, 1999.

[4] Keller, Thomas, and Hanzo, Lajos. Adaptive Multicarrier Modulation: A Handy Framework for Time-Frequency Processing in Wireless Communications. IEEE Proceedings from the IEEE 88 (2000): 609-640

[5] Lawrey, Eric. OFDM Wireless Technology. 11 May 2000. 7 November. 2000. world wide web.eng.jcu.edu.au/eric/thesis/Thesis.htm

370 Multiplex Communications

370/203 Generalized Orthogonal or Special Mathematical Techniques

370/208 Particular group of orthogonal functions (subset of 203)

708 Electrical Computers: Arithmetic Processing and Calculating

708/400 Transform (subset of 200)

708/403 Fourier (subset of 400)

708/404 Fast Fourier Transform (subset of 403)

3,488,4555 Orthogonal Frequency Division Multiplexing (Jan 6, 1970)

6,125,124 Synchronization and sampling frequency within an apparatus receiving OFDM modulated transmissions (Sept 26, 2000)

6,021,110 OFDM timing and frequency recovery system (February 1, 2000)

5,694,389 OFDM transmission/reception system and transmitting/receiving apparatus (12 , 2, 1997)

6,115,728 Fast fourier transforming apparatus and method, variable bit reverse circuit, inverse fast fourier transforming apparatus and method, and OFDM receiver and transmitter (Sept 5, 2000)

Hardware: Computer, DSP Board (dSpace or TMSC60)

Software: Matlab, dSpace Tools (Simulink), C60 Tools (C)