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Java Tools for Teaching OFDM Principles in Undergraduate Courses
Sai Zhang, Mahesh K. Banavar, Andreas Spanias, Cihan Tepedelenlioglu, Xue Zhang SenSIP Center, School of ECEE, Arizona State University, Tempe, AZ
[email protected], [email protected], [email protected], [email protected], [email protected]
Abstract—In this paper, we describe a new set of software functions and associated exercises that can be used for teaching orthogonal frequency division multiplexing (OFDM) concepts in undergraduate DSP and communications courses. These tools can be used to simulate, visualize, and analyze the performance and behavior of OFDM systems by considering different input signals and communication channels. OFDM is a compelling paradigm because of its utility in WiFi and LTE. It is also a good demonstration of the use of the FFT in a communication system. We have developed the proposed set of functions as a part of the Java-DSP (J-DSP) visual programming environment. The functions can be used in undergraduate DSP and communications courses, in order to demonstrate to students, the application of DSP concepts in a communication system, as well as concepts such as FIR filter design, properties of the DFT matrix, random signals, and circular effects.
Keywords—orthogonal frequency division multiplexing (OFDM); modulation; FFT; channel; noise
I. INTRODUCTION There is a need to present real-life scenarios and
applications for several physical and mathematical principles taught in STEM courses. In particular, the undergraduate courses on signal processing and communications are mathematically rigorous, and students are often unable to relate those concepts to practical engineering systems. Teaching the design of an OFDM system for communications can be an ideal methodology to bridge this gap. In addition to being an interesting paradigm for teaching DSP concepts, OFDM systems find widespread use in several standards for technologies such as 4G communications, digital audio/video broadcasting, ADSL, WLAN, and WiFi.
In order to teach principles of DSP and communications in undergraduate courses, the freely accessible online simulation software Java-DSP (J-DSP) (Fig. 1) was selected as the host environment. J-DSP is being used for education and research in different areas of signal processing and communications [1-4]. Simulations in J-DSP can be performed by simply placing and connecting blocks that correspond to different signal processing functions. As a result, using J-DSP requires little programming or coding experience. The intuitive interface allows for visualization of DSP concepts and students can focus more on understanding the system.
In this paper, we describe the orthogonal frequency division multiplexing (OFDM) system [5], and related function modules.
Fig. 1. Example of an OFDM simulation in J-DSP.
We will describe laboratory exercises, which are comprised of basic simulations for understanding frequency domain processing. By simulating an OFDM system using the proposed functions, students will have a better understanding of basic DSP concepts such as FIR filter design, FFT/IFFT, properties of the DFT matrix, convolution, circular effects, and random signals.
The rest of this paper is organized as follows. First, a brief description of the OFDM system is given in Section II. In Section III, we present the modules designed for the OFDM system in J-DSP. In Section IV, an exercise of the OFDM system and the analysis of the system with different parameters are shown. Assessment results are summarized in Section V, and concluding remarks are presented in Section VI.
II. OFDM PRINCIPLES We choose OFDM because of its utility as a
modulation/multiplexing system in modern communication systems, including compelling LTE and WiFi applications. OFDM shows students how the IFFT is used as a modulator. It also exposes students to channel modeling, fast deconvolution, DFT matrix properties, channel noise and circular effects.
Mobile radio communications are generally subject to multipath propagation [5, 6]. The received signal will therefore be a combination of several distinguishable signals and the channel can be modeled as a finite-length impulse response (FIR) filter, and the process of data transmission over the channel can be represented as a convolution sum of the
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transmitted signal and the filter coefficients. OFDM is one method that can be used to mitigate the effect of such multipath channels. In an OFDM system, an IFFT of the input frequency domain signal is first performed as subcarrier modulation. Then, the process of adding the cyclic prefix, filtering by the channel (linear convolution) and finally removing the cyclic prefix simulates circular convolution. By taking the FFT of the received signal, the data is transformed into frequency domain. The basic model of the OFDM system is shown in Fig. 2, and basic principles of the corresponding process can be found in [6-8].
III. OFDM MODULES IN J-DSP As shown in Fig. 3, we have developed several functions
that are designed for the simulation for OFDM systems. By using this Java tool, we will be able to introduce students to the basics of OFDM and enhance their understanding of the basic principles of signal processing and communications [9].
In Fig. 3(a), the process of OFDM is captured in the OFDM simulation shown in J-DSP. Using the SigGen(L) block, an audio signal is generated and segmented into frames for further processing. The signal is first passed though the IFFT block. The OFDM block (Fig. 3(f)), which is next, adds a cyclic prefix, simulates the channel, and then removes the cyclic prefix. The length of the cyclic prefix, as well as the length of the channel can be provided by the users. Additionally, the users can also select between randomly generated channel coefficients (Fig. 3(f)), or set their own values for the filter coefficients. The users also have the option of adding noise to the system using the Complex Noise Generator block (Fig. 3(e)).The OFDM block is followed by the FFT block to transform the signal back into the frequency domain. Finally channel equalization or channel deconvolution is achieved by dividing the FFT output by the frequency response of the channel. The Channel Equalization block is shown in (Fig. 3(c)).
In the following subsections, we will present an overview of the modules and functions developed for simulating OFDM in J-DSP.
A. The OFDM Module The OFDM block (Fig. 3(f)) performs the processes of
adding the cyclic prefix, filtering using a FIR filter, adding the transmission noise, and removing the cyclic prefix.
The channels are modeled as FIR filters of a given length. The filter coefficients can be Gaussian distributed or user defined. The block also has a second input pin for the channel noise that users can feed into the system.
In this block, users can define parameters for the FIR filter, the length of the cyclic prefix, frame size, and channel filter coefficients.
B. Complex Noise Module Using the Complex Noise block (Fig. 3(e)), complex-valued
random signals (Gaussian, Uniform or Rayleigh) can be generated, and added to the signal as noise, as an input to the OFDM block.
In the Complex Noise block, users need to set the noise type, signal length, and the mean and variance of the noise.
C. Channel Equalizer In the Channel Equalizer block (Fig. 3(c)), the output of the
FFT is fed into the upper pin and the FFT of the FIR filter coefficients (frequency response) is fed into the lower pin of the block. The block performs element-by-element division of the FFT of the signal by the channel frequency response to achieve deconvolution in time.
D. Other Modules Other modules that are used in the simulation of the OFDM
system include the FFT block and the IFFT block, which have been previously developed in JDSP [4].
IV. SAMPLE EXERCISES In this section, some parts of an exercise developed for the
OFDM system using the modules described in the previous section are described. The exercise can be used to introduce the OFDM system to the undergraduate students in signal processing and communication courses. In the exercise, by building an OFDM system, students will be able to relate the concepts learned in undergraduate courses to the real applications. The exercise helps students relate the concepts learned in STEM courses including the FFT/IFFT operations, properties of the DFT matrix, random signals, FIR filters, circular effects, convolution, and random processes. In what follows, a brief description of the exercise will be given.
In the first part of the exercise, a simulation model for the OFDM system is introduced to the students. System parameters including cyclic prefix length, channel coefficients, input signal, and noise type can be modified in the simulation. By comparing the output signal with the input signal, the OFDM system is analyzed. The simulation introduces the concept of cyclic prefix and transmissions over a wireless multipath channel to the students. In the second part of the exercise, students modify the cyclic prefix length and the order of the FIR filter to study the effects of the cyclic prefix on the performance of the system. In this way, students can learn that in OFDM, the cyclic prefix should be at least as long as the order of the filter. In the third part of the exercise, students build an audio processing model using the OFDM system. In the simulation, an 8192-sample speech signal is generated in the SigGen(L) block. The signal is processed frame-by-frame with 128 samples in each frame and a total of 64 frames, with no overlap between frames. Noise is introduced into the system to simulate a noisy channel. The Sound Player block provides students with a graphical view of the audio signal and students can also listen to the output audio signal and draw conclusions about the performance of the system.
Fig. 2. Signal flow diagram for the OFDM system.
Fig. 3. OFDM simulation in J-DSP, and related blocks.
V. ASSESSMENT RESULTS Preliminary assessments for the effectiveness of JDSP in
DSP and communications education have been conducted [4, 9]. The results show the effectiveness of using JDSP in the classroom environment and the benefits of using JDSP in undergraduate DSP and communications classes. A workshop for signal processing and communications graduate and undergraduate students will be conducted for testing the OFDM system simulation in J-DSP and assessment results will be collected. The assessment questions are based on the OFDM exercise described in Section IV. Questions that are related to the performance of the system due to different noise parameters, channel coefficients, and effects of the cyclic prefix are asked. Some questions about the general design of the modules and suggestions for improvement of the software will also be solicited. Detailed assessment results will be presented at the conference.
VI. CONCLUSIONS In this paper, we described the OFDM system simulation
using the free web-based online Java tool J-DSP. The blocks developed can be combined to create a complete OFDM system. The system will perform the functions of IFFT, addition of a cyclic prefix, channel filtering, removal of the cyclic prefix, and an FFT, all components of an OFDM system. The simulation can also include features for adding channel noise, and for channel deconvolution. Using these blocks, students can learn the concepts of IFFT-based modulation, properties of the DFT matrix, convolution and circular effects, and noise processes and statistics.
Education for STEM courses can greatly benefit from the J-DSP OFDM modules. Students can relate the basic concepts with real applications, using the JDSP-based simulations. Since using J-DSP requires no coding background, students
with limited programming ability or students from areas outside engineering can also perform the OFDM simulations.
ACKNOWLEDGMENTS The Java-DSP project was funded in part by NSF TUES
program grant DUE 0089075. The authors were also supported in part by the SenSIP Center, School of ECEE, Arizona State University.
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