IEEE Frontiers in Education Conference, Nov. 5-8, 1997, Pittsburgh, PA
D EVELOPMENT OF AN E XPERIMENTAL C OMPONENT FOR A S ENIOR C OMMUNICATION S YSTEMS C OURSE : A WEB -ASSISTED A PPROACH Bijan G. Mobasseri Department of Electrical and Computer Engineering Villanova University Villanova, PA 19085 Email:
[email protected] Laboratory Needs of A Communication Course Abstract Abstract - Instruction in communication and many other upper division EE courses have traditionally been limited to lecture sessions. With the proliferation of multimedia-capable platforms and software this situation has changed significantly. This paper documents our two year effort on the development of a MATLAB-based practicum for a senior level communication course. Detailed timetable and experimental material are presented. The second part of the paper describes integration of the web into the course content and comments on its impact.
Introduction Most Hands on, laboratory work in electrical engineering used to be confined to lower division courses such as circuits and electronics. It was not unusual at all for courses in signal processing, control and communications to have no experimental components, and with good reasons. Setting up a communication or signal processing laboratory required hardware or software that was either not available or very expensive. Even if acquired, such equipment required maintenance that went beyond typical university resources. Such conditions created a "comprehension" void in the upper division courses. With no means of illustrating the ideas, course material quickly became another exercise in mathematical manipulation of symbols, equations and graphs. With the proliferation of cheap hardware and powerful software, things began to turn around. Realizing this shift, we overhauled the entire ECE curriculum by adding a "practicum" component to every undergraduate ECE course. In this paper I will present my experience in developing a practicum component for our senior communications course, how it has faired in the past two years and the impact internet has had on it.
What are the laboratory needs of a senior level analog and digital communications course? Our senior level analog and digital communication course follows the traditional sequence of topics plus an addendum bringing in current wireless technologies. The course prerequisites are signal processing and probability and statistics, both of which are required junior courses. Simulation of communication systems and ideas necessarily requires audio-visual input/output capability. Until a few years ago, such capabilities were generally not easily and conveniently available. The simulation environment where we could efficiently invest our time and money had to meet the following criteria: • Multimedia-capability with convenient and fast audio/video I/O. • ECE-wide application domain • Established user community • Continued future development • Cross-platform interoperability Although there are powerful, communication-specific simulation tools such as SPW, they are frequently expensive to buy and maintain and have limited use across the department. With above constraints in mind, MATLAB (http://www.mathworks.com) emerged as a clear choice. The modular structure of MATLAB, particularly the recent introduction of a communication toolbox, has demonstrated that its choice was indeed a wise one. There are other similar efforts along this line. Of particular interest is that of Georgia Tech and their recent DSP First manual[1]. There are also hardware-based approaches to communication simulations. Of interest is the work by Tretter at University of Maryland [2]. Such approaches have clear advantages in so far as real-time performance is concerned but there are other practical constraints. Hardware-based laboratories require a substantially higher level of support and are more costly and difficult to install and maintain. Integration of MATLAB and DSP hardware, although doable, is by no means trivial. Recent introduction of
IEEE Frontiers in Education Conference, Nov. 5-8, 1997, Pittsburgh, PA MATLAB to C code is a step in simplifying this hardware/software link. There are also dedicated DSP boards, notably that of dSpace, that can talk to Simulink. Introducing a hardware angle to a communication lab, although of great value, will necessarily introduce additional burdens. Unless there is a reliable support structure to make everything work together, a total software solution is, in my opinion, preferable. Other notable efforts adopting MATLAB for their communications simulations are reported in [3,4] .
Structuring of the Experiments ECE 4790 is a 4 credit hours communication course taken by all seniors in the fall. It meets 3 hours per week for lecture and is augmented by a 2 hours per week "practicum" session. Both lab and lecture meet in the "Mac Room" where MATLAB can be accessed through a workgroup server. The practicum session has two main objectives, 1): make students actually implement key algorithms themselves, 2): provide a sensory feedback by working with real audio. Practicum topics and the pace they are completed closely parallel lecture material. This has actually proved to be a rather delicate task. Not every concept can be tried out in the lab and those that are implemented do not necessarily take as long as in class. TABLE 1 I NSTRUCTION AND P RACTICUM T IMETABLE INSTRUCTIONS PRACTICUM(WEEKS) (WEEKS) Review and 1 Working with real audio 2 background Channels and 1 Examining RF pulse 1 distortion Amplitude 1 Amplitude Modulation 1 Modulation Frequency 1 Frequency Modulation 2 Modulation Source coding 2 Quantization 1 Matched filtering 1 Matched filtering 1 Pulse shaping 1 Intersymbol interference 1 ISI and 1 Channel equalization 2 Equalization Digital 3 M-ary modulation 2 modulations Wireless 2 BER estimation 1 communications Total 14 Total 14
There are a number of key skills that students will be needing throughout their laboratory work. These techniques, listed below, must be covered first and gotten out of the way: • Importing digital audio from arbitrary sources into MATLAB • Power calculation. • Signal to noise ratio calculation. • Generating random noise with enough power to achieve a prescribed SNR. • Basic channel modeling and I/O computations • Power spectrum calculation • Bandwidth estimation These topics are all covered in the first practicum. The remaining practica closely parallels lecture topics. Bringing in Real Audio Availability of real audio is critical to the understanding of the material. It became clear that a single audio clip used over and over quickly becomes boring and unbearable. Therefore, a variety of sound clips are needed to keep the interest high. I have solved this problem by importing clips recorded off the air from local FM stations. They include short segments of news, talk, music etc. MATLAB 4.2 does not directly support Mac's sound files. Such recorded sounds must first be converted to AIFF format. This file is then read into MATLAB through readsound, a user written utility. Once inside MATLAB, they can be saved as .mat files and loaded like any other data. These segments are anywhere from 5 to 10 seconds. Their length is dictated by the computational power of the workstations running MATLAB and memory and software limitations of the recording machines. I have found an exuberant reaction among students when they hear a clip, just heard in the news, being played back by MATLAB. Moreover, the realization that they are now in charge and can manipulate the clip is particularly satisfying. Various test audios were used including clips from ABC Radio Network news, a familiar refrain from James Bond movies and various music clips. The First Practicum The first practicum intends to implement and teach the basic skills identified above. To be of value, this demonstration must be done within a clear and understandable framework. For this purpose, the problem of transmission of an audio signal through a communication "channel" was selected. This channel for now is no more complicated than a Butterworth low pass filter. But in the first week of the first course in communication, this is probably quite adequate. The project requires the following: 1. Listening to the original audio clip.
IEEE Frontiers in Education Conference, Nov. 5-8, 1997, Pittsburgh, PA
2. Investigation of the real audio signal in the frequency domain( learning how to get PSD) 3. Bandwidth determination (using 3-dB definition) 4. Designing of a lowpass channel in MATLAB with sufficient bandwidth 5. Addition of Gaussian noise to achieve SNR's of 0, 10, 20 and 30dB 6. Listening to the noisy signal to establish a sensory comparison to numerical SNR's 7. Passing of the signal +noise through the channel. 8. Investigation of the "received" signal including channel bandwidth effects and SNR. This evaluation is both numerical and "live" Timing the experiments is constrained in a number of ways. 1): amount of work to be completed in a reasonable time, 2): synchronism with lecture material, 3): reasonable level of complexity. The number of practica has varied from 8 to 10 over the course of a 14-week semester. Experiment sheets are divided into 4 sections; objectives, method, results and new MATLAB functions. Objectives clearly specify what needs to be accomplished, m e t h o d suggests a general approach, results section is clearly numbered so that students know what and how many outcomes they have to produce. MATLAB functions that are new to the experiment are listed at the bottom. Most work is completed during the 2 hour lab session. As a software-based laboratory, what is turned in follows the following guidelines; MATLAB code must be clearly partitioned and cross referenced to the specific items requested in the results section. Graphs and tables must also have a similar annotations. Fig. 1 is an example output of the Matched Filter experiment The way students conducted themselves throughout the 10 experiments is of interest. As MATLAB is a standard part of curriculum, seniors come with a fairly good command of the language. However, many specific skills such as generation and handling of random noise, SNR calculation, PSD and bandwidth estimation etc. are new to them. That is why the successful completion of the first experiment is critical to future work. It naturally took longer. In addition, mastering of MATLAB is an on-going process. With every new function there is a new twist in its proper use. For example, maximum deviation ratio β controls the FM bandwidth. However, MATLAB's modulate command does not accept this ratio directly. It requires an awkward manipulation to make its version of β match the corresponding textbook parameter. See Figure 2 for an example output of the FM bandwidth experiment. FM bandwidth analysis is by nature not amenable, except for trivial signals, to hand calculation (Bessel
functions etc.). MATLAB provides a unique opportunity to perform spectral analysis on an arbitrary and real life signal.
Figure 1. Illustrating matched filter concept. Noisy pulses in the middle row are identify. Peaks in the matched filter output, however, clearly point to pulse identities
The Web: Where does it come in? Internet and the world wide web have seen tremendous growth in recent years. So far, most applications involving the web involves 1): document retrieval, e.g. reading/downloading technical , recreational or business literature, 2) software download(originals and updates). Question then arises around he role of the web in classroom education. It turns out that using the web in education, in a nonredundant way, is not so trivial. A majority of course homepages across the country are announcements, schedules, test dates and syllabus; things that are rarely revisited. Therefore, the underlying challenge is the question of what the page content should be to draw students on a regular basis? To address this problem recent efforts have focused on the development of interactivity in course homepages [5,6]. We were well underway in our practicum development when the Web came along. Therefore, its integration into the big plan was not anticipated. The initial impetus for the development of a course homepage, in my case anyway, was primarily dictated by a single factor: delivery of course documents anywhere, anytime. Course homepage in its current form
IEEE Frontiers in Education Conference, Nov. 5-8, 1997, Pittsburgh, PA before class. The more innovative aspect of the page is the self-test module. Students can go through a multiple choice test and be graded on the spot. Maher[7] has reported on writing of such a script but the readily available CGI available at Selena's Sol public domain CGI scripts works just fine. A course homepage may be decorated with fancy graphics,dazzling animations and various "interactive" sections. My experience in observing students use of the page over the past year has shown that the only feature that is truly indispensable is the ability to acquire printed copies of lecture overheads, and to a lesser extent practicum, prior to class. All others have an initial novelty that fade away. The challenge in integration of the Web into classroom instruction is finding more of such applications; features that student simply cannot do without.
The Effort: What Does it Take? It is fair to ask what does it take setup and run a practicum-based course?. The answer partly depends on how much support is available? At the concept level, experiments must be identified and sequenced. Topics must mesh with the lecture and timed accordingly. The more difficult task is MATLAB implementation and subsequent interpretation of the results. This step can be the most time consuming. as initial MATLAB output is seldom in agreement with textbook results. If the faculty is the person supervising students, substantial time must be spent on running the simulation prior to class and must also be familiar with the intricacies of MATLAB itself. One glitch that cannot be resolved quickly eats up the two hour lab time leading to future planning difficulties.
References
Figure 2. FM bandwidth spectrum analysis for real signals (http://www.ece.vill.edu/~mobasser/Comm479 0/Course.html), in addition to the usual assortment of buttons for syllabus, calendar etc. has two distinguishing features, 1): Downloadable duplicates of lecture overheads and practica and 2): a self-test module. Overheads are in postscript and are available
1) J. McClellan, R. Schafer, DSP First: A Multimedia Approach, Prentice Hall, 1997 2) S. Tretter, Communication System Design using DSP Algorithms, Plenum Press, 1995 3) W. Tranter,'The role of simulation in the teaching of communications', FIE'96 Salt Lake City, Nov. 7-9, 1996 4) P. Flikkema et al, 'Template-based simulation in digital wireless communication', FIE'96 Salt Lake City, Nov. 7-9, 1996 5) L Lancor et al,'A methodology for web-based learning in engineering', FIE'96 Salt Lake City, Nov. 7-9, 1996 6) J. Rehg, 'Development of a student centered introductory computer course for delivery over the world wide web,' FIE'96 7) R. Maher, 'Simple but useful tools for interactive WWW development,'FIE'96 Salt Lake City, Nov. 7-9, 1996
