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Communication Systems Projects with

LabVIEW

By:

Ed Doering

This selection and arrangement of content as a collection is copyrighted by Ed Doering. It is licensed under the Creative Commons Attribution 2.0 license (http://creativecommons.org/licenses/by/2.0/). Collection structure revised: December 15, 2009 PDF generated: February 5, 2011 For copyright and attribution information for the modules contained in this collection, see p. 131.

Table of Contents

• Introduction
  • 1.1 Digital Communication System Simulation and Visualization 1 Simulation and Visualization of Fundamental Concepts
  • 1.2 Intersymbol Interference (ISI) and the Eye Diagram
  • 1.3 PAM Transmitter and Receiver Implementing Coherent Detection
  • 2.1 Hamming Block Code Channel Encoder 2 Channel Coding and Error Control
  • 2.2 Hamming Block Code Channel Decoder
  • 3.1 Caller ID Decoder 3 FSK Demodulation
  • 4.1 Speaker-Air-Microphone (SAM) Channel Characterization 4 Bandpass Communications Over the Speaker-Air-Microphone Channel
  • 4.2 Binary ASK Transmitter
  • 4.3 Texting Over the Speaker-Air-Microphone (SAM) Channel
  • 4.4 Introduction to the LabVIEW Modulation Toolkit
  • 5.1 General-Purpose Utilities 5 SubVI Specications
  • 5.2 Baseband Modulation and Pulse Amplitude Modulation (PAM)
  • 5.3 Bandpass Modulation
  • 5.4 Demodulation and Bitstream Regeneration
  • 5.5 Hamming Block Coding
  • 5.6 Speaker - Air - Microphone (SAM) Channel
  • 5.7 Caller ID Decoder
• Index
• Attributions.

Introduction

1

This module refers to LabVIEW, a software development environment that features a graphical programming language. Please see the LabVIEW QuickStart Guide^2 module for tutorials and doc- umentation that will help you:

• Apply LabVIEW to Audio Signal Processing
• Get started with LabVIEW
• Obtain a fully-functional evaluation edition of LabVIEW Table 1

Introduction

Welcome to Communication Systems Projects with LabVIEW, a multimedia-enhanced series of projects that explore digital communication systems through LabVIEW simulations, visualizations, and implementations of practical systems. Communication systems play an exciting role in our increasingly interconnected society. Digital commu- nication systems form the heart of computer data networks, satellite communications, mobile telephones, and wireless hand-held devices. All electrical and computer engineering programs emphasize communication systems as part of the core curriculum. Communication systems analysis and design requires a rm grasp of mathematical models, and demands mathematical skill with signals, systems, probability, and random variables. Insight and intuition, also important for the successful study of communication systems, do not always follow immediately from the mathematical presentations of traditional textbooks, however. Hands-on construction of real communication systems and interactive simulations that supplement the mathematics help to more quickly achieve insightful understanding of the myriad details involved in designing and optimizing a communications link for a given application. Communication Systems Projects with LabVIEW features ten laboratory projects based on the LabVIEW graphical dataow programming environment. LabVIEW oers an unparalleled way to directly translate communication system diagrams and mathematical descriptions into a LabVIEW program called a block diagram. The LabVIEW front panel GUI (graphical user interface) that emerges automatically as part of the programming activity enables real-time interaction with the communication system and visual- ization of the signals as waveforms, binary patterns, and text. This real-time interaction reveals connections, patterns, and often unexpected relationships  the basis of strong intuition and insight. Many of the projects emphasize listening to the signals as sound, further enhancing one's insight. Some of the laboratory projects simulate and visualize fundamental concepts such as baseband modulation, pulse shaping, intersymbol in- terference (ISI) and eye diagrams, while other projects result in fully-operational systems such as a Caller ID decoder and a text messaging system between a speaker and a microphone. (^1) This content is available online at http://cnx.org/content/m18826/1.2/. (^2) "NI LabVIEW Getting Started FAQ" http://cnx.org/content/m15428/latest/

Each project begins with an explanation of the background theory necessary to complete the project. These introductions feature narrated videos called screencasts that simulate a classroom lecture with a whiteboard visual aid. Continue by constructing a set of subVIs (LabVIEW reusable function blocks) ac- cording to precise specications. Each subVI includes a screencast video that demonstrates the LabVIEW tool in operation to introduce and explain relevant LabVIEW programming techniques for the given subVI. Once the subVIs have been built and tested individually, assemble them into a working "top-level" VI (lit- erally a Virtual Instrument, the name of a LabVIEW program). The project directions provide guidance through the complete development process, each step of the way.

To the Instructor

Communication Systems Projects with LabVIEW has been designed to augment existing commu- nication systems laboratory projects, or to serve as the complete laboratory component of an introductory engineering communication systems course. Seven guiding principles motivate the design and organization of Communication Systems Projects with LabVIEW:

1. Build the concept for deepest learning  transforming a set of ideas into a working system clearly demonstrates a rm grasp of the concepts
2. Engage the senses to develop intuition and insight  seeing signals as waveform plots, listening to signals as sound, and changing the way signals are processed through virtual knobs and slider controls all work together to enhance understanding of the system under study
3. Interact with the system to develop understanding  LabVIEW oers an unparalleled means to auto- matically generate an interactive graphical user interface as part of the programming activity
4. Motivate with "real life" activities  many of the projects culminate in practical, working systems
5. Experience impairments  once the deleterious eects of real-world constraints such as nite channel bandwidth and noise become clear through direct experience, the standard methods to mitigate those eects can be appreciated more deeply
6. Integrate teaching and instruction with project activities  each project includes numerous narrated videos to explain concepts and to demonstrate task-specic LabVIEW programming techniques; each project also includes "textbook linkages" to many popular communication systems textbooks
7. Oer learning materials in a modular and open format  each project builds on a well-dened set of building blocks; the projects can easily be modied, extended, and tailored to specic needs

Each project requires four activities on the part of the student: (1) Study the introductory material that explains theory and concepts, (2) implement several subVIs as low-level building blocks, (3) assemble the subVIs into an application VI, and (4) interact with the nished VI to explore the theory and concepts. Constructing the subVIs helps students to develop skills with a wide variety of LabVIEW programming techniques, and also helps them to establish a rm grasp on the various LabVIEW data types. The subVIs are carefully specied around standard datatypes, i.e., Boolean array for bitstreams, waveform data type for "analog" signals; successful completion of the subVIs reduces the debugging eort required for the application VIs. Many of the subVIs are reused across multiple projects. The modularity of the projects  10 projects total with a library of over 40 subVIs  allows the projects to be easily customized as necessary. An instructor's manual and complete set of application VIs and subVIs is available; please contact the author for details.

4 CHAPTER 1.^ SIMULATION AND VISUALIZATION OF FUNDAMENTAL

CONCEPTS

3. Develop a qualitative appreciation for BER and its impact on the received signal
4. Learn several ways to observe bitstreams

1.1.3 Deliverables

1. Summary write-up of your results
2. Hardcopy of all LabVIEW code that you develop (block diagrams and front panels)
3. Any plots or diagrams requested

note: You can easily export LabVIEW front-panel waveform plots directly to your report. Right- click on the waveform indicator and choose "Export Simplied Image."

1.1.4 Setup

1. LabVIEW 8.5 or later version
2. Computer soundcard
3. Speaker

1.1.5 Textbook Linkages

Refer to the following textbooks for additional background on the binary symmetric channel (also known as the discrete memoryless channel) used in this project; see the "References" section below for publi- cation details:

• Carlson, Crilly, and Rutledge  Ch 16
• Couch  Ch 7
• Haykin  Ch 10
• Lathi  Ch 15
• Proakis and Salehi (FCS)  Ch 12
• Proakis and Salehi (CSE)  Ch 9

1.1.6 Prerequisite Modules

If you are relatively new to LabVIEW, consider taking the course LabVIEW Techniques for Audio Signal Processing^4 which provides the foundation you need to complete this project activity, including: block diagram editing techniques, essential programming structures, subVIs, arrays, and audio.

1.1.7 Introduction

Figure 1.1 illustrates a generic communication system (transmitter, channel, and receiver) and a comparator to compare the original source bitstream to the output bitstream and report bit errors.

(^4) Musical Signal Processing with LabVIEW  Programming Techniques for Audio Signal Processing http://cnx.org/content/col10440/latest/

Figure 1.1: Generic communication system with comparator

This project implements Figure 1.1 at an elementary level:

1. The source is a bitstream with equiprobable 0s and 1s
2. The transmitter, channel, and receiver are lumped together as a single "black box," i.e., the internal details are hidden and only the source and received bitstreams are visible
3. The channel introduces errors according to the specied bit error rate (BER)
4. The comparator detects mismatches between the input and output bitstreams (bit errors) and reports measured BER, the ratio of the total number of actual bit errors to the total number of bits observed

1.1.8 Procedure

1.1.8.1 Build the subVIs

Build the subVIs listed below. You may already have some of these available from previous projects. Demonstrate that each of these subVIs works properly before continuing to the next part.

1. util_BitstreamFromRandom.vi (Section 5.1.1.1)
2. util_BinarySymmetricChannel.vi (Section 5.1.3.1)
3. util_MeasureBER.vi (Section 5.1.4.1)

1.1.8.2 Construct base system

1. Create the application VI SystemOne.vi pictured in Figure 1.2 by assembling the subVIs you built in the previous step. Use the default control and indicator styles for now. Expand the Boolean array indicators to show 20 elements (click on the outer frame of the indicator and drag either horizontally or vertically).
2. Try small values for length (say, 10 or 20) and various levels of bit error rate. Remember that the keyboard shortcut "Ctrl+R" runs the VI.
3. Submit a screenshot of your front panel with handwritten calculations that conrms the correct oper- ation of the measured BER output.

while-loop structure to operate the system continually, and then modify your application VI accordingly to produce SystemThree.vi.

Figure 1.4: [video] Modify base system to run continually

Experiment with SystemThree.vi:

1. Estimate the average value (mean) of the measured BER as the specied channel BER varies over the range 0 to 1.
2. Estimate the variance of the measured BER as the bitstream length changes over the range 10 to 10, (a rough guess of the spread around the mean is adequate). Feel free to increase the "millisecond multiple" constant if the loop goes too fast to see the numerical displays.

Discuss your results:

1. How is the average value of the measured BER related to the specied channel BER?
2. How is the variance of the measured BER related to the bitstream length?

1.1.8.5 Visualize the bitstreams as images

Visualizing the error bitstream as 2-D image develops a qualitative feel for the impact of bit error rate on the data output of a binary communication system. That is, what value of BER corresponds to a "high quality" image transmission? Or, what value of BER makes the received image "poor quality"? View the Figure 1.5 screencast video to learn how to reshape the error bitstream into a two-dimensional array suitable for display as a binary (2-level) image using the LabVIEW subVIs "Flatten Pixmap" and "Draw Flattened Pixmap." In addition, learn how to programmatically control the size of the front-panel image indicator using a "property node." Modify your application VI accordingly to produce SystemFour.vi.

Figure 1.5: [video] Visualize the error bitstream as a binary image

Experiment with SystemFour.vi to study the relationship between BER and image size. To begin, set the bitstream length to 1,024 to produce a 32x32 image. Set the bit error rate to 0.0001. Describe the appearance of the error bitstream as an image, and state the relative "quality" of the image (remember that an ideal error image would always be uniformly black). Now, gradually increase the bitstream length to 200,000 while watching the image. Would you still consider the image to be at the same quality level as before? What BER value do you need to obtain the same quality level you stated for the short bitstream length? Explain why a specic BER value can be considered acceptable for some types of transmitted messages and not for others.

8 CHAPTER 1.^ SIMULATION AND VISUALIZATION OF FUNDAMENTAL

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1.1.8.6 Listen to the error bitstream as sound

"Auralizing" the error bitstream as sound also develops your qualitative feel for bit error rate.

Download and run bit_errors_as_sound.vi^5. This application VI continually generates "the sound of silence" (bitstream of 0s) at the source with channel bit errors inserted according to the "BER" slider. Sound is generated in blocks (frames), and total errors within a frame are reported. The average bit errors per second is also reported. Note that the circular panel indicators use logarithmic mapping.

1. Change the bit error rate (BER) slider and listen to the bit errors.
2. Try dierent values of soundcard sampling frequency. Your soundcard may or may not support arbi- trary values, but should denitely support CD-quality (44,100 Hz) and lower sampling rates of 442 ,^100 N , where N is an integer greater than zero.
3. If possible, use a media player to play music or speech through your soundcard while the VI is running. Determine the BER values you would associate with the following qualitative labels for the noise level's impact on the music signal: none, just noticeable, tolerable, annoying, and overwhelming. Tabulate your value/label pairs.
4. For BER=0.001, describe the character of the bit error click sound as a function of sampling frequency. Propose an explanation for the change in sound.
5. Explain the role of data rate (samples per second) on the impact of bit errors. In other words, does BER tell the whole story?

1.1.9 References

1. Carlson, A. Bruce, Paul B. Crilly, and Janet C. Rutledge, "Communication Systems," 4th ed., McGraw- Hill, 2002. ISBN-13: 978-0-07-011127-
2. Couch, Leon W. II, "Digital and Analog Communication Systems," 7th ed., Pearson Prentice Hall,
   2007. ISBN-10: 0-13-142492-
3. Haykin, Simon. "Communication Systems," 4th ed., Wiley, 2001. ISBN-10: 0-471-17869-
4. Lathi, Bhagwandas P., "Modern Digital and Analog Communication Systems," 3rd ed., Oxford Uni- versity Press, 1998. ISBN-10: 0-19-511009-
5. Proakis, John G., and Masoud Salehi, "Fundamentals of Communication Systems," Pearson Prentice Hall, 2005. ISBN-10: 0-13-147135-X
6. Proakis, John G., and Masoud Salehi, "Communication Systems Engineering," 2nd ed., Pearson Pren- tice Hall, 2002. ISBN-10: 0-13-061793-

1.2 Intersymbol Interference (ISI) and the Eye Diagram

6

This module refers to LabVIEW, a software development environment that features a graphical programming language. Please see the LabVIEW QuickStart Guide^7 module for tutorials and doc- umentation that will help you: continued on next page

(^5) See the le at http://cnx.org/content/m18660/latest/bit_errors_as_sound.vi (^6) This content is available online at http://cnx.org/content/m18662/1.1/.

10 CHAPTER 1.^ SIMULATION AND VISUALIZATION OF FUNDAMENTAL

CONCEPTS

• Haykin  Ch 4
• Haykin and Moher  Ch 6
• Proakis and Salehi (FCS)  Ch 9
• Proakis and Salehi (CSE)  Ch 8
• Stern and Mahmoud  Ch 4

1.2.6 Prerequisite Modules

If you are relatively new to LabVIEW, consider taking the course LabVIEW Techniques for Audio Signal Processing^9 which provides the foundation you need to complete this project activity, including: block diagram editing techniques, essential programming structures, subVIs, arrays, and audio.

1.2.7 Introduction

Introductory digital logic courses present digital waveforms as essentially rectangular pulses. Indeed, the internal signals of a digital integrated circuit ideally exist at one of two voltage levels (high and low), with minimal time spent changing from one state to the other. Waveform displays from digital circuit simulators further emphasize the two-level rectangular shape of ideal digital signals. Rectangular pulses are not ideal for transmission through communication links, however, since commu- nication channels always restrict the bandwidth available between the transmitter and the receiver. Rectan- gular signaling pulses contain signicant spectral energy across a wide frequency range due to the step-like transition between levels, and yet most communication systems do not allocate nearly enough bandwidth to faithfully transmit these abrupt changes. Passing a rectangular pulse through a limited-bandwidth channel distorts the pulse by "smearing" it  that is, the pulse stretches out in time. The transmitter sends a series of pulses to convey the message, therefore this time smearing causes interference between adjacent time slots (or bit slots). This intersymbol interference (abbreviated ISI) adds extraneous signal energy at the exact moments when a receiver's bit sampler decides whether a received bit should be called a logic "1" or a logic "0." ISI is not the same as additive random noise, but plays a similar role by reducing the noise margin, i.e., the room for error before the receiver's bit sampler makes an error. This project studies intersymbol interference in an intuitive way by using a LabVIEW VI to simulate a pulse transmitter, nite bandwidth channel, and received signaling waveform. Rectangular pulses are considered rst to demonstrate the ISI problem, and then two alternative pulse shapes are explored as a way to minimize ISI. The eye diagram is also introduced in this project as a visual aid to present the time-domain signal- ing waveform to promote understanding of the ISI phenomenon. The eye diagram also reveals other key performance measures such as noise margin, timing jitter, and timing sensitivity.

1.2.8 ISI_and_EyeDiagram.vi

Download the LabVIEW VI ISI_and_EyeDiagram.vi^10 , an interactive tool to study various pulse shapes as they pass through a band-limited channel. Open the VI which starts running automatically, and then view the Figure 1.6 screencast video for a short orientation tour of the VI. (^9) Musical Signal Processing with LabVIEW  Programming Techniques for Audio Signal Processing http://cnx.org/content/col10440/latest/ (^10) http://cnx.org/content/m18662/latest/ISI_and_EyeDiagram.vi

Figure 1.6: [video] Orientation tour of the "ISI_and_EyeDiagram.vi" LabVIEW VI

1.2.9 Rectangle Pulse Shape

Restore the front panel controls of "ISI_and_EyeDiagram.vi" to their default values by selecting "Edit | Reinitialize Values to Defaults." Set the symbols control to 1 to produce a single rectangular pulse. The channel bandwidth should already be set to its maximum value of 0.49, which corresponds to essentially unlimited bandwidth. Note that this VI uses normalized frequency, therefore the sampling frequency corresponds to 1 and the Nyquist frequency is 0.5. Compare the "transmitted waveform" and the "received waveform" plots in the lower-right front panel. How well does the received pulse match the transmitted pulse? Also, to what extent does the received pulse "spill out" of its designated time slot? Decrease the channel bandwidth until you begin to observe noticeable pulse shape distortion. At what bandwidth does this occur? Continue decreasing the channel bandwidth. What eects do you begin to observe? Make a series of plots that show the progressive degradation of the rectangular pulse shape as the channel bandwidth is restricted. Right-click on the plot and choose "Export Simplied Image" to copy the graph to the clipboard for pasting into your report. Be sure to indicate the channel bandwidth for each plot.

1.2.10 Sinc Pulse Shape

Restore the front panel controls of "ISI_and_EyeDiagram.vi" to their default values. Set the symbols control to 1 to produce a single pulse, and set the bandwidth control to 0.02. The received pulse should show noticeable distortion. Now set the pulse shape control to "Sinc." How much distortion is evident at the receiver? How much lower can you restrict the bandwidth while still preserving the basic sinc waveform shape? The sinc function's ability to maintain its basic shape through a restricted channel bandwidth is impor- tant, but its true signicance extends beyond this fact, as explored in the next section.

1.2.11 Multiple Pulses

A transmitter converts a message, or sequence of bits, into a series of analog pulses to create the signaling waveform. A receiver recovers the bitstream by periodically sampling the signaling waveform and comparing the sample to a threshold value to decide "1" or "0." Sinc-shaped pulse do not interfere with adjacent bit slots, provided that the bit slots are sampled at the correct instant in time. To see this, reinitialize the front panel control values to their default settings, choose the "Sinc" pulse shape, and choose 2 symbols. Look carefully at the transmitted and received pulses on the lower-left front panel plots. The white trace shows the rst pulse in the sequence, while the red trace shows the second pulse in the sequence. The rst pulse has an amplitude of +1, while the second pulse has an amplitude of -1, corresponding to a bit sequence "10"; refer to the message bitstream indicator to conrm that the rst bit is T (green LED indicator active) and the second bit is F (inactive LED indicator). The waveform plots on the lower-right front panel show the actual transmitted and received waveforms, which superimpose (i.e., add) the individual pulses together. The plots on the lower-left front panel illustrate the contribution of each individual pulse.

Figure 1.8: [video] Measuring noise margin, ISI, timing sensitivity, zero-crossing jitter, and optimum sampling time using an eye diagram

1.2.13 Eye Diagram Measurements

Figure 1.9 illustrates a generic eye pattern superimposed on a measured eye diagram plot and summarizes the denition of the various performance metrics discussed earlier. Use these denitions for the following measurements.

Figure 1.9: Generic eye pattern and denition of performance metrics

14 CHAPTER 1.^ SIMULATION AND VISUALIZATION OF FUNDAMENTAL

CONCEPTS

1.2.13.1 Rectangle Pulse

Restore the front panel controls of "ISI_and_EyeDiagram.vi" to their default values, and set the symbols control to 40. Vary the channel bandwidth and observe its eect on the eye diagram plot, and then set the channel bandwidth to 0.05. Increase the eye diagram start time to 245 samples to center the eye in the plot window. Export the eye diagram plot to a piece of paper, and then use the eye diagram cursor as a tool to measure the following (show and label the relevant distances you measured on your hardcopy plot):

1. Optimum sampling time; report this as the number of samples from the nearest zero crossing
2. Peak ISI
3. Zero crossing jitter; report this as the maximum variation in time samples
4. Noise margin

1.2.13.2 Sinc Pulse

Ensure that the front panel controls of "ISI_and_EyeDiagram.vi" are the same as in the previous step, and then select the "Sinc" pulse shape. Adjust the eye diagram start time and time span to maximize the number of displayed bit intervals and also to avoid the initial startup transient that causes lines to cross through the center of the eye; also make adjustments to place the maximum eye opening at the center of the plot window. As in the previous step, export the eye diagram plot to a piece of paper, and then use the eye diagram cursor as a tool to measure the following (show and label the relevant distances you measured on your hardcopy plot):

1. Peak ISI
2. ISI at the optimum sampling time
3. Zero crossing jitter; report this as the maximum variation in time samples
4. Noise margin
5. Timing error sensitivity; report this in terms of time samples

1.2.13.3 Raised Cosine Pulse

Keep the front panel controls of "ISI_and_EyeDiagram.vi" at the same settings you used for the previous "Sinc" pulse measurements, and then select the "Raised Cosine" pulse shape. You should expect to see the maximum eye opening remain centered in the eye diagram plot. As in the previous steps, export the eye diagram plot to a piece of paper, and then use the eye diagram cursor as a tool to measure the the same ve metrics as for the "Sinc" pulse. Show and label the relevant distances you measured on your hardcopy plot. Compare your results for the raised cosine pulse and the sinc pulse. What appears to be advantageous about the raised cosine pulse shape? See the video screencast in pam_RaisedCosinePulse.vi (Section 5.2.1.1) for more background about the raised cosine pulse, the most widely-used pulse shape in digital communication systems.

1.2.14 Noise

Add random channel noise to the received waveform by moving the noise standard deviation control away from zero. Note how the eye pattern begins to close as the noise level increases. Report the noise standard deviation value at which the eye just begins to close completely for each of the three pulse shapes. Make hardcopy plots of the eye diagram for each value that you report.