TAPR Wireless Digital Communications: Design and Theory By: Tom McDermott, N5EG (n5eg@tapr.org) Supplied Software for Book -------------------------- Given the proper tools, it can be very intuitive to see the relationship between modem filter and channel responses and the resulting eye pattern, and thus modem performance. The supplied software will allow you to experiment with different modem filters and visually see the effect of the changes, plotted directly as eye patterns on screen. The software supplied on the enclosed disk was used to generate the channel response graphs, and the eye patterns in section 5 of the text. With this software, you can model many different channels and modem filter designs, and make different assumptions about the precision of the filters, the lengths of the filters, and you can model many different responses. For example, if you have a radio with known response defects, it may be helpful to model the frequency response and the resultant eye pattern from that radio. Given the measured amplitude and phase characteristics, you can decide what kind of performance degradation will occur with the use of that radio by simulating the eye pattern it will generate, and then measuring the eye closure due to intersymbol interference directly on the plotted graph. Or you can measure the jitter of the clock recovery system in your modem, and using a plotted eye pattern for your radio you can estimate the performance penalty due to the non-optimum positioning of the clock. You may wish to build a filter to compensate for the defects present in a current modem or radio design, and then model the resultant eye pattern by combing all of the channel responses. Finally, you may wish to design your own modem filters, and the supplied software will allow you to calculate good impulse response coefficients for a digital filter implementation, and then calculate the eye pattern that should result from that filter. Or, you may wish to design other filters, and the supplied software will allow you to specify some interesting filter shapes, and will determine the impulse response coefficients. The eye pattern calculation requires two steps, first inputting the frequency (and possibly phase response) of the filters and then determining the equivalent impulse response. This is done using the RAISCOS.XLS spreadsheet. Secondly, the impulse response is convolved with a pseudo-random data stream, (a pseudo-random data stream is generated, and then Œfiltered¹ by your filter), and the resultant output is plotted as an eye pattern, this involves the use of one of the ŒEYExxxx.XLS¹ spreadsheets, depending on the filter length you want to use. The procedure is described near the end of section 5, and involves some cutting and pasting between the two spreadsheets. You can easily modify the resolution and oversampling rates by changing the spreadsheets. The response of simple low-pass and high-pass filters in covered in the spreadsheet APPEN-D.XLS, which models the filters discussed in Appendix D. This spreadsheet allows modification of the pole frequencies, and observation of the amplitude and phase responses. More complex filters can be simulated directly from the transfer function of the filter. The enclosed spreadsheet shows how the transfer functions of the basic filters are mapped into the spreadsheet, and you can easily expand the spreadsheet to include more complex filters. The simulation of the filters gives a good Œintuitive¹ understanding of how the transfer function relates to the filter response. Phase locked-loops are sometimes mysterious because the properties of the feedback loop have to be understood in terms of both the amplitude and the phase of the feedback. The spreadsheet PLL.XLS provides both the open-loop and the closed-loop response of several common PLL filter functions, and allows looking at the amplitude and phase response of a phase locked loop. the critical parameters are easily varied, and the resultant response is plotted in familiar terms. You can measure you loop parameters, and then determine if the loop will be stable, or will have desirable properties. If not, its easy to change some parameters, and simulate the resultant response. Lastly, Dolph-Chebychev filters are fascinating, and not only provide unusual frequency response, but also finite impulse response duration. The enclosed spreadsheet allows modeling the frequency response and determination of the impulse response coefficients for different lengths of DC filters. Additionally, they describe the antenna currents desirable for a phased-array antenna since the mathematics behind array pattern formation is the same Fourier transform as the frequency-response / impulse response relation. Installing the Software ----------------------- The software has been compressed into a PC self-extracting archive file (.EXE). The files occupy a little over 3 megabytes of disk space when expanded. All of the EXCEL spreadsheets require Microsoft Excel 5.0 or later to run. The Excel files themselves can be run on either Macintosh or Windows versions of Excel. The program REMEZ.EXE must be run either in DOS, or from a DOS window from within Windows, it will not run on a Macintosh. Unarchiving instructions for files using a DOS based machine First, copy the file DISTRIB.EXE into the root directory of the desired destination hard drive. For example, to install the distribution on your ³C² hard drive, insert the diskette into your floppy drive, and type: c: where ³c² is the destination drive! cd \ to start from root directory copy a:distrib.exe where ³a² is the disk drive containing the distribution diskette Type 'distrib' to extract all the necessary files from the DISTRIB.EXE file. Unarchiving instructions for files using a Macintosh ---------------------------------------------------- If you are unable to read DOS formatted disks, the DISTRIB.EXE can be found on http://www.tapr.org in the publications area under products for downloading to whatever format you require. Copy the DISTRIB.EXE file on the diskette to a folder on your hard disk. Expand the DISTRIB.EXE file using StuffitExpander (freeware) and DropStuff (shareware), which are recommended. See below for a source for these programs if you do not have them already. If you are using StuffitExpander, simply drag the DISTRIB.EXE file onto the StuffitExpander icon. The archive will be expanded and placed into a folder. StuffitExpander and DropStuff may be obtained via anonymous ftp from mac.archive.umich.edu /mac/util/compression/ You'll need both: stuffitexpander3.52.sea.hqx and dropstuff3.52.sea.hqx Supplied Software Information ----------------------------- RAISCOSI.XLS Excel 5.0 spreadsheet containing raised-cosine filter frequency and impulse responses. These can be modified for custom responses, and the impulse response can be pasted into the eye pattern sheets to plot the resultant eye pattern. EYE4-9.XLS Excel 5.0 spreadsheet containing an alpha=0.4, 9-tap eye pattern plotter for 2-level eyes. You can paste a 9-tap impulse response into this sheet. EYE2-17.XLS Excel 5.0 spreadsheet containing an alpha=0.2, 17-tap eye pattern plotter for 2-level eyes. You can paste a 17-tap impulse response into this sheet. EYE2-33.XLS Excel 5.0 spreadsheet containing an alpha=0.2, 33-tap eye pattern plotter for 2-level eyes. You can paste a 33-tap impulse response into this sheet. PLL.XLS Excel 5.0 spreadsheet with open- and closed- loop plots for 3 common PLL loop filters. You can vary the gain and breakpoint frequencies, and the plots will update automatically. APPEN-D.XLS Excel 5.0 spreadsheet with simple low-pass and high-pass filter calculations using Laplace notation. You can modify the filter equations, and the plots will update automatically. DOLPH.XLS Excel 5.0 spreadsheet containing Dolph-Chebychev pulse description, including frequency and impulse response. REMEZ.EXE DOS executable program. Calculates impulse response for general low-pass, high-pass, bandpass, stopband, raised-cosine, square-root raised cosine, sinc-compensated square-root raised cosine filter. Also calculates a Hilbert-transform filter and a wideband differentiator. This program uses the Remez-exchange technique and is derived from the Parks-McClellan algorithm. The program writes its output to the standard output, so you may want to redirect it to a file. For example: REMEZ > MYFILE.DAT would place the listing into the file named MYFILE.DAT. ----------------------- Copyright © 1996 by Tucson Amateur Packet Radio Corporation This work is publication No. 96-1 of the TAPR Library, published by TAPR. All rights reserved. No part of this work may be reproduced in any form except by written permission of the publisher. All rights of translation are reserved. ISBN: 0-9644707-2-1 First Edition First Printing, 1996 TAPR 8987-309 E Tanque Verde Rd #337 Tucson, AZ 85749-9399 Office: (940) 383-0000 Fax: (940) 566-2544 E-mail: tapr@tapr.org WWW: www.tapr.org