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