A practical implementation of a PPM/FSK data transmission system using combined
spectrum and time spreading
by Karl-Max Wagner and Ikuo Oka
In the following a practical implementation of a PPM/FSK data transmission system
using combined spectrum and time spreading will be described. Due to
the complexity of the task the only practicable way is doing all the signal
processing in software using a sufficiently fast digital signal processor (
a so called ``softradio'' approach ).
2 Software implementation
2.1 Tool choice
The software was mainly written in C using gcc as a compiler. This
compiler has the advantage that it supports a multitude of targets which allows
reusing most of the code using for modelling on a workstation for the actual
signal processor code.
For the DSP used ( Texas Instruments TMS320C40  ) there is an efficient
target implentation available for gcc ( done by Michael Hayes of the University
of Caterbury, Christchurch, New Zealand ) as well as an excellent software
simulator done by Herman ten Brugge. The existence of the latter is critical
because it allows extensive source level debugging, profiling and memory usage
examination difficult to do in actual hardware ( the author actually is of the
opinion that any well done embedded software project should run as expected
for the most part when comitted to actual hardware ).
2.2 Software architecture
The task envisaged actually requires doing most of the work in the frequency
domain ( and even higher domains ). Thus, input data are collected into data
blocks of the size appropriate for the DFT used. All operations are done on
these data sets.
Modularity is always a good idea in such projects - thus most of the functionality
is collected in a file containing all kinds of frequently needed operations
as functions. In fact, it turned out that the main program consists of these
functions exclusively. The whole process is like building an electronic circuit
from IC's only. In fact, it seems as if these functions could be used in many
other communications projects - at the moment the author is considering whether
this is the beginning of a general purpose library for communications research
and development purposes.
A crucial part of the software system is a correlating AFC system in the frequency
range. It is necessary because good time spread filters are very sensitive to
frequency deviations inevitable in practical microwave communications systems.
3 Hardware Implementation
3.1 DSP subsystem
As DSP boards hema's DSP1 and DSP2 boards are used. These are based
on the Texas Instruments TMS320C40 digital signal processor with a
good amount of external RAM (between 512 kbytes and 4 Mbytes, with the possibility
to add up to 16 Mbyte of SDRAM). This is a big boon as due to the extensive
use of transforms large data sets have to be stored. External data have to be
copied to the processor's on chip memory in time (this is why extensive profiling
is required to integrate the required ``hidden DMA'' instructions at the
right place into the code).
3.2 Analog Hardware
As analog hardware the well known ``zero IF'' transceivers designed by
Prof. Matjaz Vidmar of the University of Ljubljana (Slovenia) have been used.
These transceivers are pretty much ideal for DSP use as they output / input
an I/Q baseband signal to be transposed directly to the RF signal band. They
also are well tried and tested for wideband BPSK packet radio use as well as
SSB use using the Weaver method, thus eliminating development and debugging
requirements which are not the object of the present research project.
It has been shown how the theoretical concept of  can be implemented
practically. Software implementation specific aspects, aspects of code organisation,
tool selection as well as additional subsystems indispensable under realistic
conditions have been discussed. It has also been shown how maximum use can be
made of already available hardware designsto cut down on ancillary
hardware design time.
One somewhat surprising result of the project is that the software functions
required to implement it are rather general purpose and can be reused for other
communications related projects. This might prove useful for future research
and development projects.
We want to express our thanks to the following companies for support:
Infineon Technologies, Munich, Germany for the donation of rf/microwave semiconductor
- Texas Instruments Freising, Germany for the donation of ADC's and DAC's
- Mitsubishi Electric Corp. for the donation of HEMT's
- Jinsong DUAN, Ikuo OKA, Chikato FUJIWARA, ''A Proposal of Simultaneous
Spread of PPM in Frequency and Time Axes for Adaptive CDMA'', IEICE Trans.
Comm., Vol. E00, No. 1, Jan. 1999
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