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Contained below is a copy of my undergraduate thesis on
Spread Spectrum Data Communcation. Please feel free to contact me.
Also available are schmatics and a plotting program.

Synopsis

This final year thesis project is the culmination of work done as a result of
discussions between A/Prof Sam Reisenfeld and myself in May 1994. He
convinced me that there was still a lot of work to be done with spread
spectrum technologies, as well as emphasising how important spread
spectrum is to the expansion of telecommunications in the global
environment.

This thesis report looks at the interaction between spread spectrum
technologies and the equally as new packet radio technologies. It has only
been in the last 15 years with increased computerisation that either of these
technologies have seen significant work.

This thesis also looks at some of the hardware required to implement a
spread spectrum packet transmission system. During the design process I
incorrectly assumed that PSK modems could be operated successfully at the
chip-rate. Due to factors outside my control this assumption was not
discovered until after the circuit boards had been produced.

A re-design implemented an early-late tracking loop which operates
successfully at baseband, but not with RF signals as intended. The circuit
should work with RF signals with the inclusion of ample amplification
throughout the circuit.

Therefore I am presenting a circuit which demonstrates the circuits ability
to lock to an incoming spread digital base band signal.

Most of the work on this project was done at home where I have a
reasonably well stocked workshop. However I am grateful to those on level
23 in the school of Electrical Engineering for the use of the various labs.
However I must thank here the numerous people who I have borrowed
parts and equipment from enabling me to complete this project.

Acknowledgments

Special Acknowledgment should be made to the following people who have
made this thesis possible.

Jack Heath, VK2DVH

Steve Bible, N7HPR

Clive Pickup, VK2DND

Sam Reisenfeld, VK2FPJ

Terry Behan, VK2TDQ

This is not a complete list but contains those who have contributed in
significant ways. In addition I should thank Pacific Power for their help
throughout the 5 1/2 years that I have been a Cadet with them. Their
support has enabled me to complete my degree 6 months early despite early
teething problems.

Also the members of Fisher's Ghost Amateur Radio Club (Inc) have given
me great support. Some members names appear above but these are only a
few of the many that have helped in some way.

Lastly I wish to thank all my family for all their support and
guidance.

I have made the following decisions when it comes to a Spread Spectrum
Packet network.

AX.25 is probably the protocol to use at this stage at least
for level 2 although modifications for Forward Error
Correction (FEC) would be quite required.

The network should operate on a single frequency with
transmitter power control for each packet.

Automated routing such as RSPF should be used to reduce
the Near-Far problem, but needs work to tailer it to the
needs of Spread Spectrum.

The [7,1] spreading code provides a long enough sequence
and short enough synchronisation time.

Binary Phase Shift Keying is the modulation scheme is
simple and cheap to implement. Bit-rate may be doubled by
also transmitting a quadrature signal with an additional BPSK
signal although this is not investigated.

A lot more work is required before a packet network based
on tropologies other than dedicated point-to-point links would
be feasible.

Inside any substation or power station there is a huge investment in copper
cables. Each sensor and transducer is connected to a controller with
hundreds of kilometres of wire in a site that might spread over several
square kilometres.

The cost of these cables is huge. Not only are the capital costs involved
with purchase and installation of the cables high, but also the maintenance
costs both due to aging and transients picked up on the cables causing
equipment failure.

Substations are designed with cheaper PVC cables in the switch-yard rather
than silicon. However if a transformer explodes causing hot or burning oil
to enter cable ducts it is known that all the cables will need to be replaced,
and that the substation will be out of service until this happens.

In the case of Pacific Power Western the area where Wallerawang and
Mount Piper are in close vicinity much environmental data is used by both
power stations. Even more environmental data is not collected because of
the remoteness of the sites.

Radio based telemetry is a solution to many of the situations just posed. A
radio transmitter may be placed in a sub-station yard to send telemetry to
the controller. Environmental sensors may do the same. In the case of a
power station, telemetry may go to a marshalling kiosk and then be
transmitted to the controller.

However standard radio techniques will not be reliable in an environment
such as a power or sub-station where there is a large amount of electrical
noise. This noise would cause important control information to be lost or
delayed.

But all is not lost. Once solely a military technology, Code Division
Multiple Access (CDMA) or Direct Sequence Spread Spectrum (DSSS) has
been gaining prominence as a radio transmission technique allowing high
traffic volumes to be transferred with a great immunity to
interference.

Direct Sequence Spread Spectrum modulation does not make it possible to
overcome wide band thermal noise. However it does overcome narrow
band interference with ease as well as the effects of multi-path interference.
In the power station environment there noise of all types. Switching are a
large problem although they tend to be wide bandwidth with little auto-
correlation. Modern control systems like most computerised equipment
create a large amount of highly correlated noise as do mobile
communication devices.

Throughout this thesis there is constant reference to Amateur Packet Radio.
As amateur radio operators have done much of the work on packet
technologies this is inescapable. They are also doing much of the work on
Spread Spectrum Packet Technologies because they are permitted to
experiment without the need for a special licence.

Packet Radio Networks are currently being used quite extensively although
their penetration is nowhere near that of other mobile services such as
cellular telephone communications, point to point microwave connections
and satellites.

Packet Radio is being used together with the normal voice communications
by taxi and courier companies allowing bookings to be electronically
transmitted to each vehicle. During the Gulf War, Packet Radio was used
by the United States Military to transfer commands to field officers with
Terminal Node Controller's (TNC's) connected to their secure SatComm
satellite radio's.

Except for some subtle differences with addressing in most cases the
system used by these organisations an X.25 variant known as AX.25.
AX.25, however does not make any reference to the actual physical
hardware. Provided the data is transferred end to end in packet form the
physical medium is of little concern.

The most common method used is a modified random Aloha where a
Carrier Sense is used on receivers. Commonly Narrow Band FM (NBFM)
is used with 1200 BPS FSK modulation.

The system of carrier detect is similar to that used by ethernet. However
there are some major differences. With ethernet the transmitted signal is
constantly monitored for corruption denoted as Collision Detect or CD.
There is no such facility in standard packet radio communications.

The exception is operating packet radio through a full duplex repeater. In
this case it is possible to monitor the transmitted signal. Unfortunately even
when using a full duplex repeater, the transmitted signals are seldom
monitored.

Of course there are options to AX.25 although they exhibit some problems
in terms of usage as well as standardisation. As we speak all commercial
packet radio (eg RDLAP, MOBITEX etc) uses some form of strong
Forward Error Correction (FEC).

The lack of a Forward Error Correcting code in AX.25 is one great
deficiency. The other being that it uses a 'Go Back N' retry algorithm
rather than a selective repeat algorithm. The selective repeat algorithm
would be far better in a radio environment due to the increases in spectral
efficiency.

Phil Karn has implemented TCP-IP operation over the AX.25 protocol
using the Un-numbered Information (UI) frames of AX.25. If AX.25 was
chosen as a basis for Spread Spectrum transmission it would only be useful
to encapsulate an additional protocol. Such a protocol would have FEC,
selective repeat amongst other factors.

Where many links co-exist on the one frequency there is the
tendency for one transmitter to seize the link. For a simplex channel
the p-persist has been added. The retry timer has also been modified
to allow for exponential increase in interference and high channel
usage and automatic retuning when the link improves.

Various ambiguities occur in the existing specification which need to
be removed in version 2.1. These fixes also repair the problem of
some links re-establishing themselves after a disconnect on a
marginal channel.

Not only was this the hardest problem facing the digital committee it
was also the most important. A six character callsign is a problem
requiring many users to operate illegally under reciprocal
agreements where various extensions must be added to the callsign.
In the commercial world the use of six letter is somewhat limiting.
Longer callsign fields would be a great improvement in the
commercial world. At least in Australia, many commercial callsigns
are longer than six letters.

Although in many cases there is no legal need to use the callsign it's
use is preferred. The Committee was unable to come up with a
100% backward compatible system, but was able to come up with a
fallback to the old system when required.

In association with longer frame sizes it was decided to implement a
means of negotiating various parameters such as the length of the
frame. The frame would then be able to be larger than 256 Octets
long.

In addition AX.25 Version 2 has some limitations. It is quite
suitable for data communications however it has some limitations
with respect to error correction. It uses a C.R.C. and it is fairly
reliable. However it contains no facility for error correction.

Because of this lack of error correction on receive it is proposed
that a FEC be added to the standard AX.25 packet.

Any interference caused by two stations transmitting at the same time
causes both packets to be lost in most cases. However, if one of the signals
is received at a strength much higher than the other, the signal with highest
signal level would be correctly demodulated due to the FM capture effect,
presuming that Narrow Band FM (NBFM) is used of course.

It is possible to use a full duplex repeater to listen for corrupted packets
however this becomes expensive and reduces the versatility of packetised
communications. To do this a transmitter and receiver would require a
large amount of filtering to remove the transmit signal from bleeding
straight into the receiver. Where the transmit and receive frequencies are
close this equipment is quite bulky.

In packet situations the case of a hidden transmitter is quite common. The
hidden transmitter is one which not all users can hear and are, therefore,
likely to transmit over. In rugged terrain the problem is increased.

Taking this problem of 'hidden transmitters' to a logical extension the
channel throughput approaches that of standard Aloha with just under 20%
maximum throughput. Where there are no hidden transmitters the channel
is almost constantly utilised with almost no collisions.

Due to hidden transmitters, packet networks tend to be concentrated about
hubs in each geographical area on each frequency. If they were not
grouped, stations would tend to hear only a fraction of the total number of
stations. The challenge therefore is to maintain the use of a single
frequency packet radio system while removing the limitations caused by
frequency usage.

Unfortunately having all stations in separate receive frequencies would
allow stations to independently communicate but would not solve the
problem of two users attempting to send to a station when they cannot hear
themselves.

Therefore, what would be ideal is for all transmissions to all receivers to
be totally orthogonal. However we still require a reasonable bandwidth and
to use the spectrum responsibly. By making all the transmitted signals
orthogonal we are also able to receive more than one signal detectable and
identifiable at the receiver.

One of the potential strengths of packet is as a distributed,
redundant system. Adding a repeater greatly reduces collisions, but
at a significant expense:

the repeater is a single point-of-failure, and many people
will not be able to or know how to operate without it when
the repeater dies

repeater coverage rarely stays localised. After while, a
better antenna, more power, etc. and you wind up with a
wide-coverage packet repeater that is jammed up.

In response Phil Karn of Qualcomm made the following comment also in
the Ham-Radio Digest

I happen to agree with this. Using repeaters to reduce collisions
does involve a significant opportunity cost.
Unfortunately, the alternative techniques to "do it right" are still not
yet known in the amateur service. These include:

Spread spectrum, which creates a channel that degrades
more gracefully with multiple simultaneous transmitters than
does a narrow band channel.

Strong forward error correction coding. By decreasing the
required signal-to-noise(interference) ratio, this enhances the
ability of spread spectrum to tolerate multiple simultaneous
transmitters on a channel. And by reducing the necessary
transmitter power to sustain a link, it also reduces
interference to other receivers.

Automatic transmitter power control so you never use more
power than is actually necessary to reach a particular
node.

Automatic routing algorithms with link metrics based on
power/interference estimates so that paths are chosen on the
basis of their minimum impact on overall system capacity.
That is, you would choose a path of many closely spaced
nodes over a few widely spaced nodes because the much
lower power required at each hop would more than make up
for the increased number of hops.

As Phil Karn works with cellular CDMA at Qualcomm and also has a lot
of the major work on packet radio in the last 15 years I suspect that he has
a greater grasp of the issues involved.

Taking his points individually. Firstly CDMA is a modulation technique
which degrades gracefully as is seen in the CDMA cellular telephone
system. In it the received signal may be 14 db under the interference from
another user. Additional users just add to this interference. However since
the additional users are transmitting a mainly orthogonal signal very little
of the transmitted signal causes interference.

With power control the whole packet system is not being overloaded by
those who feel that more transmit power is the answer to a busy channel.
Again taking the example of the Qualcomm CDMA cellular telephone
system, the voice is sent as packets of data. There are also cases where the
transmit power of the telephone is only 100 nW, or the received energy is
higher than the transmitted energy.

With power control all users are given equal access regardless of the
distance of the transmitter. Unfortunately in my thesis I have been unable
to implement this.

Routing is quite important to overcome the NEAR-FAR problem. Short
links must be used. To this end a protocol such as RSPF could be used.
Craig Small, VK2XLZ is completing a thesis on this at the moment. An
indication of the distance of a transmitter from the receiver can be obtained
by monitoring the BER at the receiver. The protocol needs to be modified
to channel network traffic into low BER channels if possible. The present
RSPF protocol is designed for standard packet radio networks and their
specific problems.

In the early 1980's the Tuscon Amateur Packet Radio Group in the USA
designed a series of PAD's (Packet Assembler/Disassembler) known as a
Terminal Node Controllers. To aid future expansion this TNC had an
expansion port for connecting external modems and radios.

This port contains all the clock signals a modem designer could ever want.
For this reason the author decided that the TAPR TNC should be the basis
of any packet system. This therefore really dictates the use of AX.25 as a
Level 2 Protocol.

This does not affect the ability of the system to operate with experimental
protocols which require unconnected information transfer. The UI frame in
AX.25 allows for broadcasts. Phil Karn, KA9Q has used these frames to
transfer IP datagrams.

Unfortunately the TAPR TNC-2 is getting quite old, being designed in the
mid 1980's using Zilog Z80 microprocessors. Although the Z80 was
mandatory for the CP/M operating system used on most computers of the
era, CP/M has since died and thus the Z80 family is not as popular as it
one was.

With increasing power available the traditional job done by the TNC is
often being done by the computer the TNC is connected to. The KISS
protocol (Described on the next page) is a useful machine independent
protocol for transfer between PAD (TNC) and computer.

Synchronous protocols are the most efficient to be used over radio
networks. The stop and start bits in the asynchronous data communications
would reduce the throughput of the link by at least 25% depending on the
number of stop bits used.

A version of X.25 known as AX.25 has been formulated for use on radio
networks by the American Radio Relay League (ARRL) and the Amateur
Satellite Company (Amsat). It contains a certain amount of overhead but in
most cases it is more efficient than asynchronous transmissions.

An interface from asynchronous to synchronous and back is required as
modems usually require a synchronous signal. In the 9th Computer
Networking Conference, Phil Karn proposed a standard for such an
interface and called it KISS.

KISS is based on the SLIP with special bytes for start and end of a packet.
A kiss controller simply takes asynchronous input from a computer and
converts it to synchronous transmissions and back. It also deals with setting
the speeds of transmission on the synchronous side as well as the transmit
Push to Talk (PTT) on radio links.

For many years the only options have been to use either an expensive SCC
card on your computer or a full external microcomputer known as a TNC.
In the proceedings of the 1989 ARRL computer networking conference,
Henk Peek, PA0HZP presented a universal medium speed packet interface
for the IBM-PC. It is largely based on the 8530 SCC chip from Zilog. The
circuit presented has been modified and even redesigned by others
including DRSI in their PC*PA.

The use of a programmable timer within a TNC is to control the key-up
delay of the system. In systems such as the PI card this timer is able to be
set down to 5 mSec. Relatively few radios can cope with such a short key-
up delay. In fact the author would contest that such a short period in a
Spread Spectrum situation would be almost impossible to obtain without PN
cycle times of under 2 msec. A programmable timer allows a key up delay
that is not CPU intensive which is important for DMA control.

In the past it was said that the Z-80 rules and CP/M would live forever
because of it. In the last 10 CP/M has effectively been killed off but the Z-
80 family continues to be used in many applications where cost is a
primary concern.

Standard KISS does not have the ability to route transmissions going
through it to the specific port. In this situation it is more important to have
one KISS line to each port than to multiplex both signals onto the one line.
In fact the serial links are often the slowest element in the system so by
using one port per radio there is a speed advantage.

Unfortunately there is a great problem with the IBM-PC which is prevalent
at the time of writing. The PC allows only four serial ports. In fact the
situation in most cases is that two serial ports is the limit. This provides a
problem for users.

The current state of the art in terms of interfaces is combined DSP modems
and TNC's. As would be expected with any new product in a limited
market these prices are quite high. Most TNC's are still only designed to
handle a single radio at a time, with the exception of a few top-end TNC's
catering usually to 2 radios.

Cards exist to plug into an IBM-PC although these obviously are limited by
the constraints of the computer bus, processor as well as peripherals.

In 1989 at the ARRL's 8th Computer Networking Conference in Colorado
Springs, Colorado, Roy Gould presented a study of high speed packet
radio[17]. In this article he touches on spread spectrum techniques for
packet radio operation.

He listed some of the advantages of Spread Spectrum packetised radio
as:

Immunity to man-made interference.

Security of information within the channel.

Immediate random access to the channel by a number of
simultaneous users.

Graceful degradation of the signal with overload.

He added however that a great deal of research would be needed to
determine if this was actually practical.

The network topology in relation to a Spread Spectrum Packet Radio
Network directly relates to the assignment of spreading codes. As spreading
codes are orthogonal, just as frequencies are orthogonal with Narrow Band
FM different spreading codes create different virtual links.

Several authors have provided a basis for assignments of spreading codes
within a system [22][24][30]. The three assignments suggested are random
orthogonal codes, common spreading codes and distributed assignment of
spreading codes. A last option on which I could find no references during a
detailed literature review was on individual permanent assignment of a
unique spreading code per receiver.

Where the spreading codes are anything but common to all users the
spreading code becomes an effective address where the only users receiving
the packet are those whose receiver shares the spreading code of the
transmitter. Qualcomm's CDMA system uses a common spreading code for
all users with time offsets giving addressing using the spreading code (See
Appendix 2).

Cartographers have long known that no more than 4 colours are needed to
differentiate different areas of a map. A translation may be made to Spread
Spectrum Packet Radio where very few spreading codes are required for
individual addressing using spreading codes. However assignment of these
codes is not so simple and would need some way for a new link to become
part of a changing system.

The easiest way to do this would be to broadcast using each spreading code
waiting for a response. Another option would be to have a standard
Narrow Band FM channel for assignments although this would be wasteful.

In GPS, units are transferred spreading codes via a very slow spread
spectrum link with a short sequence length [11]. It would be easy to build a
transmitter and receiver for this type of synchronisation into each unit.
Transmission of this PN sequence by the station already a part of the
network need not cause interference as the signal can be randomly
interspersed with the network traffic such as transmitting a slow speed
signal with the Qualcomm CDMA cellular telephone (See Appendix
2).

In effect, distributed code assignment creates a network of short orthogonal
links. As stated in the section on code assignment however the choice of
PN sequences is somewhat limited by the FCC in the USA.

A solution has therefore been posed whereby all users share a common
spreading code [22]. In this situation each packet effectively becomes a
broadcast just as in the present packet systems in use. However it is likely
that simultaneous transmissions will be orthogonal. This can be enforced by
time offsetting the PN codes from all users, although this adds considerable
complexity.

Kim [22] states that at the link level there are two key design parameters in
common spreading code systems to be evaluated. They are:

The expected number of packers captured at the receiver.

The allowable number of simultaneous transmission that are
supported at a specific data bit error rate and probability of
packet capture.

The author of this paper has shown that for the multiple capture model it is
possible to improve significantly system performance by using the capture
property existing in a spread-spectrum receiver.

In other words if Spread Spectrum networks are designed with common
spreading codes the receiver architecture should acknowledge this and offer
the ability to receive multiple Spread Spectrum signals at once. In fact this
is also true in the other cases of code assignment as there will often be the
case that two stations are attempting to transit to a third station.

If the receiver has multiple capture characteristics care should be taken
during the design so that signals are not multiply captured in the receiver.
The logic of the receiver should be able to skip over signals that are
already being tracked.

Comparing all the models available for code assignment it is the author's
opinion that the optimum for a packet network would be using a common
spreading code. The advantages are:

All users know the spreading code in use.

All nodes can be heard without changing spreading codes.

Receiver complexity is not increased as duplication is required
anyway.

Broadcasts are really broadcasting to all nodes that can hear the
packet.

Signals are usually orthogonal due to Auto-Correlation
properties.

There are some disadvantages. They include

Signals are not non-orthogonal at all times causing
collisions.

Security of information of the link is reduced because all users
can listen in.

Alternative Tropologies

The ideal topology for a Spread Spectrum Packet Radio network is in the
author's opinion one where there is no central hub. However there are
occasions where a central hub would be an advantage.

One example of this would be the case of a Bulletin Board System where
users are usually downloading messages and files. In this case the speed of
the down-link is most important. However the industrial environment is
getting increasingly decentralised with remote sites requiring and generating
data.

It is suggested that the spreading code on this topology be hard coded to
simplify the receiver cost.

Classical Spread Spectrum suffers when there are two transmitters in close
proximity attempting to receive a signal from a transmitter much further
away.

Aloha systems with Carrier Detect often suffer from a hidden transmitter
problem where stations on the same frequency cannot hear each other but
both transmit to a third station at the same time colliding causing packet
loss.

The Near-Far problem, if dealt with properly can actually increase the
performance of a communications system. To counter the problem of
Near/Far the systems must be designed for many small hops rather than
few large hops. Provided that each hop is acknowledged in turn, there is
likely to be less problems caused by spread spectrum packet than with
conventional modulation techniques.

But for this to work all the users on the frequency using the same coding
must accept the responsibility to re-transmit packets as required. Failing to
do this would create a situation similar to that of NBFM packet but much
worse.

In their cellular telephone systems Qualcomm overcomes the Near-Far
problem by ensuring that all nodes transmit only to the base station where
the signal is strongest. Although this adds complexity it overcomes this
problem.

In a spread spectrum packet network the protocol should contain
information allowing closed loop power control.

Qualcomm has done some pioneering work on power control of spread
spectrum signals with open and closed loop feedback. Much of this work is
covered by various patents.

In a packet network situation power control is important but more difficult
as transmissions are required to more than a single base station if the
versatility of packet protocols and topology are to be realised.

Each transmitter should only transmit as much power as it needs to close a
link. It is assumed that each node will wish to communicate to more than
one other neighbouring node. Whilst transmitting to the nearest of these
two nodes the furthest node will not be affected, but when the furthest of
the nodes is being transmitted to the nearest will be swamped by the signal
possibly losing any other signals that are being transmitted to it at that
time.

The KISS protocol discussed elsewhere allows for transmitter power control
information transferral in band along with the data. The case of transferring
the received power levels are more of a problem though. Several options
exist. The first option is to use the upper nibble of the KISS address/packet
identifier to transfer the level. However this only contains 4 useable bits,
and these may also be needed by multi-drop kiss.

A more viable alternative is to transmit the level as an 8 or 16 bit number
just before the end of frame synchronisation of the KISS packet. This
would be ignored by software that was not looking for this information,
gaining transparency to the user.

The last option would be to transfer the data as a special packet via the
KISS control packet although there is no guarantee that the correct level
would line up with the correct packet.

The following edited comment came on of the Ham-Radio[34] mailing lists
on Mon, 18 January and 8 July 1993 from Glenn Elmore, N6GN.

" I'm implementing DS spreading in my second phase of
higher speed radios which are to be part of the "layer 3 TNC"
we're working on for user access to a higher speed wide area
amateur digital network. This is being done to help combat multi-
path on less than optimum paths. I haven't yet found the limitation
of spreading codes; the particular 7,13 and 19 bit sequences
specified by the FCC, to be too much of a problem. Since I'm
already using a moderately wide information bandwidth, pushing 1
Mhz, I run out of spectrum within the band before I run out of code
length.

" I've had good luck using differential ECL logic (10116
variety) to drive DBMs directly. They have adequate current
capability along with good balance and speed. I've used this to
direct sequence modulate a variety of Schottkey diode mixers. I am
interested if you have a good and simple discrete transistor design
though.

" My spreading sequence operates synchronously with the
carrier/pilot and data clocks. Therefor, once I have acquired PN
synchronisation (by software rather than a hardware loop) and have
locked onto the pilot tone, everything stays locked and synchronous
and I also have all data clocks recovered.

" I'm generating the carrier, at 1265 Mhz, in one half of a
dual PLL chip. The second half is used to phase lock the master
VCXO (at 31.47 Mhz) to the received pilot tone. The carrier
oscillator uses a coaxial line resonator and results in very low phase
noise. See my 1988 Ham Radio Magazine microwave series for a
similar design.

" The goal of the radio is 250 Kbps data to the user. See our
paper in the 9th ARRL CNC for a description of the Hubmaster
protocol which supports this. The addition of spread spectrum and
fully synchronous and coherent radios will require some additions to
this protocol but the fundamental operation is similar. "

In the past few months, Unisys of Salt Lake City, Utah, has released an
Application Specific Integrated Circuit (ASIC) described as a "Spread
Spectrum Demodulator" [31]. This Integrated Circuit has the capability for
data rates up to 64 Mbps, chipping rates up to 32 Mcps, soft and hard
decisions, AGC and up to 48 Db processing gain. Availability of the PA-
100 integrated circuits along with the EB-100 and EB-200 development
boards is unknown. The documentation, electronically obtained, is dated
March 1995.

Put simply this integrated circuit has made the author's work on hardware
redundant except for it's educational value. The device can operate at either
RF provided the centre frequency is relatively low or at an I.F. using a
down converter. It operates by digitising the incoming waveform, tracking
it and despreading it.

According to the Technical Data Sheet and User's Guide common
applications would be Satellite modems, Personal Communications systems,
Wireless Networks and Cellular radio system. From the preliminary
documentation it appears that the development system is designed to be
used in association with microsoft windows software provided.

Interestingly the data from the manuals contains information on an epoch
for the spreading code. That is the spreading code must start on a bit
boundary, and the spreading code may be truncated to ensure this.
Unfortunately this would be equivalent to resetting the sequence making.
The epoch detection would limit the case of a symbol being transmitted
with all 1's. The author must assume that the epoch function can be over-
ridden.

Another strange detail about this ASIC is that it allows for chip-rates equal
to that of the bit-rate. In that case, the gain by using spread spectrum
technologies is certainly not as high as with a higher chip-rate.

For operation as a spread spectrum receiver a down-converter is required
to reduce the frequency of the received signal. The down-converter [32] is
able to convert the centre frequency low enough for the ADC but still high
enough to so that information is not lost in the conversion process.

Unlike the Qualcomm CDMA phone system, the PA-100 does not have
multiple fingers enabling the chip to simultaneously track multiple signals.
The PA-100 may be able to track two BPSK signals independently but is
certainly unable to track two QPSK signals independently. The lack of
multiple fingers leaves it more sensitive to multi-path interference as well
as increasing the difficulty level associated with inter-cell handoff's as
required to decrease near-far problems.

The circuit as described in the next section implements the early-late
synchroniser by delaying the incoming DAC signal and then despreading
rather than using two despreaders. The circuitry required for a despreader
is somewhat more complex and therefore probably more expensive than the
delay line.

The cost of the PA-100 is approximately $US165 with the price dropping
to about $US65 for quantities of 100 at the time of writing. The
development boards are worth about US$5000 each.

An export restriction has been placed on some of the software associated
with this integrated circuit. At the time of writing it is unsure if the
integrated circuits themselves may be exported from the USA. These export
restrictions are based on a treaty aimed at slowing the flow of technology
behind the Iron Curtain. Whilst the Iron Curtain has collapsed the
munitions export regulations have not, leaving many products with zero
export market.

It should be noted here that the same regulations apply to Australian
exports of `munitions' such as codes, ciphers and decipherers.

The preaccumulator is also processed with slightly early and slightly
late PN codes. These codes are exactly one sub-chip early and late.
In practice the incoming data from the de-spreader is subtracted
from the data delayed by two sub-chips. This can then be put
through the despreader with a PN code one sub-chip ahead of the
desired tracking point. The output has a zero-mean output in lock
conditions.

The phase/level processor accumulates the outputs of the despreader
over symbol times, scales the results, makes data decisions, and
provides outputs for use by the PN sequential detector and the phase
locked look. In addition the inputs to the phase accumulators are
inverted during the last half of the symbol time to produce a
frequency discriminator function.

This is a 1st order digital filter that may be used to form a 2nd
order timing recovery loop. The output of the filter is a sample rate
command that can be used to control an external clock generator for
generating the system clock.

The PN sequential detector is used to acquire the PN code and
monitor the signal level after code acquisition. It consists of data
removal circuitry, bias subtracter, coherent accumulator and an
acquisition/tracking controller. This circuit operates by attempting to
lock onto a signal, with the PN rate as close as possible to the
transmitted rate, and then adding slip pulses rather than modifying
the frequency of the PN signal in out of lock conditions. If the
frequency of the regenerating PN and the transmitting PN are not
similar the tracking loop can handle that.

Put simply this chip does everything that the author's thesis does, and does
it better and much faster. However it does verify that many design
decisions by the author will work in practice. The use of the slip generator
added to a PN sequence which is not locked to a carrier.

At the beginning of work on this work it was thought that Frequency
Hopping would not really be suitable for work with data communications.

Although a Frequency Hopping system might be useful in voice
communications it is less useful for data communications. In a system
without protection against multiple sequential error bits a Frequency
Hopped system would not be viable.

Frequency Hopped Spread Spectrum works on the assumption that although
some of the message is destroyed there is usually enough redundancy to
determine the message. This is certainly true for voice
communications.

When Frequency Hopped Systems are phototyped they are usually done
with a single transmitter and a single receiver with Phased Locked Loop
(PLL) frequency synthesis. Due to locking characteristics of PLL
synthesisers there is often a large period of time when the transmitted
signal's frequency is stable. On the receiver a similar problem exists where
the frequency it is attempting to receive is highly unstable.

To reduce the dead zone between frequency hops at least two PLL's are
required. One holding the present frequency and another holding the next
frequency in the hop sequence. This would reduce dead zones to the
vicinity of 1 mSec.

For low data speeds with error, correction data communications should be
possible using a Frequency Hopped Spread Spectrum system. In fact during
the Gulf War the allied forces used AFSK.

Frequency Hopping systems should become more popular in the next few
years. GSM mobile digital telephones gaining acceptance in Australia uses
a form of frequency hopping.

According to a Manager of AWA in their Military Products Division, it
should be possible in the next decade to perform digital Signal Processing
on radio frequency signals. When this happens, Frequency Hopped Spread
Spectrum for digital communications should develop beyond our wildest
dreams.

The antenna is the one component of the system where a small cost
increase can reduce the bit error rate significantly. However the antenna
system is a tradeoff between directivity, size and gain.

The directivity of the antenna system is an important factor in dimensioning
the network. Cellular Telephone systems are designed around uni-
directional base station antennas to allow maximum frequency re-use.

The gain of a transmitting antenna is only a function of the efficiency and
the directivity of the antenna. As gain of an antenna increases in one
direction, it decreases in another direction. High gain transmission antennas
will not necessarily give better results. High gain antennas will only have
this gain in a particular direction, with a very poor signal in other
areas.

Receiving antennas however do not follow this rule, and can have high
gains without the resultant minima. It therefore remains to be seen on what
type of antenna array would be required.

Whilst looking at the PN sequences available legislation must be taken into
account. It is not always possible to choose the ideal sequence or set of
sequences because of legislation.

There is some common notation for PN sequence identification. The
sequences are often generated by a shift-register using feedback. The PN
identification notation indicates which bits are modulo-2 added and fed back
to the input of the shift-register.

As an example the [7,1] sequence is generated by modulo-2 adding register
bits 1 and 7, inverting this and applying this to the input of the shift
register.

The United States has the largest English speaking Amateur Radio
population in the world. It also needs to be understood that they, as a
group, have the ability to bring spread spectrum technology to the user. At
the moment they are limited to only three sequences, [7,1], [13,4,3,1]
& [19,5,2,1]. These sequences are also suitable for the FCC Part 15
requirements for Spread Spectrum transmitters operating in ISM
bands.

This makes any form of variable assignment of sequences impossible. It is
possible to assign each station a destination address which is a time offset
from a reference sequence. In this case it would be possible to have a GPS
time code provide synchronisation information at lower bit-rates. However
this adds complexity and cost.

Under Australian law any spreading code is permitted,
although there is little use in designing a transmitting system where the use
outside Australia is limited.

The author has therefore chosen the [7,1] sequence as it is only 127 bits
long requiring a short synchronisation time. This comes to about 8 bits per
epoch, which reduces synchronisation time but has disadvantages with
interference from other users.

The following message appeared in the Packet-Radio Digest[34] list on 16
Jan 1993

I arrived at more or less the same conclusion that SS was a
good avenue for future packet development, primarily because
direct sequence spread spectrum is probably one of the cheaper
ways to 'fix' the multipath problem in high bit rate packet systems.

Rick Spanbauer, WB2CFV

SUNY/Stony Brook

Long codes are unfortunately vary difficult to synchronise to. This is
especially true in a Packet Switched Network where connections are made
when packets are needed to be sent.

Short codes are relatively easy to synchronise to but they suffer from a
problem similar to jamming. When a CDMA receiver is in search mode it
will usually lock onto the first signal it finds that has the correct signature.
It may be one of many signals transmitting at the same instant.

The following statement was made by Phil Karn of Qualcomm in
early 1993 being asked on the TCP Group[34] mailing list of the patent
situation with CDMA

Generic, basic CDMA (i.e., multiple spread spectrum
transmitters sharing the spectrum) has been around for a long time
-- since World War 2 -- so any patents on it have long since
expired. Qualcomm's patents cover only some very specific
implementation details on applying CDMA to cellular telephony,
particularly the closed-loop power control scheme.

Graphically analysing a PN sequence of 7 bits I came to the following
conclusions

The number of agreements one or more chips from perfect
correlation is equal to 3, and the disagreements are equal to 4.

The number of agreements when perfectly synchronised is equal
to 7, with no disagreements.

The following equation was derived for the case where signals are
within one chip of being synchronised.

Where

D = number of disagreements

DELTA = fraction of a chip from agreement

The correlation function can be graphed as a function of the number of
agreements, or as a function of the number of agreements minus the
number of disagreements

The closest approximation to the hardware is using the A - D since it gives
results closer to zero. The hardware does not go negative, but does go
down to 0 volts for no correlation.

If the PN sequence is inverted but synchronised, both A and A-D will give
results that indicate that no lock is close. In fact this is to be expected.
Before we can use a delay locked loop we must remove the phase
information leaving only magnitude.

Unfortunately there is no easy way to remove the phase information on a
digital baseband signal. One option is to take a digital derivative of the
difference between the incoming signal and the PN sequence. This function
will tend towards a minima under synchronisation, inverted giving a
correlation function that has phase removed.

This however requires a quite accurate local clock for the delay elements to
acquire and maintain lock. It also makes the synchronisation much more
succeptable to noise.

"A sine curve goes off to infinity or at
least to the end of the blackboard"

Prof. Steiner

Photo 1

Before coming up with a final design for the thesis many designs were
investigated. Although it is not intended to present the complete failures,
the author feels that he should indicate where mistakes were made.

The spreader and modulator simply modulo-2 added a generated PN
sequence with the incoming data, and then sent the output to an off the
shelf PSK modulator. This circuit should have worked if it had been
built.

It was intended to use an off the shelf PSK demodulator to demodulate the
modulated PN sequence. The output was then modulo-2 added with a
reconstructed PN sequence and sent to a state machine for lock detect and
data recovery. If the despreader was not locked, a 'SLIP' pulse was added
to the PN generators clock at set intervals until lock was obtained.

There were two major problems with this design. First is that generation of
the PN sequence relied on a clock from the demodulator at the bit-rate. In
addition the PSK demodulator are not intended to operate in high noise
environments which is where spread spectrum excels. PSK modems need to
find a clock to synchronise to, which would be difficult with high
interference levels. Therefore the PN generator also has a highly unstable
clock leading to more synchronisation problems.

This design might work if a transmitted reference were used such as from a
radio or television station so that a stable clock could be obtained.

Although this would have been spread spectrum, we are in fact transmitting
symbols at a rate sixteen times greater than the bit-rate. It could be shown
that there are many channel codes that could exhibit far better bit error
rates for a given signal to noise ratio.

After realising that the first design would not work, a circuit was developed
that on paper would work, using mixers and filters. Although this circuit
did not work in the lab, the reasons for this not working are not difficult to
rectify given time.

Specifically the power level to the spreading mixer is limited to 0 dBm.
Thus after the passing through mixers, splitters, pads and filters the level is
in the vicinity of -60 dBm. This level is un-suitable for the diode detector
used. To increase the level microwave amplifier circuits need to be used
throughout the RF signal paths.

The diode also appeared too insensitive to the signal level. Increasing the
level at the diode using distributed amplification should improve matters.
Should that fail, an Op-Amp configured as a precision rectifier could be
used subject to gain-bandwidth constraints.

Due to earlier problems and lead times on the production of Printed Circuit
Boards, updated design needed to be completed over a single weekend,
with no time for full circuit evaluation. Information on the losses exhibited
in the mixers and filters did not come available until after the design had
been completed

The signal level at the input to the diode detector needs to be 0.6 volts for
an un-correlated signal to about 3 volts for a correlated one.

In addition a Voltage Controlled Oscillator was implemented using an Exar
2206 VCO after the circuit was designed. During testing it was found that
the time constant of this circuit was quite large, making the VCO less than
ideal for this application. To improve this situation, the low pass filter
needed to be removed from the circuit.

This circuit also has problems with the use of a capacitor to provide
+- 25 mVolts on the I.F. input of the mixer to change the phase of the
output. Unfortunately this design was taken from another circuit with a
much higher chip rate. Unfortunately the 3db pad to ensure a correct level
and impedance limits the energy stored in the capacitor. That in turn causes
problems with the circuits operation.

A solution would be to bias one of the I.F. pins to 2.5 volts and a series
resistor between the other pin and the digital modulated PN signal.

It is interesting to note that although the author has seen no actual circuits
using the crystal filters, Ziemler[6] provides a theoretical model of the this
situation. Unfortunately the mathematics involved are quite complex.

Another circuit was developed. It relies on a TTL signal and uses early-late
correlator to synchronise to the signal. It has a slight modification in the
level detecting circuit with a 10K resistor placed in series with the diode to
improve signal levels in the digital circuitry.

This circuit only works with a received PN code that contains no data on
it. However it does prove that the delay locked loop circuit will operate
correctly given amplification.

Generating a coherent reference for demodulation of a DSSS signal is
inherently difficult due to the extremely poor signal to noise ratios. In
addition coherent early-late synchronisers have difficulty synchronising with
modulation data. Neither of these difficulties is present in non-coherent
early-late tacking loops.

Firstly the non-coherent tracker contains two energy detectors which are
not sensitive to carrier modulation or phase. With minor modifications this
loop may be used on any direct-sequence modulation scheme.

The Early-Late Tracker or Delay-Locked Loop operates by comparing a
PN sequence 1/2 a chip before and after the PN sequence of the
demodulated channel. By comparing the input signal with these two time
offset PN sequences it is possible to maintain lock on the main signal even
in case of oscillator instability and multi-path interference.

However the normal model for the Early-Late synchroniser may be unable
to lock under certain cases of oscillator frequency because the composite
early-late signal has an output of zero under lock conditions, but also at
points more than 1/2 a chip from lock.

The composite output is useful in maintaining synchronisation but less
useful in initial synchronisation. Therefore the lock condition occurs when
the correlation outputs of the early and late paths are both non-zero.

To search for a signal it is possible to use a single correlator to determine
the location of the signal. As three correlators are used in the Early-Late
synchroniser it would be trivial to use all three separately searching for the
signal drastically decreasing the search time. Once the approximate position
of the signal was located with a correlation greater than zero it would then
be possible to use all the correlators to lock the signal and maintain
synchronisation.

What is proposed is partitioning the receiver into two distinct modes, hunt
and track. During HUNT the early and late receivers are given a code that
are equidistant from each other. When one of these receivers indicate a
lock the mode changes to TRACK where the central receiver changes mode
to the code of the signal that was locked and the Early and Late receivers
return to their normal value.

This would be in effect be three tau-dither circuits searching for a signal.
When a candidate was found it would be possible to use either two
correlators in an early-late configuration in attempting to lock on or attempt
to lock using a single correlator still in tau-dither mode. This would allow
the other two correlators to look for another signal in case the one found
turns out just to be temporary noise.

However a dual mode synchroniser was not implemented due to time
constraints. A dual mode synchroniser would ideally require some
intelligent form of control circuitry such as a microcontroller.

Phase Reversal Keying (PRK) or Bi-Phase Shift Keying (BPSK) is a simple
modulation technique which involves transmitting a 0 phase shift for a 1
and a 180 phase shift for a 0. When used in conjunction with NRZI the
encoding transmission of 1 and 0 may be arbitrarily swapped.

This diagram shows all the parts of the modulator. An oscillator provides a
clock signal for the PN generator at. This PN signal is then mixed with the
carrier as well the data. In the case of transmitting this signal at RF, the
PN sequence is modulo-2 added to the data and then mixed with the
carrier.

When operating at base band, the mixer involved with the oscillator is
simply removed.

In accordance with FCC regulations in the United States of America there
is no facility in the PN generator of the modulator to reset the PN
sequence. The PN sequence is derived from a simple digital feedback loop
and is clocked at a rate of sixteen times the base clock frequency.

The sixteen times clock signal is applied to the PN. This circuit assumes
that the data only changes on the transitions of the unity clock signal.

In the circuit that I am presenting here as a working circuit the transmitted
waveform can be given by the following equation.

It should be noted that there is no data modulation being placed onto the
PN signal due to design simplification in the demodulator. Therefore the
actual transmitted signal become

The Spread Spectrum Despreader and Demodulator

Assuming that there is an accurate PN signal at the receiver, the signal
becomes

The received signal is modulo-2 added with two PN signals. These PN
signals correspond to early and late PN lock limits. The waveforms after
mixing become

where

Tc is the chip time

Te is the timing error

After filtering and differencing, the time averaged signals become

It can be seen from this equation that the error voltage to the VCO is still
dependant on the incoming modulated data. However this is what I have
implemented in hardware.

To extend this it is necessary to remove the data from the recieved
waveform. This can only be done on a digital signal by actually
demodulating the data. This involves a delay of one bit time, Tb, to make a decision on the data. We therefore gain the
modulation data one bit time too late to be able to apply it directly to the
early-late synchroniser. The block diagram appears on the next page.

Therefore we must delay the signals going to the early and late detectors by
one bit time, modulo-2 add the data and then perform the same processes
as before. This extension however has not been implemented in
hardware.

The received waveform is received. It is assumed that it has been down-
converted to the correct frequency. This signal gets mixed with the
recovered PN signal.

These signals then get passed through band pass filters with a bandwidth
sufficient so that 95% of the energy from m(t) is passed through. The loss
of the filter is being totally ignored here as it is produces a proportionality
constant.

After going through the filter, the signal out will either have a large
amplitude indicating close to synchronisation, or a low amplitude signal
indicating that the signal is completely out of lock.

This signal still has the phase (or data) information. The phase information
can be removed most easily by envelope detection of the signal. After
envelope detection this yields.

In practice the BW3dB of the filters was about 2660 Hz. At
+- 600 Hz we can assume that the signal is no more than 1dB down
from that at the centre frequency.

The equation for PSK data transmission is given as

where

It can be seen that there are two lobes with symmetry. The lobes are
centred at ± 600 Hz from the carrier frequency with the receive filter
filtering at ± 1330 Hz. Thus about 94% of the transmitted energy is
contained in the pass-band of the filter.

It can also be shown that only about 27% of a spread PN transmission
would occur within ± 1330 Hz. (The 27 becomes 13.8% if the
spreading makes ± 19K2Hz).

Thus the detected power of the data is about 5.3 dB higher than the
detected noise level assuming an equal reception power before the filter. If
the filter was approximately 1300 Hz wide rather than 2660 Hz the
detected power would have been about 8 dB higher than the power of the
spread data.

However a tighter filter would have been more expensive and resulted in
greater losses in the pass-band. The filters chosen were once sold by Dick
Smith Electronics and were on loan from Clive Pickup VK2DND. His
experiments at the CSIRO department of Applied Physics resulted in the
following results as to their characteristics

3 dB band edges 10.69483 MHz and 10.69217 MHz yielding 2660 Hz BW

7 dB insertion loss at centre frequency.

The filter is model 10622E1 KDS 6B, with a centre frequency of 10.6935
Mhz and is an 8 pole filter. This would translate to a lower sideband
normally. However in the circuit the filter is used at its centre
frequency.

Due to mass production the cost of hardware involved with television
reception is quite cheap considering the amount of circuitry involved. It
therefore makes sense to base any designs on building blocks normally
incorporated into television receiver design. Given that these transmissions
are vestibule side band, they would be ideal for spread spectrum.

A possible building block is the Dick Smith Electronics Television Field
Strength Meter. What makes this project ideal for spread spectrum
communications work is the IF output from the first tuner module. This
would enable a single tuner to be used with several IF modules. It would
also allow for a phase inverter to be placed after the tuner and before the
demodulator.

The circuit is fairly simple due to the high level of integration in the tuner
modules. A large portion of the circuit as presented is redundant in this
case as it will not be used as a field strength meter. I have modified the
circuit given to include IF in and out connections which should be
connected for normal operations. I have also modified the circuit for
connections to the AGC and the AFC.

One disappointment was the frequency range of the receiver. With a lower
limit of the UHF TV band at about 470 Mhz the receiver does not operate
in the Amateur 70 CM band. It does operate in the 50 CM band although
there is a somewhat limited life for this band.

The circuit includes a Switchmode Power Supply for generation of 36 volts
required for the tuner. This power supply appears to be well shielded and
causes no problems to the circuit.

Disappointingly the whole construction of the case is plastic. It therefore
would show little resistance to interference from nearby electric
fields.

On the IF module there is an Automatic Gain Control (AGC) input that
might need to be connected. It would be relatively simple to place a mixer
before the antenna input allowing spread spectrum demodulation.

To maintain control accurately 3 tuners would be required increasing the
cost. Alternately the early and late signals could be derived using mixers
and filters exclusively much as in the prototype design.

The demodulator is probably the most important part of the spread
spectrum system. It certainly contains the most complexity. It may be
helpful to read this description of the demodulator with reference to the
block diagram.

When a signal is received at baseband it has most of the signal about 40
kHz removed as there is little information content within this band. This
signal is then fed through a comparator for clock recovery and an amplifier
for the information recovery.

Demodulation is relatively straight forward involving the use of a Carrier
Recovery circuit and multiplying its output with the incoming signal. Then
the sum is integrated over a bit period before a decision is made at DC
where positive signals represent 1 and negative represent a 0.

This circuit does not actually recover data, although recovery of the data is
relatively simple. Again this diagram is common between both the RF
configuration and the baseband digital configuration. In the case of the RF
configuration, the mixers are balanced mixers, the filters are band pass
filters, and the detector is an envelope detector.

In the digital baseband case, the mixers are modulo-2 adders, the filters are
low pass filters as are the detectors.

The Tc/2 elements are delays of 1/2 the chip time, with the prompt PN
output between the two delay elements.

The receiver requires a 19.2 Khz signal for the PN generator clock.
Further the digital delay line for the PN output requires a clock frequency
of 8 times this. That is we require a clock of

This clock frequency is generated by a Monolithic Function Generator
integrated circuit from EXAR, driven by a filtered signal from the early
and late level detectors.

The XR-2206 is a 16 pin IC commonly used in analogue modems, although
it was more popular before modem chips such as the 7910 was released.
The design calculations following are based on the equations from the
appropriate Exar Data Book. Unfortunately the exact date of publication is
not known as only an extract was available.

The data sheets also give us an appropriate value for the resistance
R.

The following equation was incorrect in the data sheet, with misplaced
brackets. Now, including an offset the frequency is given by

We require about +- 10% frequency variation. This happens when
Vc = 0 and 6 Volts for maximum and minimum
frequencies respectively.

There are three discrete conditions found in the delay locked loop after the
modulo-2 adder despreaders. They are when

Both outputs are equal; Found when either in perfect lock or out of lock.

One the late output is high and early output is low; and visa versa.

Of course when the signal is perfectly in lock we would like the VCO
frequency to be as close as possible to the generation frequency minimising
jitter on the PN signal. Out of lock we need the frequency to be close to
the transmission frequency, with a small amount of frequency offset.

Without this small offset, the system would never lock as the loop relies on
the principle of beat frequencies. An option is to change the frequency of
the VCO by adding slip pulses when the system is out of lock.

With this in mind it should be possible to set up the system so that the free
running frequency of the system with no input is equal to the anticipated
frequency of the incoming signal. However there are two tuning
adjustments that may be made, and these are nor orthogonal.

Thus we need to set the frequency of the VCO with the early output high
and the late output low. This is the condition to tell the oscillator to slow
down. Thus we should adjust the frequency under these conditions to be
slightly lower than the centre frequency. And exchanging inputs should
return a VCO frequency slightly higher than the centre frequency.

These two frequencies are the upper and lower limits of the lock range.
Therefore they should be set wide enough to lock to the signal, and narrow
enough to minimise capture time.

Two major criterion are used to verify the operation of this circuit. They
are

Lock condition after parameter tuning

Ability to re-lock after interruption in the signal

The second of these is the most important. If a signal can be locked by an
operator manually adjusting the parameters this does not mean that the
tracking loop is operational. But if the circuit can re-lock after being
interrupted, the circuit must therefore be working correctly.

It has not been possible to verify the lock range of the circuit because of
the lack of stable test equipment. In the laboratory it has been possible to
establish lock after loss of the signal. Unfortunately the most common
reason for loss of lock appear to be transients cased by power supply
problems.

There are a number of adjustable parameters in the circuit design. First of
all there is the DC offset added for centre frequency control as a bias. The
next parameter is the frequency of the loop filter. Then comes the natural
frequency of the VFO as well as the DC reference level. Further on there
are the Tap Points for the PN code.

Not all these parameters are orthogonal. Changing either the DC offsets
will change the oscillation frequency where no signal is present. Changing
the tap points may require adjusting the frequency of the feedback loop
filter.

Van Der Jagt, L., "A Framework for Specifying a Physical Layer and
Medium for Wireless Local Area Networks," The Third IEEE
International Symposium on Personal Indoor and Mobile Radio
Communications Proceedings, pp. 149-152, Oct. 1992.

Vannucci, G. and Roman, R. S. "Measurement Results On Indoor Radio
Frequency Re-use at 900 Mhz and 18 Ghz," The Third IEEE
International Symposium on Personal, Indoor and Mobile Radio
Communications Proceedings, pp. 308-314, Oct. 1992.