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\chapter{Introduction}

This thesis primarily deals with calculation of a channel capacity for
the human body in the context of the PAN system as developed by Tom
Zimmerman and Neil Gershenfeld.  A brief description of the system
will be provided here, and a more thorough discussion is in \cite{tzpan} and \cite{tomthesis}.

\par

\begin{figure}
\begin{center}
\mbox{\epsfysize=2in \epsfbox{geom.eps}}
\end{center}
\caption{Typical PAN Geometries}
\label{pan:geom}
\end{figure}

\par

PAN, or Personal Area Network is a system for data transmission
through the human body.  Each of the transmitter and receiver is
configured with two electrodes.  One of these electrodes is in a
geometry which is more strongly capacatively coupled to the body than
to the ground.  The other electrode is less strongly coupled to the
body and more so to absolute (room) ground.  In all points of a PAN
transmission, capacative coupling dominates.  In experimental work,
typical transmission voltages were fives or tens of volts.

\par

Two typical geometries for PAN are shown in Figure ~\ref{pan:geom}.
In {\bf A}, the first receiver electrode is nearly in contact with the
bottom of the foot and the second electrode is nearly in contact with
the ground.  This provides the desired symmetry breaking about as
ideally as possible for the human body.  In {\bf B}, the first
receiver electrode is in near contact with the body and the second
electrode is somewhat further fromt he body, and hence less well
coupled to the body, and with a slightly stronger ground coupling.
This configuration is less than ideal (as will be shown in more
mathematical rigor later), however is a usable arrangement.  For both
configurations shown, the transmitter is beneath the foot, as for the
reciever in {\bf A}.

\section{Motivation for the PAN system}

The PAN system is motivated in the same way as many wireless
technologies: It aims to enable communication of a sort otherwise not
possible.  PAN has a number of advantages, disadvantages and synergies
when compared to traditional wireless, such as radio-frequency (RF) or
infra-red (IR).

\par

PAN's advantages and disadvantages primarily stem from the fact that
it is not a radiative form of communication.  Due to this, PAN
transmissions are somewhat private.  This makes it very poor for long
distance communications, but means transmissions between devices on
the body will remain private to the body, making eavesdropping
substantially more difficult.  This also resolves many channel sharing
issues, as the number of communicating devices on a body is going to
be limited to the dozens, on the high end.  This makes it much easier
to do channel allocation, as contrasted with a room size, building
size, city size or nationwide system which may have to deal with many
dozens, hundreds, thousands, or millions of devices.  Additionally,
PAN can be used in a fairly traditional network model in cooperation
with other wireless strategies, by having all devices on the body
capable of transmitting via PAN, and individual ``router'' devices
which can retransmit messages off the body via some other mechanism if
need be.

\par

A survey of applications of this system can be found in Appendix \ref{app:panapps}.


\section{Experimental and Apparatus History}\label{intro:history}

The original PAN prototype was developed by Tom Zimmerman, used
on-off-keying (OOK), and ran at 300 baud.  A transmitter and receiver
were built, and each was a one-off single task device.

\par

A second design and protoboard implementation was done by Chris
Turner.\cite{ctpan} This design also used OOK, however each device was
a tranceiver and the devices ran at 2400 baud.  The design was
based on a series of gain stages, a pair of quadrature integrators
followed by digital demodulation in a PIC microprocessor.

\par

Further development of a modified version of Turner's design was
carried out by the author.  Both OOK and differential binary phase
shift keying (BPSK) were used for encoding, with limited success.
This design was built on PCBs, however the design ultimately failed
due to noise rejection problems.  Attempts to resolve this problem
through use of an analog gain control (AGC) were made, but the AGC was
was also abandoned in favor of the final design.

\par

A final hardware design by Matthew Reynolds, employing a
phase-locked-loop (PLL) for analog demodulation was implemented using
a frequency shift keying (FSK) encoding scheme.  This design, as of
April, 1997, has shown the most promise, and has proven reliable for
speeds up to 9600 baud in certain geometries.
