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Time Division Multiplex in Amateur Radio
Steve Sampson, N5OWK, March 1992
Public Domain (p) 1992

Abstract

The current practice of one radio frequency for every
communications channel is a waste of resources. Amateurs should
begin experimenting with new modes which share a frequency among
many stations. If a channel is not carrying more than one
conversation or data transmission, it will be too inefficient for
future spectrum requirements. Radio systems ten years from now
should be designed to compress as much information into each
frequency channel as is practicable.

Introduction

Any Amateur knows that when 100 people try to call one station in
a pileup, that chaos reigns.  There are systems however, where
this is not true. One hundred different communication channels on
one frequency are possible. Rather than all 100 channels going
simultaneously, each is given a time slice to conduct its
business. This is referred to as Time-Division Multiplex (TDM) or
Time-Division Multiple Access (TDMA).

In this paper I will discuss a hypothetical 9 channel four time
slot system, more for simplicity of design than anything else. In
my petition to the FCC (dismissed) I also wanted the multiplex system
to carry data as well as voice. So I will propose here that we discuss
a system that combines data and voice over the same channels.

Multiplex Theory

A good example of TDM is your normal conversations on the local
repeater. As one station finishes, the next begins, and so on.
The time division however, is random in both length and start
time. Only one user can transmit at a time (more can transmit,
and usually do by accident, but only one person is intelligible).
The person transmitting is said to be occupying the total
bandwidth of the channel. Since the channel is busy even when the
operator is silent or composing another thought, it is also a
slow and inefficient system. The advantage of course is that
simple technology is very inexpensive, and having more spectrum
than users in most of the country, allows us to get away with it.

TIME SLOTS

If we could give each operator 10 milliseconds and then switch to
the next user, we could time share the channel. Computers do this
every day. They can have 100's of users who think they have the
computer to themselves, but each is actually being given a time
slice of the total power of the machine. As a further
improvement, these computers wait for the operator to strike a
key or request output before they even provide a time slice. So
99 terminals sitting at a desk while everyone is at lunch, demand
no computer time, while the last terminal in the basement used to
play chess by the janitor gets all the time. As these workers
come back from lunch, the janitor gets less and less time.
Worse-case is 1000 milliseconds (100 users times 10 millisecond
slices) or one second before his next time slice.

DIGITAL AUDIO

We live in a time when many things done by analog electronics in
the past, are now done by digital electronics. The analog radio
transmitter and receivers are connected to digital processors, or
merely transport digital information. Many radio stations use
Compact Disks that store digital information, and convert this to
analog for broadcast. What they are really doing is wasting their
high resolution recording by converting it to a stream of low
resolution analog audio. They could just as easily transmit this
digital directly to the user, and have the user make the
conversion with much greater fidelity.

COMPRESSION

Music is very hard to compress, because it has information during
the whole transmission. Voice on the other hand has great pauses
and vocal repetitions. The sentence "I want a cookie" can be
compressed several ways based on local dialect, but in every case
would take a fraction of the normal time to transmit. This is the
basis for getting the voice to fit into the TDM time slice. The
audio is converted to digital, analyzed, and redundant parts are
compressed. This is then transmitted during an assigned time
slot.

Your telephone for example, is multiplexed along with many other
users. The analog voice over the twisted pair is converted to 14
bit digital samples. These are then quickly coded into 8 bit
Pulse Coded Modulation (PCM) samples. This is the first step in
compressing the information. Each sample is then converted to
serial bits and dropped into the correct time slots. These are
then recombined at some distant switching center. Your voice is
first passed through a band-pass filter that removes all
frequencies except those necessary for voice. (300 to 3500 Hz).
The analog voice is then sampled at 8 kHz, producing one 8 bit
digital word every 125 microseconds. Since 8 bits times 8 kHz is
64k, this is commonly referred to as a 64 kbps interface. Notice
that it is just a bit greater than the authorized 56 kbps in Ham
radio. But you don't want to use these raw data rates for voice
communications anyway.

SYNCHRONIZATION

A good data rate to begin experimenting is 9.6 kbps. Since 9.6
kbps divided by 8 bits is 1200 bytes, I propose we design a TDM
system that converts audio to 8 bit bytes and has 4 time slots.
This produces 300 bytes per slot (some of which may be used for
synchronization or guard bytes). Each radio is assigned by the
operator to a time slot and mode. When the radio is first turned
on it looks for a data clock on channel 0. This is usually
transmit by a master time station at a high elevation (an
encoded time burst every couple of seconds). The sync clock
identifies the start of the cycle (using the masters encoded
callsign or a tone).  The radio will then automatically switch
to the Slave mode.  Alternatively the users can select one
station to be the master, and that radio will then transmit the
sync clock. At this point further radio configuration is
selected by each operator and transmissions begin. The TDM cycle
is based on 1200 bytes and lasts one second. It is repeated
again and again. Individual stations wait for their time slot
period to transmit. The first station to transmit in a time slot
sets an activity indicator and owns the slot. The slot is
relinquished after two (or more) cycles of silence unless the
activity hold switch is enabled. This last feature can allow
roundtable type communications or hold the channel slot for
direct communications between two Amateurs. As a further
safeguard, the activity hold should timeout after a few minutes
when no use is detected.  Activity hold causes a transmission of
the users call sign with no information.

The Operator Controls

In commercial systems, the radio user is not given much control
over where their transmissions will go. The Amateur however,
needs to have access to all of the radio options. They want to be
able to select a frequency, select a net, or group of users. The
control panel of a multiplexed radio should therefore have all of
these options available through keyboard control and stored in
EEPROM (Electrically Eraseable Programmable Read Only Memory) so
you don't have to type it in every time you turn on the radio:

    I.    Callsign Entry
    II.    Master Volume/Power switch
    III.    Master/Slave Entry and indicator
    IV.    Microphone Enable (M1, M2, M3, M4)
    V.    Microphone with Push-To-Talk switch
    VI.    Frequency Channel (1 - 9)

        A.    M1 Select Mode (Multiplex Slot 1)

            a.    Tx/Rx Simplex
            b.    Tx Duplex
            c.    Rx Duplex

                1.    Activity Indicator/Hold switch
                2.    Volume
                3.    Remote computer/audio Jacks

        B.    M2 Select Mode (Multiplex Slot 2)
        C.    M3 Select Mode (Multiplex Slot 3)
        D.    M4 Select Mode (Multiplex Slot 4)

Since these radios will be digital, no squelch control is needed.
There will be no output to the speaker unless the data is
decoded.  When activity is detected on the time slot, no further
transmissions are allowed.  The time slot activity indicator will
be released two cycles after end of transmission unless
retriggered.  This feature is designed to prevent interference
and loss of time slot by another station.

Our hypothetical radio is a 70cm band radio.  It operates on nine
channels:

    0.    446.050 (Synchronization Channel)
    1.    446.075
    2.    446.100
    3.    446.125
    4.    446.150
    5.    446.175
    6.    446.200
    7.    446.225
    8.    446.250
    9.    446.275

Each channel is run at 9.6 kbps and has four time slots, or 1200
bytes per second. The cycle consists of 9600 bits, or 2400 bits
per multiplex channel.  A good first experiment will be to use a
22 byte guard, 256 bytes data, and a final 22 bytes guard.

    M1.      0 -  299    300 bytes        250 ms
    M2.    300 -  599    ""            ""
    M3.    600 -  899    ""            ""
    M4.    900 - 1199    ""            ""

Each multiplex channel transmits for 300 bytes (250
milliseconds), and then must wait for 900 bytes (750
milliseconds) before transmitting again.  Using this example,
four Amateurs can conduct individual conversations or data
transfers, or alternatively one Amateur can use all four time
slots for multiple connections.

DUPLEX

A good example is a full-duplex connection between two Amateurs.
Each Ham will select a frequency and time slot for transmission,
and another pair for reception. At this point they may begin
talking as if on a telephone. Another example is a file transfer
between computers. As one computer transmits a packet of data,
the other computer will either ACK (Acknowledge) or NAK (Negative
Acknowledge) the packet. The computer can load the circuit with
packets and the remote computer will ACK/NAK without waiting for
the sender to stop and listen. If you wanted to play Flight
Simulator over the air, both systems could transmit aircraft
position simultaniously.

Another example is Direction Finding. A network of listening
stations is set up to report bearing, amplitude, and time of
detection (based on the masters time). These reports are then
sent at the assigned time slot.  Each computer can then produce a
probable location of the transmitter, as well as filter out
multipath reports by processing the report history.

SIMPLEX

In the half-duplex mode, each time slot is used round-robin. As
one Amateur finishes their transmission, the next proceeds with
theirs. The radios probably should be equipped with a tail tone
when using voice mode. The Activity indicator is dropped after
each cycle in the data mode.

For example, on channel 1 there are four multiplex programs being
conducted. On M1 a simplex voice weather net is in progress, M2
has a simplex digital packet weather roundtable, M3 has a simplex
voice swap net, and finally M4 has a simplex digital packet BBS.

RADIO SET UP

How must the radio work in order to participate in all of these
events?

The operator first selects each frequency, and then enables the
multiplex channels that they are interested in using. Usually
one frequency is used for uplink, and another (spaced away from
the other) is used for downlink. The time slot should be
different also to prevent transmitter desense of the receiver.
The decoded audio is then mixed to the speaker and also output
via a rear panel jack. The rear panel jacks are for interfacing
with a computer or standard audio levels. The radio will include
all the modems necessary to operate with another station. The
use of an external TNC (Terminal Node Controller) will not be
required.  When the Push To Talk (PTT) is enabled, the audio is
quickly digitized and compressed, and then is output during the
next appropriate time slot. Compression is achieved through a
vocoder algorithm.  There is also a digital comparison which
only passes voice above a certain threshold level. The vocoder
algorithm should be a standard 9.6 kbps LPC one. For digital
packet, the whole time slot is filled with callsign, addressing,
and data information.

Conclusion

This paper introduced and outlined a TDM system that consists of
9 channels with 4 time slots each, resulting in 36 possible
communication events in 225 kHz. The technology is available
today to accomplish and implement this design. While more
expensive at the outset than current systems, the time is fast
approaching when we can no longer afford the one channel one user
system. This proposal offers twice the events per Megahertz as
compared with current Narrow-Band FM (NBFM) systems (15 events at
5 kHz Bandwidth with 10 kHz guard bands each side, to 36 events
TDM with 25 kHz Bandwidth). Final systems could offer either more
time slots per channel, longer cycles, or even reduced bandwidth,
such as 5 kHz channels. This is all hypothetical however, as I
don't really know if nine 25 kHz channels butted up against each
other can actually be manufactured. But even with guard bands of
5 kHz between these channels results in more communications
events than NBFM.

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