From: paulf@umunhum.stanford.edu (Paul Flaherty)
Subject: Errors in the ARRL Digital Committee's Report
Organization: The Three Packeteers
Distribution: usa
Date: Thu, 30 Jul 1992 00:45:09 GMT

Summary:

There are a substantial number of flaws in the statements made by the ARRL
Committee on Amateur Radio Digital Communications regarding the technical
state of the art for High Frequency digital transmission.

>    Report and Recommendation to the ARRL Board of Directors
>
>                             by the
>
>     ARRL Committee on Amateur Radio Digital Communications
>
>                          June 13, 1992

[...]

>The State of the Art for Amateur HF Digital Operation

[...]

>As currently used all of the above modes require approximately 500
>to 1000 Hz. of bandwidth per channel except packet which requires
>2000 Hz. per channel.  Effective use of that bandwidth is terms of
>character throughput varies considerably as a function of the
>protocol used and the channel conditions.  Partly because of the
>requirement for 2000 Hz. of space per channel and partly because of
>the nature of the AX.25 protocol, the performance figures for
>packet are the poorest per unit of bandwidth of any of the
>currently used modes.  RTTY and AMTOR are better, and PACTOR is
>better still.  Clover promises to exceed the throughput per unit of
>bandwidth of any of the above modes.
>
>Tolerance to poor channel conditions also varies among the modes
>with packet having the poorest performance, RTTY next, AMTOR and
>PACTOR being very much better.

There are a number of errors in this paragraph.

1. HF Packet does not "require" 2.0 kHz of bandwidth.

HF packet is nominally transmitted as 300 baud 200 Hz shift FSK.  [CCIR86]
gives the necessary bandwidth as being between 400 and 600 Hz, with 480 Hz
being typical.  While the occupied bandwidth is somewhat larger, state of 
the art receiver filters should allow channel widths of less than 1.0 kHz.

There appears to be some confusion between "necessary" and "occupied"
bandwidth for certain modulation techniques.  The "necessary" bandwidth
is that which is required at the receiver, allowing for some intersymbol
interference.  "Occupied" bandwidth is the -60db bandwidth of the transmitted
signal.  Some compromise over the necessary bandwidth is needed to reduce
intersymbol interference at the receiver to an acceptable level, but certainly
closer to the necessary bandwidth than the occupied bandwidth.

2. AX.25 is the limiting factor in HF packet.

AX.25 does have its problems, but it is far from being the limiting factor
for HF packet performance.  Instead, several operational practices are
largely responsible for poor observed performance by HF packet.  These
practices include:

a. 300 baud symbol rate.

[Wiesner84] gives the 90% percentile Rayleigh fade as being less than 5 ms
in duration; systems with symbol rates of greater than 200 bauds will thus
be susceptible to Rayleigh fading.  Reducing the symbol rate to 150 bauds
would lead to an exponential improvement in error rate.

b. Excessively long packet sizes.

Packet lengths of O(1000) bits are not uncommon; given that most HF FSK
systems attain bit error rates of no better than 10^-3, one bit error may
be expected in such a packet.  Maximum packet size should be adjusted to
reflect the channel bit error rate, with no more than one packet in ten
expected loss.

c. Misaligned receivers and transmitters.

FSK filter center frequencies in modern receivers and transmitters are often
misaligned with modem center frequencies.  Many operators "solve" this problem
by simply operating AFSK with 2400 Hz SSB filters, clearly not an optimal
solution.

d. Poor persistence implementation and carrier detection in multiuse channels.

Many TNCs do not properly implement P-persistence backoff algorithms for
collision avoidance, and more still do not provide true carrier detection.

e. Lack of coordination and channelization.

Lack of coordination and channelization has led to situations where two
channels overlap in frequency; since collision detection depends on the 
ability to detect modem carrier, the channel capacity is halved.

f. Poor utilization of windowing.

Common HF packet practice is to transmit one packet per frame; four or more
should be transmitted in order to reduce channel turnaround overhead.

All of these operating practices have simple remedies which fall under
the "Good Amateur Practice" clause of Part 97.

3. Comparing HF packet to other HF digital modes is fatally flawed.

Comparisons of digital transmission systems must be based on equally 
attained error performance and throughput.  AMTOR Mode A, for example,
nets 15 bits per 450 ms cycle, for a net throughput rate of less than
34 bits per second, on an errorless channel.  The comparison to RTTY,
which has no error protection, is clearly orthogonal.

Finally, while this report mentions CLOVER, it clearly fails to consider the
measured performance as reported in [HENRY92].  In particular, CLOVER provides
an adaptive transmission scheme to provide throughputs varying from 31.25 to
750 bps depending on conditions, in less than 500 Hz of occupied bandwidth,
which would allow 500 Hz channelization.


REFERENCES
----------

[CCIR86] _Recommendations and Reports of the CCIR, 1986_, XVIth Plenary
	Assembly, Dubrovnik, 1986, Volume III (Fixed Service at Frequencies
	Below About 30 MHz).

[WIESNER84] Wiesner, Lothar.  _Telegraph and Data Transmission over Shortwave
	Radio Links_, 1984, Siemens.

[HENRY92] Henry, B., and Petit, R.  _CLOVER: Fast Data on HF Radio_, CQ,
	May 1992, p.40.

--
-=Paul Flaherty, N9FZX/5W1JX  | "What a difference an hour can make."
->paulf@Stanford.EDU          |		-- Vernor Vinge