[Ground-station] "Lab Call" - uplink transmit and receive - discussion on packet format

[Michelle Thompson]

Something we must produce is an end-to-end demo of Phase 4 Ground.

We've got a few of the hardest and largest pieces solved, but there is a
lot else that must come together for a demonstration to happen.

Paul KB5MU is writing a document of the steps required from here to get a
demonstration working.

This week I've been working on uplink transmit. There isn't anything that
exactly matches up 4-ary minimum shift keying (MSK). There are advantages
to using this scheme. It's constant modulation, has reduced sideband power,
and is spectrally efficient.

There are some disadvantages. Both MSK requires more power to transmit
compared to QPSK, an adaptive equalizer is probably going to be needed at
the receiver, and more inter symbol interference may occur.

The GMSK blocks are what GNU Radio people say to use for MSK. The G stands
for Gaussian filtering. This is done before modulation stage. GMSK is used
in GSM.

If we can't just use the GMSK blocks and remove the filter, then we'll need
to customize some blocks for this.

So far this week I've gotten back up to speed with phase shift keying,
reviewed timing recovery, constant modulus types of equalization, and
frequency and phase recovery. This is on the receiver side and is important
in the downlink.

There's been some improvements over the past year in the mainline code base
of GNU Radio for clock recovery.

More soon - I'll share all the flowgraphs and I'll undoubtedly have a lot
of questions.

Paul is working on getting the test plan published.

David Viera and David Fanon were looking to help with this, and of course
we'll need more people digging into the uplink. If you're adept with the
GMSK blocks already, then your help will be appreciated here.

Uplink packets will have content that supports authorization and
authentication.

Stations that comply with the air interface can transmit through the
satellite. The station is configured with an amateur radio callsign, plus a
Secondary Station Identifier (SSID) of TBD bits to allow a particular
licensee to operate multiple simultaneous stations, and with the
ARRL-issued private key and certificate for that callsign. In addition, the
station must generate a token of TBD bits chosen at random. Every
transmitted uplink frame contains in its header the callsign, SSID, and
token.

The callsign and SSID will be retransmitted in the downlink frame header,
but the token is never transmitted on the downlink. Since it is fairly
difficult to intercept an uplink transmission, this makes it difficult for
an impostor to hear another station's authenticated callsign:SSID and start
using it.

The satellite/Groundsat stores the token, the claimed call sign, the
claimed SSID associated with the call sign, and a time stamp. This is a
tuple that forms the rows of a database.

(satellite time stamp : callsign : SSID : token)

When each uplink frame is received by the satellite, it decides whether to
accept it for retransmission on the downlink, or discard it. It may also
choose to initiate an authentication transaction with the ground station.
Alternately, or in addition, a Ground Control Station may initiate an
authentication transaction with any or all active station(s). Unless and
until an authentication transaction with a given station has been attempted
AND FAILED, the satellite must accept its frames for retransmission (unless
that station has already been blacklisted for another reason).

This achieves two important goals. Communication is unimpeded, and the loss
of Ground Control Stations can be well-tolerated. When Ground Control
Stations are not required for normal communications, system durability and
reliability is greatly increased.

When authentication is not frame-by-frame, or required to initiate the
process of accessing and transmitting through the satellite, efficiency and
performance are greatly increased. In particular, the very first message
from a station is not delayed for formalities or blocked entirely; in an
emergency situation this could be essential.

The risk of bad actors is recognized, but management of bad actors is
achieved through Black Listing known bad actors after complaint or system
statistics or some other method reveals that the problem is Misuse.

More soon!

-Michelle W5NYV
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