[amsat-bb] JAMSAT report, language support for GRC, Phase 4 Ground update+strategy+test plan, GNU Radio Conference

Sat May 11 16:15:17 UTC 2019

My daughter Geneva and I had a wonderful time at JAMSAT Symposium in March
2019! There was a wide variety of talks about so many different payloads, a
very special banquet dinner, adventures in Kyoto and Osaka, visits to ham
radio stores, getting to see a new ICOM radio up close, lots of Pokemon, a
Fire Festival, and making so many new friends. We were welcomed and will
never forget the hospitality.

A big part of Symposium was the GNU Radio Workshop by Imamura-san. It was
an honor to share how we on Phase 4 Ground use GNU Radio in our
presentation on Sunday morning.

GNU Radio is a digital signal processing framework for software-defined
radio. It’s the software that tells the hardware in your radio what to do.
We need to be able to quickly and easily set up a software-defined radio to
do whatever modulation and coding we want during development of new
payloads and new ground stations. In many cases, GNU Radio is the exact
right tool for this job.

GNU Radio Companion is a Graphical User Interface that allows us to drag
and drop functions onto a canvas. We click block outputs to connect to
block inputs. When we do this, it creates a directed graph that implements
radio functions. The signals flow from beginning to end. Each block
modifies the signal, as if it was part of a circuit. The flow graph looks
something like a block diagram combined with a software flowchart. GNU
Radio has software variables. It can adapt to signal conditions and user
input.

The workshop at JAMSAT was held after the last talk on Sunday. It was
several hours of hands-on training. Participants brought their own
computers, installed GNU Radio, and created useful radio flow graphs that
worked with real hardware. Several experiments were done in order.
Imamura-san kept everything organized through a set of projected slides
that had clear instructions. Optimizations and customizations were made so
that participants could see how they can use GNU Radio to achieve their
goals. The hardware included RTL-SDRs and Plutos. Imamura-san also
demonstrated a live video transmission from the podium.

Something that I noticed in the workshop was how much easier it would be if
this community could use the software if it was in Japanese. I decided to
see what was required. After some homework and reading, I got a very simple
example working and understood the basic workflow. I asked JAMSAT if they
were interested in a localized version of GNU Radio Companion. JAMSAT
responded positively and work began in earnest! It started to become clear
that after this initial groundwork, that additional languages would be much
easier. I reached out to AMSAT-DL next, and got an enthusiastic response
there too. As of today, both German and Japanese are in progress! I
received an inquiry about Portuguese following the initial announcement
that the main menus had successful translations in testing. It's now on the
list after we fully test localization for German and Japanese.

While at JAMSAT, I was able to give a status update for Phase 4 Ground.

Phase 4 Ground is a broadband digital microwave system for both terrestrial
and space use. It complies with both ITAR and EAR open source and public
domain carve-outs, so it’s open to participation worldwide. All engineering
is published as it’s created. All are welcome to participate.

Phase 4 Ground is best suited for GEO and HEO satellite missions. The
uplink is frequency division multiple access. We use a 5GHz uplink. The
regenerative repeater payload receives the uplink signals, digitizes them,
multiplexes them, and processes them into DVB-S2 and DVB-S2X frames. The
downlink is 10GHz. DVB-S2 is Digital Video Broadcasting Satellite 2nd
edition. The X stands for extensions down in to Very Low SNR modulation and
codings. Very Low SNR is of interest to hams, so we include the extension
to the main standard DVB-S2.

We use both pilots and short frame lengths in order to make the receiver
implementation as easy as possible. Pilot tones are optional, and there are
medium and long frames available in the standard.

There is a recommended order to receive DVB-S2/X frames. The first stage of
the demodulator is symbol timing recovery. We have to figure out the best
possible time to measure, or take a snapshot, of the received signal. The
receiver does not know the phase of the transmitter clock. We will not be
automatically coordinated with it. It's not just a problem with phase,
either. The receiver and transmitter may disagree in terms of the period of
the clocks, or we might have jitter, or we might have drift. We have to
analyze the received waveform and synchronize our receiver clock to the
transmitter clock that is “hidden” in the received signal. Then, once we
are synchronized, we sample that symbol and report the results.

Doing this gives us a reliable value for the received symbol. Now that we
have a series of received symbols, we have to figure out the start of the
frame. This is done in DVB-S2 (and many other protocols) by sending a fixed
pattern at the start of every frame. For DVB-S2, this is called a Physical
Layer Start of Frame sequence. It’s 26 symbols long. This is what we look
for. Once we see it, we know where the start of the frame is!

Frame synchronization can be done in several ways. There are two different
methods described in the implementation guidelines for DVB-S2/X. One is
relatively simple, using shift registers. The other is bit more complex,
using state machines. There are advantages to using the state machine
method, but it’s more complicated and expensive. The shift registers is
simple and cheap, but gives up a bit of performance. This is the constant
balance in digital communications. Performance comes at a cost!

Right after frame synchronization, we correct for carrier frequency error.
First we do a coarse correction. This can be done with a delay-and-multiply
frequency error detector. Then we do a fine correction. This can be done
with something like a feed-forward estimation algorithm. Coarse correction
is in the MHz, and fine correction is the hundreds of kHz.

Next, we do phase recovery. This is to fix any residual frequency offset
from the coarse and fine frequency offsets. Phase 4 Ground will support all
the modulation and codings of DVB-S2/X, but we expect lower order
modulations to be more heavily used. This means that a pilot-assisted
maximum-likelihood (ML) feed-forward estimator will be the most useful. If
you compute the average phase of each pilot field, then you can subtract
this out and improve the signal. Higher-order modulations will need another
feedback loop.

Automatic gain control is next. AGC can be done in many ways. One way to do
it depends on the pilot symbols in DVB-S2/X standard. These symbols are
sent at regular intervals to provide a known easy-to-receive signal. We use
these known pilot symbols in order to determine the amplitude
multiplication factor for the rest of the signal. Pilot symbols are
optional in the DVB standard, but Phase 4 Ground requires them. When the
pilot symbols are on, the AGC is listening. When the pilot symbols are off,
the AGC turns off, and the information from the AGC is used.

After AGC, the constellation is decoded. DVB-S2 has a lot of them! There
are many techniques to get the bits from the constellations. GNU Radio has
a very versatile and powerful constellation block.
Instead of the usual MPEG transport stream (DVB-S2 is for satellite TV, so
the content is usually broadcast television signals), we use the more
flexible Generic Stream Encapsulation standard from DVB.org. This means we
have less overhead and complexity, and can handle any digital traffic that
the amateur operator wants to transmit. It’s just a digital pipe.

Phase 4 Ground uses GNU Radio extensively in research and development as
well as for archiving and publishing our work. GNU Radio is not just a tool
to figure things out, but is also a way to define a reference design for
the radio.

Because Phase 4 Ground is not a bent pipe, the payload is more complex.
This complexity needs to be fully tested on the ground before risking large
digital circuits in space.

All the uplink channels are received with a polyphase filter bank. The
current polyphase filter bank implementation in GNU Radio needs some
updates in order to achieve the speeds and performance that we want. This
is an active area of research and development. There have been three
efforts over the past three years by various groups that have attempted to
update and improve the existing working polyphase filter bank in GNU Radio.
There is some recent good news here! EJ Kreinar reported that his open
project (Theseus Cores) has an open source FPGA polyphase channelizer! This
works with RFNoC. We will experiment with this in our payload design. This
could fill in a big piece of the system that we need. Check it out here:
http://gitlab.com/theseus-cores/theseus-cores

Ron Economos and Paul Williamson successfully implemented GSE in GNU Radio
and in Wireshark. This made it possible to do transport layer testing. Ron
Economos is the lead author of the DVB blocks in GNU Radio. Improvements to
GSE continue today. The current focus is improving internetworking
functions so that large amounts of data can be more easily handled. We
intend to use multicast IP as much as possible, and making sure GSE
integrates well with multicast IP is important. The path forward with that
is in our test plan. You can find our test plan here:

https://github.com/phase4ground/documents/blob/master/Papers_Articles_Presentations/Papers/P4G%20Testbed%20Proposal.md

The error correction in DVB-S2/X is state of the art. There are not many
other error correcting codes that are better than Low Density Parity Check
+ BCH. This is a concatenated digital code specified by the DVB standard
for S2 and T2 transmissions. We have two open source implementations of
LDPC decode for DVB-S2/X. The first one is for graphical processing units
and was written by Charles Brain. It was demonstrated at 2017 AMSAT-NA
Symposium and at several events following.

The second open source implementation is in C by Ahmet Inan and can be
found here: https://github.com/xdsopl/LDPC
This version has been incorporated into GNU Radio by Ron Economos. This can
be found here: https://github.com/drmpeg/gr-dvbs2rx

The next step for LDPC is to implement and publish an open source version
for FPGA.

GNU Radio is very important for our voice codec work, uplink modulation
experiments, and trying out authentication and authorization schemes. GNU
Radio allows us to use a wide variety of off the shelf hardware to achieve
things that were not possible only a few short years ago. The GNU Radio
community has been welcoming, helpful, supportive, friendly, and a source
of continually amazing software-defined radio advancements.

GNU Radio has an annual conference. In 2018, we held a week-long “Block
Party” for DVB-S2/X. We had fun, set up multiple demos, explained DVB-S2/X,
made the case for open source LDPC, and made progress on DVB-S2 correlates
and GSE. Phase 4 Ground made significant progress due to the generous
support of the conference organizers and the community.

Learn more about the conference here:
https://www.gnuradio.org/grcon/grcon19/

Registration for 2019 is open. The conference will be held September 16-20,
2019 in Huntsville, AL, USA. There is a poster session, proceedings, talks,
workshops, contests, and social activities. The theme for 2019 is Space
Communications! There are special gifts for space themed content. If you
have a GNU Radio project that you want to share, consider making a
presentation at or sending a poster to GNU Radio Conference 2019.

The collaboration between Phase 4 Ground and JAMSAT has been absolutely
stellar and we all look forward to continued enjoyment and success. Next
generation payloads will be more complicated with multiplexing and advanced
digital techniques. We all need to be able to work together,
internationally. Open source and public domain is the best way forward.
Phase 4 Ground and Open Research Institute are entirely dedicated to making
this happen. We will be keeping the momentum and progress going. ORI is
proud to be an affiliate member of Open Source Initiative
https://opensource.org/

Join the Phase 4 Ground team! Our work benefits all AMSAT organizations.
Our mailing list can be found at our website https://openresearch.institute/

Write me at w5nyv at arrl.net if you want to join our Slack account. This is
where daily engineering discussions take place.

More soon!
-W5NYV