[Ground-station] P4B Regenerative Repeater Likelies

Wally Ritchie wally.ritchie at gmail.com
Sat Mar 2 17:28:57 EST 2019


I've begun some work on an Internet based simulator for a p4b geosat. It
focuses on the Narrowband Multiplexor entity that relays the data from the
uplinks to the DVB-S2 downlink. (More on this in a few days in another
thread).

In the process I've collected a random assortment of "sort of consensus"
thinking as I recall it from the slack channels over the last year or two.

Please comment here or on slack regarding any that are not actual likelies
or are not understood.

Wally WU1Y


1. The Downlink will likely have power output of 10W – 100W into a
hemispherical beam.
2. The Downlink will likely have symbol rate of 1 Mbaud to 10 Mbaud.
3. The downlink will likely have a saturated and unfiltered final.
4. The downlink will likely provide a DVB-S2 carrier.
5. The downlink will likely use GSE supporting multiple streams and
ACM/VCM.
6. The primary stream will likely be a flow of UDP broadcast packets at 25
pps. These will carry the relayed narrowband channel data and system data.
7. System data will likely include both telemetry and operational data
(e.g. logs) and will be unencrypted except where necessary.
8. A narrow band multiplexor (NBM) entity will likely combine the system
streams and the relayed narrowband channels into a single UDP packet flow
which will have priority over other flows.
9. Wideband channels will likely be supported and relayed in separate IP
packet flows (or perhaps repeated BBFRAMES).
10. System data may in some cases utilize separate IP packet flows.
11. Narrowband uplinks will likely be handled by a multi-channel receiver
with 64 – 128 channels spaced at 25kHz (or somewhere between 12.5 to
75kHz). Note that with 64 25kHz channels the narrowband input spectrum
would be 1.6MHz.
12. Earth stations will likely require a sub-ppm accuracy station clock
with Frequency Accuracy and Stability on the order of 100ppb for 25kHz
channels. The clock may have to be synchronized to the downlink. Note that
1ppm at 5GHz corresponds to 5kHz frequency shift.
13. Uplinks will need to control power output, frequency, and timing to
arrive in their assigned uplink slot.
14. Narrowband Uplink modulation will likely be some form of GMSK or QPSK
and could vary over the lifetime of the repeater.
15. Narrowband Uplink PSD will need to maintain power in the adjacent
channels below specified tbd limits.
16. Uplink spurious outputs both in-band and out-out of band will need to
be kept below specified tbd limits.
17. The possible (and wildly optimistic) presence of other satellites with
5GHz uplinks as close as 2 degrees should be recognized.
18. Narrowband Uplink transmissions will likely be framed in 40ms slots
matching the NBM frame rate.
19. Narrowband Uplink symbol rates will likely be on the order of 5000 baud
to 2000 baud for 25KHz channel spacing.
20. Narrowband Uplink transmissions from active stations will be continuous
back to back frames. Where the frame does not carry relayed speech (e.g.
silence intervals) identification or alternate data may be transmitted and
relayed.
21. Frames containing station identification will likely be FEC coded (e.g.
Golay Coding).
22. Preambles on each frame will likely be required for synchronization and
adaptive equalization.
23. Longer preambles may be required on the initial frames of a
transmission.
24. Identification will likely be required on the first frame of a sequence
and periodically during a continuous sequence.
25. Data within a frame will only be relayed through the NBM when fully
conforming to the rules.
26. Where the receiver is unable to synchronize and decode a frame,
parametric data related to the transmission will likely be substituted for
the missing payload.
27. Dedicated channel sets on both band edges will be used for testing and
commissioning.
28. Stations will not be permitted to transmit on regular channels until
they have been commissioned using automated procedure that take place using
the commissioning channels. Any transmission by an un-commissioned station
will be considered intentional interference.
29. A cease transmission command will likely be provided in the NBM output
requiring all stations to immediately and temporarily cease transmission.
This might be used to support processes to identify and locate interference
or unauthorized transmissions.
30. Commissioning will likely involve short transmission bursts on the
order of 3 to 5 frames in a test channel. Commissioning transmissions will
likely be separated in time (many seconds for an individual station).
31. Commissioning channels will likely dedicate one or more adjacent guard
channels to be used in the commissioning process (e.g. proving adjacent
channel power, frequency, and frame alignment measurement to verify that
the station is within tbd limits.
32. Allocation of a commissioning channel to a station will likely involve
a fair hash algorithm based on offered demand and capacity. Transmission
Collisions will be possible and expected during commissioning.
33. Allocation of channels to commissioned stations will likely use a
channel allocation protocol operating within single channels (as opposed to
the multiple channels used for commissioning/test). Note that a station
cannot be identified until its transmissions can be decoded by the
receiver.
34. Once a station is able to transmit a compliant signal in the
commissioning channel it will acquire a secure access token through
interaction with the server. This will likely involve authentication. The
token will be used to identify the station in transmissions. The receiver
will likely substitute the station's actual callsign for the token in the
downlink.
35. The NBM output configuration will likely be flexible allowing for a
variable number of active uplink channels of various payload rates with an
aggregate relayed payload on the order of 256kps.
36. The NBM will likely support Nx800 channel allocations with N <= 12
(i.e. 800 to 9600bps).
37. During any given frame (UDP packet), an NBM channel segment may carry
Uplink Payload, Uplink Identification (Station ID and payload type),
Parametric Receiver Data, or System Data.
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