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Add the white paper digi-contest.txt.
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Joe Taylor
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lib/digi-contest.txt
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276
lib/digi-contest.txt
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Possible FT8 Enhancements for Contesting
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----------------------------------------
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In addition to all the standard FT8 messages, FT8 DXpedition Mode
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defines a new message type to convey messages like this example
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acknowledging completion of a QSO with K1ABC and initiating a QSO with
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W9XYZ:
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K1ABC RR73; W9XYZ <KH1/KH7Z> -11
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With 15s T/R sequencing and otherwise using standard FT8 messages,
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this feature allows QSO rates up to 120/hour with one Tx signal. The
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callsign enclosed in angle brackets is sent as a 10-bit hash code.
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High QSO rates are also desirable for contest operating, but some
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details are quite different from the DXpedition case. Contesting is a
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many-to-many (as opposed to many-to-one) activity. We distinguish
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between "Run stations" and "S+P stations" rather than between "Fox"
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and "Hounds".
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An optimal sequence of messages suitable for contesting looks
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something like the list in Table 1, where {exch} represents the
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required contest exchange. With 15 s transmissions and a steady
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stream of callers, messages like these can support QSO rates
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approaching 120/hour.
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Table 1. Example sequence of FT8 contest messages
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-------------------------------------------------------------------------
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Run station S+P stations
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-------------------------------------------------------------------------
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1. CQ K1ABC
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2. K1ABC W9XYZ, K1ABC G4AAA, ...
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3. W9XYZ K1ABC {exch}
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4. K1ABC W9XYZ {exch}
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5. TU; G4AAA K1ABC {exch}
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6. K1ABC G4AAA {exch}
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7. TU; VE2BBB K1ABC {exch}
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8. K1ABC VE2BBB {exch}
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9. ...
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-------------------------------------------------------------------------
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In some circumstances one or both station callsigns may safely be
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taken as known by context. High-rate contest transmissions in SSB,
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CW, and RTTY can therefore be considerably shortened with no resulting
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ambiguity for attentive operators. CQing stations need not include
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their own callsign in every transmission, while S+P stations may send
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only their own callsign at first, as in line 2 of Table 2, and then
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only the contest exchange, as in line 4.
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Table 2. Abbreviated contest messages
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-------------------------------------------------------------------------
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Run station S+P stations
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-------------------------------------------------------------------------
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1. CQ K1ABC
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2. W9XYZ, G4AAA, ...
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3. W9XYZ {exch}
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4. {exch} (sent by W9XYZ)
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5. TU; G4AAA {exch}
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6. {exch} (sent by G4AAA)
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7. TU; VE2BBB {exch}
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8. {exch} (sent by VE2BBB)
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9. TU; CQ K1ABC
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10. ...
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-------------------------------------------------------------------------
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There would be no advantage to such message brevity with FT8. FT8
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transmissions are of fixed duration, by design; and the AP decoder can
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treat the home callsign and previously decoded callsigns as
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hypothetically given, thereby making the effective code rate lower and
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sensitivity up to 4 dB better.
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Required exchange information for some relevant contests is
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illustrated in Table 3, along with a breakdown of bit requirements for
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each component of the exchange. Lower-case letter-number combinations
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such as r1, r3, s7,... in the table suggest the meanings and indicate
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the number of bits required to convey each part of the exchange.
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Further details are given below the Table. Parameter T1 is the total
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number of exchange bits, and T2=T1+56 is the number of bits for the
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full message, including two standard 28-bit callsigns.
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Table 3. Examples of required contest exchanges {exch}
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------------------------------------------------------------------------------
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Event Exchange Example Bits T1 T2
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------------------------------------------------------------------------------
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ARRL RTTY US/Can: rpt state/prov R 579 MA r1 r3 s7 11 67
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DX: rpt serial R 559 0013 r1 r3 n12 16 72
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Field Day US/Can: OpClass Section R 6A EMA r1 n5 c3 s7 16 72
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DX: OpClass DX R 1A DX r1 n5 c3 s7 16 72
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CQ WPX RTTY RST + serial R 589 0013 r1 r3 n12 16 72
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CQ WW RTTY US/Can: RST CQZ state/prov R 579 8 NJ r1 r3 z6 s6 16 72
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DX: RST + CQzone R 559 3 r1 r3 z6 10 66
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ARRL VHF+ grid4 R FN42 r1 g15 16 72
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EU VHF+ rpt serial grid6 R 590013 IO91NP r1 r3 n12 g25 41 97
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------------------------------------------------------------------------------
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Meaning and number of bits for each exchange component:
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c3 Operating class (A-F)
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g15 grid4
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g25 grid6
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n12 Serial number
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n5 Number of transmitters (0-31)
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r1 acknowledgment of received exchange
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r3 3-bit report (0-7 ==> -24 to +18 dB, effectively "S2 to "S9")
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s6 US state or Canadian province (48+14=62)
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s7 ARRL/RAC Section (83 sections)
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z6 CQ zone
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------------------------------------------------------------------------------
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How best to accommodate all these possibilities within the 72+3-bit
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FT8 message payload? Let i3 (aka "i3bit") denote message type, with
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available range 0-7. Types 0 and 1 are already used for standard
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JT-style structured messages and free text. Examples of suggested new
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message types 2 through 6 are summarized in the Table 4.
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Table 4. Proposed FT8 message types
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------------------------------------------------------------------------------
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i3 Example message Bits i72 Total Special purpose
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------------------------------------------------------------------------------
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0 K1ABC W9XYZ EN37 28 28 15 0 72 Standard message
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0 FREE TEXT 71 1 72 Free text
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1 K1ABC RR73; W9XYZ <KH1/KH7Z> -11 28 28 10 6 72 DXpedition Mode
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2 W9XYZ K1ABC x16 28 28 16 72 Contesting
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3 TU; G4AAA K1ABC x16 28 28 16 72 Contesting with "TU;"
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4 <K1ABC> W9XYZ/R R x25 17 28 1 1 25 72 Rovers, Grid6
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5 <K1ABC> PJ4/W9XYZ 17 49 66 Compound TxCall
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6 PA9XYZ R 590003 IO91NP 28 1 3 12 24 68 EU VHF contest
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7 tbd...
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The first callsign in a message can also be "CQ" and a few other
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special tokens. Type 3 messages are the same as type 2 except for
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including "TU;", the completion-of-QSO indicator. Message fragments
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x16 and x25 represent generic contest exchanges. A "contest template"
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will define the specific source encoding/decoding needed for each
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event.
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Suggested message types 4 and 5 use a 17-bit hash for the first
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callsign. I'm imagining that we'd start with a 32-bit crc and then
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use its remainder after dividing by the prime number 131063. Values
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less than 131063 will be the desired hash code, and the nine values
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131063-131071 can be assigned special meanings such as CQ, QRZ, etc.
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Type 4 messages identify the transmitting station as a Rover and can
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also accommodate 6-digit grid locators. Type 5 messages allow the
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transmitting station to send a full compound callsign with add-on
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prefixes up to 4 characters and suffixes up to 3. Compound callsigns
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are also permissible for the hashed callsigns in message types 4 and
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5.
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Contest Operating
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-----------------
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Operating in this proposed FT8 Contest Mode would be similar to that
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in current RTTY contests. CQing stations will be distributed over 20
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kHz or more on each band, perhaps at ~500 Hz separation. They will
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respond to callers on their own frequency +/- ~200 Hz. Thus, a CQing
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station and callers should occupy no more than 500 Hz total bandwidth.
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CQers might always transmit at Tx audio frequency 1750 Hz and
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configure their FT8 decoders to respond to signals between 1500 and
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2000 Hz. S+P stations will typically work their way up or down the
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band, perhaps in steps of 2 or 3 kHz, looking for unworked CQers. The
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FT8 Contest GUI will offer special features for CQ mode and S+P mode
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that make such conventions easy to follow.
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Some Pertinent Questions
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------------------------
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1. FT8 is a good mode, but does it make for a good contesting mode?
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The model described above maxes out at 120 QSOs/hour. (One can
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imagine SO2R or even SO3R extensions, doubling or tripling that
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limit.) Should we consider T/R sequences of 10s, or even 5s, to get
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potentially higher rates? Should we consider giving up synchronized,
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fixed-length sequences entirely, and use operator-determined start
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times and transmission lengths? That's a significant departure from
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all existing WSJT-X modes.
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2. How much automation should be permissible?
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We're not aiming to make a contesting robot. We want something that
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rewards operator and station performance. The recently introduced FT8
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DXpedition Mode offers the "Fox" operator a list of decoded "Hounds"
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sorted by Call, Grid, S/N, Distance, or Random order. Hounds must
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initiate their calls to Fox, and Fox must manually select each Hound
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to be called. Otherwise, QSOs proceed with standard FT8
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"auto-sequencing". Is this model acceptable?
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3. How much bandwidth? How much sensitivity?
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RTTY signals use bandwidth ~220 Hz and require S/N around -5 dB or
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better, measured in a 2500 Hz bandwidth. FT8 uses signal bandwidth 50
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Hz and reaches threshold sensitivity between -20 and -24 dB, depending
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on how much a priori (AP) information is available. Shorter
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transmissions conveying the same messages would increase the
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bandwidth, S/N threshold, and potential QSO rate proportionally. Has
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FT8 already hit the "sweet spot" of such trade-offs?
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4. Suitable power limits?
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WSJT-X modes are designed as weak-signal modes. They have strong FEC
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and don't suffer from partial or corrupted copy. Sensitivity already
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beats RTTY by 15-19 dB, so arguably it makes sense for an FT8 contest
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or contest category to be limited to 100 W or even QRP power levels.
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5. What software should provide the operator's GUI?
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Things are described above as though WSJT-X, the software that
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introduced FT8, would be used for contest operating. WSJT-X can send
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QSO information to N1MM+ and other logging programs, so they could be
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used in combination with WSJT-X. Alternatively, we could set things
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up so that N1MM+ is the control program and FT8 encoding/decoding is
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provided by a plug-in "MMFT8" similar to MMTTY. Operators already
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into serious RTTY contesting would probably like the N1MM-in-control
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option. However, existing FT8 users might be more comfortable with
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WSJT-X in full control. Moreover, WSJT-X offers full support for
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Linux and MacOS, which N1MM+ does not.
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########################################################################
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More thinking, beyond the above...
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FT8 currently uses LDPC(174,87), where 87=72+3+12. Suggest going to
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LDPC(174,91), with 91=72+5+14. This would give us 32 message types
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rather than 7, and stronger suppression of false decodes. SNR penalty
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would be 10log(91/87)=0.2 dB.
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MSK144 currently uses LDPC(128,80), where 80=72+8. Suggest going to
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LDPC(128,87), with 87=72+5+10. SNR penalty is 0.4 dB.
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ARRL RTTY Roundup
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-------------------------------------------------------------------------
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Run station S+P stations
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-------------------------------------------------------------------------
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1. CQ K1ABC
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2. K1ABC W9XYZ, K1ABC G4AAA, ...
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3. W9XYZ K1ABC 579 MA
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4. K1ABC W9XYZ 599 WI
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5. TU; G4AAA K1ABC 559 MA
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6. K1ABC G4AAA 579 0013
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7. TU; VE2BBB K1ABC 599 MA
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8. K1ABC VE2BBB 599 QC
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9. TU; CQ K1ABC
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-------------------------------------------------------------------------
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NA VHF Contest
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-------------------------------------------------------------------------
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Run station S+P station i3
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-------------------------------------------------------------------------
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1. CQ K1ABC/R FN54 0
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2. K1ABC W9XYZ EN37 0
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3. W9XYZ K1ABC R FN54 2
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4. K1ABC W9XYZ RR73 0
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5. CQ K1ABC/R FN54 0
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-------------------------------------------------------------------------
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1. CQ K1ABC FN42 0
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2. <K1ABC> W9XYZ/R EN47 5
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3. W9XYZ K1ABC R FN42 0
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4. DE W9XYZ/R RR73 0
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5. CQ K1ABC FN42 0
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-------------------------------------------------------------------------
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EU VHF+ Contest
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-------------------------------------------------------------------------
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Run station S+P station i3
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-------------------------------------------------------------------------
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1. CQ G4ABC IO91 0
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2. G4ABC PA9XYZ JO22 0
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3. PA9XYZ 590003 IO91NP 6
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4. G4ABC R 570007 JO22DB 6
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5. PA9XYZ G4ABC RR73 0
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