Page 28 January February 2018 TCA

6) Receive,
synchronize, and
demodulate to yield n
symbols, some of which
might be in error.
7) Decode n received
symbols to recover k
error‑free message
symbols.
8) Decompress k
symbols to recover
original message in
human‑readable form. Figure 2: Simulated signals for an unmodulated carrier, a 25 WPM CW signal, and the WSJT-X
9) Deliver message to receiving user. slow modes WSPR, JT9, JT4, FT8, QRA64A, and JT65. The slow modes are shown in their
“A” submode, in increasing order of occupied bandwidth. All signals have S/N of –10 dB
The most crucial are steps 3 and 7. in a 2,500 Hz reference bandwidth. The vertical extent of the waterfall corresponds to 50
Step 7 likely requires the most seconds. Two successive FT8 transmissions are shown.
computational resources.
37 × 36 × 10 × 27 × 27 × 27, or somewhat Error-Correcting Codes
When developing a protocol, we want to over 262 million. The numbers 27 and 37
choose an efficient code that maximizes arise because in the first and last three Different codes, modulation schemes,
the probability of recovering transmitted positions a character may be absent, or a and synchronizing patterns have been
messages even when the received letter, or perhaps a digit. Because 2 is adopted for each protocol in WSJT-X.
28
codeword is corrupted. It’s also greater than 268 million, 28 bits are The goal has been to optimize each
important to consider likely types of enough. Similarly, the number of four‑digit mode’s effectiveness for a particular type
fading, Doppler spread, and interference Maidenhead grid locators on Earth is of propagation. To some extent, the final
that may occur on the targeted 180 × 180 = 32,400, which is less than code choices also reflect our own
propagation paths. We need an efficient 2 , so a grid locator can be encoded incomplete but growing familiarity with
15
decoding algorithm that can be executed uniquely with 15 bits. historical developments in communication
in reasonable computing time and will theory. JT65 uses a Reed-Solomon code,
ensure that false decodes are rare. More than six million of the possible and JT4, JT9, and WSPR all use a robust
28‑bit values are not needed for standard convolutional code, first implemented for
The WSJT-X Protocols call signs. A few of these slots have been ham radio use by Phil Karn, KA9Q.
2,3
Message Structure assigned to special message These are among the best‑known types
components, such as CQ, DE, and QRZ. of error‑correcting codes, and they have
Steps 2 and 8 involve lossless CQ may be followed by three digits to been studied thoroughly for over half a
compression and decompression of data. indicate a desired call‑back frequency. century. Our latest modes use state‑of‑
This process is called source encoding In the meteor‑scatter mode MSK144, if the‑art codes that are close to the
the message. WSJT-X protocols JT4, JT9, KA2ABC transmits on say 50.260, and forefront of this research field. MSK144
JT65, QRA64, and MSK144 all use sends the message “CQ 290 KA2ABC and FT8 use low-density parity check
structured messages that source‑encode FN20,” it means that he or she will listen (LDPC) codes and QRA64 a Q-ary
human‑readable information for basic on 50.290 and respond there to any repeat-accumulate (QRA) code, a
contacts into packets of exactly kq = 72 replies. A numerical signal report of the particular type of non‑binary LDPC code.
bits. The packets contain two 28‑bit fields form ±xx or R±xx can be sent in place of Full technical specifications for each
normally used for call signs and a 15‑bit a grid locator. As originally defined in mode can be found in the WSJT-X
field for a grid locator, signal report, JT65 mode, the numerical signal report User Guide and our openly available
acknowledgment, or 73. One additional values “xx” lie in the range –30 to –01 dB. source code.
4,5
bit is used to flag packets encoding Recent program versions accommodate
arbitrary alphanumeric text, up to 13 reports between –50 and +49 dB for all Protocol Details for Slow Modes
characters. Special cases allow efficient modes except JT65. A country prefix or Figure 2 shows an example of each slow
encoding of other information, such as portable suffix may be attached to one of mode on the WSJT-X waterfall display.
add‑on call sign prefixes (ZA/KA2ABC) the call signs. When this compound call This also includes an unmodulated carrier
or suffixes (G8XYZ/P). The aim is to sign feature is used, the additional and a 25 WPM CW signal. The signals
compress the most common messages information is sent in place of the grid were generated with a key‑down signal‑
used for minimal contacts into fixed‑ locator, or by using some of the six million to‑noise ratio of –10 dB in a 2,500 Hz
length 72‑bit packets. FT8 uses similar available slots mentioned above. reference bandwidth. Among the WSJT-X
source encoding, but provides additional modes, WSPR has the narrowest
flexibility and room for growth by adding Our compression algorithm supports occupied bandwidth, 5.9 Hz, and JT65
three extra bits that can be used to define messages starting with CQ AA through has the widest at 177.6 Hz. JT4, JT9,
up to 14 enhanced message types. CQ ZZ. Such messages are encoded by JT65, and QRA64 use 1‑minute timed
sending the pseudo‑call signs E9AA
Why 28 bits for a call sign, and 15 for a through E9ZZ in place of the first call sign sequences of transmission and reception,
grid locator? A standard Amateur call sign of a standard message. Upon reception, synchronized approximately with UTC.
consists of a one‑ or two‑character prefix, these calls are converted back to the form FT8 uses 15‑second sequences, and
at least one of which must be a letter, CQ AA through CQ ZZ. This allows users WSPR uses 2 minutes.
followed by a digit and a suffix of one to to send directed CQ messages, such as Some design parameters of the slow
three letters. Within these rules, the CQ DX, CQ EU, or CQ VT. modes are summarized in Table 1.
number of possible call signs equals
26