Page 27 January February 2018 TCA
P. 27
Work the World with WSJT-X
Part 2: Codes, Modes and Cooperative Software Development
The second of a two-part tutorial about the Joe Taylor, K1JT, Steve Franke, K9AN and Bill Somerville, G4WJS
most popular digital software in use today,
from the people who designed and
developed it. Part 1 was published in the
January-February 2017 TCA. The articles are
being reprinted with permission from the
September 2017 and October 2017 QST,
with thanks to the Amateur Radio Relay
League.
Part 1 of this two‑part article covered
topics that highlight the capabilities of
weak‑signal communication program
1
WSJT-X. This software package
provides tools for a wide range of
Amateur Radio activities including
low‑power DXing, meteor scatter,
moonbounce and precise frequency
measurement – all of them possible with
relatively modest station equipment. Figure 1: Block diagram showing steps in a typical digital communication system.
Based on modern communication and
information theory, the WSJT-X protocols Offset Quadrature Phase‑Shift Keying Reception of transmitted symbols
and software can boost your signal’s (OQPSK) with waveforms shaped to requires accurate synchronization of time
effective reach by the rough equivalent of maintain a constant envelope. All other and frequency between transmitting and
10‑15 dB of added signal strength. modes use frequency‑shift keying receiving stations. To make this possible
(FSK), with a different tone frequency to with typical Amateur station equipment,
This concluding Part 2 outlines some represent each allowed symbol value. each WSJT-X protocol includes a unique
digital communication theory Binary modulation (m = 1) implies synchronizing pattern: a sequence of
fundamentals, including examples to transmitting one bit at a time. Modulation known symbols interspersed with those
make the discussion accessible to most schemes with larger m are used to an carrying message information. The
Amateurs. We compare the eight weak‑ advantage in all but one of the WSJT-X software demodulation algorithm starts
signal protocols in WSJT-X and explain modes. by looking for the known pattern, thereby
how their impressive performance is determining any frequency and time
achieved. Finally, we describe the tools Important benefits can be gained by offset, as well as the locations of
and informal cooperative practices used adding controlled redundancy to a digital boundaries between received symbols.
for creating the WSJT-X software. We message so that transmission errors can
think it’s important that dedicated be recognized and corrected. Simple Specifically, the JT65 mode uses a
enthusiasts devote their algorithmic and repetition of each symbol is a trivial form (63,12) code with q = 6 and thus
programming skills and interests to the of redundancy. But much more powerful Q = 2 = 64. Its code rate is k/n = 0.19,
q
good of the hobby. We have found that redundancy can be arranged by mapping and its modulation uses m = 6 and thus
many other Amateurs would like to have a each sequence of k message symbols in 2 = 64 tone frequency‑shift keying, with
m
deeper understanding of how these a controlled way into a unique and longer one additional tone used for
weak‑signal protocols work, and how they sequence of n symbols called a synchronization.
were developed. We hope this article codeword. This technique is called
satisfies that desire. forward error correction (FEC). The Let’s divide the process of transmission
and reception into a sequence of
Digital Communication Fundamentals WSJT-X protocols use block codes in independent steps (see Figure 1).
which the values of n and k are fixed, and
Digital communication conveys digital labeled as (n,k) codes. An integer The steps correspond roughly to
identifiable blocks of the WSJT-X source
information from an originating source to parameter q can be used to define the code. Steps 1 to 5 take place at the
one or more destinations. Here the range of available symbol values for a transmitting station, and steps 6 to 9
digital information is modulated onto a code, analogous to the m we used for at the receiving end.
q
carrier and transferred over a radio modulation schemes. Parameter Q = 2 is
channel. The basic unit of transmitted then referred to as the alphabet size of 1) Generate a message.
data is a channel symbol. The symbols the code. The code symbol values range
represent numbers, in turn comprised of from 0 up to Q – 1, and each codeword 2) Compress message to k symbols of
bits. The modulator may transmit m conveys kq message bits. The amount of q bits per symbol.
information bits in each symbol, using redundancy is characterized by the ratio 3) Add error‑correcting redundancy to
2 different waveforms to represent n/k, and its reciprocal k/n is the code rate. produce codeword of n symbols.
m
symbol values from 0 up to 2 – 1. The The mathematics underlying design of
m
different waveforms might have distinct such k‑to‑n mapping schemes and their 4) Add synchronizing pattern and
amplitudes, phases, frequencies, or corresponding n‑to‑k reverse modulate onto a carrier.
shapes. The WSJT-X waveforms are transformations forms a major branch of 5) Transmit modulated waveform over a
made of sinusoids with constant modern communication theory. radio channel.
amplitude. The MSK144 protocol uses
25
Part 2: Codes, Modes and Cooperative Software Development
The second of a two-part tutorial about the Joe Taylor, K1JT, Steve Franke, K9AN and Bill Somerville, G4WJS
most popular digital software in use today,
from the people who designed and
developed it. Part 1 was published in the
January-February 2017 TCA. The articles are
being reprinted with permission from the
September 2017 and October 2017 QST,
with thanks to the Amateur Radio Relay
League.
Part 1 of this two‑part article covered
topics that highlight the capabilities of
weak‑signal communication program
1
WSJT-X. This software package
provides tools for a wide range of
Amateur Radio activities including
low‑power DXing, meteor scatter,
moonbounce and precise frequency
measurement – all of them possible with
relatively modest station equipment. Figure 1: Block diagram showing steps in a typical digital communication system.
Based on modern communication and
information theory, the WSJT-X protocols Offset Quadrature Phase‑Shift Keying Reception of transmitted symbols
and software can boost your signal’s (OQPSK) with waveforms shaped to requires accurate synchronization of time
effective reach by the rough equivalent of maintain a constant envelope. All other and frequency between transmitting and
10‑15 dB of added signal strength. modes use frequency‑shift keying receiving stations. To make this possible
(FSK), with a different tone frequency to with typical Amateur station equipment,
This concluding Part 2 outlines some represent each allowed symbol value. each WSJT-X protocol includes a unique
digital communication theory Binary modulation (m = 1) implies synchronizing pattern: a sequence of
fundamentals, including examples to transmitting one bit at a time. Modulation known symbols interspersed with those
make the discussion accessible to most schemes with larger m are used to an carrying message information. The
Amateurs. We compare the eight weak‑ advantage in all but one of the WSJT-X software demodulation algorithm starts
signal protocols in WSJT-X and explain modes. by looking for the known pattern, thereby
how their impressive performance is determining any frequency and time
achieved. Finally, we describe the tools Important benefits can be gained by offset, as well as the locations of
and informal cooperative practices used adding controlled redundancy to a digital boundaries between received symbols.
for creating the WSJT-X software. We message so that transmission errors can
think it’s important that dedicated be recognized and corrected. Simple Specifically, the JT65 mode uses a
enthusiasts devote their algorithmic and repetition of each symbol is a trivial form (63,12) code with q = 6 and thus
programming skills and interests to the of redundancy. But much more powerful Q = 2 = 64. Its code rate is k/n = 0.19,
q
good of the hobby. We have found that redundancy can be arranged by mapping and its modulation uses m = 6 and thus
many other Amateurs would like to have a each sequence of k message symbols in 2 = 64 tone frequency‑shift keying, with
m
deeper understanding of how these a controlled way into a unique and longer one additional tone used for
weak‑signal protocols work, and how they sequence of n symbols called a synchronization.
were developed. We hope this article codeword. This technique is called
satisfies that desire. forward error correction (FEC). The Let’s divide the process of transmission
and reception into a sequence of
Digital Communication Fundamentals WSJT-X protocols use block codes in independent steps (see Figure 1).
which the values of n and k are fixed, and
Digital communication conveys digital labeled as (n,k) codes. An integer The steps correspond roughly to
identifiable blocks of the WSJT-X source
information from an originating source to parameter q can be used to define the code. Steps 1 to 5 take place at the
one or more destinations. Here the range of available symbol values for a transmitting station, and steps 6 to 9
digital information is modulated onto a code, analogous to the m we used for at the receiving end.
q
carrier and transferred over a radio modulation schemes. Parameter Q = 2 is
channel. The basic unit of transmitted then referred to as the alphabet size of 1) Generate a message.
data is a channel symbol. The symbols the code. The code symbol values range
represent numbers, in turn comprised of from 0 up to Q – 1, and each codeword 2) Compress message to k symbols of
bits. The modulator may transmit m conveys kq message bits. The amount of q bits per symbol.
information bits in each symbol, using redundancy is characterized by the ratio 3) Add error‑correcting redundancy to
2 different waveforms to represent n/k, and its reciprocal k/n is the code rate. produce codeword of n symbols.
m
symbol values from 0 up to 2 – 1. The The mathematics underlying design of
m
different waveforms might have distinct such k‑to‑n mapping schemes and their 4) Add synchronizing pattern and
amplitudes, phases, frequencies, or corresponding n‑to‑k reverse modulate onto a carrier.
shapes. The WSJT-X waveforms are transformations forms a major branch of 5) Transmit modulated waveform over a
made of sinusoids with constant modern communication theory. radio channel.
amplitude. The MSK144 protocol uses
25
