MSK441
Techinqucially speaking this is a form of PSK modulation called OQPSK or Offset Quadature Phase Shift Keying
Little about OQPSK
Offset quadrature phase-shift keying (OQPSK) is a variant of phase-shift keying modulation using 4 different values of the phase to transmit. It is sometimes called Staggered quadrature phase-shift keying (SQPSK).
Difference of the phase between QPSK and OQPSK
Taking four values of the phase (two bits) at a time to construct a QPSK symbol can allow the phase of the signal to jump by as much as 180° at a time. When the signal is low-pass filtered (as is typical in a transmitter), these phase-shifts result in large amplitude fluctuations, an undesirable quality in communication systems. By offsetting the timing of the odd and even bits by one bit-period, or half a symbol-period, the in-phase and quadrature components will never change at the same time.
MSK441 Protocol
Standard MSK144 messages are structured in the same way as those in the slow modes, with a 72 bits of user information. Forward error correction is implemented by first augmenting the 72 message bits with an 8-bit cyclic redundancy check (CRC) calculated from the message bits. The CRC is used to detect and eliminate most false decodes at the receiver. The resulting 80-bit augmented message is mapped to a 128-bit codeword using a (128,80) binary low-density-parity-check (LDPC) code designed by K9AN specifically for this purpose. Two 8-bit synchronizing sequences are added to make a message frame 144 bits long. Modulation is Offset Quadrature Phase-Shift Keying (OQPSK) at 2000 baud. Even-numbered bits are conveyed over the in-phase channel, odd-numbered bits on the quadrature channel. Individual symbols are shaped with half-sine profiles, thereby ensuring a generated waveform with constant envelope, equivalent to a Minimum Shift Keying (MSK) waveform. Frame duration is 72 ms, so the effective character transmission rate for standard messages is up to 250 cps.
Contest Mode in MSK144 conveys an additional acknowledgment bit (the “R” in a message of the form W9XYZ K1ABC R FN42) by using the fact that meteor scatter and other propagation modes usable with MSK144 are generally effective only out to distances of order 2500 km. To convey the message fragment R FN42, WSJT-X encodes the locator as that of its antipodes. The receiving program recognizes a locator with distance greater than 10,000 km, does the reverse transformation, and inserts the implied “R”.
MSK144 also supports short-form messages that can be used after QSO partners have exchanged both callsigns. Short messages consist of 4 bits encoding a signal report, R+report, RRR, or 73, together with a 12-bit hash code based on the ordered pair of “to” and “from” callsigns. Another specially designed LDPC (32,16) code provides error correction, and an 8-bit synchronizing vector is appended to make up a 40-bit frame. Short-message duration is thus 20 ms, and short messages can be decoded from very short meteor pings.
The 72 ms or 20 ms frames of MSK144 messages are repeated without gaps for the full duration of a transmission cycle. For most purposes, a cycle duration of 15 s is suitable and recommended for MSK144.
The modulated MSK144 signal occupies the full bandwidth of a SSB transmitter, so transmissions are always centered at audio frequency 1500 Hz. For best results, transmitter and receiver filters should be adjusted to provide the flattest possible response over the range 300Hz to 2700Hz. The maximum permissible frequency offset between you and your QSO partner ± 200 Hz.
