The WSPR Protocol
The type of radio emission is "F1D", frequency-shift keying. A message contains a station's callsign, Maidenhead grid locator, and transmitter power in dBm. The WSPR protocol compresses the information in the message into 50 binary digits. These are encoded using a convolutional code with constraint length K=32 and a rate of r=1/2. The long constraint length makes undetected decoding errors less probable at the cost, that the highly efficient Viterbi algorithm must be replaced by a simple sequential algorithm for the decoding process.
- Standard message: callsign + 4-digit locator + dBm (e.g., K1ABC FN20 37)
- Messages with a compound callsign and/or 6-digit locator use a two-transmission sequence. The first transmission carries compound callsign and power level, or standard callsign, 4-digit locator, and power level; the second transmission carries a hashed callsign, 6-digit locator, and power level. Add-on prefixes can be up to three alphanumeric characters; add-on suffixes can be a single letter or one or two digits.
- Standard message components after lossless compression: 28 bits for callsign, 15 for locator, 7 for power level, 50 bits total.
- Forward error correction (FEC): non-recursive convolutional code with constraint length K=32, rate r=1/2.
- Number of binary channel symbols: nsym = (50+K-1) * 2 = 162.
- Keying rate: 12000/8192 = 1.4648 baud.
- Modulation: continuous phase 4-FSK, tone separation 1.4648 Hz.
- Occupied bandwidth: about 6Hz
- Synchronization: 162-bit pseudo-random sync vector.
- Data structure: each channel symbol conveys one sync bit (LSB) and one data bit (MSB).
- Duration of transmission: 162 * 8192/12000 = 110.6 s.
- Transmissions nominally start one second into an even UTC minute: e.g., at hh:00:01, hh:02:01, etc.
- Minimum S/N for reception: around –28 dB on the WSJT scale (2500Hz reference bandwidth).