What is the length of an FT8 transmission cycle?

FT8, one of the most popular digital modes for weak-signal HF communication, uses a fixed transmission sequence duration of exactly 15 seconds. This block includes about 12.6 seconds of actual transmission followed by a short window for decoding and processing the signal. Because of this strict timing, both the transmitting and receiving stations must have closely synchronized computer clocks (usually within a second of each other) to decode the signals successfully. It is important to remember that the "8" in FT8 refers to its 8-tone frequency shift keying modulation, not the duration of its cycle.