HIGH FREQUENCY COMMUNICATIONS

摘要

A slow speed hf communications modem employs frequency and time diversity to overcome transmission problems due to noise, multipath etc. A digitally encoded signal is transmitted consecutively on five complementary 2-tone channels distributed within the hf band. A unique start-of-message (SOM) signal is generated (9) to precede a message transmission and an end-of-message signal (EOM) follows the message. The SOM signal is detected (18) to produce an accurate timing pulse (20) which initiates a clock in the receiver modem 3 arranged to sample each channel tone at times appropriate to receiving message signals. The noise in each channel tone is also sampled before (pre-data bit) and after (post-data bit) a signal data bit and the measured pre- and post-data bits are combined to assign a weighting factor to the associated message data bits. By providing SOM and EOM signals the modem is able to operate unattended.