Reliable vocoder data transmission over fading HF channels

The quality of shortwave channels may vary considerably due to the time-specific changes of the transmission conditions. So the correct transmission of a continuous data stream, e.g. digitized speech signals output by a vocoder cannot be guaranteed under adverse conditions even by means of complex error correction measures or costly high transmitter power. This results in the loss of vocoder data-the transmission of speech information is partly or fully interrupted which impairs intelligibility. For this reason we propose a voice transmission system as a solution to the problem of correct voice data transmission. It is a method that offers quasi-real-time behaviour for channels that are subject to no interference or only slight interference, and that offers complete and correct data transmission with reduced real-time behaviour (mailbox operation) in the presence of strong or long-term interference and fading. To reach intelligibility at the transmitter end, the vocoder data are first divided into intelligible blocks, e.g. into coherent words between two speech pauses and stored in a buffer. The blocks are then transferred to the lower layers of the ISO/OSI reference model where an ARQ protocol is employed which leads to correct data transmission. At the receiver end, the blocks can then be listened to individually or as a whole and, if necessary, repeated once or several times (mailbox operation). This method was tested over a radio link of 800 km between Rostock University and Rohde & Schwarz Munich laboratory. 1200 bps vocoders from Comsat Co. and XK 2100 transceivers with packet radio protocol (PRP) RSX.25 from Rohde & Schwarz were used.