Abstract: The invention relates to apparatus for detecting the presence or the absence of a signal carrier wave received by a receiver. The carrier wave is modulated by a digital signal at a symbol clock frequency. According to the invention, the apparatus includes:a non-linear circuit receiving the received signal or the received signal after it has been frequency transposed, the non-linear circuit supplying in particular a signal whose frequency corresponds to the symbol clock frequency;a narrow-band filter centered on the symbol clock frequency and supplying a filtered signal;detection means for detecting the filtered signal and for supplying a detection signal; andcomparator means for comparing the detection signal with a reference value, the output signal from the comparator means constituting a signal indicating the presence or the absence of the carrier wave.

Abstract: In a convolutional coding and Viterbi decoding system transparent to phase skips of .pi. and .pi./2 with applications in TDMA transmission, the transmitter 3 includes a demultiplexer receiving a digital bit stream and operating at the bit timing rate, a QPSK differential coder receiving the demultiplexed bit streams and supplying two coded bit streams, two convolutional coders each supplying one code word, and two multiplexers each receiving one code word from a respective convolutional coder and supplying one component of a symbol to be transmitted. The receiver generates two received symbol components which are applied to two demultiplexers each supplying one code word. Each bit of a code word is applied to a single Viterbi decoder. A QPSK type differential decoder supplies two decoded bit streams which are then multiplexed.