Introduction to digital communications. Signals and spectra. The digitization of analog signals. Selecting and quantization. Digital transmission in the baseband. Pulse-code modulation. A receiver with a custom filter. The correlation receiver. Detection of digital signals in the presence of AWGN. The probability of error. Interference of symbols. The Nyquist theorem. Shaping pulses and equalization. The concept of signal space, orthogonal signals. Digital modulations in the transposed range (ASK, PSK, FSK). Coherent and incoherent detection. Detection of signals in channels with AWGN. Mean error probability per symbol. The probability of error for different modulation methods. Spectrally efficient modulation. The stream in an extended spectrum. Direct sequence. Frequency hopping. Techniques of multiple access (TDMA, FDMA, CDMA).