Alternatives in digital communications

The following major interrelated aspects in digital communications are described: source encoding, channel encoding and transmission, and receiver methods. Methods for digitizing speech and video are summarized. Digitized voice can be transmitted at 10 kb/s, while phone video requires 1.544 Mb/s. The major aspects of both forward-error control (FEC) and automatic request for repeats (ARQ) are considered. Power-limited forward channels are distinguished from bandwidth-limited ones. Signal distance concepts and maximum-likelihood detection principles provide the framework for describing both block-coded and convolutional-coded signaling. Convolutional coding with Viterbi decoding appears to achieve the most cost-effective forward-error control. The improved FEC techniques benefit the transmission of real-time digitized voice and video and any communication over severely power-llmited links. ARQ methods continue to be effective and practical for most cases where the delay associated with retransmission can be tolerated.