Pseudo-Random Binary Sequences and Data Scramblers

Abstract

This chapter begins a series on digital communications. DSP technology has made a dramatic impact on digital communications, particularly narrow band systems like voice-band telephone line modems and cellular telephones. In 1970, a plain 9600 bps telephone line modem was the size of a big microwave oven and cost at least $15,000. It was basically just a data pump with no extra features. Now a state-of-the-art V.90 56 kbps modem can be bought for less than $100 and fits in a small box or on a small card. In addition, this modern modem has many features like data compression, error detection and correction, trellis coded modulation, fax modes, automatic dialing, network management functions, a secondary channel, and the ability to do most of the past popular modem standards ranging from speeds of 300 bps up to 33,600 bps. It is now possible to concurrently run at least 12 full duplex V.90 modems in a single state-of-the-art DSP core and chips with multiple cores are in development. These high-end chips will be used in remote access servers (RAS) by Internet service providers for voice over IP (VOIP) and modem pools. Because of the flexibility of the software approach to implementing signal processing algorithms with DSP’s, new theoretical developments have almost instantaneously been included in commercial telephone line modems. These techniques have later found their way into higher speed systems that use greater channel bandwidths like high speed digital subscriber lines (DSL), microwave systems, and satellite communications.