Filtering and equalization for digital transmission

Abstract

ICITAL transmission systems of all types are of great current interest as we embark on the information age. For any new transmission system, the topology, components, and system parameters are determined by many and various techniques. They are chosen not only by explicit, deductive technical reasoning, but also by the art of engineering, the desire for continued product line compatibility, patents possessed or not possessed, and new technology. Nonetheless, most digital transmission systems are made up using one or more of the following components: a bandpass filter, low-pass filter, Nyquist filter, slope equalizer, cable equalizer, and transversal equalizer. These items are the subject of this article. It is ironic that digital transmission systems have a preponderance of such components-all necessary for analog signal conditioning of the digitally transmitted signal. In the myriad of digital transmission systems, two distinct approaches to filtering and equalization predominate: one for baseband systems and the other for frequency translated, or modulated, signal systems. This division results from the need to have a competitive system transmission efficiency to cost ratio: where transmission efficiency is transmitted bit rate/channel bandwidth (BR/BW). For baseband systems, as might be expected, the difficulties, and thus cost, of filtering and equalization increase with increased transmission efficiency. To obtain a favorable BR/BW to cost ratio, the bandwidth, an available quantity, is usually sacrificed to keep system costs low. Signal modulation is required when it is desired to simultaneously transmit several baseband signals over common transmission facilities using a frequency division multiplex scheme. A modulated signal system is also necessary when the transmission vehicle has a significant low-frequency limitation; as does, for example, the general customer premises-to-customer premises telephone connection. In such systems, costs start off high because of the additional modulation circuitry. Money can then be further spent on filtering and equalization to allow for increased transmitted bit rate in order to provide a more reasonable BR/BW to cost ratio. Based upon the above premise, the text expands further on the differences between filters and equalizers used for baseband and modulated digital transmission systems.