have a prestored table derived from a calibra- Finally, Section 2.4a.8 presents a summary of the chapter. tion process, and then adjust passive components to match the tuning recommendations of the table, or it will sense the VSWR and adapt the tuning elements until 2.4a.2 Hardware architecture a minimum VSWR is attained. Some modern antennas include a number of passive The underlying hardware structure for a system provides components spread over the length of the radiating the maximum bounds for performance. The goal of this elements that are able to present reasonable VSWR Carrier Symbol Recovery Recovery Symbol Filter Network Demodulator Detector Application Processing Stack Carrier Symbol Phase Recovery Recovery Detector Symbol Network Filter Demodulator Detector Application Processing Stack Figure 2.4a-1 (a) Data flow and component structure of a generalized coherent radio receiver. (b) Data flow and component structure of a generalized coherent radio receiver designed for digital systems, with sampling as a discrete step (see Design Choices section for an explanation). The software defined radio as a platform for cognitive radio CHAPTER 2.4a performance without active tuning. The best such units upon to provide one of these control strategies for an ex- today span a frequency range of nearly 10:1. However, ternally connected antenna, and SO must provide in- for radios that are expected to span 2 MHz-2 GHz, it is terfaces to control external RF devices. Experience has reasonable to expect that the radio will need to be able to shown Ethernet to be the preferable standard interface, SO control a switch to select an appropriate antenna for the that remote control devices for RF external adapters, frequency band in which the transceiver is currently switches, PAs, LNAs, and tuners can readily be controlled. operating. Where beam antennas are used, it may also be necessary for the radio to be able to manage beam RF front-end pointing. In some cases, this is accomplished by an antenna rotator, or by a dish gimba