Wireless Transceiver Design
Wireless Transceiver Design
1.1 Radio Frequency Systems
Radio frequency (RF) systems are an essential part of our everyday life. They provide wireless connectivity for diversified applications, such as short-range car/door openers and wireless earphones, medium-range digital systems such as routers for computer data links, and remote-piloted vehicle controls, or long-distance communication systems such as cellular phones, and satellite networks. The required characteristics of wireless transceivers, however, are strongly dependent on the nature of the target system in which the equipment is intended to operate. In this introductory chapter, we provide a detailed overview of several important RF systems, with the purpose to provide the reader with a basic background on the different architectural and operational requirements, which directly dictate the various transceiver design strategies discussed in the chapters to follow.
1.1.1 Conceptual RF system
An RF system consists essentially of five major components, as shown in Figure 1.1 .
Transmitter : Accepts at its input the information to be transmitted. Generates an RF signal embedding the input information. "Boosts" the RF signal to a suitable power level. The RF signal is routed to the antenna port.
Transmit antenna : Serves as the mediator between the transmitter and the transmission medium. Its purpose is to make sure that all the RF signal power present at the antenna port, leaves the transmitter, enters the transmission medium, and propagates in the desired direction.
Transmission channel : Is the medium separating the transmitter from the receiver. The RF signal must cross it in order to reach the receiving antenna. Usually the transmission medium consists of air or vacuum, but it may be solid or liquid as well. While propagating through the transmission medium, the RF signal loses its strength, and becomes weaker and weaker as it proceeds through the medium.
Receive antenna : Serves as the mediator between the transmission medium and the receiver. Its purpose is to capture as much as possible of the incident (weak) RF signal power remaining after crossing the medium, and convey it to the input of the receiver.
Receiver : Accepts the RF signal captured by the antenna. Extracts the information embedded in it. The information is routed to the receiver output.
Figure 1.1 One-way RF system.
The system of Figure 1.1 is one-way. However, adding an identical RF system in the opposite direction yields a two-way RF system, as shown in Figure 1.2 . The transmitter/receiver combination is termed a "transceiver". The antenna may transmit and receive simultaneously, while the transmitter and receiver are operating independently from each other.
Figure 1.2 Two-way RF system.
1.1.2 The frequency spectrum
For various reasons, not all RF frequencies are equally well-fit for implementing different RF systems. For instance, since the optimum physical dimensions of transmit and receive antennas are directly related to the frequency and must be made larger as the frequency becomes lower, it follows that at low frequencies the antenna size becomes impractical for use in mobile systems such as cellular. In contrast, as the frequency becomes higher, the antennas may be made smaller, but the power losses and Doppler fading through the medium increase, which limits the transmission range and the travelling speed. It follows that choice of the RF frequency range is application dependent and the number of useful RF channels is limited. Several RF system architectures, such as the cellular architecture, have been developed in order to overcome the frequency shortage.
1.1.3 Cellular concept
The cellular concept i