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Introduction

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 is of great importance. Many modern RF system architectures are based on it, thus we find it appropriate to discuss it briefly here. As pointed out in the previous section, the number of available frequencies for mobile applications is limited. With reference to Figure 1.3, assume that a multitude of mobile users are found simultaneously in the region of area A. Further assume that there are N available RF channels and all the users connect to each other through a central base station that is located at a favorably high spot to provide appropriate geographical coverage. It follows that the system capacity is limited to simultaneous users per square meter. Clearly such an architecture is limited and cannot support large communication systems in large coverage areas.

Figure 1.3 Limited capacity RF system.

Now, with reference to Figure 1.4, assume that we divide the same area A into separate sub‐areas, named “cells”. At the center of each cell we place a base station that transmits with power sufficient to cover its own cell, but low enough so that it cannot be received in the adjacent cells. The base stations are all connected to each other by physical lines interconnected by a central computer that acts as a switch. Now, assume that we arrange the cells in regular patterns of cells, called clusters, each consisting of K adjacent cells.

Figure 1.4 The cellular principle.

Since, there is virtually no interference between cells belonging to different clusters, we can use all the N frequencies within each cluster. If the whole coverage area consists of M clusters, it follows that now the system capacity increases to C = N/(A/M) = MN/A, a factor of M. However, the problem remaining is how to prevent the mobile users from losing communication when passing from cell to cell. To see how the issue is solved, assume that the base stations in the various cells continuously report to the central computer how well they receive the mobile subscribers passing nearby. Assume now that a mobile subscriber is connected to the base station of cell #1 and is approaching cell #2, while travelling away from cell #1. At a certain point the user will begin to lose communication with cell #1, while the link with cell #2 becomes stronger. Since the central computer is aware of the scenario, at a certain point it will instruct the mobile user to leave the channel of cell #1 and connect to a free channel of cell #2. This process is called a “handoff” and allows the mobile subscribers to pass from cell to cell without losing communication. The cellular architecture was made possible by the advent of microprocessor components, which allowed introducing enough intelligence within the mobile equipment so to be able to instruct it how to handle the handoff process.

1.2 Detailed Overview of Wireless Systems and Technologies

1.2.1 System types

Wireless communications using electromagnetic waves began at the end of the nineteenth century with Tesla, Popov, and Marconi. Marconi sent the first wireless signals (Morse code). In his first experiments, Marconi used a wavelength (λ) much longer than 1 km, and it was in 1920 that he discovered short waves with λ ≈ 100 m.

World War 2 gave rise to many advances in development of wireless communication systems, especially in the fields of RADAR (RAdio Detecting And Ranging), wireless data transmission, and remote sensing. Since then, wireless communication has been evolving continuously, significantly affecting many different aspects of our life. Standardization of the communication technologies, an important step in development of communication systems and services, started with the advent of commercial TV in the 1940s, when the first TV standards were introduced. The development of mobile communications was rather slow till the 1970s, when enabling technologies were developed for reliable, compact RF circuits and modules.

Today, wireless communication systems are very ubiquitous, providing a wide variety of highly reliable services. A broad range of systems and services have been developed, paving the way for implementation of wireless communication systems: satellite communications, radio and TV broadcasting systems, mobile phones, wireless LANs, wireless sensor networks, and so on.

The rapid growth of wireless systems implies an increased demand for spectrum, making spectrum allocation a key issue for the further extension of existing communication services and the development of new ones.

The challenge in the design of communication systems is the efficient use of the allocated resources, that is, power budget and available bandwidth, to provide high‐quality communications in terms of bit error rate (BER) and data rate (measured in bits per second, bps). In the case of wireless communications, the design of such systems is even more challenging due to the fact that wireless channels are subject to dynamic fast environmental changes.

No single technology can provide a proper and optimal solution for all desired wireless applications. Wireless communication systems/networks can be generally divided into three main categories, where each category aims to address specific needs. The division is based on the coverage range: wireless personal area network (WPAN), wireless local area network (WLAN), and wireless wide area network (WWAN). The system’s range determines its latency.

WPANs, such as Bluetooth, provide...