Part IV MULTIPLE ACCESS AND ADVANCED TRANSCEIVER SCHEMES 363
9.3 Antennas for Base Stations
9.3.4 Impact of the Environment on Antenna Pattern
The antenna pattern of BS antennas is usually defined for the case when an antenna is in free space, or above an ideally conducting plane. This is what is measured in an anechoic chamber, and is also the easiest to compute. However, at its location of operation, a BS antenna is surrounded by different objects of finite extent and conductivity. The antenna patterns in such surroundings can deviate significantly from theoretical patterns. The antenna mast, which is usually metallic, and thus highly conductive, can lead to distortions. Similarly, roofs made out of certain building materials, like reinforced concrete, can distort antenna patterns (see Figure 9.10).
Antennas 177
Ref = 20 10 dB / division
Y
X
X-Y-Pattern
Figure 9.10 Antenna pattern of an omnidirectional antenna close to an antenna mast. Distance from the antenna mast 30 cm. Diameter of the mast: very small (solid), 5 cm (dashed), 10 cm (dotted).
Reproduced with permission from Molisch et al. [1995]©European Microwave Association.
Ground Ground
−400
−20 0 20
Gain/ (dB)
22.5
θ/(degree) 45 67.5 90 Concrete
(a) (b) (c)
hb hb hroof
db db
Geome ry (a)
Geome ry (b)
Figure 9.11 Distortion in the vertical antenna pattern. (a) Idealized geometry. (b) Real geometry, including the roof. (c) Distortions in antenna patterns. Properties of all materials: relative dielectric constantεr=2: conductivity 0.01 S/m;db=20 m.
Reproduced with permission from Molisch et al. [1995]©European Microwave Association.
Distortions in the vertical pattern are plotted in Figure 9.11. They arise from two effects: the fact that the (finite extent) roof is much closer to the antenna than the ground and that the dielectric properties of the roof are different from those of the ground.
The presence of a human body also distorts antenna patterns. One way of taking this into account is to consider the antenna and the human body as a “superantenna,” whose characteristics (efficiency, radiation pattern) can be measured and characterized in the same manner as “regular”
antennas. Figure 9.12 shows example measurements of antenna patterns by a human head and body.
20 15
10 150 5
120 90
30 30 60
120
150
180 0
90
Average gain (dBi)
Talk position (free) Talk position (phantom) Browse position (free) Browse position (phantom) Figure 9.12 Pattern of antenna distortion by human body in talk and data-browsing position.
Reproduced with permission from Harryson et al. [2010]. Copyright IEEE.
Further Reading
For a general introduction to antenna theory, we just refer to the many excellent books on antenna theory: Balanis [2005], Kraus and Marhefka [2002], Ramo et al. [1967], and Stutzman and Thiele [1997], as well as the somewhat more advanced text of Collin [1985]; the latter also devotes a separate chapter to the properties of receiving antennas. For more details on antenna specifically for wireless communications, see Godara [2001] and Vaughan and Andersen [2003]. Antennas for the MS are discussed in Fujimoto [2008] and Hirasawa and Haneishi [1991]. Antenna design for BSs is surveyed in the monograph of Chen and Luk [2009] and in the conference paper by Beckman and Lindmark [2007]. Finally, phased array antennas are treated in detail in Hansen [1998] and Mailloux [1994]. Discussions on beam tilting can be found in Manholm et al. [2003]. The impact of the human head and body on antenna characteristics is discussed, e.g., in Ogawa and Matsuyoshi [2001], Kivekaes et al. [2004] and [Harryson et al. 2010].
For updates and errata for this chapter, see wides.usc.edu/teaching/textbook
Part III
Transceivers and Signal Processing
The ultimate performance limits of wireless systems are determined by the wireless propagation channels that we have investigated in the previous parts. The task of practical transceiver design now involves finding suitable modulation schemes, codes, and signal processing algorithms so that these performance limits can be approximated “as closely as possible.” This task always involves a tradeoff between the performance and the hardware and software effort. As technology progresses, more and more complicated schemes can be implemented. For example, a third-generation cellphone has a computation power that is comparable to a (year 2000) personal computer, and can thus implement signal processing algorithms whose practical use was unthinkable in the mid-1990s. For this reason, this part of the book will not pay too much attention to algorithm complexity – what is too complex at the current time might well be a standard solution a few years down the road.
The part starts with a description of the general structure of a transceiver in Chapter 10. It describes the various blocks in a transmitter and receiver, as well as simplified models that can be used for system simulations and design. Next, we discuss the variousmodulation formats, and their specific advantages and disadvantages for their use in a wireless context. For example, we find that constant-modulus modulation methods are especially useful for battery-powered transmitters, since they allow the use of high-efficiency amplifiers. Building on the formal mathematical description of those modulation formats, Chapter 12 then describes how to evaluate their performance in terms ofbit error probability in different types of fading channels. We find that the performance of such systems is mostly limited by two effects: fading and delay dispersion. The effect of fading can be greatly mitigated bydiversity, i.e., by transmitting the same signal via different paths. Chapter 13 describes the different methods of obtaining such different paths, e.g., by implementing multiple antennas, by repeating the signal at different frequencies, or at different times. The chapter also discusses the effect that the diversity has on the performance of the different modulation schemes.
Negative effects of the delay dispersion can also be combated by diversity; however, it is more effective to use equalization.Equalizers do not only combat intersymbol interference created by delayed echoes of the original signal, but they make use of them, exploiting the energy contained in such echoes. They can thus lead to a considerable improvement of performance, especially in systems with high data rates and/or systems operating in channels with large delay spreads.
Chapter 16 describes different equalizer structures, from the simple linear equalizers to the optimum (but highly complex) maximum-likelihood sequence detectors.
W ireless Communications, Second Edition Andreas F. Molisch
© 2011 John Wiley & Sons Ltd. ISBN: 978-0-470-74187-0
Diversity and equalizers are not always a sufficient or effective way of improving the error prob- ability. In many cases,codingcan greatly enhance the performance, and provide the transmission quality required by a specific application. Chapter 14 thus gives an overview of the different cod- ing schemes that are most popular for wireless communications, including the near-optimum turbo codes and Low Density Parity Check (LDPC) codes that have drawn great attention since the early 1990s. These coding schemes are intended for correcting the errors introduced on the propagation channel. The chapter also describes the fundamentals of information theory, which establishes the ultimate performance limits that can be achieved with “ideal” codes. A different type of coding is source coding, which translates the information from the source into a bitstream that can be trans- mitted most efficiently over the wireless channel. Chapter 15 gives an overview ofspeech coding, which is the most important type of source coding for wireless applications. Finally, Chapter 16 describes equalization, i.e., methods for compensating for delay dispersion of the channel
Modulation, coding, and equalization for wireless communications are, of course, strongly related to digital communications in general. This part of the book isnotintended as a textbook of digital communications, but rather assumes that the reader is already familiar with the topic from either previous courses, or one of the many excellent textbooks (e.g., [Proakis 2005], [Barry et al. 2003], [Sklar 2001], [Anderson 2005]). While the text gives summaries of the most salient facts, they are rather terse, and only intended as a reminder to the reader.
10
Structure of a Wireless Communication Link