Wireless Communications Principles and Fundamentals

2 Wireless Communications Principles and Fundamentals 2.1 Introduction Wireless networks, as the name suggests, utilize wireless transmission for exchange of information The exact form of wireless transmission can vary For example, most people are accustomed to using remote control devices that employ infrared transmission However, the dominant form of wireless transmission is radio-based transmission Radio technology is not new, it has a history of over a century and its basic principles remain the same with those in its early stage of development In order to explain wireless transmission, an explanation of electromagnetic wave propagation must be given A great deal of theory accompanies the way in which electromagnetic waves propagate In the early years of radio transmission (at the end of the nineteenth century) scientists believed that electromagnetic waves needed some short of medium in order to propagate, since it seemed very strange to them that waves could propagate through a vacuum Therefore the notion of the ether was introduced which was thought as an invisible medium that filled the universe However, this idea was later abandoned as experiments indicated that ether does not exist Some years later, in 1905 Albert Einstein developed a theory which explained that electromagnetic waves comprised very small particles which often behaved like waves These particles were called photons and the theory explained the physics of wave propagation using photons Einstein’s theory stated that the number of photons determines the wave’s amplitude whereas the photons’ energy determines the wave’s frequency Thus, the question that arises is what exactly is radiation made of, waves or photons A century after Einstein, an answer has yet to be given and both approaches are used Usually, lower frequency radiation is explained using waves whereas photons are used for higher frequency light transmission systems Wireless transmission plays an important role in the design of wireless communication systems and networks As a result, the majority of these systems’ characteristics stem from the nature of wireless transmission As was briefly mentioned in the previous chapter, the primary disadvantage of wireless transmission, compared to wired transmission, is its increased bit error rate The bit error rates (BER) experienced over a wireless link can be as high as 10 23 whereas typical BERs of wired links are around 10 210 The primary reason for A BER equal to 10 2x means that out of 10 x received bits is received with an error, that is, with its value inverted 26 Wireless Networks the increased BER is atmospheric noise, physical obstructions found in the signal’s path, multipath propagation and interference from other systems Another important aspect in which wireless communication systems differ from wired systems, is the fact that in wired systems, signal transmissions are confined within the wire Contrary to this, for a wireless system one cannot assume an exact geographical location in which the propagation of signals will be confined This means that neighboring wireless systems that use the same waveband will interfere with one another To solve this problem, wavebands are assigned after licensing procedures Licensing involves governments, operators, corporations and other parties, making it a controversial procedure as most of the times someone is bound to complain about the way wavebands have been assigned Licensing makes the wireless spectrum a finite resource, which must be used as efficiently as possible Thus, wireless systems have to achieve the highest performance possible over a waveband of specific width Therefore, such systems should be designed in a way that they offer a physical layer able to combat the deficiencies of wireless links Significant work has been done in this direction with techniques such as diversity, coding and equalization able to offer a relatively clean channel to upper layers of wireless systems Furthermore, the cellular concept offers the ability to reuse parts of the spectrum, leading to increased overall performance and efficient use of the spectrum 2.1.1 Scope of the Chapter The remainder of this chapter describes the fundamental issues related to wireless transmission systems Section 2.2 describes the various bands of the electromagnetic spectrum and discusses the way spectrum is licensed Section 2.3 describes the physical phenomena that govern wireless propagation and a basic wireless propagation model Section 2.4 describes and compares analog and digital radio transmission Section 2.5 describes the basic modulation techniques that are used in wireless communication systems while Section 2.6 describes the basic categories of multiple access techniques Section 2.7 provides an overview of diversity, smart antennae, multiantenna transmission, coding, equalization, power control and multicarrier modulation, which are all techniques that increase the performance over a wireless link Section 2.8 introduces the cellular concept, while Section 2.9 describes the ad hoc and semi ad hoc concepts Section 2.10 describes and compares packet-mode and circuitmode wireless services Section 2.11 presents and compares two approaches for delivering data to mobile clients, the pull and push approaches Section 2.12 provides an overview of the basic techniques and interactions between the different layers of a wireless network The chapter ends with a brief summary in Section 2.13 2.2 The Electromagnetic Spectrum Electromagnetic waves were predicted by the British physicist James Maxwell in 1865 and observed by the German physicist Heinrich Hertz in 1887 These waves are created by the movement of electrons and have the ability to propagate through space Using appropriate antennas, transmission and reception of electromagnetic waves through space becomes feasible This is the base for all wireless communications Electromagnetic waves are generated through generation of an electromagnetic field Such a field is created whenever the speed of an electrical charge is changed Transmitters are Wireless Communications Principles and Fundamentals Figure 2.1 27 Wavelength and amplitude of an electromagnetic wave based on this principle: in order to generate an electromagnetic wave, a transmitter vibrates electrons, which are the particles that orbit all atoms and contain electricity The speed of electron vibration determines the wave’s frequency, which is the fundamental characteristic of an electromagnetic wave It states how many times the wave is repeated in one second and is measured in hertz (to honor Heinrich Hertz) Higher vibration speeds for electrons produce higher frequency waves Reception of a wave works in the same way, by examining values of electrical signals that are induced to the receiver’s antenna by the incoming wave Another fundamental characteristic of an electromagnetic wave is its wavelength This refers to the distance between two consecutive maximum or minimum peaks of the electromagnetic wave and is measured in meters The wavelength of a periodic sine wave is shown in Figure 2.1, which also shows the wave’s amplitude The amplitude of an electromagnetic wave is the height from the axis to a wave peak and represents the strength of the wave’s transmission It is measured in volts or watts The wavelength l and frequency f of an electromagnetic wave are related according to the following equation: c ¼ lf ð2:1Þ where c is a constant representing the speed of light The constant nature of c means that given the wavelength, the frequency of a wave can be determined and vice versa Thus, waves can be described in terms of their wavelength or frequency with the latter option being the trend nowadays The equation holds for propagation in a vacuum, since passing through any material lowers this speed However, passing through the atmosphere does not cause significant speed reduction and thus the above equation is a very good approximation for electromagnetic wave propagation inside the earth’s atmosphere 2.2.1 Transmission Bands and their Characteristics The complete range of electromagnetic radiation is known as the electromagnetic spectrum It comprises a number of parts called bands Bands, however, not exist naturally They are Wireless Networks 28 Figure 2.2 The electromagnetic spectrum used in order to explain the different properties of various spectrum parts As a result, there is not a clear distinction between some bands of the electromagnetic spectrum This can be seen in Figure 2.2, which shows the electromagnetic spectrum and its classification into several bands As can be seen from the figure, frequency is measured on a logarithmic scale This means that by moving from one point to another on the axis, frequency is increased by a factor of 10 Thus, higher bands have more bandwidth and can carry more data However, the bands above visible light are rarely used in wireless communication systems due to the fact that they are difficult to modulate and are dangerous to living creatures Another difference between the spectrum bands relates to the attenuation they suffer Higher frequency signals typically have a shorter range than lower frequency signals as higher frequency signals are more easily blocked by obstacles An example of this is the fact that light cannot penetrate walls, while radio signals can The various bands of the spectrum are briefly summarized below in increasing order of frequency Of these, the most important for commercial communication systems are the radio and microwave bands † Radio Radio waves occupy the lowest part of the spectrum, down to several kilohertz They were the first to be applied for wireless communications (Gugliemo Marconi sent the first radio message across the Atlantic Ocean in the early 1900s) Lower frequency radio bands have lower bandwidth than higher frequency bands Thus, modern wireless communications systems favor the use of high frequency radio bands for fast data services while lower frequency radio bands are limited to TV and radio broadcasting However, higher frequency radio signals have a shorter range as mentioned above This is the reason that radio stations in the Long Wavelength (LW) band are easily heard over many countries whereas Very High Frequency (VHF) stations can only cover regions about the size of a city Nevertheless, reduced range is a potential advantage for wireless networking systems, since it enables frequency reuse This will be seen later in this chapter when the cellular concept is covered The LW, VHF and other portions of the radio band of the spectrum are shown in Figure 2.3 The HF band has the unique characteristic that enables worldwide transmission although having a relatively high frequency This is due to the fact that HF signals are reflected off the ionosphere and can thus travel over very large distances Wireless Communications Principles and Fundamentals Figure 2.3 29 The various radio bands and their common use Although not very reliable, this was the only way to communicate overseas before the satellite era † Microwaves The high frequency radio bands (UHF, SHF and EHF) are referred to as microwaves Microwaves get their name from the fact that they have small wavelengths compared to the other radio waves Microwaves have a large number of applications in wireless communications which stem from their high bandwidth However, they have the disadvantage of being easily attenuated by objects found in their path The commonly used parts of the microwave spectrum are shown in Figure 2.4 † Infrared (IR) IR radiation is located below the spectrum of red visible light Such rays are emitted by very hot objects and the frequency depends on the temperature of the emitting body When absorbed, the temperature increases IR radiation is also emitted by the human body and night vision is based on this fact It also finds use in some wireless communica- Figure 2.4 The various microwave bands and their common use Wireless Networks 30 † † † † tion systems An example is the infrared-based IEEE 802.11 WLAN covered in Chapter Furthermore, other communication systems exchange information either by diffused IR transmission or point-to-point infrared links Visible light The tiny part of the spectrum between UV and Infrared (IR) in Figure 2.4 represents the visible part of the electromagnetic spectrum Ultraviolet (UV) In terms of frequency, UV is the next band in the spectrum Such rays can be produced by the sun and ultraviolet lamps UV radiation is also dangerous to humans X-Rays X-Rays, also known as Rontgen rays, are characterized by shorter frequency than gamma rays X-Rays are also dangerous to human health as they can easily penetrate body cells Today, they find use in medical applications, the most well known being the examination of possible broken bones Gamma rays Gamma rays occupy the highest part of the electromagnetic spectrum having the highest frequency These kinds of radiation carries very large amounts of energy and are usually emitted by radioactive material such as cobalt-60 and cesium-137 Gamma rays can easily penetrate the human body and its cells and are thus very dangerous to human life Consequently, they are not suitable for wireless communication systems and their use is confined to certain medical applications Due to their increased potential for penetration, gamma rays are also used by engineers to look for cracks in pipes and aircraft parts Signal transmission in bands lower than visible light are generally not considered as harmful (e.g UV, X and gamma rays) However, they are not entirely safe, since any kind of radiation causes increase in temperature Recall the way microwave ovens work: Their goal is for food molecules to absorb microwaves which cause heat and help the food to cook quickly 2.2.2 Spectrum Regulation The fact that wireless networks not use specific mediums for signal propagation (such as cables) means that the wireless medium can essentially be shared by arbitrarily many systems Thus, wireless systems must operate without excessive interference from one another Consequently, the spectrum needs to be regulated in a manner that ensures limited interference Regulation is commonly handled inside each country by government-controlled national organizations although lately there has been a trend for international cooperation on this subject An international organization responsible for worldwide spectrum regulation is the International Telecommunications Union (ITU) ITU has regulated the spectrum since the start of the century by issuing guidelines that state the spectrum parts that can be used by certain applications These guidelines should be followed by national regulation organizations in order to allow use of the same equipment in any part of the world However, following the ITU guidelines is not mandatory For spectrum regulation purposes, the ITU splits the world into three parts: (i) the American continent; (ii) Europe, Africa and the former Soviet union; and (iii) the rest of Asia and Oceania Every couple of years the ITU holds a World Radiocommunication Conference (WRC) to discuss spectrum regulation issues by taking into account industry and consumer needs as well as social issues Almost any inter- Wireless Communications Principles and Fundamentals 31 ested member (e.g scientists and radio amateurs) can attend the conference, although most of the time attendees are mainly government agencies and industry people The latest WRC was held in 2000 in which spectrum regulation for the Third Generation (3G) of wireless networks was discussed 3G wireless networks are covered in Chapter Several operators that offer wireless services often exist inside each country National regulation organizations should decide how to license the available spectrum to operators This is a troublesome activity that entails political and sociological issues apart from technological issues Furthermore, the actual policies of national regulation organizations differ For example, the Federal Communications Commission (FCC), the national regulator inside the United States licenses spectrum to operators without limiting them on the type of service to deploy over this spectrum On the other hand, the spectrum regulator of the European Union does impose such a limitation This helps growth of a specific type of service, an example being the success of the Global System Mobile (GSM) communications inside Europe (GSM is described in Chapter 4) In the last year, the trend of licensing spectrum for specific services is being followed by other countries too, an example being the licensing by many countries of a specific part in the GHz band for 3G services Until now, three main approaches for spectrum licensing have been used: comparative bidding, lottery and auction Apart from these, the ITU has also reserved some parts of the spectrum that can be used internationally without licensing These are around the 2.4 GHz band and are commonly used by WLAN and Personal Area Networks (PANs) These are covered in Chapters and 11, respectively Parts of the 900 MHz and GHz bands are also available for use without licensing in the United States and Canada 2.2.2.1 Comparative Bidding This is the oldest method of spectrum licensing Each company that is interested in becoming an operator forms a proposal that describes the types of services it will offer The various interested companies submit their proposals to the regulating agency which then grades them according to the extent that they fulfill certain criteria, such as pricing, technology, etc., in an effort to select those applications that serve the public interest in the best way However, the problem with this method is the fact that government-controlled national regulators may not be completely impartial and may favor some companies over others due to political or economic reasons When a very large number of companies declare interest for a specific license, the comparative bidding method is likely to be accompanied by long delays until service deployment Regulating organizations will need more time to study and evaluate the submitted proposals This increases costs of both governments and candidate operators In the late 1980s, the FCC sometimes needed more than three years to evaluate proposals Comparative bidding is not thought to be a popular method for spectrum licensing nowadays Nevertheless, inside the European Union, Norway, Sweden, Finland, Denmark, France and Spain used it for licensing spectrum for 3G services 2.2.2.2 Lottery This method aims to alleviate the disadvantages of comparative bidding Potential operators submit their proposals to the regulators, which then give licenses to applicants that win the lottery This method obviously is not accompanied by delays However, it has the disadvan- 32 Wireless Networks tage that public interest is not taken into account Furthermore, it attracts the interest of speculator companies that not posses the ability to become operators Such companies may enter the lottery and if they manage to get the license, they resell it to companies that lost the lottery but nevertheless have the potential to offer services using the license In such cases, service deployment delays may also occur as speculators may take their time in order to achieve the best possible price for their license 2.2.2.3 Auction This method is based on the fact that spectrum is a scarce, and therefore expensive, resource Auctioning essentially allows governments to sell licenses to potential operators In order to sell a specific license, government issues a call for interested companies to join the auction and the company that makes the highest bid gets the license Although expensive to companies, auction provides important revenue to governments and forces operators to use the spectrum as efficiently as possible Spectrum auctions were initiated by the government of New Zealand in 1989 with the difference that spectrum was not sold Rather, for a period of for two decades, it was leased to the highest bidder who was free to use it for offering services or lease it to another company Despite being more efficient than comparative bidding and lotteries, auction also has some disadvantages The high prices paid for spectrum force companies passed on high charges to the consumers It is possible that the companies’ income from deployed services is overestimated As a result companies may not be able to get enough money to pay for the license and go bankrupt This is the reason why most regulating agencies nowadays tend to ask for all the money in advance when giving a license to the highest bidder Since 1989 auction has been used by other countries as well In 1993, FCC abandoned lotteries and adopted auction as the method for giving spectrum licenses In 2000 auction was used for licensing 3G spectrum in the United Kingdom resulting in 40 billion dollars of revenue to the British government, ten times more than expected Auctioning of 3G spectrum was also used inside the European Union by Holland, Germany, Belgium and Austria Italy and Ireland used a combination of auction and comparative bidding with the winners of comparative bidding entering an auction in order to compete for 3G licenses 2.3 Wireless Propagation Characteristics and Modeling 2.3.1 The Physics of Propagation An important issue in wireless communications is of course the amount of information that can be carried over a wireless channel, in terms of bit rate According to information theory, an upper bound on the bit rate W of any channel of bandwidth H Hz whose signal to thermal noise ratio is S/N, is given by Shannon’s formula:
 S W ¼ Hlog2 1 ð2:2Þ N Equation (2.2) applies to any transmission media, including wireless transmission However, as already mentioned, Equation (2.2) gives only the maximum bit rate that can be achieved on a channel In real wireless channels the bit rates achieved can be significantly lower, since Wireless Communications Principles and Fundamentals 33 apart from the thermal noise, there exist a number of impairments on the wireless channels that cause reception errors and thus lower the achievable bit rates Most of these impairments stem from the physics of wave propagation Understanding of the wave propagation mechanism is thus of increased importance, since it provides a means for predicting the coverage area of a transmitter and the interference experienced at the receiver Although the mechanism that governs propagation of electromagnetic waves through space is of increased complexity, it can generally be attributed to the following phenomena: free space path loss, Doppler Shift which is caused by station mobility and the propagation mechanisms of reflection, scattering and diffraction which cause signal fading 2.3.1.1 Free Space Path Loss This accounts for signal attenuation due to distance between the transmitter and the receiver In free space, the received power is proportional to r 22, where r is the distance between the transmitter and the receiver However, this rule is rarely used as the propagation phenomena described later significantly impact the quality of signal reception 2.3.1.2 Doppler Shift Station mobility gives rise to the phenomenon of Doppler shift A typical example of this phenomenon is the change in the sound of an ambulance passing by Doppler shift is caused when a signal transmitter and receiver are moving relative to one another In such a situation the frequency of the received signal will not be the same as that of the source When they are moving towards each other the frequency of the received signal is higher than that of the source, and when they are moving away from each other the frequency decreases This phenomenon becomes important when developing mobile radio systems 2.3.1.3 Propagation Mechanisms and Slow/Fast Fading As mentioned above, electromagnetic waves generally experience three propagation mechanisms: reflection, scattering and diffraction Reflection occurs when an electromagnetic wave falls on an object with dimensions very large compared to the wave’s wavelength Scattering occurs when the signal is obstructed by objects with dimensions in the order of the wavelength of the electromagnetic wave This phenomenon causes the energy of the signal to be transmitted over different directions and is the most difficult to predict Finally, diffraction, also known as shadowing, occurs when an electromagnetic wave falls on an impenetrable object In this case, secondary waves are formed behind the obstructing body despite the lack of line-of-sight (LOS) between the transmitter and the receiver However, these waves have less power than the original one The amount of diffraction is dependent on the radio frequency used, with low frequency signals diffracting more than high frequency signals Thus, high frequency signals, especially, Ultra High Frequencies (UHF), and microwave signals require LOS for adequate signal strength Shadowed areas are often large, resulting in the rate of change of the signal power being slow Thus, shadowing is also referred to as slow fading Reflection scattering and diffraction are shown in Figure 2.5 In a wireless channel, the signal from the transmitter may be reflected from objects (such as hills, buildings, etc.) resulting in echoes of the signal propagating over different paths with Wireless Networks 34 Figure 2.5 Reflection (R), diffraction (D) and scattering (S) different path lengths This phenomenon is known as multipath propagation and can possibly lead to fluctuations in received signal power This is due to the fact that echoes travel a larger distance due to reflections and they arrive at the receiver after the original signal Therefore, the receiver sees the original signal followed by echoes that possibly distort the reception of the original signal by causing small-scale fluctuations in the received signal The time duration between the reception of the first signal and the reception of the last echo is known as the channel’s delay spread Because these small-scale fluctuations are experienced over very short distances (typically at half wavelength distances), multipath fading is also referred to either as fast fading or small-scale fading When a LOS exists between the receiver and the transmitter, this kind of fading is known as Ricean fading When a LOS does not exist, it is known as Rayleigh fading Multipath fading causes the received signal power to vary rapidly even by three or four orders of magnitude when the receiver moves by only a fraction of the signal’s wavelength These fluctuations are due to the fact that the echoes of the signal arrive with different phases at the receiver and thus their sum behaves like a noise signal When the path lengths followed by echoes differ by a multiple of half of the signal’s wavelength, arriving signals may partially or totally cancel each other Partial signal cancellation at the receiver due to multipath propagation is shown in Figure 2.6 Despite the rapid small-scale fluctuations due to multipath propagation, the average received signal power, which is computed over receiver movements of 10–40 wavelengths and used by the mobile receiver in roaming and power control decisions, is characterized by very small variations in the large scale, as shown in Figure 2.7, and decreases only when the transmitter moves away from the receiver over significantly large distances Multipath propagation can lead to the presence of energy from a previous symbol during the detection time of the current symbol which has catastrophic effects at signal reception ... reflected off the ionosphere and can thus travel over very large distances Wireless Communications Principles and Fundamentals Figure 2.3 29 The various radio bands and their common use Although... into account industry and consumer needs as well as social issues Almost any inter- Wireless Communications Principles and Fundamentals 31 ested member (e.g scientists and radio amateurs) can... lower, since Wireless Communications Principles and Fundamentals 33 apart from the thermal noise, there exist a number of impairments on the wireless channels that cause reception errors and thus