volume i: wireless communications - editorial digital … i: wireless communications ... lopment and...

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Volume I: Wireless Communications

Concept Map

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Volume I

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Multicarrier

Systems

Orthogonal

Sequences

TreendsChannel

Capacity

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This book will be used both in professional and graduate level courses, and is intended to cover the topics of different personal communications and wireless communications courses offered for undergraduate and graduate

programs in the area of engineering and information technology. We mainly want to have an e-book that has interactivity to enable the reader to follow-up the deve-lopment and analysis of the phenomena that characterize the wireless media, with informative capsules that can be consulted by the reader without losing the objec-tive. The book has the mathematical formality required for the graduate level, but the reader can follow up on issues with a different depth for the professional level. The book features informative capsules not only with the theoretical develop-ment of the subject, but also with examples that readers can follow as a guide for their activities. The main themes of the wireless communications are co-vered in such a way that the reader can take advantage of the theoretical re-search and apply it either to analyze different technologies or to develop future technologies.

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eBook introduction

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The book covers the topics of the cellular concept and wire-less signal propagation in the

early chapters with real and practical examples for the study of propagation. The book also develops the base of the currently used digital modulations. Aspects of channel capacity and di-versity are presented to complete the theoretical part of the communications system. Spread spectrum, multicarrier and OFDM technologies are also intro-duced so the reader can see the trends and technologies. In the chapter of trends there are aspects of interferen-ce, cooperation, and location that are still being researched for their develop-ment and application in reconfigurable networks. It is also the intention of the book to relate to the didactic technique that is used, for example, the course has been implemented with various techniques over several semesters, which include techniques such as pro-blem-based learning (PBL), or project-base learning (POL) and most recently that of research-based learning (RBL).

Multicarrier Systems Chapter 7

Orthogonal Sequences Chapter 6

ChannelCapacity Chapter 8

Digital Communications

Review Appendix A

Detection, Estimation

and Modulation Appendix B

Trends Chapter 9

Reconfigurable Networks

Cooperative Wireless SystemsRIFD

Position

Location

Cognitive

Radio

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eBook introduction

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Graphic Organizer


Cellular

Concept

Chapter 1. The Cellular Concept

Hexagonal

Geometry

Interference

Capacity

Frequency

reuse

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Chapter 1. The Cellular ConceptIntroduction

The Cellular Concept

This chapter introduces the cellular concept, which represents the base on which wireless and mobile communications have been developed. Some comparisons of different technologies are discussed and the issues on network design and evaluation are treated in the following sections.

1.1 History and Technologies

Historically speaking, several important events have made possible the wireless systems that we have today. Among those events, we can recall the formulation of the four laws of James Clark Maxwell in 1864 that describe how the radiofrequency signals propagate along the wire-

less medium. In 1876, we have the invention and patent of the telephone by Alexander Graham Bell; in 1887 Heinrich Rudolph Hertz elaborates the experiment that detects the electromagnetic waves produced by the four laws previously formulated by Maxwell, now the frequency is measured in Hertz. In 1896 Guglielmo Marconi implements the first wireless receiver, gets the patent for the telegraph in 1897. In 1900 the first transatlantic transmission takes place. In 1921, radio in a broadcasting system of at most 2 MHz in frequency is introduced in the city of Detroit in the U.S.A., Amplitude Modulation (AM) is used. In 1935 Frequency modulation is demonstrated for the first time by its inventor Edwin Armstrong. During the Second World War, Spread Spectrum (SS) appears, specifically in Frequency Hopping (FH) form.

In 1947, AT&T Bell Labs introduces the cellular concept, but it is until 1963 that the same AT&T Bell Labs demonstrate the operation and feasibility of the first cellular system. During the 1980’s, we have different systems with analog modulation that provide cellular service, appearing in the world. In 1981 Nordic Mobile Telephone Service (NMTS) appears in Sweden, in 1983 in Chicago, the Advanced Mo-bile Phone System (AMPS) appears after the Federal Communications Commission (FCC) allocated frequencies to be used for cellular service. In the United Kingdom in 1985, a similar system appears, it is called Total Access Communications System (TACS). These history facts and information can be consulted at IEEE Global History Network, (2011). w

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After saturation and roaming problems ap-pear in those systems, instead of allocating more frequencies, technology evolution was seen as the solution. This evolution was called 2G and it consisted basically in changing to digital wireless communications systems, so in 1991 we have the introduction of TDMA (Time Division Multiple Access) and the use of microcells; in 1992 GSM appears in Europe and in a very short time span 80 million users had GSM phones. In 1994 CDMA (Code Division Multiple Access) systems had the first trials, this system is now known as cdmaO-ne. After this evolution, GSM was improved to provide higher data rates, and later its evolution appears as EDGE, one of the systems currently in operation. On the other hand, CDMA evolves towards 3G as cdma2000 and EDGE towards WCDMA (Wideband CDMA), the former having backward compatibility with cdmaOne, the latter with EDGE.

During the first half of the 20th century, wireless systems in operation consisted mainly of broad-casting systems such as the AM and FM radio, and television. These systems did not allowed response from users; hence delivery of informa-tion was only in one direction. Wireless commu-nications modify this idea to provide service with information delivery both ways. This characteris-tic needed to satisfy customers, thus the service provided had to accomplish at least the same qua-

Chapter 1. The Cellular Concept1.1 History and Technologies

lity as that provided by the conventional wireline telephone network. But in order to achieve such quality, different features needed to be known in order to propose solutions that would make the wireless system withstand the competition of con-ventional telephones.

Among the comparable issues of these net-works we have that in the wireline network, ge-nerally, the bandwidth is abundant, in contrast, in wireless networks the bandwidth is scarce and re-gulated. In wireline networks we have the possibi-lity of transmitting at very high data rates such as Gigabits per second (Gbps), whereas in wireless networks we have low data rates, and especially for mobile services with rates of up to Megabits per second (Mbps), in other words, three orders of magnitude less.

In wireline networks, depending on the transmission techniques, the problem of interfe-rence could be negligible, and there is no need to have a line-of-sight between transmitter and receiver affecting signal reception. Also when a wireline network needs to be implemented in an indoor scenario, there is no problem; one only needs to deploy the required wiring in order to achieve connectivity with the adequate quality. In contrast, an indoor wireless network does not have a straightforward solution and quality can decrease by many things since signal propaga-

tion is strongly affected by the environment. In wireline networks, information travels along wi-res and in order to be stolen, physical intrusion needs to be done, on the other hand, in wireless networks information travels through the air, in an open environment where anyone could “hear” what others are saying.

In wireline networks, users without mobility can access computers in different geographic areas other than their own, whereas in wireless networks we need standards, as well as agree-ments among service providers and sometimes among different governments so that a user can access service or information from other geogra-phic areas. With respect to size of the user de-vice, in wireline networks size does not matter, users can have large devices such as desktop computers, but in wireless networks users need small and compact devices that provide not only portability, but also mobility.

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Even though all these issues just discussed could be seen as problems, there have been solutions that have made possible the networks and devices we currently have. For example, the scarce bandwidth has been trea-ted with frequency reuse techniques that allow us to repeat frequencies in areas that satisfy distance and interference criteria. Frequency planning helps to decrease interferen-ce, also new receiver design techniques supports interfe-rence mitigation techniques. Compression algorithms have helped to achieve higher data rates, and coding increases signal quality. For privacy and security, we have cryptogra-phy and authentication algorithms. For indoor penetration we have new receiver technologies that work with higher frequencies, and in order to have small-sized devices with a high duration battery, power control techniques are used.

Also, battery technology is evolving providing longer times of energy.These solutions that have been implemented in different technologies, required to find out ways to manage other problems as well, for example, the frequency reuse created the need to have call handoffs, which consist of the possibility of transferring a call from an antenna servicing the call, to another; power control algorithms are complex, and the use of emergency services due to numbering tech-niques was not easy to introduce. However, solutions have been found and implemented, and now we have services that are comparable in quality to those provided by wireline networks.

With this short discussion on the evolution of wireless systems, we can see that what defines the technology con-sists of two things:

Frequency reuseAssignments of the same frequen-cy to two different antennas located at a minimum separation to avoid co-channel interference.

Glossary

Click on each characteristic.


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The original cellular system, e.g., AMPS, consisted basically of Frequency Division Multiplexing (FDMA) and FM, whereas the digital systems evolution considered changing towards TDMA and CDMA and digital modulation which are more robust and allow the operation of the system at higher noise levels. All these techno-logies based their multiple access schemes on conflict-free pro-tocols with static allocation of the resource, i.e., once a resource or channel is assigned to a user, the channel will be in the user’s possession until the user decides to free such resource, see Ble-cher, F.H. (1980).

To illustrate how these multiple access techniques work, we can consider that we have a frequency band available with a channel division where each channel is assigned to a user, on demand. Once the channel is assigned, no other user can access such channel until the current user finishes the session. In Figure 1.1, we can see the representation of FDMA with 6 channels in the frequency domain, i.e., at most, the system will support six simul-taneously active users.

In contrast, we can see in Figure 1.2 the TDMA technique where a new domain is added to the multiple access scheme, providing new possibilities. When time is considered, the system can provi-de the same frequency to different users, where each of them will use such frequency for a fixed amount of time, allowing another user to communicate via the same frequency. The original TDMA system consisted in a synchronized time division of each frequen-cy in three parts. This division allows the use of the resources in a simultaneously form to three times those users in FDMA, hence, capacity of the system is increased. GSM also uses TDMA, but it considers a division of the channel into eight time-slots.

Figure 1.1 FDMA

Figure 1.2 TDMA


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The third multiple access scheme, CDMA, is also based on a division of the frequency domain in channels, but it allows the simultaneous use of each frequency by different users as long as each user is identified by a code. These codes have properties, such as orthogonality, to be intro-duced later, that let the receivers identify and separate the signals from all the active users. One of the main differences of CDMA is that its main limitation is noise or interference, whereas for TDMA and FDMA, the main limitation is dimensional since once the resources are used completely, no more users can be accommodated to receive service. CDMA would still accommodate more users as long as the noise levels permit signal reception. In Figure 1.3, we can see that capacity is increased since the number of codes used would allow us to allocate the same frequency to users in the same amount, see Rappaport, T. (2002).

A wireless system consists of Base Stations (BS) covering areas whe-re users demand service through wireless devices. A base station is the set of hardware and software together with the transmission antennas that are installed at certain places to transmit a signal that covers an area of the network; such area is what we usually call a cell. The base station provides the connectivity to a core network that will make possible local and long distance calls; its hardware and software work together to regis-ter users and control the active calls. BSs are equipped to perform call transfers and system measurements to ensure service quality. The wire-less devices or handsets that users carry are what we call Mobile Stations (MS) and usually work under a specific set of standards in order to provi-de connectivity and network services to the users by wireless links to the BSs. Such MSs have applications previously programmed that access, through a wireless channel, the service via a BS which is connected to a core network. This basic system is illustrated in Figure 1.4

Figure 1.3 CDMA

Figure 1.4 A basic wireless system


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Communication from the BS to the Mobile Station (MS) is called the forward link or downlink, and from the MS towards the BS is called reverse link or uplink. These two links achieve communication through two channels at least, depending on the system used. The traffic channel in charge of transmitting the information to be communicated; and the control channel, used to transmit orders such as user update identification. This is shown in Figure 1.5. Parameters that are commonly transmitted are the MIN (Mobile Identification Number) and the ESN (Electronic Serial Number), which are used to authenticate users, see Tse, D. (2005).

Technologies that have been used, that are currently in operation and that will be introduced in the future are specified through standards. In the wireless telecommunications industry, several organizations have been in-volved in the generation of standards and recommendations. For example, the International Telecommunications Union, (ITU 2011), has within its or-ganization ITU-R, in charge of issuing the recommendations for spectrum management, see Cave, M. and Doyle, C. (2007), and Wither, D.J. (2000), as well as the definition of parameters and recommendation of services for those frequency bands. Other organizations such as the American National Standards Institute (ANSI, 2011), European Telecommunications Standards Institute, ETSI, the Institute of Electrical and Electronics Engineering, IEEE (2011), and the Telecommunications Industry Association, TIA (2011), have also generated standards currently in use such as the WiFi standard which is IEEE 802.11a/b/g/n, or the standards for 2G CDMA known at the beginning as IS-95, today known as cdmaOne by TIA.

In North America, several standards have been in use, such as the AMPS and its narrowband version NAMPS, the cdmaOne and the DCS-1900, which is the American GSM technology, all of these issued by TIA. cdmaOne evol-ves towards a 3G technology known by its standard cdma2000. In Europe, development started with ETACS and its narrowband version NMT-900, later due to several problems such as telephone numbering system, lack of roa-ming agreements, and high service demand, GSM was adopted in the entire continent. The current evolution of GSM is for EDGE in 2.5G and WCDMA in 3G.


Figure 1.5 Communications between BS and MS

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Also, the wireless local area networks have evolved at the point that technologies offer very good quality and the network services that users get from desktops. Mobility and portability is still an issue that is explored, although some standards have been developed in this direction. Among the known stan-dards besides the WiFi, we have the IEEE 802.15.1 better known as Bluetooth, IEEE 802.15.3 also known as Ultra Wide Band (UWB), and the IEEE 802.16 known as WiMAX. WiMAX is one where mobility is being proposed as a competitive solution for classic cellular networks. In terms of the topics related to this book, two of the main characteristics of these standards are their transmission techniques and their multiple access schemes. Most of them use the concept of spread spectrum, to be treated thoroughly in chapters 5 to 7 of this book. Spread spectrum is used when information being transmitted occupies a large amount of frequencies in the transmission medium; the purpose of this spreading is generally for protection and robustness of the transmission being made. Some of the technologies that will be covered in the chapters mentioned are Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spec-trum (FHSS), Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), see Goldsmith, A. (2005), Molisch, A. (2007), and Tse, D. and Viswanath, P. (2005).


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1.2 Frequency Reuse and the Cellular Concept

Before discussing the frequency reuse concept, we need to recall that the frequency bands used in the wireless systems are the radio frequencies which

can be located in the electromagnetic spectrum just below the infrared frequencies. This is shown in Figure 1.6, whe-re we can also see that frequency in Hz increases towards the right side, whereas wavelength increases towards the left side due to their corresponding inverse relation given by

In (1.1), c is the speed of light, 3x108 m/s, and ƒ is the frequency in Hz of the signal being transmitted. Also, we can see in Figure 1.6 that the band of interest consists of tho-se frequencies between 10 MHz and 300 GHz, in terms of wavelengths we have from 1 mm up to 30 m, see Couch II, L.W. (2006) and Wither, D.J. (2000).

Some services have specific frequency bands allocated. A summary of those services and their frequency bands are in Table 1.1, the frequencies shown may vary from country to country given their own regulation and policies.

Frequency reuseAssignments of the same frequen-cy to two different antennas located at a minimum separation to avoid co-channel interference.

Glossary

Figure 1.6 Electromagnetic spectrum

Chapter 1. The Cellular Concept1.2 Frequency Reuse and the Cellular Concept

(1.1)

Table 1.1 Frequencies allocated by services

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The cellular networks or wireless networks organized by cell sites, come from the idea of frequency reuse. Frequency reuse has been done since the early days of wireless transmissions, for example with the AM broadcasting system in 1921, where it was seen that different radio stations needed to be allocated different frequencies, and that if a frequency was already in use, it could be allocated once more if the new site was separated from the original site using that frequency certain distance. In the broadcasting system we have typical distances of hundreds of kilometers, but in the PCS systems of today we have distances that could go for a few units of kilometers or less depending on the technology and services provided.

One of the advantages of reusing a frequency is that of increasing the possibility of having such frequency available for another user, thus increa-sing the number of simultaneous users in the network. In Figure 1.7 we can see a BS that is allocated a frequency band that consists of three channels, hence, the maximum number of simultaneous users would be of three for that BS.

If we would like to increase the number of simultaneous users in the net-work of Figure 1.7, we would need to reuse at least one of the frequencies allocated. In order to do it, we need to divide the coverage area into smaller areas called cells, and each of those cells need to have a BS similar to the one in the original network, but with a smaller transmission power (see Figu-re 1.7).

In Figure 1.8, we can see that channel 1 was reused in the same original coverage area, achieving an increase in the number of simultaneous users from three to four. The cellular concept consists in dividing the area to be serviced, into smaller areas called cells and installing a BS with an antenna in each of these cells. The frequency reuse is determined by the geometry used to divide the area.


Figure 1.7 Network with no reuse of frequencies

Figure 1.8 Network with frequency reuse

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Considering a planar area, we can divide it into cells by using different geometrical figures, but only three regular geometric figures will not have overlapped areas or areas not covered among the cells. These figures are the equilateral triangle, the square and the regular hexagon as shown in Figure 1.9. In this figure, we have a hypothetically area covered with tho-se regular geometric figures defined by the same radius, i.e., the distance from the center of the geometric figure to one of the vertices of the figure. As we can see, the regular hexagons achieve a smaller number of BSs in the same area, thus producing a reduction on the initial investment that a company might need to do. Also, the hexagons are the closest figure in shape, from those three, to that of a circle, and since it is assumed that the signals transmitted from a BS antenna will propagate equally to the surroundings defining a circular coverage area, the regular hexagon is the closest to the coverage area shape. A story tells that since Mr. Hooke when seeing through his microscope saw shapes similar to those regular hexagons and he called those figures cells, what we are going to design with those regular hexagons is called a cellular network. Figure 1.9 Cellular geometry


The cellular concept was designed to satisfy objectives to achieve large capacity networks, but with an efficient use of the spectrum. The main idea was to provide services to mobile and portable units, and to have service availability widely. In terms of network operation, quality of service has always been a challenge since the conventional service is the point of comparison, but mobility is one advantage with no competitor.

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where n is the Path Loss Exponent (PLE) that depends on the environment, PT is the power at which the BS antenna transmits, Pr(d) is the re-ceived power at a distance d from the BS, see Mo-lisch, A. (2007). Sometimes we will have a mea-surement at a different distance, say d0, which can be used to obtain the power at a distance d. So for example, we can have at a distance d0

and carrying out the ratio of Equation (1.3) to Equation (1.4), we obtain:

1.3 Propagation Basics, PLE

In this section, we introduce the basic idea of propagation modeling since it will be used to evaluate the quality of a network design through

the use of the signal to interference ratio. A tho-rough treatment of propagation will be provided in Chapter 2.

In general, propagation models are only valid for those distances, which are in the far-field re-gion, which is defined by the far-field distance from the transmitting antenna. The far-field dis-tance is determined by the following expression

where l is the largest physical linear dimen-sion of the transmitting antenna. In general, we will have that the distance dF >> l and that dF >>λ. see Rappaport, T.S., (2002).

The main objective of a propagation model is to provide a prediction of the amount of signal power or signal strength at a point in the far-field region of the coverage area separated a distance d from the transmitting BS. The propagation mo-del most widely used is the following:

Chapter 1. The Cellular Concept1.3 Propagation Basics, PLE

(1.2)

(1.3)

(1.4)

(1.5)

(1.6)

With Equation (1.5), we can obtain a simplified propagation model based on a distance d that does not need the parameter PT to find the recei-ved power which is given by

Figure 1.10 presents graphically equations (1.3) – (1.6) where a mobile station is in the far field region separated a distance d from the BS transmitting antenna, see Goldsmith, A. (2005).

With the simplified models introduced in this section, we will be able to evaluate the interfe-rence in a cellular network given its geographical structure. In the following sections, the hexagonal geometry is introduced and evaluated by using these models.

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1.4 Hexagonal Geometry

The hexagonal geometry consists of a group of regular hexagons covering a planar area. It was used in the design of cellular networks in Mac Donald, V.H. (1979), and we present the main results in this section. The purpose of such geometry is to determine which hexagons or cells will be assigned the same set of frequencies, thus applying

the concept of frequency reuse to increase capacity. In order to establish such geometry, one needs to consider a set of axis at sixty degrees instead of the orthogonal set of axis usually used. Since it is needed to calculate distances in the hexagonal geometry and these depend on the hexagon size and radius R, we present these measurements in a typical hexagon to be considered in the cellular network with the drawing of Figure 1.11.

Hexagonal geometryCoordinate system where the axes are separated by 60 degrees instead of being orthogonal.

Glossary

In such figure, we can see that the distance from the center to any of the vertices is what we defi-ned as the radius R, each of the sides of the regular hexagon measures R in length. One of the impor-tant distances to be used in the calculations is the one shown as d, i.e., the distance from the center to any side of the hexagon in a perpendicular direc-tion. This distance is given by

Figure 1.10 Propagation Fundamentals

(1.7)

Chapter 1. The Cellular Concept1.4 Hexagonal Geometry

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13 Glossary

The hexagonal geometry can be seen in Figure 1.12, where a group of hexa-gons is drawn with a reference axis. These axes identify the centers of the hexa-gons, and by looking at the drawing, we can see that the separation of any pair of centers of adjacent hexagons is given by 2d, where d is given in (1.7)

The origin of the hexagonal axis is fixed at a hexagon center, and then each of the centers along an axis will be assigned a number as shown in Figure 1.12. So for example, the identification of cell with coordinates (2, 1), as shown in such figure, defines a distance D that needs to be calculated, this distance is known as the Reuse Distance, and defines the distance at which co-cells need to be separated. The reason to calculate distance D is that this methodology will help us find out which hexagons will be assigned the same set of frequencies. In Figure 1.12, the cell at coordinates (2, 1), will be a co-cell of the cell at the origin (0, 0) since it will have the same set of frequencies. Although the selec-tion of coordinates, e.g., (2, 1) in the drawing can be arbitrary, it depends on the interference that can be supported and the technology to be used, i.e., better technology implies better performance and more robustness against interferen-ce, hence co-cells can be closer from one another. If coordinates (2, 1) were chosen, then we need to repeat the process of identifying the next co-cell by rotating the axes and selecting the (2, 1) co-cell. This is done by using each and every side of the hexagon at the origin. At the end of this method, we will end up with six co-cells arranged in a ring or tier surrounding the center cell as shown in Figure 1.13. Note that in this figure, we used the same coordinates (2, 1) to ob-tain the co-cells, i.e., the same strategy of selecting the first co-cell needs to be followed when determining the other five co-cells. Since co-cells use the same set of frequencies or channels, they are called co-channel cells. Note that when selecting the first co-cell in Figure 1.12, one can decide to take the two steps in the direction i, and one step in the direction j as shown (60 degrees to the left), or one step 60 degrees to the right; if you decide to do this, then all the co-cells need to be determined using this method.

Figure 1.11 Hexagon Measurements

Figure 1.12 Hexagonal geometry


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ClustersGroups of cells that use up the com-plete set of frequencies.

Glossary

In Figure 1.13, we indicated the cell at the center as cell A, and the co-cells in the first tier also as cell A since they will have the same frequencies. Now in order to determine another group of co-cells, say co-cells B, you need to move the axis to consider another center cell, this new center cell will be an adjacent cell of the first center cell you started with. Repeat this for co-cells C, D, etc. With this procedure, you will define some geometrical figures such as those shown in Figure 1.13. These groups of cells are known as clusters.

1.4.1 Clusters.Figure 1.13 shows a group of seven

cells in the center, i.e., a center cell and 6 adjacent cells, and similar groups of cells can be seen with other colors su-rrounding the first one. Note that at the end, the position of the co-cells will be the same in each of these groups of cells, i.e., cell A will always be at the center of these geometrical figures. Each of these groups of cells is what is called a cluster. In the case of figures 1.12 and 1.13, when we chose coordinates (2, 1) to determine the co-cells, a seven-cell cluster is the result. Therefore, the choice of coordinates will determine the shape and size of the clusters.

In Figure 1.14, we have three of the most used clusters in cellular network design. The cluster will be determined by the choice of coordinates, by the radius of the hexagons, and thus by the reuse distance D.

Looking back at Figure 1.12, let us calculate the distance D for the general case of coordinates (i, j). To do this, we need to have in mind the distance d obtained from Figure 1.11. Thus, we see that to calculate the distance D of the co-cell at coordinates (i, j), we have a triangle where one side is i, and the other side is j, but the distance i contains the parameter d of Figure 1.11.

Figure 1.13 Co-cells in hexagonal geometry


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In fact, one can see that from point 0 to point 1 along the axis, the distance covered is 2d, thus, applying the triangle identity to obtain the reuse distance, we get

Now, the reuse distance D will be given by

In the second equality of (1.9), we use Equation (1.7) to substitute for the value of d in terms of R. The final result of (1.9) contains the parame-ter N which is the number of cells in the cluster. Also, in Mac Donald, V.H. (1979), the parameters i, and j are called the shift parameters. Thus, the number of cells in a cluster in the hexagonal geometry is given by

See that in Equation (1.10), you could have substituted coordinates (1, 2) and obtained the same result of N=7. We define the parameter co-channel reuse ratio Q as the ratio of the reuse distance D to the cell radius R, and this can be obtained from (1.9) as follows

Figure 1.14 Cell clusters

(1.8)

(1.9)

(1.10)

(1.11)


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If D is increased, then co-channel interference will be de-creased since co-cells will be more separated and this will increase the parameter Q, thus increasing Q will decrease interference. A decrease in Q with no change on R implies a decrease in N by (1.11), and a decrease in D, also co-cells will be closer producing more interference. Then, we can say that the parameter Q is linked to quality of service given by interference. Since the shift parameters determine the cluster size N and this determines Q, we obtain Table 1.2 of the co-channel reuse ratio Q in terms of i, j, and N by using (1.10) and (1.11) as follows

Recall that the values of N shown in Table 1.2 can be ob-tained for different values of i and j, for example, i=2, j=1 will give N=7 as well. Once decided the shift parameters to be used for the network design, clusters are determined and the frequencies are allocated to the cells. It is impor-tant to say that the whole set of frequencies is used in each

cluster, i.e., if the network has N cells in each cluster, and the total number of channels in the frequency band availa-ble is M, then the number of channels per cell in a cluster Z is given by

Now, if the network consists of k clusters, then the total number of channels in the network C is given by

In (1.13), C is what is known as the network capaci-ty, and it is defined as the number of simultaneous active users that the network can accommodate. Hence, a network with k clusters will multiply k times the number of channels available in the frequency band. The frequency reuse fac-tor is given by 1/N. In Figure 1.15 we have a network with five clusters, each one of three cells, see Rappaport, T.S. (2002).

(1.12)

(1.13)

Co-channel interferenceAmount of power received from a sig-nal being transmitted at the same fre-quency through a different antenna.

GlossaryChapter 1. The Cellular Concept1.4 Hexagonal Geometry

Table 1.2 Co-channel reuse ratio Q

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Figure 1.16 Scenariosfor handoffs

Figure 1.15 Cellular networlk with clusters

Having a cellular network in an area as shown in Figure 1.15, will make users change from cell to cell when mobility is present. If such users have calls active, then their call needs to be kept active by changing the control of it from BS antenna to BS antenna, which implies the change of fre-quency channels. The process of passing on the call control from BS to a neighboring BS is called a handoff, and it is explained in the following subsection.

1.4.2 Handoffs.

Handoffs are the transfer of control from one BS to a neighboring BS, along with a change of frequency channel for an active call due to mobility or signal strength conditions. Handoffs are generally implemented through the traffic or voice channel (recall Figure 1.5), and are executed if a servicing BS receives a weak signal from an MS, see Goldsmith, A. (2005), and Molisch, A. (2007).

When a BS detects such weak signal from an active call in its coverage region, then it triggers a procedure for the handoff. The reason behind such weak signal might be because the MS is approaching the cell boundary or it could be because the MS is within an area where propagation of the signal is such that strong losses to signal power are present, see Figure 1.16.


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The types of handoff can be defined in terms of the parameter used to make decisions; in this way we can find handoffs based on signal stren-gth and those based on carrier-to-interference ratio. There can be others that combine some pa-rameters, but broadly speaking those mentioned are the basic ones. In general, the process starts when a servicing BS, say cell A, perceives weak signal strength from an MS; see Figure 1.17.

Assume that such MS is moving toward the cell boundary and getting closer to another cell, say cell B. BS A reports such event to the sys-tem and the system starts an identification proce-dure looking for the new BS to serve such MS. Once the new BS is identified, it is reported of the handoff event to take place. In those FDMA and TDMA systems, the MS will break commu-nication first with the servicing cell A, in order to establish communication with the new cell B. In general this is because the MS needs to change its modulator’s frequency and cannot be synchro-nized to two frequencies at the same time. This handoff is called a hard handoff. After the handoff event takes place, the new BS cell B will continue serving the MS. In CDMA, since all the MS are using the same frequency and the identification is done by different codes, the MS will establish communication with the new cell before breaking communication with the old cell, this is called a soft handoff.

Figure 1.17 Hard handoff


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The handoff event is produced by the variation of signal strength as time evolves as shown in Figure 1.18.

The graph in Figure 1.18 shows the handoff scenario of Figure 1.17, where cell A is servicing the MS and as time evolves, its signal strength falls below a threshold value of Ph which triggers the new BS identification procedure, indicated in the figure as the handoff region. The MS will continue to be served by cell A although it is expected that another cell will capture the signal from that MS at a strength that increases over another threshold Pmin, this cell B re-ports the event to the system and the system starts organizing those cells reporting the same situation so that a list of candidate cells is made. The call will continue to be served by cell A until the signal strength falls below Pmin, moment at which in another cell, say cell B, the signal strength of the MS is received with power above Ph, then the handoff is executed and frequencies are changed. It is important to mention that every time the signal strength falls below or increases above a threshold, timers are considered in order to be certain that the signal strength is being affected in such way and it is not caused by some rapid fluctuations commonly found in multipath propagation environments.

Figure 1.18 Signal strength during handoffs


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1.5 Frequency Bands and AMPS

Frequency bands used in wireless systems depend on the speci-fic country regulation, for example, in Mexico the B band of the 800 MHz frequency band, shown in Figure 1.19, has been assig-

ned to the incumbent service provider Telcel, Band A was assigned to the remaining service providers. Such regulation in general depends on recommendations issued by the International Telecommunication Union (ITU, 2011), and neighboring countries agreements in terms of frequency bands and services to be offered. Although at the beginning regulation was considered in terms of technologies, nowadays what determines frequency band use is service. Technology to provide such service is left to be decided by the service providers.

In this section we briefly show the cellular frequency band and some of the channel assignments that have been used for services such as that of the AMPS system, Blecher, F.H. (1980).

The AMPS system is a cellular system that works with an FM transceiver with individual channel bandwidth of 30 kHz. This system worked in the 800 MHz band, with forward-reverse link separation of 45 MHz. In Figure 1.19, we show a representation of the frequency band used by AMPS.

In Figure 1.19 we can see the frequency bands of the uplink (MS – BS) and the downlink (BS – MS) used, separated by 45 MHz as pre-viously mentioned. The bands are divided and identified as the A band and the B band. Each of these blocks has the channel numbers below their identification in parentheses. Note that the numbers in parenthe-ses are the same for the uplink as for the downlink. The frequency of these bands is marked with numbers of the horizontal frequency axis in MHz. When the AMPS service started to have saturation problems due

to traffic growth, the Federal Communications Commission provided the bands A’ and A’’ to those companies delivering service in band A, and band B’ to those in band B. At the same time it was determined the need to evolve to some better technologies, and this evolution resulted in TDMA and CDMA with the correspon-ding 2G systems in operation such as GSM and cdmaOne.

Chapter 1. The Cellular Concept1.5 Frequency Bands and AMPS

Figure 1.19 800MHz frequency band

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In Figure 1.20, we can see the representation of one of the bands, either uplink or downlink band. The figure contains the amount of Hz that each part of the bands has, and shows as well signaling chan-nels used in the AMPS system, 21 channels for the A band and 21 for the B band. The reason of having 21 signaling channels is that the clusters used were of 7 cells and each cell was sectorized in three sectors, which gives 21 channels. These signaling channels are those that are used as control channels for establishment and control of the communication.

A service provider in the 800 MHz band would be occupying one of the bands, i.e., 12.5 MHz. For an AMPS system consisting of clusters of seven cells with channels of 30 kHz, and since the frequency band contains guard channels of 10 kHz, we have that the total number of channels available for such cluster is

And since 21 of those channels are for signaling and control, we have that the total number of voice channels in an AMPS system for a cluster is 395 channels. These 395 channels would be divided evenly among the seven cells in the cluster. This frequency assign-ment can also be carried out in networks with another cluster size, for example, consider the network in Figure 1.21 where three clus-ters of size four conform the network.

Figure 1.20 800MHz band uplink or downlink

Chapter 1. The Cellular Concept1.5 Frequency Bands and AMPS

(1.14)

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The cells in Figure 1.21 are identified. Assume that we have a frequency band where 36 channels have been pre-viously numbered, i.e., channels 1, 2, 3, …, 36. Since each cluster will have the total number of channels of the fre-quency band assigned, such assignment will be done ac-cording to Table 1.3 where such table is filled by columns. This assignment order will help prevent adjacent frequen-cies to be assigned to the same cell; so for example, we have channels 1 and 2 in cells 1 and 2, respectively. This assignment takes care of the adjacent channel interferen-ce. The hexagonal geometry and cluster formation will take care of the co-channel interference, see Rappaport, T.S. (2002), and Tse, D. and Viswanath, P. (2005).

Table 1.3 Channel assignment for a four-cell cluster.

Figure 1.21 Network for channel assignment

Adjacent channel assignment for a four-cell cluster.Amount of power perceived by an antenna from signals being transmit-ted at adjacent frequencies.

co-channel interferenceAmount of power received from a sig-nal being transmitted at the same fre-quency through a different antenna.

GlossaryChapter 1. The Cellular Concept1.5 Frequency Bands and AMPS

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1.6 Interference and SINR

In a wireless communication system, transmit-ted signals are subject to different impairments while traveling through the air. One of these

impairments comes from those signals similar to the one being transmitted, but from unknown sources that are occupying the same frequency as the intended transmission. These undesired signals are superimposed to the desired trans-mitted signal and the receiver has this superpo-sition as input. The set of superimposed signals occupying the same frequency as the desired transmission is what we call interference.

Interference is important to consider as a qua-lity of service parameter since it will determine if calls can stay active, and will also play an impor-tant role in coverage determination and capaci-ty in general. In order to obtain interference, we will consider a scenario such as that in Figure 1.22 where a cluster of size seven is used, but it could be consider another cluster size without loss of generality. The scenario presented in the figure is for the downlink or communication from BS to MS. If the network has more tiers than that shown in the figure we will only consider the first one, since it is the ring that contains co-channel cells that are closer in distance to the center cell, hence providing the maximum influence in inter-ference. Also Figure 1.22 shows what we consi-

Chapter 1. The Cellular Concept1.6 Interference and SINR

der the worst case, i.e., the MS being serviced at the center cell is found at the maximum sepa-ration distance from its BS which is the radius R. This worst case scenario also considers that the six co-channel cells in the first tier are transmit-ting at the same frequency as that used by the MS, hence affecting with interference its signal reception. Figure 1.22, also considers approxi-mate distances as in Mac Donald, V.H. (1979), and Rappaport, T.S. (2002). In order to calculate interference we need to use a propagation model such as that in Equation (1.3).

In communication systems, it is convenient to have parameters for comparison in order to eva-luate their performance in a fair way. One of these parameters in wireless systems is signal-to-inter-ference ratio (SIR), which is the ratio of the power of the desired signal to be received and the un-desired signals perceived by the receiver. SIR is usually presented in units of decibels or dB. The calculation of the SIR is what interests us, and it is determined by the MS received power of the signal that comes from its BS antenna, divided by the signal power received by the MS from the co-channel cell antennas transmitting the same fre-quency. This power received is calculated using Equation (1.3), which is used to get SIR as follows

Figure 1.22 Downlink scenario for interference calculations

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In Equation (1.15), n is the Path Loss Exponent (PLE) that depends on the environment, D is the co-cell separation distance, R is the cell radius and PT is the power at which the BS antenna transmits. n general, when the path loss exponent n is not known, in the case of mobile networks, n is fixed at 4. Now, Equation (1.15) could be modified by using the parameter Q=D/R, which together with the value of n=4, gives the result

If we consider the number of cells in the cluster as in Section 1.4.1, N, Equation (1.16) results in 18 dB of SIR when N=7, and Q=4.58. This 18 dB is the level needed in order to have calls active, if during an active call SIR falls below 18 dB, then the call will be dropped.

(1.15)

(1.16)

Chapter 1. The Cellular Concept1.6 Interference and SINR

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1.7 Erlang Capacity

The capacity of networks is measured by the number of simultaneous active users that can be supported, but when we talk

about traffic, there is an inherent random beha-vior of users that will let us calculate in terms of users and calls per time unit, the capacity. This is what we call the Erlang capacity of the network. Recall that one unit of erlangs is equivalent to a channel being used uninterruptedly during one hour by one user, and then we say that such user contributes to the network with one erlang of traffic.

In order to calculate such capacity, a grade of service given by the blocking probability of the re-source being shared is considered. This blocking probability represents the proportion of time that the resource being shared has no channels avai-lable to connect more users; in other words, the proportion of time that a user requesting service finds the resource full, with the result of being de-nied service until some resource is liberated.

We can consider the network scenario as that shown in Figure 1.23 where three cells are shown and cell i has Ci channels assigned, and users generating traffic in such cell produce a new call arrival process which is Poisson with rate λi calls per hour. This rate can consider handoffs as shown in the figure. It is also assumed that a user

that occupies a channel in cell i, will remain with the channel for an average time duration given by 1/µi hours per call. With these assumptions, we can say, in a simplified way, that each cell of the network will behave as a resource with finite ca-pacity that can be evaluated in terms of blocking probability by the Erlang B Formula, Tanenbaum, A. (2010).

The Erlang B formula applied to cell i, i.e., the blocking probability of cell i, is given by

(1.17)

Figure 1.23 A three-cell network for erlang capacity calculations

Chapter 1. The Cellular Concept1.7 Erlang Capacity

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The quotient λi /µi is a quantity that has units of erlangs. It is important to obtain the typical behavior of a user in order to calculate the number of users supported, so for example, one can consider that the user typically generates three calls every four hours, i.e., it will contribute with a parameter λ=0.75 calls per hour. Also, one can consider that the user has call duration of 2 minutes, i.e., 1/µi =2/60 hours per call. As an example, we can consider a cell with C=56 channels assigned, and say that you want to have a grade of service given by B= 0.02, i.e., 2% of the calls being offered will be blocked and 98% will be carried. Then, the amount of erlangs that can be supported at 2% of blocking with 56 channels is 45.9 erlangs. This number is obtained either by nume-rical evaluation of Equation (1.17) or by using tables of the Erlang B formula. The typical user that we just described contributes with λ/µ =0.025 erlangs, i.e., each user will represent 25 merlangs of traffic being offered. Now, since we support 45.9 erlangs with the 56 channels at 2% blocking, by dividing 45.9 into 0.025, we get 1836 users. This number is called system penetration, and allows us to determine a number of users that can be located in the coverage of the BS of the cell that has 56 channels. Another quantity of interest would be the number of calls per hour in that cell. In this case, this number would be calculated as 45.9(60)/2 which gives 1377 calls per hour in that cell site, see Rappaport, T.S. (2002), and Tse, D. and Viswanath, P. (2005).

Chapter 1. The Cellular Concept1.7 Erlang Capacity

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1.8 Capacity and Quality Improvements

There are several methods used to increase signal quality or system capacity. The improvement of signal quality allows us to increase the number of simultaneous users since interference would be decreased, see Rappaport, T.S. (2002), and Tse, D. and Viswanath, P. (2005). The methods that are used in this direction are

Cell splitting Technique where a cell is subdivided into smaller cells with antennas of less transmission power to increase capacity.

Frequency reuse Assignments of the same frequen-cy to two different antennas located at a minimum separation to avoid co-channel interference.

SectorizationTechnique that uses antennas that transmit the signal in concentrated areas defined by angles such as 120 degrees.

Glossary

Click on each method.

Chapter 1. The Cellular Concept1.8 Capacity and Quality Improvements

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Chapter 1. Integrating Exercise

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Chapter 1. Integrating Exercise

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Chapter 1. Conclusion

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Consider a system that uses an FDD frequency band of 12.5 MHz. Each user gets a 30 KHz channel for communication. Assume a cellular network with hexagonal cells, where each cell has an area of 8 km2. The network covers a total area of 4,000 km2. The average number of calls per user during the busy hour is of 1.2 calls per user, and the average holding time of a call is of 100 seconds. It is expected to have a grade of service of 2% blocking and a frequency reuse factor of 7 cells, in other words, the cluster size is seven. Solve the following:

This chapter introduced the Cellular Concept, which is the base for the design, planning and implementation of mobile wireless networks. The concepts of hexagonal geometry and frequency reuse were presented to design clusters of

cells for network organization and frequency assignment. The basics of propagation were used for the calculation of co-channel interference and the signal-to-noise ratio.

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A B C D E F G H I J K L M N Ñ O P Q R S T U V W X Y Z

Chapter 1. Glossary

Page. 1 of 2

Cell splitting, sectorization and cluster size were discussed as quality and capacity improvements.

The cellular networks form the base to develop communications with new mobile technologies with infrastructure. This hexagonal cellular struc-ture of the network will be used in the following chapters to analyze propagation, modulation and new technologies.

AAdjacent channel interference

Amount of power perceived by an antenna from signals being transmitted at adjacent frequencies.

CCell splittingTechnique where a cell is subdivided into smaller cells with antennas of less transmission power to increase capacity.

ClustersGroups of cells that use up the complete set of frequencies.

Co-channel interferenceAmount of power received from a signal being transmitted at the same frequency through a di-fferent antenna.

FForward channelLink established from base station antenna to mobile device.

Frequency reuse

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Chapter 1. Glossary

Page. 2 of 2

Assignments of the same frequency to two different antennas located at a minimum separation to avoid co-channel interference.

HHexagonal geometryCoordinate system where the axes are separated by 60 degrees instead of being orthogonal.

OOmnidirectional antennaAntenna that transmits the signal with equal power in every direction produ-cing a radiation pattern as a sphere.

RReverse channelLink established from mobile device to base station antenna.

SSectorizationTechnique that uses antennas that transmit the signal in concentrated areas defined by angles such as 120 degrees.

A B C D E F G H I J K L M N Ñ O P Q R S T U V W X Y Z

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Chapter 1. Resources

Page 1 of 2

» Cellular Concepts and Basics (2011). In Radio-Electronics. Retrieved from http://www.radio-electronics.com/info/cellulartelecomms/cellular_concepts/mobile-basics-concepts.php

http://www.radio-electronics.com/info/cellulartelecomms/cellular_concepts/mobile-basics-concepts.php

http://www.radio-electronics.com/info/cellulartelecomms/cellular_concepts/mobile-basics-concepts.php

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eBook introduction .......................................................................................... iiiChapter 1. The Cellular Concept ......................................................................v

The Cellular Concept.................................................................................11.1 History and Technologies .............................................................11.2 Frequency Reuse and the Cellular Concept ................................81.3 Propagation Basics, PLE ...........................................................111.4 Hexagonal Geometry .................................................................12

1.4.1 Clusters. ..........................................................................141.4.2 Handoffs. ..........................................................................17

1.5 Frequency Bands and AMPS .....................................................201.6 Interference and SINR ...............................................................231.7 Erlang Capacity ..........................................................................251.8 Capacity and Quality Improvements ..........................................27

Chapter 1. Integrating Exercise ...............................................................28Chapter 1. Conclusion .............................................................................29Chapter 1. Glossary ................................................................................30Chapter 1. Resources .............................................................................32

Chapter 2. The Mobile Radio Channel ...........................................................33The Mobile Radio Channel ......................................................................34

2.1 Large Scale Propagation............................................................362.1.1 Free space and 2-ray models. .........................................392.1.2 Empirical models of path loss. .........................................492.1.3 Examples of empirical models of path loss. .....................522.1.4 Empirical path loss for wireless channels. .......................572.1.5 Stochastic approach to path loss and outage. .................59

2.2 Small Scale Propagation ............................................................622.2.1 Doppler spread. ...............................................................63

2.2.2 Delay spread. ...................................................................652.2.3 Coherence time and coherence bandwidth. ....................672.2.4 Level crossing rate and average fade duration. ...............69

2.3 Impulse Response, Channel Sounding ......................................702.4 Statistical Channel Models .........................................................732.5 Single Input Single Output (SISO) Modeling .............................742.6 Markov Modeling of Fading Channels........................................77

Chapter 2. Integrating Exercise ...............................................................80Chapter 2. Conclusion .............................................................................81Chapter 2. Glossary ................................................................................82Chapter 2. Resources .............................................................................83

Chapter 3. Digital Modulation over Wireless Channels ..................................84Digital Modulation over Wireless Channels .............................................85

3.1 Modulation for Wireless Channels .............................................853.1.1. QPSK and its variations. .................................................853.1.2. Continuous-phase modulation. .......................................98

3.2 Quadrature Amplitude Modulation (QAM) ..................................993.3 Performance over AWGN Channels ........................................1023.4 Performance over Fading Channels ........................................105

3.4.1 Rayleigh fading. .............................................................1073.4.2 Rice fading. ...................................................................108

Chapter 3. Integrating Exercise .............................................................110Chapter 3. Conclusion ........................................................................... 111Chapter 3. Glossary ..............................................................................112Chapter 3. Resources ...........................................................................113

Chapter 4. Diversity Techniques in Fading Channels ...................................114Diversity Techniques in Fading Channels .............................................115

Index

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4.1 Types of Diversity ..................................................................... 1164.2 Rake Receiver..........................................................................120

4.2.1 Combining methods. ......................................................1214.2.2 Selection combiner.........................................................128

Chapter 4. Integrating Exercise .............................................................132Chapter 4. Conclusion ...........................................................................133Chapter 4. Glossary ..............................................................................134Chapter 4. Resources ...........................................................................135

Chapter 5. Spread-Spectrum Systems .........................................................136Spread-Spectrum Systems ...................................................................137

5.1 Fundamentals of TH, FH and DS Spreading ...........................1375.2 PN sequences ..........................................................................149

5.2.1 Generation, properties and DS spreading. ....................1495.2.2 Maximum length and other sequences. .........................150

5.3 Spread Spectrum .....................................................................1585.3.1 Discrete-time approach. .................................................1605.3.2 Advantages: Interference, MAI, and multipath. ..............163

Chapter 5. Integrating Exercise .............................................................172Chapter 5. Conclusion ...........................................................................174Chapter 5. Glossary ..............................................................................175Chapter 5. Resources ...........................................................................176

Glossary .......................................................................................................179References ...................................................................................................183Index .............................................................................................................187Legal Terms ..................................................................................................189

Index

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©Vargas Rosales, César; Rodríguez, José Ramón Rodríguez Cruz

Volume I: Wireless Communications / César Vargas Rosales, J.Ramón Rodríguez

p. 189 cm.

1.Sistemas de radiocomunicación

2. Sistemas móviles de comunicación

I. Rodríguez Cruz, José Ramón

LC: TK5103.2 Dewey: 621.384

eBook edited, designed, published and distributed by the Instituto Tecnológico y de Estudios Superiores de Monterrey.

No part of this publication may be reproduced or transmitted in any form or by any means without permission in writing from the Instituto Tecnólogico y de Es-tudios Superiores de Monterrey.

Copyright © Instituto Tecnológico y de Estudios Superiores de Monterrey, Mexico. 2011. Ave. Eugenio Garza Sada 2501 Sur Col. Tecnologico C.P. 64849 | Monterrey, Nuevo Leon | Mexico.

ISBN Pending

Preliminary Edition: December, 2011.

Legal Terms

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