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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Features of cellular communication. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Mobile telephony The rapid development of new telecommunication technologies in the 80s and 90s was associated with the active use of the achievements of science and technology in the branches of telecommunications, information technology and electronics. One of the priority technologies is cellular communication, which is gaining more and more popularity among the population and is developing at a very rapid pace.
"Don't turn your back on a possible future until you're sure there's nothing there for you to learn."
world championship Like those readers who prefer to read the magazine from the end, let's jump ahead and say the main thing: today there is no type of telecommunication that would develop as quickly as wireless radiotelephony, and especially cellular communications serving mobile users. The number of such users is increasing annually by about 40% and has recently confidently crossed the 350-million mark. It should be noted that certain types of cellular mobile networks show even more impressive growth rates. In the world, the annual increase in GSM network subscribers is approximately 70% (gsmworld.com), and for cdmaOne networks, known to us as CDMA / IS-95, it has reached almost 160% over the past year (cdg.org)! Cellular subscribers are expected to reach 1 billion sometime in 2002-2003. Although not much time has passed since the moment when cellular networks left the stage of scientific and technical developments on the commercial market (the turn of the 70-80s), the cell phone has now ceased to be a symbol of prestige and has become a tool that allows more efficient use of the working time, promptly manage technological, economic and other processes. At the same time, cellular networks not only grow in breadth, but also steadily increase the number of additional services provided. If we take the density of cellular networks, which many developed countries have reached (data from British Telecom Engineering), then the undisputed leader among them is Finland, where about 70% of the population have cell phones. The neighbors in Scandinavia are a little behind (from 50 to 60%), between which a piece of China (the former Hong Kong) has wedged. Six more countries, including Australia and Japan, have crossed the 30% threshold, and a whole group of countries, including the UK and the US, have come close to it. Approximately 20% indicators are in Germany, France, Spain, Canada and a number of other countries. However, one can list the successes of mobile communications for a long time, but the information already given is enough to draw some conclusions. All of the above indicates that in most of these countries the density of radiotelephone lines has already reached the level of line density of traditional telephony (approximately 50-60%), or at least half of it. Many of these countries have either very large territories, or very large populations, or both. Therefore, even 5% in Brazil and 3% in China, excluding Hong Kong (what a small figure and at the same time a monstrously large number of users) turn out to be more significant than, for example, 27% somewhere in the Netherlands. In general, cellular communication has already become a product of mass consumption and continues to increase its growth rate. Even the most immediate forecasts (2003) are impressive: in terms of population coverage, a whole group of countries will come close to 90%. and the former Hong Kong (now a special administrative region of the PRC) will be the first territory where this figure reaches 95%! Almost two dozen countries in terms of density will be in the range of 60 ... 95%. However, even very developed countries with relatively large territories will not advance so far: the USA - 53%, Germany - 47%, Canada - 30%, Brazil - 22%, China - 8%. Interestingly, the pioneers of mobile communication in the face of the UK and the US will be behind countries such as Austria, Ireland and Greece. Perhaps in a few years we will see how the groups of users of traditional and cell phones are almost equal in number. By the way, today there are about 1 billion wire telephone lines all over the world. Since there are enough reasons to doubt the usual primacy of wired telephony, foreign analysts are beginning to consider cellular communications as a service equal to wired, and some even give it primacy. the rapidly developing global market for cellular services attracts the attention of many companies. For example, only in Russia about two hundred operating companies are involved in this business. All the accelerating pace of development of society urgently requires the receipt of more personalized services, namely: the possibility of a telephone conversation (and not only) with any subscriber, anywhere and at any time. Note that the last phrase has truly become the motto of cellular communications these days. And not only her. Now let's take a closer look at the components of the cellular communications market and at some historical stages of both its technical and organizational development. But first, a small lyrical digression. About the beginnings When touching on a large topic, it is useful to start with terminology in order to slightly streamline the knowledge that readers managed to get earlier from other sources, as well as to outline the boundaries of the narrative. In connection with the foregoing, first of all, we note that cellular communication is a type of highly mobile radio communication, which is distinguished primarily by the mass service of subscribers in a limited area. At the same time, historically, cellular communication has gradually expanded the scope of service of the public telephone network (PSTN). In fact, the term "cellular communication" itself is a generally accepted abbreviation for the service received using deployed cellular mobile networks based on the corresponding systems. Thus, this term characterizes mobile communication. Such lengthy discussions are necessary so as not to confuse the cellular network and cellular communications, because the first means a method of providing radio coverage of a certain territory (respectively, it can be used to provide both mobile and fixed communication services), and the second - a service in the form of transmitting and receiving information exclusively between mobile subscribers of the cellular network. And today it can be not only speech, but also data, of course, and the Internet. It should be remembered that any radio communication system that works with a subscriber radio terminal with an omnidirectional antenna is more or less mobile (and only with a rigid fixation of the antenna, the movement of the subscriber is impossible), because the user can move within the coverage area of the base station (BS) and be served by it. Therefore, such communication systems can be divided into those that serve sedentary subscribers who have the ability to move during a communication session only within the service area of one BS (these systems can rather be called fixed), and those that allow a highly mobile subscriber to carry out a continuous communication session with movement (including very fast, even at a speed of 100 km/h or more) from the service area of one BS to the area of another BS and further within the entire territory covered by the network. This is what true cellular communication is. From the foregoing, it can be concluded that the concept of "fixed communication" is often used incorrectly here, and if a radio communication system consists of only one long-range base station, then it can usually provide mobile radio communications in its service area. If the base station is located on a satellite, then we will take an important step towards creating a network of global (planetary) satellite communications. But let's go down to the ground and now let's deal with the "honeycomb". Cellular networks got their name in accordance with the cellular principle of organizing communication, which is as follows. In the service area of the cellular network, the required number of the already mentioned base transceiver stations (BS) is installed, each of which has its own relatively small radio coverage area, somewhat overlapping the service area of neighboring BSs (to ensure continuous radio coverage of the territory). Answering the reader's logical question: "Why is this necessary?" - Many authors of works on cellular communications often give approximately the following arguments: "to provide subscribers with the opportunity to move", "to implement the electromagnetic compatibility of neighboring cells" by "them" to improve communication quality ". All this, of course, is true, but, firstly, fixed-line network operators are also interested in good radio coverage, and, secondly, objectively, any cellular network operator is always interested in having fewer cells in its network. Even better, the network generally consists of only one cell, because this is the simplest and cheapest. Another thing is that this is usually not possible in practice, and when trying to simultaneously serve a significant number of subscribers grouped in a relatively small area, it inevitably becomes necessary to have an appropriate number of radio communication channels. And the radio spectrum, as you know, is a natural resource exploited by many users, which is always in short supply. Including those who organize a lot of radio interfaces between base stations and subscriber terminals (cellular phones). Thus, the main task that any cellular network solves is the economical multiple use of the same radio frequency resource allocated to the network operator to cover as many subscribers as possible. This approach makes it possible to solve the problem of truly queuing subscribers at a serious commercial level. About the benefits of bees Now a few words about the cells themselves, or, as they are also called, cells. Why, by the way, honeycombs? Yes, because the radio coverage area around the BS in the general case is a circle. And of the equilateral figures inscribed in a circle (triangles and squares do not count), only hexagons can cover any surface without breaks. Therefore, they are used as a mathematical model of a cellular network. It is also clear that at one time, much earlier than the radio operators, the bees guessed this. The size of cells / cells of a cellular radiotelephone network in practice can range from tens of meters to tens of kilometers, however, their smallest varieties, organized in places of special congestion of subscribers (for example, business centers), are often called according to their size: pico or micro cells. In order to increase subscriber capacity, cells can be implemented in a multi-sector design (usually with three or six sectors), which is equivalent to an economical organization instead of one cell, respectively, three or six. In this case, the BS is a block of several transceivers, each of which, with its directional antenna, serves the corresponding sector of the cell. It remains to add that one cell of the cellular network, together with the BS installed in it, is usually called by foreign experts "cell site". A very important task facing the operator of many cellular networks for any purpose is RF cell planning. Indeed, if the radio interface technology of a cellular network provides for the separation of radio channels by frequency, then neighboring cells cannot operate on the same radio frequencies, and they can be reused only by stepping back a certain distance, depending on the technical characteristics of the BS. This situation is very common, but also very unpleasant for the operator, because the need to organize a new cell entails the need for frequency rescheduling of extensive network fragments. However, not everyone is concerned about these issues. We will talk about this a little later. In pursuit of a subscriber The next one is very important. let's even say that the defining concept in cellular communications is the so-called relay-race transfer of control between cells when a subscriber moves from one cell to another. It is this ability that distinguishes cellular communications from wireless communications in general. This control is provided not by the BS, but by a specialized switching center, which is the core of the network and to which all BSs are connected. At the same time, this center, which in the general case is a specialized exchange. included in TfOL. The switching of control is associated with the organization of a new communication channel and the transfer of a specific telephone connection to it. It can be done in different ways, called "hard" or "soft", which depend on the technologies used in the network and will be discussed below. For now, we'll just focus on the name. Historically, this switching process is more commonly referred to as "hand-over" in Europe and "hand-off" in North America. Therefore, having encountered such discrepancies, one should not be surprised. Considering hand-off (we will focus on a shorter name), one cannot ignore the case when a subscriber moves from the service area of one cellular network to the service area of another. But different networks have different owners-operators. The process (as well as the possibility) of transferring control between cells / networks of different owners not only received the name "roaming" (tramp) abroad, but also firmly established itself in the Russian spelling as "roaming". Of course, on the one hand, and hand-off is nothing more than roaming between neighboring cells, however, signalers mean much more by roaming, namely, inter-operator interaction, for which, of course, not only technical characteristics and network standards must match , but appropriate agreements on tariff policy, mutual settlements, etc. must be concluded. In addition, cellular networks may not border on each other, which suggests that roaming is generally the ability to receive services in the networks of other operators (with which service contracts) using the same cell phone. It remains to be added that roaming can be automatic, when the process of switching from one operator to another is noticed by the subscriber only by the corresponding indication on the subscriber terminal, and by the received bill. Non-automatic roaming implies prior notification of the operator by the subscriber about the possibility of traveling outside the network with all the attendant inconveniences, the degree of which depends on the promptness and capabilities of the operator. Well, the most versatile and unobtrusive, but also the most inconvenient roaming is just a change of cell phone when leaving the range of "one's own" network. However, if we cannot use our subscriber terminal, then this is not roaming at all (although some operators offer just this type of roaming!). Radio interface technologies Cellular communication is characterized by the possibility of multi-access, which implies the simultaneous transmission of information through one device by many users in a common communication channel. In this case, the division of the common channel can be carried out by frequency (FDMA - Frequency Division Multiple Access), time (TDMA - Time Division Multiple Access) and code ( CDMA - Code Division Multiple Access). The features of these technologies are clearly shown in the figure.
In frequency division, the transmission spectrum is divided into sections allocated to different users. Only this method can be used with analog cellular. Therefore, all known analog cellular communication standards are based on it: NMT, AMPS, TACS, etc. The disadvantages of analog systems are now obvious: poor noise immunity and the associated low quality of voice transmission, inefficient use of scarce radio spectrum, lack of protection from listening, etc. It should also be said that analog systems reached their peak of development in 1993, after which there is a steady decline in the number of their subscribers, and in a few years they should generally leave the communications market due to loss of interest from operators and subscribers. The analog standard in the world was and still is AMPS. The other two methods are used with digital technology and usually in combination with frequency division. In the case of time division multi-access, multiple subscribers transmit their messages on the same radio frequency, but at different times, which allows you to increase the amount of voice traffic and get a number of other advantages specific to digital communication systems. This method is based on such well-known narrow-band digital cellular communication standards as GSM and its DCS variety, as well as D-AMPS, which has become a logical development (improvement) of the AMPS standard. There are also Japanese standards, but their scope is limited exclusively to the national territory. The principle used in CDMA multi-access systems is to expand the spectrum of the original information signal (in our case, speech) with the help of a special signal with a unique envelope shape, which is a kind of code. As can be seen, in this case, all subscribers operate on the same radio frequencies, as a result of which the CDMA cellular network does not need to spend much effort on frequency planning. In the receiver, the signal received on the other side is processed using an identical code, as a result of which the original information signal is restored. At the same time, the signals of other users for this receiver continue to be extended and are perceived by it only as "white noise", which is the "softest" interference that least interferes with the normal operation of the receiver. Of course. to this we shall return. And in order to popularly explain the principle of operation of such a system, we will refer to one very successful allegory "for experts and not so", which, explaining the basics of CDMA technology, is usually offered by Motorola. Imagine a room in which many pairs of people are talking to each other at the same time, and in different languages. Each of them understands his interlocutor well, and all extraneous conversations are perceived as a kind of background that does not particularly interfere with the conversation. That, in fact, is the whole principle. It's a matter of technique. Author: A.Golyshko, Moscow
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