The history of telecommunications includes several periods, the first of which began in the Stone Age when people learned to speak. It was followed by postal services, pigeon service, telegraph service, telephone and radio communications, and by the end of the 20th century there appeared another one ??“ cellular communications. It is not necessary to describe the differences between cellular communications and other means of communications as long as the average reader has already got an idea about the space propagation of radio waves and is likely to experience their influence feeling the vibration of a mobile phone in the pocket.
A mother of children who are playing in a sandpit wouldn??™t cry for them from the balcony but would rather call them using her cellular phone. Business partners are always available for each other. A young man sitting in a park with his notebook and cellular phone wouldn??™t carry thousands kilometers of wires in order to connect to an Internet provider. All this has become possible when using a mobile cellular communications, the creation, evolution and the future of which will be discussed further.
The reason for creation and evolution of cellular communication was the necessity for communications to become countrywide and affordable. The first attempt to implement mobile telephony was made in 1946 in St.Lewis (USA) when a simple 6-channel transceiver was created. Only 24 years later scientists started the development of the ???wireless??? telephony again. The architecture and performance principles of cellular network were clearly described in 1975. In the result of almost 10-years work there appeared NMT-standard (Nordic Mobile Telephone), which was intended to work in a 453.0-457.5 MHz band and had 180 channels 25 KHz each. In 1978 NMT was introduced as a standard by Denmark, Finland, Iceland, Norway and Switzerland.
A modification of NMT-900, which performed at 900 MHz and had some advantages in comparison with the first standard NMT-450, emerged a bit later. In particular, there appeared an opportunity to produce portable mobile phones and also the quality of communications increased.
By the beginning of the 80s there were telephones constructed, which let to make calls to another phones with the aid of base stations. These devices pertain to the first generation of cellular communications, or 1G.
One of the first of the NMT-450-standard telephones by Nokia
Mobile telephones of the first generation were no less than a size of a suitcase and included a base and a separate handset, which was impossible to carry. Nowadays it is hard to imagine such a wizardry, which weighed several kilos, emanated 20-30 watt signal and had a 30-40 cm antennae. The maximal voice bitrate was 9.6 Kb/s, and data rate was 1.9 Kb/s. On a European market the NMT-standard was introduced by somewhat different standards: AMPS (Advanced Mobile Phones Service) and TACS (Total Access Communications System). The disadvantages of analogue standard NMT are high level of radiation, interference sensitivity and a low level of privacy of the transmitted data.
2G standard was introduced to solve the problems of cellular communications of the first generation, the development of which began long before the public implementation of NMT-networks.
In 1982 on the European Conference of Postal and Telecommunications Administrations there was formed Groupe Speciale Mobile, or GSM, the aim of which was to develop a reliable and simple system of terrestrial mobile communications. The system had to provide high quality of the transmitted voice data, simple construction and maintenance of cellular network, support of working with portable devices, such as PDA, notebooks etc., possibility to realize not only local, but international roaming, openness for the development of new services, compatibility with ISDN systems and effective air usage.
The right to proceed with the development of the standard was imposed on ETSI (European Telecommunication Standards Institute). In the middle of 1990 there were introduced specifications of standard GSM-900, which then meant Global System for Mobile communications. Then there was evolutional standard GSM-1800, which used radio frequency 1800 MHz. The increase of the radio signal frequency two times let to increase density of subscriber base four times.
GSM-900 applies restrictions on the remoteness of mobile phone from the base station, which cannot exceed 35 km even if the signal power is enough. This is because packets with overcome life-span are culled. In GSM-1800 the coverage is even less: about 6-10 km. NMT doesn??™t have such restrictions and in perfect conditions it is possible to realize communications at 200 km (experimentally proved). It is needed to say that reducing coverage two times lets to increase density of subscriber base two times. There is a special configuration of base stations, Extended Cell, where the distance may be increased up to 70-100 km, but the amount of simultaneously serviced subscribers cannot exceed 2-3
The uniqueness of the new GSM standard is that it is the first one to use digital cellular system as distinct from the existing analogue systems. Its practical usage was started in 1991. The data bitrate in the network increased from 1.9 Kb/s to 14 Kb/s. That let to use mobile phone, which by that time could be named that, modem or fax, and afterwards to use WAP-services.
Primarily GSM was very flexible in the amount and diversity of services. Due to the new standard there appeared Short Message Service, or SMS. This service let to exchange short text messages up to 160 symbols. Based on SMS protocol there broadcasting appeared ??“ news and other information was broadcasted to all the subscribers of the network. An additional flexibility was achieved by using SIM-cards (Subscriber Identity Module), which let to assign to the net not the telephone itself but a small module, which included international user??™s identifier of mobile services.
TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access) and PDC (personal Digital Cellular) have had a wide distribution apart from GSM.
In July of 1989 TDMA was accepted by a range of large mobile companies in the USA as a replacement of the existing analogue standards. That can be counted as the first commercial maintenance of such network.
TDMA is a technology of transmitting digital signal. TDMA lets to avoid interference and lets to use radio signal most effectively. The principle of operation is as follows: when calling, a telephone is given a frequency channel, which is divided into equal timeslots. When a telephone transmits an encoded audio signal over a channel, all the other telephones are waiting for their turn. In the next timeslot another telephone transmits and so on. Timeslots have such length, that lets to share the air imperceptibly for human ear. The advantages of TDMA:
- the most effective, simplest and recompensed standard for the transfer from analogue transmission of AMPS signal to digital;
- data transmission from 64 Kb/s up to 120 Mb/s;
- no interference from other telephones;
- prolonged time for working from accumulators, which is because the signals are transmitted periodically, in contradiction to 1G networks, where signal is transmitted all the time;
- high network capacity.
Along with the advantages there are disadvantages of the technology:
- impossible in some cases to continue talk in the time of switching between base stations;
- possibility of interference from buildings because of the simultaneous transmission of the signal to several available base stations.
TDMA is compatible with many different cellular networks standards, and its principles are used in several third-generation networks technologies. Its flexibility, reliability, scalability influenced on the distribution of the standard, which is partially used in European GSM and 3G and 3.5G technologies.
CDMA is thought to be an obvious competitor of TDMA. The principle, which is used in signal transmission in such networks, pertains to military technologies and is used in military satellites. CDMA networks appeared in the USA a few months after TDMA, and they still exist and are constantly modernized. The increased network capacity and easiness of creation was the reason for quick spreading of the standard, and by the year 2003 the amount of its users was more than 150000000.
The principle of CDMA performance lets to avoid interference, to increase the number of simultaneously talking subscribers and to protect the transmitted information. During a call the encoded information is simultaneously transmitted from a telephone to several available base stations, which lets to choose the most ???rightful??? received packet. Apart from that, the transmitted signal is spread in frequency-time space, in contradiction with other radio technologies, where all the signal power is concentrated on the chosen frequencies or time intervals. In fact, only noise will be heard in the air, from which it is very hard to select wanted signal. That is why the technology has a very high protection level from overhearing. The data transmission bitrate varies from 9.6 Kb/s to 1.23 Mb/s.
CDMA technology has almost no disadvantages, provides high reliability of the voice and data transmission, required data protection level and sutureless switching between base stations. The principles of CDMA networks are widely used in third generation networks.
The coming of the second generation networks to market and a rising necessity of the mobile access to Internet were the reason for development of cellular communications in the direction of a bitrate increase. The development of 2.5G standards, which are, in fact, a buildup for second generation networks, started in 1985. The 2.5G standard in known to us as GPRS (General Packet Radio System), EDGE (Enhanced Data for Global Evolution), 1xRTT (2.5G CDMA data service). The maximum data transmission bitrate, which is possible to achieve using these standards, is 384 Kb/s.
GPRS technology lets to simultaneously talk on a telephone and to transmit data. It has become possible because of a more effective use of radio band and new coding methods. One more feature of the service is that it??™s not necessary to call to a provider every time. Telephone is always online, but it transmits data only when there is such need. This working scheme lets to simultaneously serve much more clients than when using second generation GSM networks.
As any other system, GPRS has its own disadvantages:
- Maximum theoretical speed, which is possible with GPRS-connection, is 172.2 Kb/s. The real bitrate rarely exceeds 48-50 Kb/s, which is because of redundancy of the transmitted data that provides protection from errors and includes control information;
- GPRS uses ineffective modulation technology GMSK (Gaussian minimum-shift keying). A more modern service EDGE uses a better technology 8 PSK (eight-phase-shift keying), that lets to achieve higher bitrates of data transmission..
The main advantage of 2.5G is a possibility to develop and apply completely new net services, the performance of which would be simply impossible in previous generation of standards. The average bitrate of 48 Kb/s for GPRS is enough for communication using IRC or IM-client, which, in practice, is much more convenient than interchanging SMS, internet-surfing using web-browsers and even for online video- or radio-broadcasting. But the main thing which 2.5G networks let to accomplish was the beginning of an integration process of Internet and cellular networks.
Quickly increasing needs in broadband access to Internet were the reason to a further development of cellular communications. As a result of 3G and 3.5G networks development there several different standards emerged: CDMA (Code Division Multiple Access) 2000, UMTS (Universal Mobile Telecommunications Service) and WCDMA (Wide CDMA). The third generation networks are as well officially called IMT-2000.
The family of standards was to guarantee a high bitrate of a symmetric and asymmetric data transmission inside the network, support of packet and circuit modulation for integration with IP-networks, compactness and effective usage of granted spectrum, possibility of global roaming.
There is a diversification in data transmission bitrate depending on a subscriber??™s mobility:
traverse speed <3 km/h
up to 2.048 Kb/s
traverse speed<120 km/h
up to 144 Kb/s
global satellite coverage
up to 64 (144) Kb/s
The coming of the third generation networks let to develop innovation services, the implementation of which in 2G and 2.5G networks was simply impossible because of low radio channel bandwidth capacity and low effectiveness from using QoS (Quality of Service).
- mobile and quick Internet access;
- mobile television;
- enhanced audiodata quality calls;
- new way of the development of mobile electronic commerce;
- mobile communication with corporate networks;
- possibility of presenting internet-services to mobile cellular communication users.
The effectiveness of the 3rd generation networks is proved by the high quest in the countries where they are widely used. Having low self-cost of the equipment, a subscriber can get Internet access at 2 Mb/s, a possibility to make videocalls and watch favourite TV-channel in a spare-time. One of the aims, which are to be achieved by 3G networks, is global coverage, when a subscriber stays in the coverage zone anywhere on Earth. The possibilities are really unique, taking into consideration the specifics and high technologies development level in CIS.
The manufacturers of mobile telephones played big role in quick development of 3G networks. They embedded possibility to work with UMTS, CDMA2000 and WCDMA into telephones of 2G standards. Thus, buying an ordinary business-class telephone a user got possibility to work in networks both of 2G and 3G. Such approach could have enforced the transfer from 3G to 4G. That is why in the nearest future it is possible that there are such novelty at the mobile techniques market.
Nowadays 3G networks are widely distributed in Eastern Asia and America. Experts presume that number of WCDMA network subscribers will increase up to 90 million people by the year 2007, mostly in Japan and European countries. Wholesale implementation of UMTS, CDMA2000 and WCDMA will happen not before the year 2008. Before that time cellular communications service providers will be able to do ???rearmament??? of the equipment park, whilst service users will get telephones which support new technologies. At present, the developed European countries have already started to present communications services with the 3G technologies??™ aid, but the cost is still high.
While waiting for the appearance of 4G networks at the market, the existing 3G standards are being improved. NTT DoCoMo company promises to start a commercial maintenance of HSPDA, which is 3.5G, by the year 2007. It will make it possible to receive data from a satellite at bitrates up to 14 Mb/s; and by the end of the year the emergence of the equipment is preplanned. This equipment will be able to increase this bitrate up to 100 Mb/s. Nowadays many cellular communications operators have already built and launched HSPDA networks in testing modes. It is worth saying that due to its flexibility this technology will not leave the market in the nearest 5-6 years until public implementation of 4G cellular communications.
Growth of Internet, of quality and quantity of interactive services presumes simultaneous widening of communication channels which bind user with World Wide Web. Now, when data transmission bitrates are measured in Mb/s and Gb/s, the preceding technologies have gone into history and there are new appearing which let to integrate all the devices and accomplish centralized control. Such services as online-broadcasting of popular TV and radiochannels, VoIP-telephony are becoming more and more traditional and available for the user. Despite the fact that the amount of users of broadband access to Internet is still a small part of the whole, this amount is growing up all the time. There were official research results presented on a IDF conference in 2006 in Moscow, which show quick dynamics of Internet growth, which testify of its high being in demand.
The developed data transmission technologies for 3G networks in some cases are not able to manage tasks. Providing in case of an ordinary user the data transmission delays might not matter, but in case of business applications a 1-second delay might cost thousands and even million of dollars. In such situations UMTS, CDMA2000 and WCDMA are not enough for the high-growing need. The using of 802.11 a/b/g standard, which could??™ve improved the situation a bit, is restricted by several values, the main of which is the distance to the access point.
One of the main aims which were counted when developing 4G standards family is the integration of all the kinds of communications in one structure, the scheme of which is presented on the picture below.
Networks integration scheme
WiFi networks, 3G, satellite video and audio broadcasting, WiMAX networks and 4G networks will communicate through a set of devices called Internet Gateway Router, the task of which is to produce the environment for a transparent usage of the technologies mentioned above and connection of the Internet networks with terrestrial telephone lines. It will let to use not the expensive technologies for transmitting huge volumes of data, but the existing IP-networks. This will influence the cost of service of cellular networks.
There were several obstacles on the way of developing 4G. These were because of the search of new effective means of using the air, algorithms of compression and data transmission. Initially cellular networks were designed for voice transmission, that is why simply increasing equipment powers was not enough. Main innovations in 4G cellular communications are: Variable-Spreading-factor Spread Orthogonal Frequency Division Multiplexing, VSF-Spread OFDM, and Multiple Input Multiple Output, MIMO. VSF-Spread OFDM lets to use simultaneously several frequency bands, which gives opportunity to increase bandwidth capacity of the channel several times. MIMO lets to transmit information through several routs for or from base stations.
Experts promise public distribution of 4G technology in 2006-2007, but there??™s no talking about public commercial maintenance. It is assumed that not before 2010 the cost decrease to $100 is not expected, so the main users of 4G networks will be large enterprises and corporations. At this stage the main rival of 4G networks is wireless data transmission standard WiMAX, or IEEE 802.16. Before the appearance of the devices working with a mobile realization of WiMAX 802.16e, the cost of which would be compared with 3G networks equipment, wide availability of the standard among average users is not expected. Not everyone could give $600-1000 for a set of client equipment, getting vague profits in bitrate and quality of service. In the first place it is connected with the realization of WiMAX technology which has some disadvantages. They should be taken into account in a big city with a high population density.
Now we will present some numbers in order to verify the above said. Coverage radius of one WiMAX base station in a city is 4 km in average. Such territory might be inhabited by approximately 180000-240000 people. If one of users is given a 1 Mb/s channel, then maximum number of simultaneously serviced clients would be approximately 1900 people from 240000.
In some regions before the realization of WiMAX there existed networks, namely pre-WiMAX, which hasn??™t become popular because of high cost of a slightly known subscriber??™s equipment.
In that situation 3G and 4G networks are far more attractive for an ordinary user. Despite the fact that the provided services - UMTS, WCDMA and CDMA2000 let to achieve a bitrate only up to 2 Mb/s in comparison with a rival - WiMAX, it is in most cases not enough for a comfortable Internet surfing and access to provided nowadays services. The realization and implementation of 4G networks will let to transmit data several times faster than it is possible to do now. For example, when testing the first 4G network by NTT DoCoMo company, the bandwidth capacity of communication channel was 1Gb/s in subscriber??™s static state. Such bitrate is explained in a way that in 4G networks only effective packet data transmission is used, in contradiction with the existing packet transmission comprised with the voice traffic transmission. This let to abandon ???braking??? technologies and to replace them with a more productive, having saved and improved their functionality
According to the latest researches, while testing 4G network there was achieved 1Gb/s bitrate to a subscriber moving with a speed under 20 km/h, which is quite high for the existing technologies.
Gigabit wireless networks are already near. Along with a global roaming they will bring huge data transmission bitrates, perfect quality of the transmitted voice and video, will let to integrate the existing networks in one complicated structure. Any person, who has got a mobile phone, will be able to put in to global processes brought by 4G cellular communications.
What will 4G cellular networks introduce to us? Nothing revolutionary, except for the improving of services, presented by 3G cellular communications. The enhanced quality of videocalls and voice, higher Internet access bitrates, global roaming, corporate networks communications. In any case it is hard to imagine how to use 100 Mb/s, which are selected for a telephone. In conditions of quite high costs of Internet access in our country an ordinary user will not be able to pay for such a bandwidth of the channel, while for the connection of the existing corporate networks 5-10 Mb/s is enough, which are possible to provide by pre-WiMAX and WiFi. That is why it is possible to assume that 4G was being developed for a long perspective and all its resources wouldn??™t be depleted for a long time.
During the last 20 years cellular communications was being transferred from an ordinary primitive means of communications into a high-speed technology able to make people??™s life easier and better. The coming of 4G networks might be counted as a new milestone in the evolution not only in cellular communications but in the life of many people who will get a new level of mobility and availability. Business will become truly controlled due to high data transmission bitrates with minimal delays, which will let to avoid faults and to prevent critical situations. Life without wires ??“ that is the motto for the next millennium.