# Overview of Mobile Communications



## عماد نبيه (19 يناير 2010)

* Overview of Mobile Communications*

Recent years, which are characterized by booming social progress resulting from economic and technological development, especially in the fields of computer, program controlled switching and digital communications, have witnessed rapid growth of the mobile communication system and its extensive applications in the society thanks to its remarkable features and advantages. Now there are over one hundred million mobile subscribers in the world, and this number is expected to increase to two hundred million at the end of this century. Wireless communication is superior to wired communication in development potential, because it not only provides ordinary telephone services, but also can or will provide various services beyond wired communication to meet the subscribers’ demands.
Mobile communications aims to enable you to communicate with any of others whenever you want and wherever you are.
From the communication network perspective, a mobile network can be regarded as extension of a wired communication network, so it is composed of wireless and wired parts.
The wireless part provides access of subscriber terminals by transmitting voice and data reliably in air using limited frequency resources. The wired part performs the network functions, including switching, subscriber management, roaming, and authentication, which form the Public Land Mobile Network (PLMN).
According to the realization means, the land mobile communication is classified into analog mobile communication and digital mobile communication.
The mobile communication system can be divided into three phases according to its development courses and directions since 1940s.​* First Generation – Analog Cellular Mobile System*

The first generation of mobile telephone system adopts the cellular networking technology. The cell concept was put forward by Bell Labs and afterwards researched in many places throughout the world. As the first trial run network was commissioned in Chicago, America had its first cellular system AMPS (Advanced Mobile Phone Service) in 1979.
Nowadays, this kind of system that is still working practically with large capacities includes:
1) AMPS in North America
2) NMT-450/900 in North Europe
3) TACS in Great Britain. Their operating frequencies linger around 450MHz and 900MHz, with a carrier frequency separation below 30 kHz.
In view of the characteristics of mobile subscribers, a mobile communication system must not only implement automatic intra-area, cross-area and cross-exchange channel switching, but also handle roaming subscribers’ (including calling and called parties) calls. Therefore, a mobile communication system hopes for both a standard open interface with the public network and an open development interface. Since mobile communication is based on the fixed telephone network, the analog mobile communication networks are quite different in networking, and the total capacities are limited considerably.
Although the analog cellular mobile communication system will continue to develop at a certain rate in the coming several years, the limitation of analog mobile communication results in the following fatal disadvantages:
A) No public interface between the systems.
B) Unable to keep pace with rapid digitization of the fixed network to develop digital bearer services.
C) Unable to meet the large-capacity demands due to its low frequency utilization rate.
D) Vulnerable to wiretapping and “faking” due to its low security utilization rate.
These fatal disadvantages will obstruct further development of the analog cellular mobile communication system, so it will be replaced gradually by the digital cellular mobile communication system. However, the networking technology in the analogy system is still applied to the digital system.​* Second Generation – Digital Cellular Mobile Communication System*

To overcome the shortcomings of the analog systems such as TACS, the mobile telephone system was developed in 1990s, featuring digital transmission, time division multiple address (TDMA), and narrowband code division multiple address (N-CDMA). It is referred to as second generation of mobile telephone system, and has two typical classifications of products.​* TDMA System*

Of the TDMA series, the mature and typical systems include GSM in Pan-Europe, D-AMPS in USA, and PDC in Japan.
1) Regarding D-AMPS, its technical standards were made in 1989 by EIA (Electronics Industries Association) of USA, and its commercial application began in 1993. Derived from AMPS, it combines analog and digital features, and has complex base stations and mobile stations.
2) As for Japan’s JDC (renamed as PDC now), its technical standards were made in 1990, and came into use in 1993, but limited to Japan proper.
3) GSM (Group Special Mobile) of CEPT (Conference of European Postal and Telecommunications Administrations) made the first phase of standards (phase1) for GSM in 1988, with an operating frequency of about 900MHz, applied commercially in 1990. In the same year, at the request of Great Britain, the GSM specification of 1800MHz came into being.
These three products share the features of digitization, TMDA, better voice quality than the first generation, good security, data transmission, and automatic roaming.
The three systems have their own advantages. For example, PDC has a high utilization rate of frequency spectrum, and D-AMPS has the largest capacity. But GSM has the most mature technology, and it is based on OSI and has its technical standards open, so it has the largest development scale.​* N-CDMA System*

N-CDMA is developed on the IS-95 basis by Qualcomm. The North American digital cellular system specifications were made by Telecommunications Industry Association of USA, and its systematic research began in 1987. These specifications were accepted by EIA in 1990. Since North America already has a unified AMPS analog system, this digital cellular system was designed in dual modes. Later, its frequency band was extended to 1900MHz, namely the PCS1900 based on N-CDMA.​* Third Generation - IMT-2000*

The increasing number of subscribers and fast growth of digital communication reveal gradually the shortcomings of the second generation of mobile telephone system. First of all, the frequency band is too narrow to provide broadband information services, such as high-speed data, low-speed image, and television image. Besides, GSM, though referred to as Global System for Mobile communication, has not realized actual global roaming yet, and is not applied extensively in some countries like USA and Japan, where there are a great number of mobile telephone subscribers. The development of science & technology and communication service necessitates an integrated service system combining all the current mobile telephone system functions and various value-added services, so the International Telecommunications Union required commercialization of the third generation of mobile communication system, or IMT-2000, in 2000. Its major features include:
1) With various systems integrated.
2) High consistency in world-wide design.
3) Compatibility between the services in IMT-2000 and fixed networks.
4) High quality.
5) World-wide use of small portable terminals.
Typical 3rd generation mobile communication system technologies:
There are two standards:
1) The broadband cdmaOne proposed by Qualcomm, compatible reversely with the IS-95 system.
It is recommended to use multi-level DS-CDMA, with an RF channel bandwidth of 1.25/10/20MHz and a PN code frequency of 1.288/3.6864/7.3728/14.7456Mbps. The use of multiple levels aims to divide 5MHz into three channels of 1.25MHz to be compatible backward with IS-95 for sharing and overlapping.
On the basis of IMT-2000 network development, America considers to evolve the current function-layered network mode to an end-to-end client-server mode, with the broadband packet switching network as the core.
2) The UMTS standard compatible reversely with GSM, including two sub-solutions:
l Japan’s W-CDMA
NTT DoCoMo, the biggest mobile telephone operator in Japan, proposes the coherent multi-rate wideband CDMA (W-CDMA). As Japan’s 2nd generation mobile telephone system failed to become a global standard, Japan is determined to put its 3rd generation IMT-2000 network technology solution onto a course of global cooperation. Supporting ITU’s IMT-2000 family and interface concepts, Japan follows on purpose the cooperation means of radio transmission technology to support Europe’s GSM UMTS network concept. Now Ericsson and some other companies cooperate with NTT DoCoMo to advocate that the radio transmission technology adopts W-CDMA, and the core network inherits the GSM network platform, for the purpose of evolution from GSM to the 3rd generation IMT-2000.
l Europe’ TD-CDMA
In Europe, Siemens and Alcatel propose TD-CDMA. This solution integrates FDMA/TDMA/CDMA. It is characterized by expansion of channel separation to 1.6MHz, but its frame structure and time slot structure are the same as those of GSM. The expansion factor is 16, which can support 8 subscribers per time slot. Since each time slot has only 8 subscribers (code division), Joint Detection is applicable, so that fast power control is unnecessary and inter-code interference is reduced. Additionally, TDD (Time Division Duplex) is applicable as well. The mobile station will employ dual-band mobile telephones in order to be compatible with GSM on the network and signaling layers.
This solution facilitates smooth transition from GSM to the 3rd generation, and thus is supported by many GSM suppliers.
IMT-2000 frequency spectrum allocation:
According to the prescriptions of the World Radiocommunication Conference 1992, the IMT-2000 frequency spectrum is allocated as follows:
Uplink frequency band: 1885~2025MHz; Downlink frequency band: 2110~2200MHz.
Mobile satellite service frequency band: 1980~2010MHz; 2170~2200MHz.
The allocation above shows that the uplink and downlink frequency bands are asymmetric, so some systems proposes to provide services by means of TDD using asymmetric frequency bands. However, each country or region has different considerations in IMT-2000 frequency spectrum allocation, and it is impossible for them to follow this allocation completely.​


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