Mobile communication facilities have become commonplace in many parts of everyday life. Cordless telephones and pagers are approaching everyday usage by a large number of people. The various acronyms of the different systems are numerous and one would be forgiven for not keeping abreast of the most recent systems to emerge on the market. The European communication market is setting the goals for future personal mobile communication. In mobile communication of the future "the users will be able to make or receive calls anywhere and at anytime should they so choose". Integrated communication systems are becoming increasingly important in mobile information systems. Almost all existing communication links at present could be replaced by cordless links. The benefits of such replacement e.g mobility, flexibility, reduced installation costs etc are enormous. Cordless telephones and pagers have been used within buildings for many years, but have been used randomly and without regard for the interference they caused. Quality of service of such systems is the major limiting factor in their widespread use. Radio data systems allows for the possibility of indoor wireless LAN's. (Local Area Networks).
However, indoor wireless systems are not without their problems and systems in existence have a very simple structure and their performance varies depending on environmental conditions. Multipath adversely affects the received signal and depends on building layout and structure. Up to now indoor systems were usually custom - designed to suit the particular building but, standards are beginning to emerge as systems move from analogue to digital. Future mobile radio communications systems will be integrated into the digital telecommunications network (ISDN) resulting in a variety of effective services will be available. In effect the ultimate goal is to have a Universal Mobile Telephone System (UMTS).
Before this dream is turned into a reality there are many problems to be overcome namely: cost, availability, regional standards and regional collaboration. The difficulty which faces radio communication is the limited available frequency spectrum. The need for efficient spectrum utilisation is essential if mobile communications is to make a major impact.
It is important to realise that rapid technological progress results in the growth of new telecommunications services, such as mobile communication systems. The dissemination of these new technologies is accelerated by deregulation and privatisation, which encourages new and competitive markets.
All wireless communication is based upon the application of a radio operating on an assigned frequency. The assigned frequency is used to broadcast and receive messages. The effectiveness of the radio transmission is a function of the antenna, transmitting power, and interference encountered along the transmission path.
The transmission characteristics of a radio wave and its attenuation in a particular environment depends upon its frequency. The radiation frequency will determine the size and type of aerial and the technology used in the transmitter and receiver. Unlike other media, such as cable, the radio spectrum is finite and, therefore, is closely controlled. It is allocated on an international basis in order to ensure priorities are satisfied, and maximum use is made of the available spectrum. Figure 2 shows the Electro-magnetic Spectrum and the frequencies used in radio communication.
The problem with radio communication is that the spectrum available is finite and cannot be automatically increased as demand increases with the growth of a system. Cellular radio is one means of providing such efficiency in that it enables high density frequency reuse. In order to provide a good grade of service in a cellular system the maximum number of channel frequencies must be available where the calling rates are high. Where calling rates are low a smaller number of channels is required. The size of an area in which a specific number of channels is used is variable. A high calling area, such as a large city, is divided into a number of small cells in which a number of different frequencies are used. Adjacent to these are cells with transmitters operating on different frequencies. The transmitters in adjacent cells do not interfere with each other. At some suitable distance the frequencies can be reused in other cells.
First generation mobile radio systems provide cellular, cordless, paging and Private Mobile Radio (PMR) facilities using separate networks and terminals with different standards for different countries. Present day second generation cellular phones are adopting common standards throughout Europe and cordless and cellular systems are now beginning to converge. A major difference between first and second generation technology is the change from analogue to digital technology. This digital technology is leading to an overlap between mobile systems, but also to integration with fixed network systems. It is not surprising that third generation telecommunication systems are attempting to converge to a single system. The increasing number of users of the various systems has resulted in these services being offered at higher frequencies. As new systems emerge at these higher frequencies there is increasing overlap in the services provided for narrowband voice-based transmission. There is evidence of a steady migration towards a single universal system based on narrowband technology. The mobile environment can be seen as a tree type structure, with systems such as DECT, GSM and PCN, providing branches. Whether this tree converges or diverges is of major concern to the telecommunications standards committees.
GSM (known as Global System for Mobile Communication) is a second generation cellular radio system designed to provide a common standard throughout Europe. It allows the user to roam through various different countries and maintain a quality of service that is only possible using digital technology. GSM operates at a frequency of 900MHz and was designed mainly for voice transmission.
Digital European Cordless Telecommunications (DECT) was designed to allow inexpensive but good quality handsets and Base Stations. In particular, the DECT design is very suitable for high capacity, wireless Private Exchange systems. Using a relatively high bit-rate, 1.2Mbit/sec, means that one Base Station can support up to 12 channels. Time division duplexing is used, thus simplifying frequency planning. Each time slot carries a preamble for demodulation purposes, signalling and user data. DECT does not provide extensive error protection on the voice channel, since users are likely to be static. But if the channel quality deteriorates the handset will hand over to a new channel.
The European Radio Message System (ERMES) was initiated in 1987 by CEPT. A study group under the European Telecommunications Standards Institute (ETSI) was set up to produce specifications for a western European paging system. The target date for initial service was January 1993. ERMES will have a roaming facility and the user will specify the area to which calls should be transferred for a defined period. Ascending categories of calls are tone-only, numeric message (maximum 20 digits) and text message (maximum 400 ASCII characters).
The universal mobile telecommunication system will evolve from contemporary mobile communication systems. Cordless and cellular systems are converging into a form of personal communication. UMTS will be an improvement of both systems. Because of the wide variety of systems available and being developed, one would be forgiven for asking why another system is needed, since we already have a very good cellular system in GSM, and an excellent cordless system in DECT, not to mention ERMES and other systems. The answer is of course that the GSM's and DECT's of this world will not be static; they will be continuously developed and improved. The constant adjustment and refinement would lead to systems that would become very diverse. The objective of UMTS is that everybody can afford to buy and use a personal telephone, which has the facility that it will work everywhere. For such a mass market it needs to be dependable, which means that the systems will have enough capacity for everyone wanting to use the system. Finally the system must be multi-purpose in that the personal telephone can be used at home, in the office or on public systems..
Satellite communications have moved away from the access of very large bandwidth usage to the individual user, giving far greater services for both voice and data applications. Commsat’s Immarsat division now sells access to its satellites on a call-by-call basis. There are several manufacturers of briefcase satellite telephones, which allow calls to be made anywhere in the world. Global Position System (GPS) have created a range of navigational and range measurement systems and services using this unique satellite constellation.
Indoor Wireless Local Area Networks (IWLAN's) are quickly becoming a very attractive method of communicating data and control information throughout factories, offices etc. Apart from the portability of the mobile unit it provides installation of a system that is often more cost-effective then a wired system. Wireless LAN's provide a flexible alternative to traditional, cable-based networks. It allows for movement of network nodes, thus permitting alterations of the network structure as required. While wireless LAN's share many features with standard networks, they take many different approaches to the question of node addressing and network security. Two new technologies in existence are photonic and spread-spectrum communications. The latter is gaining in strength as interest increases for indoor wireless communication. Generally one does not query which type of network is required, but what kind of network services are needed.
The modulation technique used in mobile communication systems is exactly the same as broadcast radio, namely Frequency Modulation (FM). For the mobile radio environment other techniques are finding widespread use. Spread spectrum techniques have been used for about three decades as a means for communications in a hostile environment. In particular, direct sequence and frequency hopping spread spectrum systems afford the opportunity for some measure of protection against intentional jamming. When the hostile jamming environment is removed and spread spectrum is examined primarily in terms of CDMA and mutual interference effects, the issues for design and evaluation are shifted drastically. Recently, spread spectrum systems have been given a second look from the perspective of applications which require exactly these features. Entirely new opportunities such as personal communication networks (PCN) are "optimized" for most efficient use of the spectrum and other features in these non-hostile environments.
Wireless networking products are currently available from a number of companies. The cost and sophistication of these products vary widely. The vast majority of the systems are based on spread spectrum techniques. The first generation of wireless LAN's used voice-band modems and walkie-talkie's and transmitted at less than 9600bps. The second generation are operating using spread spectrum and have bit rates of the order of 100Kbps. The next generation will probably still use spread spectrum but will use more advanced signalling processing and/or sectored antennae and would have 1 - 20 Mbps data rates.
While Europe has been the leader in Mobile Voice communication, America has taken the lead in Wireless LAN technology. This is partly due to the FCC allocating spectrum space in the Industrial, Scientific & Medical (ISM) bands at 900MHz, 2.4GHz, 5.5GHz and 18GHz for the application of Wireless LAN's. Almost all of the American systems are based on "Spread Spectrum" (SS) techniques, this is very different from narrow band schemes which are used in all voice mobile systems. Details of the current status of wireless LAN's presently on the market or about to emerge are outlined in the table above.
Spread spectrum is also suitable for network controller applications to cope with a noisy environment due to uncontrolled A.C. power lines. The ability of a transmitted signal to survive power-line noise partly depends on its power spectral density. To cope with power-line wiring, Echelon have developed direct sequence power line Integrated circuits, which combine spread spectrum technology and several advanced signal-processing and error correction techniques.
The area of personal communications is expanding at a rapid rate. In this decade we have seen the introduction of the GSM Pan European Digital Cellular Radio Network and also a new Digital European Cordless Telephone (DECT). The Universal Mobile Telecommunications System (UTMS) is not too far away. Other systems such as (PCN) Personal Communication networks and (PCS) Personal Communication Services are emerging each day, as mobile communication finds new areas to expand into. The cell sizes in such a system will be variable according to requirements. Future mobile communication systems are likely to operate at frequencies above 1GHz because of a shortage of spectrum at lower frequencies. The services provided will cover voice data and possibly video.
Sean McGrath is a Lecturer in Telecommunications. He has research interests in mobile radio communications systems, spread spectrum advanced modulation techniques, and adaptive equalisation and simulation of communication systems.
