From the customer’s point of view, it is mobile systems that are perhaps the most exciting telecommunications development since the invention of the telephone. The developments in optical fiber technology might sound very impressive and the statistics involved are mind-boggling but the average subscriber does not usually appreciate the full extent of the benefts at a personal level. The developments in this field are taking place mainly behind the scenes, and are not really tangible service improvements to the subscriber.
The pocket telephone, on the other hand, is a revelation that is center-stage and whose benefits can be instantly appreciated by anyone who purchases one of these devices. The cellular telephone industry has experienced explosive growth over recent years. It is an area of telecommunications that has benefited not only the developed world, but also many developing countries. From the service provider’s point of view, cellular systems are very fast to install compared to extending new cables to customer premises. When a leading telecommunications company forecast in 1996 that by the year 2000 there would be 350 to 400 million mobile radio units worldwide, there was widespread disbelief. So far, all forecasts of this nature have turned out to be conservative.
Cellular mobile telephone systems are not easy to classify. They could be considered as part of the local loop because they extend out to the subscriber handset, or because of the long distances that can be bridged between a fixed
The analog systems that have been around for a number of years are giving way to digital technology. Narrowband TDMA is currently seeing widespread deployment, and CDMA started deployment in 1996. Significant equipment incompatibilities are encountered when goingfrom one system to another or trying to incorporate two or three of the same network.
Many of these problems stem from the difference in outputs. High-mobility cellular radios transmit at relatively high power in the region of 1 to 10 W, whereas the latest low-mobility portable units transmit at relatively low power levels of 1 to 10 mW. While this is fine for the customer, it makes coexistence of the two systems a network planner’s nightmare High tier is a term often associated with high-mobility systems, whereas Iow tier is associated with low-mobility systems.
In summary, cellular telephony is the culmination of several technologies which have progressed in parallel over the past two decades. In fact. the progress has been so rapid that the standards bodies have had difficulty organizing meetings fast enough to determine standards that are consistent with the new technology.
Types of Mobile Radio Systems
Historically, technology has lagged behind the design of mobile telephone ystems, and it was only by 1983 that the first good-quality systems were put into operation. Since those early, low-capacity pioneering systems subscriber demand has mushroomed, despite the higher cost of calling from a mobile telephone. The different types of mobile radio systems, in terms of frequency spectrum usage. They differ primarily in modulatic technique and carrier spacing.
Analog FM. The first-generation cellular systems in operation were analog FM radio systems that allocated a single carrier for each call. Each carrier frequency modulated by the caller. The carriers were typically spaced at 25-kl
FDMA systems resemble analog FM, with the exception that the carrier is modulated by a digitally encoded speech signal. The bandwidth of each carrier is similar to the analog FM systems (typically 25 kHz).
Digital narrowband TDMA.
TDMA systems operate with several customers sharing one carrier. Each user is allocated a specific time slot for transmission and reception of short bursts or packets of information. The bandwidth of each carrier is typically 200 kHz, and the total bandwidth available is in the region of 10 to 30 MHz, so many FDMA carriers each contain several customers on a TDMA shared basis. This access combination allows a reasonably large channel capacity in the region of 500 to 1000 channels, before frequency reuse.
In the United States, for example, the 824- to 849-MHz frequency band is allocated for one-way transmission from the base station to the user, and the 869- to 894-MHz band is allocated for transmission from the user to the base station. To enable two competitive systems to operate simultaneously, only half of each of these bands is available to each operator. Each system therefore has 12.5 MHz available for transmission and 12.5 MHz for reception. Each of these 12.5-MHz bands is subdivided into several carriers in an FDMA manner. Each carrier is operated in a TDMA mode having time slots for voice or data channels. Digital wideband (spread spectrum). One form of digital wideband operation is CDMA.
In these systems there is a single carrier that is modulated by the speech signals of many users. Instead of allocating each user a different time slot, each is allocated a different modulation code. Mobile users in adjacent cells all use the same frequency band. Each user contributes some interfering energy to the receivers of fellow users, the magnitude of which depends on the processing gain. In addition to interference from users within a given cell, there is also interference from users in adjacent cells. The distance between adjacent cell users attenuates the interference considerably more than users within the same cell. Frequency reuse is therefore unnecessary. Consequently, each cell can use the full available bandwidth (12.5 MHz) for CDMA operation.
Analog Cellular Radio
Analog cellular systems were used exclusively in the early days of mobile communications. Although they are being superseded by digital technology, a large number of systems are still in service and will probably remain in use for several years to come.
Analog FM cellular radio systems are relatively old technology (1980 to 1985) in this fast-paced industry. These are the first-generation cellular radio systems. However, it is informative to discuss some of their features briefly, because they provide insight into how future systems are evolving. Analog cellular radio was initially designed for vehicle-mounted operation. By 1990, already more than 50 percent of mobile radios (stations) in most networks were hand-held portables, and the demand was growing.
As far back as 1979, Bell Labs designed and installed a trial cellular mobile system called the Advanced Mobile Phone Service (AMPS). This was really the birth of cellular radio in the United States, and is still the basis of the analog systems in operation today. The AMPS system uses the hexagonal cell structure, with a base station in each cell.
The cells are clustered into groups of seven cells (i.e., a seven-cell repeat pattern). AMPS covers large areas with large-sized cells, and high-traffic-density areas are covered by subdividing cells.
Sectorization is also used to enhance capacity. The overall control of the system is by a mobile telephone switching office (MTSO) in each metropolitan area. This digital switch connects into the regular telephone network and provides fault detection and diagnostics in addition to call processing. The mobile unit was originally installed in a car, truck, or bus. The frequencies for AMPS are 870 to 890 MHz from base to mobile, and 825 to 845 MHz from mobile to base. Each radio channel has a pair of one-way channels separated by 45 MHz.
The spacing between adjacent channels is 30 kHz. The AMPS system uses FM with 12-kHz maximum deviation. FM has a convenient capture mechanism. If a receiver detects two different signals on the same frequency, it will lock onto the stronger signal and ignore the weaker, interfering signal.
Mobile units are microprocessor controlled. The MTSO periodically monitors the carrier signal quality coming from the active mobile. If, during a call, a mobile moves to the edge of a cell boundary and crosses the boundary, the signal quality from the adjacent cell gradually becomes better than the existing service provider, so handoffs initiated. The handoff command is a "blank and burst" message sent over the voice channel to the designated cell.
A brief data burst is transmitted from the base providing service to instruct the mobile microprocessor to retune the radio to a new channel (carrier). The voice connection is momentarily blanked during the period of data transmission and base station switching. This interruption is so brief it is hardly noticeable, and most customers are unaware of its occurrence.
All of the call setup is done by a separate channel. There are dedicated signaling channels that transmit information only in the form of binary data. These channels are monitored by all mobiles that do not have a call in progress. When a mobile is first switched to the on mode or is at the end of a call, it is in the idle state. It scans the frequencies used for call setup and monitors the one providing it with the strongest signal. Each cell has its own setup channel. The mobile periodically makes a scan to see if its change of position has made the
The AMPS system has nationwide roaming capability. This is possible by cooperation between the service providers in different parts of the country. The AMPS system has been very successful, but its main disadvantage is that its total system capacity is inferior to the more advanced digital cellular radio systems.
The AMPS cellular radio network structure.
Digital Cellular Radio
Digital cellular radio systems can be divided into two categories, narrowband and wideband. Narrowband systems are often considered to be the second generation of cellular radio. Although the digital narrowband TDMA systems in North America and Europe have developed along similar lines, there remain many features that are different. Because of its global success, in this text the main focus of attention for digital narrowband TDMA cellular radio will be the European-designed system called GSM, to which the U.S. system called 0IS-54 will be compared.
The acronym GSM originally stood for the French name Groupe 011" Speciale Mobile, the planning organization that did much of the groundwork for the TDMA cellular system.
GSM now stands for global system for mobile communications, A description of its features serves to highlight some of the intricacies of present-day cellular radio systems.
GSM operates in the primary spectrum range of 890–915 MHz (uplink) and 935–960 MHz (downlink), with subsequent adaptations to operate in 1800 MHz (Digital Cellular System or GSM 1800) and 1900 MHz (Personal Communications Services or GSM 1900).
GSM 450 and GSM 800 (part of the IS-136(TDMA cellular standards) 850 Band) are planned to utilize the 450 MHz and 800 MHz spectra in the future.
GSM is the basis of a powerful family of platforms for the future – providing a direct link into next generation solutions including GPRS (General Packet Radio Services) EDGE (Enhanced Data for GSM Evolution) and 3G(3GSM).In the tables below will show frequency and the speed for the wireless transmission.
GSM terminals may incorporate one or more of the GSM frequency bands listed below to facilitate roaming on a global basis.
In the above bands mobile stations transmit in the lower frequency sub-band and base stations transmit in the higher frequency sub-band.
Theoretical vs. Actual Wireless Transmission Speeds
|Generation||Technology||Theoretical Top Speed||Avg. Delivered Speed|
|1G||AMPS||19.2 Kbps||Less than 9 Kbps|
|1G||CDPD||19.2 Kbps||9.2 Kbps|
|2G||TDMA, CDMA, iDEN, GSM||19.2 Kbps||9.6-19.2 Kbps|
|2.5G||GPRS||115 Kbps||20-40 Kbps|
|3G||1xRTT||153 Kbps||60-80 Kbps|
|3G||EDGE Phase II||384 Kbps||80-100 Kbps expected|
|3G||1xEV-DO||2.4 Mbps||200-300 Kbps|
|3G||W-CDMA||384 Kbps||200-300 Kbps|
|3G||1xEV-DV||4.8 Mbps||200-300 Kbps|
The Wireless Generation is a function of speed and maturity of technology and is usually representative of a family of similar technologies, Theoretical Throughput is the best-case attainable speed over the network, and is typically 50 to 100% faster than real-world performance. 3G An industry term used to describe the next generation of public wireless voice + data networks. To qualify as 3G, a network must meet certain requirements for speed, availability, reliability and other criteria set forth by the International Telecommunications Union. There are many 3G network technologies being developed, generally they are packet-based "always on" networks.
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