A mobile telephone, commonly known as a mobile phone, is a portable device used for connecting to a telecommunications network. Its primary purpose is to transmit and receive voice, video, or other data. Mobile phones are designed to connect to the public switched telephone network (PSTN) through cellular telephone systems or global satellite-based telephony.
Cellular Telephones: Enhancing Mobility
Cellular telephones, often referred to as cell phones, are portable devices that offer mobility to users, whether they are in a vehicle or on foot. These devices utilize radio waves for communication, enabling users to have a significant degree of mobility within a specific coverage area. This area can range from a few city blocks to hundreds of square kilometers. Initially, the first mobile and portable subscriber units for cellular systems were large and heavy. However, advancements in component technology have led to significant reductions in size and weight. This section discusses the concept of cell phones and the evolution of cellular systems.
Cellular Communication: Key Characteristics
All cellular telephone systems share several fundamental characteristics, as outlined below:
Cellular Structure: The geographic area covered by a cellular system is divided into smaller areas known as cells. While hexagons are commonly used to represent cells on maps, the actual cell shapes are irregular due to the behavior of radio waves.
Base Stations: Communication with mobile or portable devices within a particular cell is directed to a base station responsible for serving that cell.
Frequency Reuse: To maximize spectral efficiency, the limited transmitting power of battery-operated devices allows for the reuse of specific sending and receiving frequencies in other cells within a larger geographic area. This increases the capacity of the cellular system.
Seamless Handoff: When a mobile device moves from one cell to another during an ongoing call, a central controller called the mobile telephone switching office (MTSO) automatically redirects the call without noticeable signal interruption. This process is known as handoff, ensuring uninterrupted communication for mobile subscribers.
Cell Splitting: As the demand for radio channels within a particular cell exceeds its capacity, the cell can be divided into smaller cells. Each smaller cell has its own base station and central controller. The radio-frequency allocations are adjusted to accommodate the increased number of cells, allowing cellular systems to serve densely populated areas efficiently.
By employing frequency reuse and cell splitting, cellular systems distinguish themselves from other wireless telephone systems. These concepts enable cellular providers to cater to large metropolitan areas with a substantial customer base, ensuring effective coverage and service for hundreds of thousands of users.
Development of Cellular Systems
Introduction of Mobile Telephone Service (MTS) and Improved Mobile Telephone Service (IMTS)
In 1946, the American Telephone & Telegraph Company (AT&T) introduced mobile telephone service (MTS) in the United States, which marked the beginning of connecting mobile transmitters and receivers with the public switched telephone network (PSTN). The MTS system required users to manually search for an available channel and speak with a mobile operator who dialed the call over the PSTN. The system allowed simple communication, meaning only one party could speak at a time. In 1964, AT&T launched the improved mobile telephone service (IMTS), which offered full duplex operation, automatic dialing, and automatic channel searching. Initially, there were 11 channels available, but in 1969, an additional 12 channels were made accessible. However, the IMTS system faced limitations due to the high demand for limited channels and the need for high-powered base-station antennas.
Development of the Advanced Mobile Phone System (AMPS)
To address the limitations of the IMTS system, AT&T and Motorola collaborated to develop a truly cellular system known as the advanced mobile phone system (AMPS). Introduced in 1983, AMPS was based on 666 paired voice channels spaced every 30 kilohertz in the 800-megahertz frequency range. Unlike the previous systems, AMPS employed analog frequency modulation (FM) and supported subscriber units for both automobiles and pedestrians. The launch of AMPS was a success, with 200,000 subscribers within the first year and over 2,000,000 subscribers five years later.
Methods for Increasing Capacity
Anticipating service shortages, the American cellular industry proposed various methods to increase capacity without requiring additional spectrum allocations. Motorola introduced narrowband AMPS (NAMPS) in 1991, which divided each existing 30-kilohertz voice channel into three 10-kilohertz channels. This resulted in 2,496 channels, significantly increasing capacity compared to the 832 channels available in AMPS systems. Another approach, developed by the Telecommunications Industry Association (TIA) in 1988, utilized digital modulation, digital voice compression, and time-division multiple access (TDMA) to allow for three new voice channels in place of one AMPS channel.
Additionally, Qualcomm, Inc. developed a third approach in 1994, which was later adopted as a standard by the TIA. This approach utilized code-division multiple access (CDMA), a form of spread spectrum multiple access, combining digital voice compression and digital modulation. The CDMA system offered 10 to 20 times the capacity of existing AMPS cellular techniques.
Deployment of Improved-Capacity Cellular Systems
All of the improved-capacity cellular systems mentioned above were eventually deployed in the United States. However, since they were incompatible with one another, they coexisted with the older AMPS standard instead of replacing it.
Development of Cellular Systems-Introduction of Cellular Systems
The deployment of cellular systems began with the Japanese system in 1979, followed by the Nordic mobile telephone (NMT) system in 1981 and the total access communication system (TACS) in 1983 in the United Kingdom. These analog cellular systems were incompatible with each other, leading to the need for standardized systems.
Emergence of a Global System for Mobile Communications (GSM)
In 1988, the European Community public telephone bodies introduced the digital global system for mobile communications (GSM). GSM was the first system that allowed seamless operation for cellular users across European countries using the same equipment. Its success led to widespread adoption throughout Europe.
Evolution to Second Generation (2G) Systems
The analog cellular systems of the 1980s became known as “first-generation” (1G) systems. In the late 1980s and early 1990s, digital systems referred to as “second-generation” (2G) systems, emerged. These systems enabled data services and applications such as internet browsing, text messaging, and image transmission. Notably, the iMode application launched in Japan in 1999, attracting over 35 million users within three years.
Development of Third Generation (3G) Systems
Starting in 1985, the International Telecommunication Union (ITU) study group began considering specifications for Future Public Land Mobile Telephone Systems (FPLMTS). These specifications formed the basis for “third-generation” (3G) cellular standards known as IMT-2000. 3G systems were based on CDMA technology and aimed to support voice, data, and multimedia services. The first 3G service was introduced in Japan in October 2001 by NTT DoCoMo, with subsequent launches in various countries.
Advancements to Fourth Generation (4G) Systems
The increasing demand for handling larger amounts of data led to the development of “fourth-generation” (4G) technology. In 2008, the ITU established requirements for 4G systems, including high data rates. Two technologies, LTE-Advanced (Long Term Evolution) and WiMAX, were recognized as meeting the requirements. TeliaSonera, a Swedish telephone company, introduced the first 4G LTE network in Stockholm in 2009.
Airborne Cellular Systems
There are two types of in-flight telephone systems for commercial aircraft: terrestrial-based and satellite-based. These systems allow passengers to make telephone calls to the Public Switched Telephone Network (PSTN) while in the air.
The North American Terrestrial System (NATS) was introduced by GTE Corporation in 1984 in the United States. By the 1990s, it was installed in over 1,700 aircraft, with ground stations providing coverage across the United States and southern Canada. The second-generation system, GTE Airfone GenStar, used digital modulation. In Europe, the European Telecommunications Standards Institute (ETSI) adopted the Terrestrial Flight Telephone System (TFTS) in 1992, operating in specific frequency bands and requiring ground stations to be spaced every 50 to 700 km.
Satellite-based systems aim to enhance mobile telephone connectivity by providing global coverage, especially in areas not served by terrestrial cellular networks.
A) Geostationary Satellite Systems
Inmarsat satellites have been used in airborne cellular systems for some time. However, these satellites are in geostationary orbit, approximately 35,000 km above the Earth’s surface. Communication with these satellites requires high transmitting power and large antennas, resulting in noticeable delays during voice conversations.
B) Low Earth Orbit (LEO) Satellite Systems
LEO satellite systems offer an alternative to geostationary satellites. These satellites orbit at an altitude of less than 1,600 km and are not geostationary, providing global coverage through a constellation of satellites. Communication between satellites allows seamless handoff of calls, ensuring uninterrupted service even if a satellite moves out of range.
Iridium System: Designed by Motorola, the Iridium system was the first commercial LEO satellite system. It consisted of 66 satellites orbiting Earth in six planes. Launched from May 1997 to May 1998, commercial service began in November 1998. Each satellite transmitted 48 spot beams to Earth and communicated with other satellites via radio “crosslinks.” Despite initial financial difficulties, Iridium Satellite LLC acquired the assets of Iridium LLC and continued to provide global communication services.
Globalstar: Another LEO system, Globalstar, launched 48 satellites around the same time as the Iridium constellation. It started offering service in October 1999 but faced bankruptcy in February 2002. However, a reorganized Globalstar LP continued to provide services.
These satellite-based systems aimed to provide worldwide connectivity, particularly in areas without existing cellular infrastructure. While some systems faced challenges, they contributed to the advancement of mobile communication technology.