5.1 FROM CELLULAR TO RECONFIGURABLE NETWORKS
Wireless communication systems have evolved during the last years at an incredible paceófrom simple analog techniques (first generation [1G]) to digital wireless systems (second generation [2G]) for personal use, and during this decade to some multimedia communication through third-generation [3G] wireless systems, as well as the sharing of resources having communication links traversing different networks. This section highlights the evolution, developments, and trends of wireless network technologies.
Wireless networks have evolved from several inventions throughout history, but one of the most important developments that form the bases of the systems that we know and use these days is the cellular concept. In 1933, the Federal Communications Commission (FCC) authorized the use of four channels in the 30- to 40-MHz range. During World War II, spread spectrum was used.
In 1945, AT&T Bell Labs began experiments with the use of higher frequencies with the goal of improving mobile services and of introducing the cellular concept, where the network service area is divided into smaller regions called cells and each one of these cells is served by a low-power transmitter. In 1963, AT&T Bell Labs demonstrated the first cellular system.
In 1972, Motorola demonstrated the cellular telephone handset to the FCC. In 1974, the 800- to 900-MHz portion of the UHF band was allocated for cellular use. In 1983, the first commercial cellular system became operational in Chicago,the start of theAdvanced Mobile Phone System (AMPS). Nordic Mobile Telephone Service (NMTS) in 1981 and Total Access Communication System (TACS) in 1985 became operational, in Scandinavia and the United Kingdom, respectively.
In 1991, technology was introduced as well as the first validation of the U.S. Time Division Multiple Access (TDMA) standard. Also in 1991, the initial validation of Code Division Multiple Access (CDMA) started. Groupe Speciale Mobile (GSM)was introduced as the Pan-European Cellular System in 1992, and in 1994 cellular digital packet data systems were introduced. In 1994, the first personal communications services calls were placed (TDMA and CDMA).
The concept of personal communication systems comes to mind whenever we talk about a wireless network, so that the paradigm of point-to-point communication becomes person-to-person anywhere communication. Traditional networks were designed for a single service, voice. When information theory started gaining fame, people at remote sites needed to share information and so networks had to provide another service, data transmission. Demand for new services such as image retrieval and video transmission increased and with a little patience one can get these services with acceptable quality.
The new era of communication networks comprises all such services at high-quality levels and for large numbers of users with small portable and mobile communication devices. Modern communications systems exploit traditional communication technologies such as twisted pair and coaxial cable, and integrate new technologies such as fiber and wireless links (cellular, satellite, etc.), which in turn provide mobility to users to achieve a true person-to-person anywhere communication.
The development of wireless communications has determined industry technology standards that depend on multiple access techniques of transmission media, the frequency band used, type of modulation, channel bandwidth, and 5.1 From Cellular to Reconfigurable Networks 155 type of service provided. The techniques used are Frequency Division Multiple Access (FDMA), TDMA, and CDMA. In FDMA, a channel of those available from the frequency band is occupied by the user. In TDMA, time is divided into slots and each customer uses one slot to communicate in a channel; the same channel can be used by other customers as long as they communicate in another time slot. In CDMA, all customers use the same channel, but they are identified by a code that is unique for each user and that does not interfere with those of others.
The modulation techniques most widely used are based on type of system. For example,in an analog system FM is used;for digital systems,several modulation schemes have been used: quadrature phase shift keying (QPSK), frequency shift keying (FSK),Gaussian minimum shift keying (GMSK), Ŗ/4 differential PSK (DPSK), binary PSK (BPSK), quadrature amplitude modulation (QAM), and so on. Most of these are treated in digital communication books (e.g., [21, 32, 67, 82]. The services provided are basically cellular, personal communication systems (PCSs), paging, cordless, and specialized mobile radio (SMR).
The first U.S. cellular telephone system was deployed by Ameritech in Chicago in late 1983. It used the standard advanced mobile phone system (AMPS), for which the FCC had just allocated 40 MHz of spectrum in the 800- MHz band. Each channel, one-way, has a bandwidth of 30 kHz, so the duplex channel has a total bandwidth of 60 kHz; therefore, the total 40 MHz contain 666 duplex channels. The modulation technique used is FM with a multiple access scheme of FDMA. Hence, this is an analog system.
The channels in the 800-MHz band are numbered from 1 to 1023, excluding the numbers 800 to 989. If the reverse channel uses channel n, from the reverse channel band, the forward channel will use the nth channel from the forward channel band. The forward and reverse channel center frequencies are separated by 45 MHz.