Digital technology offers the opportunity for improved transmission in cellular systems. This is due to powerful error detection and recovery techniques, which can be used to counter the debilitating effects of noise, fading and interference. Digital technology also provides the basis for security in the forms of encryption and authentication. Finally, digital technology requires less in the way of mobile transmit power, which increases battery life in portable mobile units.
Digital cellular technologies also offer the promise of effective data transmission via cellular services. Although their vocoders prohibit the use of conventional modems, recent extensions to standards provide low-throughput data traffic in either a circuit-switched mode or via a digital control channel. Packet-switched data services are also being developed by the proponents of digital cellular standards.
However, the primary motivations for the digital cellular standards are unrelated to data. Development of the North American digital standards was motivated by the need for increased capacity in light of the 40-plus percent compounded growth rate in AMPS penetration during the 1990's. Overseas, development of the GSM standard was motivated by the desire to unify cellular service across European national boundaries.
Once the commitment to digital cellular voice standards was achieved in the various standards bodies, it was quickly recognized that digital services could include much more than mere capacity enhancement. Data applications, secure channels and enhanced voice services such as caller identification are now possible with the new digital standards.
Before presenting the primary digital cellular technologies, understanding the basic differences between FDMA, TDMA and CDMA is essential. As depicted in Figure 1.4, a frequency division multiple access (FDMA) system, such as AMPS, separates individual conversations in the frequency domain-different conversations use different frequencies (channels). In this depiction, the frequency domain is represented by the vertical dimension and the time domain is represented by the horizontal dimension.
Figure 1.5 shows how time division multiple access (TDMA) systems, such as IS-54/136, GSM or PDC, separate conversations in both the frequency and time domains; each frequency (channel) supports multiple conversations, which use the channel during specific timeslots. Typically there is a maximum number (3 in the example) of conversations which can be supported on each physical channel.
Figure 1.6 shows how frequency-hopping code division multiple access (CDMA) systems, such as spread spectrum wireless LANs, separate conversations in both the frequency and time domains. By rotating conversations through frequencies (channels) on a synchronized basis, each conversation experiences a variety of channel conditions. This rotation through the frequency set also tends to reduce the interference levels.
Figure 1.7 shows how direct sequence CDMA systems, such as IS-95, separate conversations on the basis of something entirely different than frequency or time. It's hard to show in a time versus frequency diagram.