Three multiple access schemes are in use today, providing the foundation for mobile communications systems:
- Frequency Division Multiple Access (FDMA), which serves the calls with different frequency channels.
- Time Division Multiple Access (TDMA), which serves the calls with different time slots.
- Code Division Multiple Access (CDMA), which serves the calls with different code sequences.
All three technologies are widely used in cellular networks. FDMAis still used on some first-generation cellular analog networks, such as Advanced Mobile Phone Service (AMPS)1 and TACS (Total Access Communications System).1 TDMAis used on second-generation digital cellular networks, such as North American Digital Cellular and Global System for Mobile (GSM) communications.
CDMAis also used on second-generation digital cellular networks, such as PCS 1900. Both TDMAand CDMAhave been enhanced to support emerging third-generation networks. Of the three, FDMAis the simplest and still the most widespread technology in use today for mobile communications. For example, FDMAis used in the CT2 system for cordless telecommunications.
The familiar cordless phone used in the home is representative of this type of system. It creates capacity by splitting bandwidth into radio channels in the frequency domain. In the initial call setup, the handset scans the available channels and locks onto an unoccupied channel for the duration of the call. The traditional analog cellular systems, such as those based on AMPS, also use FDMAto derive the channel.
In the case of AMPS, the channel is a 30-kHz “slice” of spectrum. Only one subscriber at a time is assigned to the channel. No other conversations can access the channel until the subscriber’s call is finished, or until the call is handed off to a different channel in an adjacent cell. The analog operating environment poses several problems. One is that the wireless devices are often in motion.
Current analog technology does not deal with call handoffs very well, as evidenced by the high incidence of dropped calls. This environment is particularly harsh for data, which is less tolerant of transmission problems than voice. Whereas momentary signal fade, for instance, is a nuisance in voice communications, it may cause a data connection to drop.
Another problem with analog systems is their limited capacity. To increase the capacity of analog cellular systems, the 30-kHz channel can be divided into three narrower channels of 10 kHz each. This is the basis of the narrowband AMPS (N-AMPS) standard. However, this band-splitting technique incurs significant base station costs, and its limited growth potential makes it suitable only as a short-term solution.
While cell subdivision often is used to increase capacity, this solution has its limits. Since adjacent cells cannot use the same frequencies without risking interference, a limited number of frequencies are being reused at closer distances, which makes it increasingly difficult to maintain the quality of communications. Subdividing cells also increases the amount of overhead signaling that must be used to set up and manage the calls, which can overburden switch resources.
In addition, property or rights of way for cell sites are difficult to obtain in metropolitan areas where traffic volume is highest and future substantial growth is anticipated. These and other limitations of analog FM radio technology have led to the development of second-generation cellular systems based on digital radio technology and advanced networking principles.
Providing reliable service in this dynamic environment requires digital radio systems that employ advanced signal processing technologies for modulation, error correction, and diversity. These capabilities are provided by TDMAand CDMA.
FM systems have supported cellular service for nearly 20 years, during which demand has finally caught up with the available capacity. Now first-generation cellular systems based on analog FM radio technology are rapidly being phased out in favor of digital systems that offer higher capacity, better voice quality, and advanced call handling features. TDMA- and CDMA-based systems are contending for acceptance among analog cellular carriers worldwide.