Second generation (2G) mobile communication systems have several limitations including network capacity and data rate. To satisfy the increasing demand for higher data rate, tighter data security, larger network capacity and support of various multimedia applications, the International Telecommunication Union (ITU) has defined a set of requirements, which specify what is needed from the next generation (3G) mobile systems. Emphasis was given to the following points: (a) high data rate, greater than 2 Mbps, (b) simultaneous support of voice and data capability, (c) high speech quality, (d) channel switching and packet switching transfer, (e) symmetrical and asymmetrical data transfer (IP-services), (f) low round-trip packet delay (below 200 ms), (g) seamless mobility for voice as well as for packet data applications, (h) high spectrum efficiency, and (i) inter-working with the existing networks (GSM/GPRS). To satisfy these requirements, 15 different proposals came up worldwide, out of these ten were related to the terrestrial segment; the other five were for satellite systems. These system proposals were tested and evaluated by the ITU and finally six different systems were incorporated into the International Mobile Telecommunications at 2000 MHz (IMT-2000) family.
For the terrestrial segment, these proposals can be divided roughly into four categories.