In 1946, the first mobile telephone service was introduced in twenty five major American cities.The features of the conventional mobile radio telephony was simple to design.

Each system used a single, high-powered transmitter and large tower to cover distances of over 50 km as shown in Figure 3. It initially used 120 KHz of RF bandwidth in a half-duplex mode. (push-to-talk release-to-listen systems).The large RF Bandwidth was used because of the difficulty in mass producing tight RF filters and low noise front end RF Amplifiers. In 1950, the channel bandwidth was cut in half to 60 KHz due to improved VLSI technology. By mid 1960s, the channel bandwidth again was cut to 30 KHz. Also in 1950s and 1960s, automatic channel trunking was introduced in IMTS (Improved Mobile Telephone Service).(Concept of trunking will be introduced later). With IMTS, telephone companies started offering full duplex, auto-dial, auto-trunking phone systems. However it became saturated very early.