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All the key inventions for cellular radio networks were made before the breakup of the Bell System. The first demonstration of the Motorola hand held was in 1973.

https://en.wikipedia.org/wiki/Mobile_phone

https://en.wikipedia.org/wiki/History_of_mobile_phones

In December 1947, Douglas H. Ring and W. Rae Young, Bell Labs engineers, proposed hexagonal cells for mobile phones in vehicles.[17] At this stage, the technology to implement these ideas did not exist, nor had the frequencies been allocated. Two decades would pass before Richard H. Frenkiel, Joel S. Engel and Philip T. Porter of Bell Labs expanded the early proposals into a much more detailed system plan. It was Porter who first proposed that the cell towers use the now-familiar directional antennas to reduce interference and increase channel reuse (see picture at right)[18] Porter also invented the dial-then-send method used by all cell phones to reduce wasted channel time.

In all these early examples, a mobile phone had to stay within the coverage area serviced by one base station throughout the phone call, i.e. there was no continuity of service as the phones moved through several cell areas. The concepts of frequency reuse and handoff, as well as a number of other concepts that formed the basis of modern cell phone technology, were described in the late 1960s, in papers by Frenkiel and Porter. In 1970 Amos E. Joel, Jr., a Bell Labs engineer,[19] invented a “three-sided trunk circuit” to aid in the “call handoff” process from one cell to another. His patent contained an early description of the Bell Labs cellular concept, but as switching systems became faster, such a circuit became unnecessary and was never implemented in a system.

A cellular telephone switching plan was described by Fluhr and Nussbaum in 1973,[20] and a cellular telephone data signaling system was described in 1977 by Hachenburg et al.[21]