# How Early Cell Phones Handled Calls Across Different Towers > This patent describes a system for early portable phones to automatically find the strongest signal from a base station and switch channels as the user moves, reducing battery drain and interference. - **Patent:** US 3906166 - **Original title:** Radio telephone system - **Owner:** Motorola Inc - **Granted:** 1975 - **Status:** Public domain (expired) - **Times cited:** 206 - **Field:** telecommunications, consumer_electronics, software, semiconductors ## What it does This patent describes a "portable radio telephone system" (Claim 1) designed for mobile communication. It uses multiple base stations, each with a "predetermined coverage area," that transmit on "outgoing signalling channels" and "outgoing communications channels." Smaller "receiver sites" are strategically placed around these base stations to pick up signals from portable units. When a portable unit (Claim 2) needs to communicate, its "portable receiver" scans various "outgoing signalling channels" from different base stations. A "signal strength detector" then identifies the strongest signal, and "logic means" automatically tunes the "portable transmitter" to the correct "incoming signalling channel" associated with that strongest signal. For example, if you were making a call while moving between city blocks, your phone would continuously monitor which cell tower offered the best connection and seamlessly switch to it, ensuring your call remained uninterrupted. ## What it does NOT cover - Does not cover systems where the portable unit cannot automatically adjust its transmitting frequency based on the strongest received signal (Claim 2). - Does not cover systems without multiple, smaller "receiver sites" distributed within a larger base station coverage area (Claim 1). - Does not cover systems that lack a "scanning means" in the portable receiver to sequentially check different outgoing signalling channels (Claim 2). - Does not cover systems where the base station does not compare signal strengths from its various receiver sites to determine the strongest signal from a portable unit (Claim 1). - Does not cover portable units with a transmission range equal to or greater than the base station transmitter range (Claim 2). ## The clever bit The novelty lies in the system's ability to automatically hand off a portable unit between different base station coverage areas by continuously monitoring signal strength and retuning both the portable unit and the base station connection. This also includes dynamically reducing the portable unit's power to save battery and minimize interference. ## Real-world examples 1. Early analog cellular networks (1G) 2. Modern cellular networks (2G, 3G, 4G, 5G) 3. Mobile phone handovers between cell towers 4. Dynamic power control in mobile devices ## Why it matters This patent, assigned to Motorola, describes foundational technology for cellular communication. Martin Cooper, one of the inventors, is widely recognized for making the first public handheld cellular phone call in 1973. The system's ability to manage calls across different base stations and optimize portable unit power was crucial for making mobile phones practical and user-friendly, laying the groundwork for modern cellular networks. ## Frequently asked questions ### What does How Early Cell Phones Handled Calls Across Different Towers cover? This patent describes a system for early portable phones to automatically find the strongest signal from a base station and switch channels as the user moves, reducing battery drain and interference. ### Who owns patent US 3906166? Motorola Inc owns this patent, granted in 1975. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 3906166 cited by? This patent has been cited by 206 later patents that build on its ideas. ### What problem does this patent solve? This patent, assigned to Motorola, describes foundational technology for cellular communication. Martin Cooper, one of the inventors, is widely recognized for making the first public handheld cellular phone call in 1973. The system's ability to manage calls across different base stations and optimize portable unit power was crucial for making mobile phones practical and user-friendly, laying the groundwork for modern cellular networks. ### What does this patent NOT cover? Does not cover systems where the portable unit cannot automatically adjust its transmitting frequency based on the strongest received signal (Claim 2). **Full plain-English explainer:** https://patentbrief.org/patent/us/3906166/cellular-mobile-phone-radio-telephone **Original patent:** https://patents.google.com/patent/US3906166 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Phones Automatically Find and Update Network Lists Using CDPD](https://patentbrief.org/patent/us/5920821/use-of-cellular-digital-packet-data-cdpd-communications-to-convey-system-identification-list-data-to-roaming-cellular-subscriber-stations) — A 1995 system for helping mobile phones automatically update their preferred network lists and find the best service provider using a data-only channel. - [How Bluetooth Creates Wireless Networks with Unique Addresses](https://patentbrief.org/patent/us/6590928/bluetooth-frequency-hopping) — This 2003 patent describes how Bluetooth devices use a master device's address and clock to create a unique, hopping radio channel for communication and build a network map. - [How to Seamlessly Switch Video Streams for Many Viewers](https://patentbrief.org/patent/us/6732183/video-and-audio-streaming-for-multiple-users) — This patent describes a computer system that allows an administrator or viewer to smoothly switch between different video or audio sources for many people watching at the same time, without interrupting their viewing experience. - [How Almon Strowger Invented the Automatic Telephone Switch](https://patentbrief.org/patent/us/447918/strowger-automatic-telephone-exchange) — An 1891 patent for an automatic telephone exchange that allowed callers to connect to each other without needing a human operator. - [How Marconi Patented Early Wireless Telegraphy Signals](https://patentbrief.org/patent/us/586193/radio-wireless-marconi) — Guglielmo Marconi's 1897 patent for sending electrical signals through the air to enable early wireless communication.