800 MHz Trunked Radio Frequency Offset for Unauthorized Access Prevention
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Solution Overview
Problem
Unauthorized devices, such as stolen or reprogrammed radios, can easily operate on 800 MHz trunked radio communications systems, posing a security concern by accessing channels 559 to 320, which existing systems fail to effectively prevent.
Innovation Solution
Implementing a method where receiving devices calculate and tune to specific transmit and receive frequencies based on assigned voice channels using the formula TX frequency = Base Frequency + [(559-N)*Channel Spacing], preventing unauthorized access by misinterpreting channel grants for inactive channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If radios are made inexpensive and easily reprogrammable, then accessibility and ease of operation improve, but security and system reliability deteriorate due to unauthorized access
Solution Approach 1:
The patent changes the frequency calculation parameters by introducing an offset value that modifies how receiving devices calculate transmit and receive frequencies from channel numbers. This parameter change ensures that unauthorized devices using standard formulas will calculate incorrect frequencies and cannot access the system, while authorized devices with the correct offset can operate normally.
2Ease of manufacture
If standard frequency calculation methods are used, then device compatibility and ease of manufacture improve, but vulnerability to unauthorized access increases
Solution Approach 1:
The patent applies preliminary anti-action by pre-configuring authorized receiving devices with an offset value that counteracts the frequency calculation method used by unauthorized devices. This preemptive measure ensures that when unauthorized devices attempt to connect using standard formulas, they are blocked before they can cause harm, while authorized devices are already prepared with the correct compensation.
3Device complexity
If channel assignments are made sequentially without modification, then system simplicity and ease of operation are maintained, but security against reprogrammed radios is compromised
Solution Approach 1:
The patent modifies the frequency calculation parameter by introducing an offset value that is added to the channel number before calculating the transmit frequency. This simple parameter change maintains the sequential channel assignment structure while effectively blocking unauthorized devices that do not have the offset value, thus protecting the system without significantly increasing complexity.
Data Source
AI summary
A receiving device in a trunked radio communications system determines a transmit frequency, wherein the transmit frequency is calculated by subtracting a voice channel number from 559, multiplying by a channel spacing and adding a base frequency to yield the transmit frequency, if the voice channel number is between 559 and 320. The receiving device determines a receive frequency by 851.0250 MHz+[(559−a voice channel number)*12,500 Hz], if the voice channel number is between 559 and 320. In any case, the voice channel number is communicated in an outbound signaling message defined by Motorola's 3600-baud radio trunking protocol.