The VHF radio can overcome line-of-sight transmission by "repeaters," which pick up weak signals, amplify and retransmit them on a different frequency. However, repeaters can interfere with each other unless they are far enough apart or transmit on sufficiently separated frequencies. In this paper, we propose such a system using the continuous tone-coded squelch system (CTCSS) to mitigate interference problems. This system associates to each repeater a separate sub-audible tone that is transmitted by all users who wish to communicate through that repeater. The repeater responds only to receive signals with its specific tone.
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