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In a duplex radio communication system, where you have two walkie-talkies trying to communicate with each other through a repeater, it is possible for both walkie-talkies to receive signals from the repeater but not hear each other when they are in close proximity. This situation can occur due to the way duplex communication systems are designed and the concept of the "duplex offset."
Here's an explanation of why this happens: 1. Duplex Communication and Frequency Offset: In a duplex communication system, one radio (walkie-talkie) transmits on one frequency while the other radio receives on a different frequency. This is necessary to avoid interference because a single radio can't transmit and receive on the same frequency simultaneously. 2. Duplex Offset: To prevent interference and allow for simultaneous communication, there is typically a frequency offset between the transmit and receive frequencies. This offset ensures that the two radios don't interfere with each other while communicating through a repeater. 3. Communication through the Repeater: When you transmit from one walkie-talkie to the repeater, the repeater receives your transmission on one frequency and then retransmits it on another frequency. The second walkie-talkie receives the repeater's transmission on this different frequency. 4. Close Proximity Issue: When both walkie-talkies are very close to each other, their own transmissions can interfere with each other due to "near-far" interference. The transmitted signal from one walkie-talkie can be strong enough to saturate the other walkie-talkie's receiver, making it unable to hear the distant transmission from the repeater. To address this issue and allow for simultaneous communication between the walkie-talkies in close proximity, some advanced systems use technologies like Time-Division Duplex (TDD) or digital signal processing to manage transmissions more effectively. These systems allow the walkie-talkies to switch quickly between transmit and receive modes, reducing the chances of self-interference. However, in traditional analog duplex systems, when both walkie-talkies are very close together, they may have difficulty hearing each other due to the near-far interference problem. In such cases, it's often better to communicate directly on a simplex frequency without using the repeater when you are in close proximity.