| 摘要 |
Spread Aloha Multiple Access (SAMA) or other multiple access hubs provide feedback to each RTU on the frequency error in its transmission, so that each RTU can change its frequency output to the correct transmission frequency. At any time, and especially when a RTU has not transmitted for a time or is not transmitting, or during transmission, the hub requests that an RTU broadcast a pure tone of frequency. That pure tone produces a beat with the hub's detector. The hub filters the beat frequency and passes it to a comparator that produces a signal waveform. The period of the waveform is measured by the hub. The hub calculates the error in the RTU's carrier and sends a correction value for the RTU to use when transmitting. The hub measures slow drift in the carrier during continued transmission from a RTU and provides corrections to the RTU on the fly.
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