| Narrow band modem |
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The more and more bigger need for new frequencies and the saturation on the current bands force the administration to reduce the channel bandwidths for increasing their numbers. Thus, since several years, the channel bandwidth has passed from 100kHz to 50, 25 and 12.5 kHz now. People are talking about 8.33 kHz (25/3) and the trend will continue. For the data transmission, the necessary bandwidth depends on the desired transmission speed, of the mode and the used modulation index. Also, for transmitting faster with bandwidth more and more narrow, new modulation process had appeared such as FFSK, GMSK, DPSK, QPSK, etc… For instance, a GMSK modulation doesn't allow to go over 9,600 bauds for a 25kHz bandwidth. |
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