When using the baseband service to send passband signals, it seems that using a preamble code is not supported. Am I just not proficient enough in using Unetstack, or is it that in the communication process implemented by Unetstack, baseband signals are generally used instead of passband signals because baseband signals have a lower sampling rate and are more cost-effective to represent?Does this also mean that in certain scenarios of using UnetStack for communication, if we want to send a passband signal, we can first convert it to a baseband signal, send it through the baseband service, and then at the receiving end, convert the received baseband signal back to a passband signal? Is my understanding correct?
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The baseband representation is more efficient for representing communication signals and is mostly the recommended approach for transmitting and receiving arbitrary signals using UnetStack. In practice, you don't lose anything by converting a passband signal to baseband and then transmitting, as most of the modems will only be able to transmit in the communication band and any signal in the communication band can be represented as a baseband signal faithfully.
The reason preambles are not supported in passband is that they are defined in baseband and inserted before passband conversion. When you pass in a passband signal directly, the conversion is skipped and so the preamble cannot be inserted for you. You can, however, include a passband version of the preamble manually inside your passband signal, and it will be detected on the peer modem.