The spectrum emission mask of a transmitter can be broken up as follows:

Necessary bandwidth = width of the frequency band which is theoretically just sufficient to ensure the transmission of information.
Occupied bandwidth = width of the frequency band such that, below the lower and above the upper frequency limits, the mean powers emitted are each equal to a specified percentage β/2 of the total mean transmitted power. The value of β/2 shall be taken as 0.5%.
Out-of-band emission = emission on frequencies immediately outside the channel bandwidth which results from the modulation process and non-linearity in the transmitter, but excluding spurious emissions.
Spurious emission = emission on frequencies which are outside the channel bandwidth and the level of which may be reduced without affecting the corresponding transmission of information, thus excluding out-of-band emission.
Unwanted emissions = consist of out-of-band emission and spurious emission.

Figure 376: Illustration of a spectrum emission

The necessary bandwidth shall be considered to determine the maximum co-channel interference level that is acceptable by the receiver and the occupied bandwidth shall be considered to determine in which part of the channel 99% of the transmitted power is concentrated.

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