High Level and Low Level Modulation
You know for wireless communication is suitable for long distance communication. In wireless, for data transmission modulation become essential to avoid interference and to reduce antenna size significantly. Especially, in modulation process, we translate the low frequency baseband signal to higher frequency by modulating with high frequency carrier signal. For a typical communication system we generate the high frequency (carrier) signal by using local oscillator. Source signal or message signal is modulated with local oscillator. Then modulated signal is transmitted thru antenna.
Low Level Modulation
In low level modulation, message signal is modulated with local oscillator that produces high frequency. Then the frequency of message signal is translated to much higher frequency. Then the modulated signal passes thru wideband amplifier.
High Level Modulation
In high level modulation, source or message signal is passed thru wideband amplifier, then the signal is modulated with local oscillator.
What is Wideband Amplifier
For often, signal is amplified in communication process so that signal can reach at receiver with acceptable energy level. For the name 'wideband amplifier' we can primarily guess that it can pass signal with large bandwidth. In case of modulation process, i.e., for amplitude modulation (AM), we know after modulation it contains upper side band and lower side band at both side of carrier frequency. That posses large amount of bandwidth. So, it is essential to pass thru wideband amplifier so that all available frequencies can pass thru amplifier and get amplified.
MATLAB Code for Comparison of High Level and Low Level Modulation