Communication systems: analog & digital communication system

Definiton of analog system and digital communication system
* source: Digital and Analog Communication Systems(7th) by Leon W. Couch
     - an analog information source produces messages that are defined on a continuum.
     - a digital information source produces a finite set of possible messages.
     - an analog communication system transfers information from an analog source to the intended receiver.
     - a digital communication system transfers information from a digitla source to the intended receiver.

Communication system block-diagram model
     - regardless of the particular application, all communications systems involve 3 main sub-systems: the transmitter, the channel, and the receiver.

     - source: the message information(m(t)) may be in analog or digital form, depending on the particular system, and it may represent audio, video, or some other type of information.
     - Tx signal processor: the signal-processing block at the transmitter conditions the source for more efficient transmission. For example, in an analog system, the signal processor may be an analog LPF. In a hybrid system (analog system + digital system), the signal processor may be an analog-to-digital converter (ADC).
     - Tx carrier circuit: it converts the processed baseband signal into a bandpass that is appropriate for the tranmission medium of the channel. If the channel propagates baseband signals, no carrier circuits are needed.
     - Channel: Channels may be classified into 2 categories: wire and wireless.
     - Rx (carrier circuit + signal processor): the receiver takes the corrupted signal at the channel output and converts it to a baseband signal that can be handled by the receiver baseband processor.

* FYI: digital communication system
     - digital communication system의 경우, transmitter 내부의 연산과정 중 ADC(in general, signal processor)을 통과한 digital modulating signal은 carrier circuit을 통과한 뒤 channel로 들어간다. 여기서 carrier circuit을 통과한 modulated signal은 신호가 가진 내용은 digital 이며, 신호의 waveform은 analog 형태이다. 결국 외향적으로 전송되는 signal의 형태는 analog signal이든 digital signal 이든 모두 analog waveform 이다. [refer. digital modulated signal: binary & multilevel modulating signal]

     * digital signal의 경우, ADC에서 이루어지는 좀더 상세한 signal processing 과정은 'Sampler-Quantizer-Encoder' 이며, 최종 단계 encoder 이후의 신호의 형태는 내용적으로 0 or 1의 형태이며, 다양한 modulation scheme에 따라 다양한 waveform으로 전송이 된다.
     * digital signal의 경우, AD conversion 에 대한 개략적인 개념적 접근은 "digital and analog communication systmes (specific chap. 3)"에서 알아본다. "digital signal processing" 에서는 이런 조작과정 결과물(digital signal) 과 이를 처리할 system(digital system) 사이에서 일어나는 다양한 부분을 조금더 detail하게 알아본다.