COMMUNICATION SYSTEM

When someone sends a message to a remote person, say voice, this message will be transmitted over the media as an electric signal. The device that performs this transformation when the transmitted message is a voice will be a microphone. This is the first step for transmitting the message by the communication system. This electric message is a voltage signal which varies with time.
Once the electric message is produced, the message is now prepared for processing by the communication system. The first step for processing may be amplification if the signal is weak. Note that the resulting signal is now analog meaning that it changes in continuous range. The modern systems must first digitize the signal before transmitting it. this have many advantages including easy of implementing the electronic circuits, and less effect for the noise. The digitization process is done by processes called sampling and quantization. Sampling mean to send just samples of the signal at regular interval, while quantization mean to approximate theses samples to the nearest point in the interval to which it belongs. After approximating the samples to predetermined points, it is converted to binary signal by encoding it(transferring it to bits). This will produce pulse signals to be sent.
A common process called modulation must be done first on the binary pulses before sending it. This process has many purposes, but the most important one is to be radiated by the antenna more efficiently. There is a law in antenna theory which states that the length of the antenna must be comparable with wavelength of the signal. The frequency of the voice is in the range of 200-3400 Hz. Thus it will have a large wavelength and the antenna will be large. By modulating the signal with a high frequency, the resulting signal will have lower frequency and the antenna will be small which will be practical to implement.
The resulting modulated signal is now ready to be sent. This signal is fed to the antenna processing subsystem to be radiated by the antenna. Note that if the signal is sent over a wire, it can be sent without modulation or in the baseband. The modulation process is needed only when antenna is used for transmission.
Once the signal is received at the receiver the operations is performed at the reverse order until the original signal is received.