With the advancement of communication technologies, modern digital communication systems have replaced analog modulation-based systems. Digital communication systems offer significant advantages over analog systems, including:
Pulse Modulation Power Efficiency: In pulse modulation, the transmitted power is concentrated into short pulses, unlike the continuous transmission in analog modulation.
Multiplexing Capability: The gaps between pulses can be filled with pulses from other message signals, allowing multiple information signals to be sent over a single communication system.
Advancements in Integrated Circuit Technology: Rapid developments in integrated circuit technology have made the implementation of digital communication circuits increasingly easier.
Improved Noise Immunity: Digital systems exhibit superior resistance to noise compared to analog systems.
In digital communication systems, information is typically in an analog form, such as voice or images. The first step in digital communication is converting this information into digital pulses. These pulses are transmitted from the sender and then converted back into analog information at the receiver.
Various modulation methods are used to prepare analog information for digital transmission, each with its corresponding demodulation system. Common digital communication systems include:
PAM (Pulse Amplitude Modulation): Modulates the amplitude of pulses to represent the information signal.
PCM (Pulse Code Modulation): Encodes the amplitude of the analog signal into a series of coded pulses.
PWM (Pulse Width Modulation): Modulates the width of pulses to convey information.
PPM (Pulse Position Modulation): Modulates the position of pulses relative to a reference to encode information.
ASK (Amplitude Shift Keying): Modulates the amplitude of a carrier signal to represent digital data.
FSK (Frequency Shift Keying): Modulates the frequency of a carrier signal to transmit digital information.
PSK (Phase Shift Keying): Modulates the phase of a carrier signal to encode data.
Delta Modulation: Encodes the difference between successive samples of the analog signal.
QPSK (Quadrature Phase Shift Keying): A form of PSK that uses four distinct phase shifts to represent data.
Bit: A bit is an electrical signal representing binary information, typically with a digital '1' indicating the presence of voltage and a digital '0' indicating the absence of voltage. Each '1' and '0' in an information signal corresponds to one bit. Eight bits constitute one byte (B). For example, the signal '1001000011111010' is 16 bits or 2 bytes.
Bits Per Second (bps): The rate of information transmission is measured by the number of bits transmitted per second, denoted as bps.
Baud: This term is commonly used to express the signaling rate of devices like modems. It represents the number of signal units transmitted per second. For instance, if a device sends information coded with 2 bits per signaling unit, 1 baud equals 2 bits.
Baud Rate: The baud rate is the number of signal changes or symbols transmitted per second over a communication channel. In the RS-232 standard, which operates on a one-bit-per-signal-change principle, a baud rate of 9600 corresponds to transmitting 9600 data bits per second. If each bit requires two signal changes (as in NRZ coding), a baud rate of 9600 would result in transmitting only 4800 bits per second.
Bit Error Rate (BER): In digital communication, BER refers to the ratio of incorrectly received bits to the total number of transmitted bits.
Channel: The medium through which information is sent to the receiver is called the channel. In modern data transmission, twisted pair cables (UTP-STP), fiber optic cables, and wireless communication are commonly used.
Channel Capacity: This term denotes the maximum number of bits that can be transmitted through a channel.
Noise: Various types of noise and their formulas are discussed in analog communication topics. In digital communication, noise can be categorized into two groups: internal (system-generated) and external (environmental).
Encoding: Digital encoding defines how data bits are represented in the physical communication medium. An effective digital encoding technique should meet the following criteria:
Bandwidth Efficiency: Utilize minimal bandwidth to allow multiple signals to be transmitted simultaneously over the communication channel.
Low DC Level: Maintain a low direct current (DC) level to reduce attenuation over long distances, as high DC levels are more susceptible to signal degradation.
Polarity Independence: Ensure the signal is not affected by the physical characteristics of the transmission medium, such as when transmitted over a two-wire cable.
Encoding Methods:
NRZ (Non-Return to Zero): In this basic method, '0' bits are represented by 0V, and '1' bits by a positive voltage.
RZ (Return to Zero): '0' bits are represented by 0V, while '1' bits are represented by a positive voltage for the first half of the bit duration and 0V for the second half.
NRZI (Non-Return to Zero Invertive): A '0' bit is represented by no change in voltage. A '1' bit is represented by a change in voltage: if the previous voltage was 0V, it changes to positive; if it was positive, it changes to 0V.
AMI (Alternate Mark Inversion): '0' bits are represented by 0V, while '1' bits are represented alternately by positive and negative voltages.
PE (Phase Encode, Manchester): '0' bits are represented by a positive voltage in the first half of the bit duration and a negative voltage in the second half. '1' bits are represented by a negative voltage in the first half and a positive voltage in the second half.
These encoding methods are fundamental to digital communication systems, each offering unique advantages depending on the application and transmission requirements.
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