Baseband Transmission

Introduction:

In telecommunications and signal processing, the term baseband is commonly used to refer to the signals having frequency range from 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting close to zero. Baseband signals can be described as lowpass or non-modulated signals, and modulated ones are termed as passband, bandpass, carrier-modulated or radio frequency (RF) signals

Baseband Transmission:

Digital transmission is the transmission of electrical pulses.  Digital information is binary in nature in that it has only two possible states 1 or 0.  Digital signals are infact sequences of bits encode data (e.g., text characters). Digital signals are commonly referred to as baseband signals. In order to successfully send and receive a message, both the sender and receiver have to agree how often the sender can transmit data (data rate). 

Digital baseband transmission is also known as line coding. It aims at transferring a digital bit stream over baseband channel, typically an unfiltered wire, contrary to passband transmission, also known as carrier-modulated transmission.Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel.

A baseband channel or lowpass channel is a communication channel that can transfer frequencies that are very near zero.

Line Coding:  Line coding is the process of converting of binary data, a sequence of bits into a digital signal.Line Coding of the bit stream is required to make its spectrum suitable for the channel response. Also to ensure the presence of frequency components to permit bit timing extraction at the receiver.

Different line coding schemes have been developed to use with different systems. Depending on the requirements of a systems a specific line coding scheme is adopted. Each line coding has its set of advantages and disadvantages. Basically Line codings are compared on the basis of following important characteristics

1. Signal level versus data level.
2. Pulse rate versus bit rate,
3. DC components.
4. Self synchronization

Most desirable line code must have following properties

• Transmission Bandwidth: It should be as small as possible
• Power Efficiency: It should also be as small as possible for a given Bandwidth and Probability of Error. 
• Error Detection: It should have Error detection as well as error correction capability.
• DC component: DC component must be zero. If the channel is AC coupled, the PSD of the line code signal should be negligible at frequencies near 0.
• Synchronization: It should carry adequate timing information must be able to self synchronize. it should also prevent long strings of ones and zeros

Different Line coding schemes and their specific properties will be discussed in next post