Introduction: Line coding is the process which converts digital data to digital signals. It converts sequence of bits to a digital signal suitable to be transmitted over the channel. The line coding helps to make digital communication system bandwidth and power efficient. Moreover it helps in synchronization at the receiver by eliminating lons string of ones and zeros.

There are different types of line coding which include unipolar, polar, bipolar, multilevel (2B1Q, 8B6T, 4D-PAM5) and multitransitional (MLT-3) etc.

## What is Bipolar Line Coding?

This encoding technique has three voltage levels to represent binary data which are positive, negative and zero or DC level. This signal is also known as duo-binary signal or bipolar. There are three types of bipolar coding used for data communication viz. AMI, B8ZS and HDB3.

As shown in the figure, in AMI form of Bipolar encoding, binary zero ('0') is represented by 0 voltage where as binary one ('1) is represented by alternating positive and negative voltages. The Bipolar coding technique uses different types of pulses based on which it is known as NRZ (Non Return to Zero) or RZ (Return to Zero).

Following parameters of Bipolar coding ('AMI') are used for comparison with other line codes.
• Transmission of DC component: No
• Signaling Rate : 1/Tb
• Noise Immunity : High
• Synchronizing capability : Very Good
• Bandwidth required : 1/2Tb
• Crosstalk : Low

Bipolar line coding

As shown, Bipolar NRZ coding uses zero voltage to represent binary zero and +ve pulse and -ve pulse to represent alternating binary ones during entire bit period ('Tb'). Hence pulse duration and symbol bit duration are equal in NRZ type.
Example of Bipolar NRZ coding :
Input : Binary data [1 0 1 0 0 1 1 1 0]
Output : [+ve pulse, DC voltage, -ve pulse, DC, DC, +ve pulse, -ve pulse, +ve pulse, DC ]

As shown, Bipolar RZ coding uses same DC to map binary zero and +ve pulse or -ve pulse with transition to DC at the center of the bit period is used to map binary one. Hence pulse duration is half of symbol bit duration in RZ type.

Example of Bipolar coding (RZ) :
Input : Binary data [1 0 1 0 0 1 1 1 0], Bit period = Tb
Output : [+ve pulse (for Tb/2 duration) DC (over Tb/2), DC voltage (Tb),
-ve pulse (Tb/2), DC(Tb/2), DC(Tb), DC(Tb), +ve pulse (Tb/2) DC (Tb/2),
-ve pulse (Tb/2), DC (Tb/2), +ve pulse (Tb/2), DC (Tb/2), DC (Tb) ]

## Benefits or advantages of Bipolar Line Coding

Following are the benefits or advantages of Bipolar Line Coding:
➨It is very simple line coding technique to implement.
➨Low frequency components are not present in the bipolar encoded signal.
➨It occupies low bandwidth than unipolar and polar NRZ types.
➨Signal droop issue is not observed in bipolar signal and hence it is suitable for transmission over AC coupled lines.
➨Single error detection is possible using bipolar coding technique.

## Drawbacks or disadvantages of Bipolar Line Coding

Following are the drawbacks or disadvantages of Bipolar Line Coding:
➨No clock signal is present for use.
➨Long string of binary data with continuous 1's and 0's causes loss of synchronization.