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RZ vs NRZ | Comparison between RZ and NRZ Line Coding

This page compares RZ vs NRZ and mentions comparison between RZ and NRZ pulse shapes. The RZ and NRZ line coding is used for digital communication and optical Duobinary transmission system.

Introduction:
There are two types of signals viz. digital and analog. Digital signals are represented in the form of binary zeros and ones. Line coding techniques such as unipolar, polar and bipolar are used to transmit digital data using digital signals. The RZ and NRZ pulse shapes are used in these techniques to minimize ISI (Inter Symbol Interference) by avoiding distortion and overlapping of consecutive pulses.

What is Line Coding ?

Line coding is a technique which represents digital data into digital signals. Using this coding, it is possible to map sequence of bits into digital signal.

Line encoder and Line decoder
Figure-1 : Line Encoding and Decoding

Line encoder does line coding at the sender side as shown in figure-1. The same digital signal is decoded at the receiver side by line decoder. Line encoder converts digital data to digital signal format and line decoder does the reverse.

Following are line coding properties.
• It reduces required bandwidth by transmitting many bits using single signal pulse.
• It makes system power efficient for given bandwidth.
• It reduces error probability.
• Long string of ones and zeros are avoided.
• Technique such as bipolar coding has error correction capability.

Let us understand Unipolar, Polar and Bipolar signaling types.
• Unipolar : In the unipolar signaling type, binary one ('1') is encoded as presence of pulse where as binary zero ('0') is encoded as absence of pulse. Hence it is known as "ON-OFF Keying". It is categorized into unipolar NRZ and unipolar RZ types as described below.
• Polar : In the polar signaling type, one ('1') is encoded as positive pulse where as zero ('0') is encoded as negative pulse. It is further categorized into polar NRZ and polar RZ types.
• Bipolar : In the bipolar signaling type, there are three voltage levels viz. positive, negative and zero. Binary '0' is encoded as neutral zero voltage. Binary '1' is encoded either as positive pulse or as negative pulse based on its alternate positions. It is also called Duobinary signal. Binary ones are mapped by alternatine positive and negative voltages. Hence it is called as Alternate Mark Inversion (AMI). It is further categorized into Bipolar NRZ and Bipolar RZ.

NRZ Pulse Shape | Non Return to Zero

In NRZ line coding, binary 1 is represented by positive voltage and it does not return to zero during its bit period T0 where as binary 0 is represented by zero voltage.

Unipolar NRZ

Unipolar NRZ
Figure-2 : Unipolar NRZ coding

As shown in the figure, in unipolar NRZ, binary '1' is represented by pulse with 'V' amplitude and binary '0' is represented by absence of any pulse.

Polar NRZ

Polar NRZ
Figure-3 : Polar NRZ coding

As shown in the figure, in polar NRZ, binary '1' is represented by positive pulse with 'V' amplitude and binary '0' is represented by negative pulse with 'V' amplitude.

Bipolar NRZ

Bipolar NRZ
Figure-4 : Bipolar NRZ coding

As shown in the figure, in bipolar NRZ, binary '0' is represented by zero voltage level. Every alternate binary ones are represented by sequence of positive and negative pulses with the same amplitude 'V'. Here pulse duration and symbol bit duration are equal.

Advantages of NRZ line coding

Following are the benefits or advantages of NRZ line coding.
➨It is simple line coding technique than RZ type as pulse does not return to zero while mapping binary data (1's and 0's).
➨For unipolar NRZ signaling less bandwidth is needed.
➨In polar NRZ and bipolar NRZ, no low frequency components are present in the signaling waveforms after mapping.

Disadvantages of NRZ line coding

Following are the drawbacks or disadvantages of NRZ line coding.
➨Presence of low frequencies may cause droop in the signal waveforms.
➨No error correction is done.
➨Long string of ones and zeros lead to loss of synchronization.
➨No clock is available.

RZ Pulse Shape | Return to Zero

In RZ line coding, pulse which represents 'binary signal' returns to zero or ground potential after the half bit period.

Unipolar RZ

Unipolar RZ
Figure-5 : Unipolar RZ coding

As shown in the figure, binary '1' is represented by pulse with high to low transition. Here during entire bit period, initially pulse remains high in the first half period and returns to zero in the next half bit duration. Binary zero is represented as absence of pulse.

Polar RZ

Polar RZ
Figure-6 : Polar RZ coding

As shown in polar signaling, binary 1 is mapped by pulse with positive to negative transition and binary '0' is mapped by pulse with negative to positive transition. Here for binary '1' initially for first half bit period pulse remains high and for second half bit period it remains low. For binary '0' initially for first half bit period, pulse remains low and for second half it remains high.

Bipolar RZ

Bipolar RZ
Figure-7 : Bipolar RZ coding

In this signaling type, binary 0 is represented as zero voltage. Alternate Binary ones are represented by positive and negative pulses with transition in the center from High->Low and Low->High respectively. For example, binary data pattern is say 101001110. Here first binary '1' is represented by pulse with positive voltage in the first half bit period and than returns to zero in the next half bit period. Next is binary 'zero' which is represented by no voltage for the entire bit duration. Next is binary 'one' which is represented by pulse with negative voltage in the first half and returns to zero voltage in the next half bit period. The mapping continues for alternate ones in the digital data. Here pulse duration is half of symbol bit duration compare to Bipolar NRZ type.

Advantages of RZ line coding

Following are the benefits or advantages of RZ line coding.
➨It is simple line coding technique.
➨In polar RZ and bipolar RZ, no low frequency components are present.
➨Bipolar NRZ/RZ signaling waveform occupies lower bandwidth than unipolar NRZ and polar NRZ waveforms.
➨Signal drooping does not happen in Bipolar coding. Hence this line coding is suitable for data transmission over AC coupled lines.
➨Single error detection is possible using this line coding technique.

Disadvantages of RZ line coding

Following are the drawbacks or disadvantages of RZ line coding.
➨Signal droop occurs when signal is non-zero at 0 Hertz.
➨Unipolar/Polar RZ occupy twice bandwidth than Unipolar/polar NRZ respectively.
➨No error correction.
➨No clock is available.
➨Loss of synchronization occurs due to long string of ones and zeros in the binary data.


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Line coding techniques

Difference between Unipolar Polar and Bipolar coding    RZ vs NRZ vs Manchester coding    RZ vs NRZ pulse shapes    Advantages and disadvantages of NRZ encoding    RZ encoding    2B1Q coding    8B6T coding    4D PAM5 coding    MLT-3 coding    4B/5B encoding    8B/10B encoding    R8ZS scrambling    HDB3 scrambling   

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