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Baseline wandering in digital transmission | Line coding to eliminate

This page describes Baseline wandering issue found in digital transmission of data. It mentions line coding techniques used to eliminate baseline wandering.

Introduction: For data communication, digital data in the form of binary ones and zeros are represented by digital signals (i.e. pulse shapes) before transmission over channel. Based on line coding techniques, this digital data is mapped in different signal waveforms.
Unipolar : Binary one is encoded as presence of pulse and binary zero as absence of pulse. It is called "ON-OFF Keying".
Polar : Binary '1' is encoded as positive pulse with 'V' amplitude where as binary '0' is encoded as negative pulse with 'V' amplitude. Bipolar : Binary '0' is encoded as DC voltage where as Binary '1' is encoded as either positive pulse of negative pulse based on its alternate positions. It is called AMI.

All these line coding techniques are further divided into NRZ or RZ types based on pulse used to represent binary bit. In NRZ, binary data is represented by pulse with high or low level throughout the bit duration. In RZ, pulse remains at high (+V) or low level (-V) for half bit duration and than returns to zero for the rest of the half bit duration. The transition occurs exactly at the center of the bit period.

Following figure-1 depicts unipolar NRZ signal waveform and its normalized power calculation. Normalized power is double in unipolar NRZ compare to polar NRZ. Due to this reason, NRZ is not used in data communication domain.

Unipolar NRZ power

Following figure-2 depicts polar NRZ signal and its sub types viz. NRZ-L and NRZ-I. As shown, voltages on both side of the time axis is used.

Polar NRZ-L and Polar NRZ-I

As shown, in NRZ-L also known as NRZ-Level, level of the voltage determines value of bit. In NRZ-I also known as NRZ-Invert, change or lack of change in the level of voltage determines value of bit. If there is no change than the bit is 0 and when there is a change, the bit is 1.

What is Baseline Wandering

Average signal power at the receiver is taken as "baseline". Value of data element at the receiver is determined against this baseline reference. If there are long string of ones and zeros in the data, it drifts baseline and makes it tough for the receiver to decode the data correctly. This drift in the baseline is known as "baseline wandering".

Let us understand issue of Baseline wandering in NRZ-L and NRZ-I.
➨Due to long string of 0's and 1's in NRZ-L, average power becomes high and receiver finds it difficult to differentiate bit value.
➨In NRZ-I, long sequence of 0's cause the baseline wandering.

Line coding technique to eliminate baseline wandering

Manchester and differential manchester techniques are used to eliminate baseline wandering. Manchester encoding combines concept of RZ and NRZ-L. In Manchester encoding, bit duration is divided into two halves. In the first half, voltage is at one level and in the second half it is at the other level. Transition at the center of bit period helps in synchronization.

Manchester vs Differential Manchester

Differential Manchester encoding combines RZ and NRZ-I. There is transition at the center of the bit period. Moreover bit values are determines at the beginning of the bit. If next bit is zero ('0'), there is transition and if next bit is one ('1') there is no transition.

Manchester line coding overcomes issues observed in NRZ-L where as differential Manchester overcomes that observed in NRZ-I.

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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